Medicine Lodge Thrust System, 

East-Central Idaho and 

Southwest Montana

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1031

Medicine Lodge Thrust System, 

East-Central Idaho and 

Southwest Montana

By EDWARD T. RUPPEL

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1031

A summary of geologic data on 

a major segment of the North 

American Cordilleran fold 

and thrust belt

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1978
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 77-600066

First Printing 1978 

Second Printing 1983

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

Washington, D.C. 20402 

Stock Number 024-001-03039-0
CONTENTS

Abstract ................................................

Introduction ............................................

Summary of earlier studies ...........................

Acknowledgments ...................................

Revised definition of the Medicine Lodge thrust system .....

The mapped location of the Medicine Lodge fault ......

Beaverhead Mountains ...........................

Lemhi Range ....................................

Stratigraphic changes across the Medicine Lodge thrust 

system ............................................

Precambrian X ..................................

Precambrian Y ..................................

Precambrian Z ..................................

Cambrian .......................................

Ordovician ......................................

Silurian .........................................

Devonian .......................................

Upper Paleozoic .................................

Mesozoic ........................................

Page

1

1

I

3

3

3

3

8

9

9

9

9

9

9

II 

11

11

12

Page

The Lemhi arch ......................................... 12

The Beaverhead Formation and the age of the Medicine Lodge 

thrust system ......................................... 14

Deformation in the allochthon ............................ 15

Some regional relations, implications, and problems ........ 16

The northward extension of the Medicine Lodge thrust 

system ............................................ 16

Thrust slices of lower plate rocks in the upper plate .... 16

The Beaverhead pluton and its relation to thrust faulting 16 

The relation of volcanic rocks and the thrust trace ..... 18

The relation of intrusive igneous rocks and associated

mineral deposits to the Medicine Lodge thrust system 18 

Petroleum and natural gas resources beneath the 

Medicine Lodge thrust system ...................... 20

Summary ............................................... 20

References cited ......................................... 21

ILLUSTRATIONS

FIGURE 1. Index map, east-central Idaho and southwest Montana ...........................................................

2. Map showing trace of Medicine Lodge thrust system, east-central Idaho and southwest Montana ...................

3. Map showing locations of allochthon and autochthon, Medicine Lodge thrust system ..............................

4. Map showing approximate location of Lemhi arch ..............................................................

5. Schematic diagram of Medicine Lodge thrust system and its relation to the Montana disturbed belt and the Sapphire 

tectonic block ............................................................................................

Page

2

4

10

13

17

TABLE

TABLE 1. Comparison of formations across the Medicine Lodge thrust system, east-central Idaho and southwest Montana

Page

6

in

MEDICINE LODGE THRUST SYSTEM,

EAST-CENTRAL IDAHO AND

SOUTHWEST MONTANA

By EDWARD T. RUPPEL

ABSTRACT

The Medicine Lodge thrust system, exposed in east-central Idaho 

and southwest Montana, is a major segment of the North America 

Cordilleran fold and thrust belt on which Precambrian and Paleozoic 

rocks have been telescoped and transported far east of their depositional area. The fault has overlapped rocks that initially were 

deposited in differing sedimentary environments on opposite sides of 

the northwest-trending geanticlinal Lemhi arch. The nature of the 

arch itself has been obscured by later thrusting, but it can be roughly 

reconstructed as a major landmass recurrently uplifted from Precambrian Y time at least through the Mesozoic. Although sections of 

the large landmass have been identified in the past as small islands, 

the landmass was instead a southern extension of Belt island, and it 

exerted a major influence on sedimentation patterns.

The thrust has obscured recognition of correlative sedimentary 

rock units. It also appears to have controlled the distribution of 

volcanic rocks in satellite centers of the main field of Tertiary Challis 

Volcanics. The thrust system has exerted a major influence on the 

emplacement of granodiorite -quartz monzonite stocks and related 

mineral deposits in east-central Idaho. Recognition of the fault as a 

major influence in intrusive activity and deposition of metallic 

mineral deposits suggests new possibilities for mineral resource exploration in this region. And consideration of the stratigraphic, 

paleogeographic, and structural framework of sedimentary rocks 

beneath the thrust suggests the possibility of petroleum and natural 

gas resources in the region north of the Snake River Plain.

INTRODUCTION

The Medicine Lodge thrust fault system of eastcentral Idaho and southwest Montana has long remained one of the least known segments of the North 

America Cordilleran fold and thrust belt, and its 

singularly significant role in confusing the geology of 

this part of the Rocky Mountains has not been understood. As a result, most stratigraphic studies in this 

region have failed to consider that great masses of sedimentary rocks have been telescoped and transported 

far east of their depositional sites; structural studies 

have not considered the pervasive effects of major 

thrusting; and the controlling influence of thrust faults 

on the emplacement of intrusive igneous rocks and related mineral deposits has not been recognized.

Recent field studies indicate more clearly the full significance of this major fault system, and suggest that 

most geologic problems in this region cannot be 

resolved without a clear understanding of Medicine 

Lodge thrusting. They show that the Medicine Lodge 

thrust system underlies much of southwest Montana 

and all of east-central Idaho, and that the possible overthrust distance, although uncertain, could be as much 

as 160 km.

SUMMARY OF EARLIER STUDIES

Kirkham (1927, p. 26-27) first mapped the trace of 

the Medicine Lodge fault near Medicine Lodge Creek, 

Idaho (fig. 1), north of the Snake River Plain, and 

clearly recognized its regional importance. Later the 

thrust system was traced into southwestern Montana 

by Sloss and Moritz (1951, p. 2160). Scholten, Keenmon, 

and Kupsch (1955, p. 382) mapped several thrusts in 

the Beaverhead and Tendoy Mountains, and traced the 

main Medicine Lodge thrust and its branches from the 

Idaho-Montana State line to Horse Prairie, Mont. Maps 

by Lowell (1965) and Myers (1952) extended other 

thrust faults from Horse Prairie northward into the 

Pioneer Mountains east of the Big Hole Basin. Later 

mapping by Staatz (1973) and by me in the present 

study, extended the Medicine Lodge thrust into the 

Beaverhead Mountains and the Lemhi Range. In all, 

the trace of the thrust system now has been mapped for 

a distance of more than 200 km.

At the time the fault was mapped near Horse Prairie, 

Lowell and Klepper (1953) also mapped and named the 

Beaverhead Formation, which they considered to be a 

syntectonic conglomerate related to thrust faulting. 

The tectonic significance of the Beaverhead Formation 

has recently been discussed in detail by Ryder and 

Scholten (1973), Ryder and Ames (1970), and Wilson 

(1970).

Stratigraphic and lithologic differences in rock units 

of similar age in the upper and lower plates across the

l
MEDICINE LODGE THRUST SYSTEM, EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

114 113

Gibbonsville 

{_ Big Hole 

Basin

Warm Springs Creek 

THOUSAND

EXPLANATION

. Approximate trace of Medicine 

Lodge thrust system, dashed 

where partly concealed or 

inferred, teeth on up thrust 

block

High-angle reverse fault;

arrow on upthrown block

Mountain areas

10

I I

20

I

30 40 KILOMETERS

I I

row indicates source <

direction of Medicin

Lodge thrust

FIGURE 1. Index map, east-central Idaho and southwest Montana. Base modified from U.S. Geological 

Survey, 1:500,000, Idaho, 1968, Montana, 1966. Mountain areas are patterned.
REVISED DEFINITION OF THE MEDICINE LODGE THRUST SYSTEM

thrust are of critical importance in understanding the 

magnitude of thrusting. Such differences are summarized in this report, and more detailed descriptions 

of the various formations are available in published 

reports by Sloss and Moritz (1951), Scholten, Keenmon, 

and Kupsch (1955), Myers (1952), Shannon (1961), 

Lowell (1965), Huh (1967), Mamet, Skipp, Sando and 

Mapel (1971), Mapel, Read, and Smith (1965), Luchitta 

(1966), Hait (1965), Ross (1961, 1962), Ruppel (1968, 

1975), Ruppel, Watts, and Peterson (1970), Ruppel, 

Ross and Schleicher (1975), and Embree, Hoggan, 

Skipp, and Williams (1975). Regional differences in 

lithology of Paleozoic rocks in east-central Idaho and 

adjacent Montana were discussed by Scholten (1957), 

who also showed some thickness data on isopachous 

maps. Ross (1962) summarized the Paleozoic stratigraphy of central Idaho in some detail.

Earlier studies of mining districts also have contributed information useful in understanding the 

effects of thrusting, for many mineral deposits are 

localized by the Medicine Lodge thrust. Additional information on these mining districts can be found in the 

following reports: Little Eightmile district (Thune, 

1941; Umpleby, 1913; Staatz, 1972,1973); Nicholia and 

Birch Creek districts (Shenon, 1928; Scholten and 

Ramspott, 1968; Anderson and Wagner, 1944); Gilmore 

and Spring Mountain districts, (Umpleby, 1913; Hait, 

1965; Ruppel and others, 1970); Blue Wing district 

(Callaghan and Lemmon, 1941); Dome district (R. A. 

Anderson, 1948; Ross, 1933, 1961); Pope-Shenon mine 

(Ross, 1925).

ACKNOWLEDGMENTS

A significant part of the necessary background for 

this summary of geologic data on the Medicine Lodge 

thrust system was provided by the works of Scholten 

(1968,1973), Scholten and Ramspott (1968), and Ryder 

and Scholten (1973). I am indebted to J. E. Harrison, 

M. R. Klepper, S. S. Oriel, C. A. Wallace, and I. J. 

Witkind for many discussions on the concepts 

developed in this paper, and to David A. Lopez for his 

help in mapping part of the trace of the fault. 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, Idaho.

REVISED DEFINITION OF THE 

MEDICINE LODGE THRUST SYSTEM

I propose that the name Medicine Lodge thrust 

system be applied to the major system of flat faults of 

eastern Idaho and southwestern Montana, a fault 

system that is a major segment of the North America 

Cordilleran fold and thrust belt, and that is comparable

to the Bannock system in southeastern Idaho and 

western Wyoming and to the Montana disturbed belt of 

west-central and northwestern Montana.

The known main trace of the Medicine Lodge fault 

system extends northward in the Beaver head Mountains from the margin of the Snake River Plain to the 

west margin of the Big Hole Basin, Mont. (fig. 2). It is 

also widely exposed in the central part of the Lemhi 

Range, west of the Beaverhead Mountains, as a result 

of uplift on younger, steep normal faults. The 

stratigraphic and petrologic changes across the thrust 

along its trace indicate that rocks from very different 

sedimentary facies have been tectonically overlapped 

(table 1). The rocks tectonically displaced in the thrust 

plate were deposited in Precambrian and Paleozoic geosynclines in what is now central and western Idaho. 

The rocks tectonically overlapped were deposited in a 

relatively shallow marine embayment or seaway east of 

the main Cordilleran geosyncline. Movement on the 

thrust probably occurred mostly in Late Cretaceous and 

Paleocene time and, as indicated by geologic and 

radiometric evidence, it was completed by early Eocene 

time. The total eastward displacement of the upper 

plate, or allochthon, possibly was as much as 160 km 

(Ruppel, 1975, p. 16).

THE MAPPED LOCATION OF 

THE MEDICINE LODGE FAULT

BEAVERHEAD MOUNTAINS

In the Beaverhead Mountains, the long but interrupted trace of the Medicine Lodge fault is known 

mainly from reconnaissance mapping, supplemented in 

only a few areas by more detailed geologic maps. In the 

vicinity of Medicine Lodge Creek, Idaho, in the 

southernmost Beaverhead Mountains, Carboniferous 

rocks of the White Knob Limestone are thrust over syntectonic red conglomerate of the Beaverhead Formation, and over Paleozoic and Mesozoic sedimentary 

rocks (Scholten and others, 1955, p. 382; Ryder and 

Scholten, 1973, p. 783).

Farther north in Medicine Lodge Creek, Mont., near 

Maiden Peak, and near Bloody Dick Creek (Scholten 

and others, 1955; M'Gonigle, 1965; Coppinger, 1974, p. 

167) (figs. 1, 2), sedimentary rocks of the Lemhi Group 

of Precambrian Y age (Ruppel, 1975, p. 15-16) are 

thrust over Precambrian X crystalline metamorphic 

rocks.

The Medicine Lodge thrust fault has been recognized 

(M. H. Staatz, oral commun., 1973) and mapped in 

reconnaissance in Black Canyon, in the Beaverhead 

Mountains west of the upper part of Horse Prairie 

Creek, and mapped as the Peterson Creek fault on the 

west side of the Beaverhead Mountains (Staatz, 1973).
4 MEDICINE LODGE THRUST SYSTEM, EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

114°00' 45, 30- 15/ 113«00' 45' 112°30'

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fVYbEN CREEK^--'--.--'St'STOfc» B'_^^=^^^r^T^r^^y^^;^:^ -?^miijm *^m$$^ m//;mmmm 

;'/.•':•;v-: ; /:!^fej•'•'•:•''••'•'•'•:•'-'•.•• CREEK'^ 

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CANYON district '/. 

'•' .•.'.•-STOCK ;
REVISED DEFINITION OF THE MEDICINE LODGE THRUST SYSTEM

EXPLANATION

Tertiary Challis Volcanics and 

Medicine Lodge Volcanics 

of Scholten and others, 1955

Tertiary granodiorite and 

quartz monzonite

Silurian and Ordovician Beaverhead 

granitic pluton, and other granitic 

rocks

Palaozoic rocks

Swauger Formation and 

Lamhi Group

Tertiary Challis Volcanics, Precambrian

Trace of Medicine Lodge thrust system 

(teeth on overthrust plate), dashed 

where inferred, at shallow depth, or 

concealed by Challis Volcanics or 

surficial deposits

High-angle reverse fault, arrow 

on upthrown block

Range-front normal fault, shown 

only in cross section, location 

approximate, arrows show 

direction of movement

High-angle reverse fault as shown on 

cross sections, location approximate, 

arrows show direction of movement

Area of brecciation and shearing

Paleozoic rocks, Pracambrian Swauger 

Formation and Lemhi Group

'Of Scholtin and othirs, 1955 VERTICAL EXAGGERATION x2

C

METERS

4500-

30OO1500-

0-

LEMHI

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MEDICINE

YV

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BEAVERHE

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THRUST *"

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VERTICAL EXAGGERATION *2

FIGURE 2. (left and above). — Generalized geologic map of the Medicine Lodge fault trace, east-central 

Idaho and southwest Montana, showing associated stocks, plutons, and mining districts; with 

schematic cross sections A—A', B-ff-ff', and C-C", showing interpreted relations at depth of 

Medicine Lodge thrust. Younger normal faults that break the Medicine Lodge thrust are known or 

inferred from geologic field studies, but are not shown on the map. Datum of cross sections is mean 

sea level. Base modified from U.S. Geological Survey 1:250,000, Dillon, 1955, Dubois, 1955.
TABLE 1. — Comparison of formations across the Medicine Lodge thrust system, east-central Idaho and southwest Montan

[Figures following formation descriptions are approximate thicknesses, in meters]

Age

Mesozoic

Paleozoic

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian

Mississippian

Autochthon

Bannack — Argenta area 

Colorado Formation 

sandstone and shale 45 m 

Kootenai Formation 

sandstone and shale 365 m

Morrison Formation 

shale and sandstone 

Ellis Group

Bannack — Argenta area 

Thaynes Formation \ 

Woodside Formation I 275 -550 m 

Dinwoody Formation '

Tendoy Mountains 

Colorado Group equivalents 

sandstone and shale 1 ,065 m 

Kootenai Formation 

sandstone and shale 455 m

90m 

15 -90 m

Tendoy Mountains 

Thaynes Formation \ 

> 455m 

Dinwoody Formation '

Phosphoria Formation and equivalent rocks, 

sandstone, phosphatic shale 

and phosphate rock, chert, 

limestone and dolomite 100 -260 m

Quadrant Quartzite, 

sandstone and quartzite 

Amsden Formation, 

limestone and sandstone

825m 

30 -90 m

Big Snowy Formation, 100- m in 

limestone and shale Tendoy Mountains 

Madison Group 

Mission Canyon Limestone » 610 m (as much as 

Lodgepole Limestone ] 1,035 m in Argenta area)

Medicine Lodge Thrust System

Allochthon

None

None

South Beaverhead Mountains

Dinwoody Formation 35 m 

(Local only)

South Beaverhead Mountains 

Phosphoria Formation and 

equivalent rocks, 

cherty phosphatic about 

limestone and shale 100 m

South Beaverhead Mountains 

Unnamed 

Upper sandstone \ 

Alternating limestone, 1 about 

dolomite, and > 900 m 

sandstone 1 

Lower sandstone /

South Beaverhead Mountains 

Big Snowy Formation, 

limestone and shale 200 m 

Surrett Canyon Formation, 

limestone 65 m 

South Creek Formation, 

limestone 90 m 

Scott Peak Formation, 300 -400 m 

Middle Canyon Formation 

limestone, sandstone, \ 

and siltstone 1 

Unnamed Lower \ about 

Mississippian 1 1,000 m 

shale and limestone /

Lemhi Ran

None

South Lem

Unname

Limes

san

South Lem

Unname

Limes

South Lem

Surrett 

limes

South C

limest

Scott Pe

limes

Middle 

limest

and s

McGow

shale 
Precambrian

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3 CL 

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Continued — Paleozoic 

Cambrian

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System Thrust Lodge Medicine . 

None know

35'

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§' CQ

F< Swauger Gro Lemhi siltit and 

^ "«. 

§ 6* 

D3 5t 

1 O 

zite 0-3,050m juartzite 

m7,620 

Wilbert Formation, and quartzite conglomerate

o

do

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w

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^* c *^ *^ ^* fS s* 

CL 3 O'S? ^^ S

ftS" * 1 S. 3 H" Ops 

2L Q B °" 3" 3- w 

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3 5' S, S" g 

— Ran Lemhiost River 

Mountains Beaverhead 

None known

i

Area Bayhorse 

Cash CreekQuartzite

CO 

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J

CDW

3

Mountain Form, Saturday dolomite gray Kinnikinic Quartzite vitreous light-gray quar Summerhouse Formation sandstone andshale, dolom calcareous and

§ i ? 

ens

< O3 

en Dolomite ofsome reports),

_ 425m 

f*

El 370 m 

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Beaverhead DiithMountains) 

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Beaverhead Mountains Formation Jefferson dolomite

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Lemhi Range

Three FoksFormatio (?)

shale andlimestone 

Jefferson Formation Gilmore (inarea), 

limestone limebreccia, and sandston dolomite, 

Middle Devonian channel sandstone

3 » (B 

§(B

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1 CD 1 I-

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3 33 

NOIUNM3CIQ3SIA3H JO 3HX aNIOIQaiM aOQOl J-SHHHX JM3XSAS 
8 MEDICINE LODGE THRUST SYSTEM, EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

In this area the allochthonous rocks are part of the 

Lemhi Group and the autochthonous rocks are part of 

the Yellowjacket Formation (Ruppel, 1975, p. 21). 

Farther north along the west flank of the Beaverhead 

Mountains, the fault has been mapped between Reese 

Creek and Agency Creek, where the fault breccia is 

mainly of Big Creek and Gunsight Formations of the 

Lemhi Group and is above the Yellowjacket Formation.

From Agency Creek northward to Rathburn Gulch in 

the Beaverhead Mountains, and west of Bloody Dick 

Creek the fault is not known to be exposed. Rocks of the 

Yellowjacket Formation, beneath the fault, crop out 

from the Peterson Creek segment of the fault, across 

Lemhi Pass, at the head of Agency Creek, and northward along the west flank of the Beaverhead Mountains to Freeman Creek. The rocks along the crest and 

east flank of the mountain range are not known, but 

the presence of Lemhi Group rocks in glacial deposits at 

the head of Bloody Dick Creek suggests that the thrust 

fault is present at depth west of Bloody Dick Creek, and 

could be exposed in places not yet mapped.

At the head of Freeman Creek, the Yellowjacket Formation is faulted against quartzite that probably is part 

of the Wilbert Formation; the steep west-dipping Miner 

Lake fault that separates the two formations is thought 

to be a high-angle reverse fault (MacKenzie, 1949, p. 

34 -36). The Medicine Lodge thrust presumably is present at depth east of the Miner Lake fault, beneath the 

Wilbert Formation rocks. The Miner Lake fault continues to the northwest into the upper part of Carmen 

Creek, where it separates light-colored feldspathic 

quartzite of the Big Creek Formation, east of the fault, 

from Yellowjacket rocks to the west; the Medicine 

Lodge thrust probably is beneath the Big Creek 

quartzites. Both the Yellowjacket and Big Creek have 

been intruded by the complex granodiorite Carmen 

Creek stock, but the relation of the stock to the Miner 

Lake fault is not known.

Tucker (1975, p. 136-146) considered the Beaverhead Divide fault zone, which he mapped along the Continental Divide south of the Freeman Creek area 

(MacKenzie, 1949), to be a southern extension of the 

Miner Lake fault and a link between the Miner Lake 

fault and the Bloody Dick Creek fault of Coppinger 1 

(1974, p. 167). The Beaverhead Divide fault zone as 

thus extended is a high-angle reverse fault, but Tucker 

suggested that it was originally a low-angle thrust fault, 

placing the Yellowjacket Formation over the Big Creek 

Formation. The relation of the Beaverhead Divide fault 

zone to the Medicine Lodge thrust fault is unknown, but 

most probably the Medicine Lodge thrust zone simply is 

broken by the high-angle Beaverhead zone, and the 

thrust is concealed to the east beneath the upper plate 

rocks of the Big Creek Formation and is eroded away to 

the west.

North of Carmen Creek, the quartzite and siltite of 

the Yellowjacket Formation crop out in the canyon of 

the Salmon River and its North Fork, and are overlapped by Challis Volcanics. The Challis is broken to the 

east by steep normal faults that are part of the range 

front system, and quartzite of the Big Creek Formation 

is exposed on the high peaks in the Beaverhead Mountains. The Medicine Lodge fault is beneath the Big 

Creek rocks, but no exposures of the fault are known.

At Rathburn Gulch, near Gibbonsville, Idaho, the Big 

Creek and Gunsight(?) Formations of the Lemhi Group 

overlie the Yellowjacket Formation. The contact has 

been interpreted as a gradational, depositional one (Anderson, 1959, p. 18; Ruppel, 1975, p. 6), but reconnaissance mapping shows that the supposed contact dips 

gently west, across bedding, and that Lemhi Group 

quartzites are extensively brecciated. The field relations suggest that the exposures near Gibbonsville are 

the northernmost known outcrops of the Medicine 

Lodge fault.

LEMHI RANGE

In the Lemhi Range (figs. 1,2), the main thrust zone 

is exposed in several places beneath complexly 

deformed rocks (Ruppel, 1968). Here, as in the western 

part of the Beaverhead Mountains, repeated exposures 

of the fault result from comparatively recent uplift of 

these block-faulted mountain ranges, and the thrust 

system has been folded and repeatedly broken by 

several generations of younger, high-angle faults (Ruppel, 1964). The best exposures of the Medicine Lodge 

fault system are on the northeast flank of Mill Mountain (in the central part of the Lemhi Range), and 

northwest from Hayden Creek and the Hayden Creek 

window to the Poison Peak area and the west flank of 

Twelvemile Creek. These are the westernmost known 

exposures of the Medicine Lodge thrust fault system. At 

all of the localities allochthonous rocks immediately 

above the thrust are part of the Lemhi Group or 

Swauger Formation, and autochthonous rocks beneath 

the thrust are part of the Yellowjacket Formation. The 

thrust zone ranges in thickness from about 50 m to 

more than 300 m, and includes stretched, intensely 

brecciated, and mylonitized rocks that grade upward 

into less internally sheared, but complexly folded and 

faulted rocks of the upper plate. West and south of Mill 

Mountain, as in many other places, the thrust zone is 

concealed by Challis Volcanics.

The thrust zone is exposed southwest of Gilmore in a 

small window at Squaw Creek where the Kinnikinic 

Quartzite of Middle Ordovician age is thrust over the 

Yellowjacket Formation. The entire Lemhi Group and 

Swauger Formation, about 10,000 m thick, are missing 

and the Kinnikinic Quartzite is intensely shattered. 

The widespread distribution of shattered Kinnikinic in
REVISED DEFINITION OF THE MEDICINE LODGE THRUST SYSTEM

outcrops in Sawmill Canyon west of Squaw Creek suggests that this wide basin is underlain at shallow depth 

by the fault, beneath blanketing Challis Volcanics. The 

thrust also might be exposed farther south in Basinger 

Canyon, where descriptions by Umpleby (1917, p. 23) 

suggest that the lower part of the Lemhi Group is missing above Yellowjacket-like rocks.

The exposures of the fault in the Lemhi Range suggest that it lies at relatively shallow depth beneath the 

central part of the range. Aeromagnetic evidence (U.S. 

Geological Survey, 1971) also suggests a magnetic surface which slopes gently westward and which probably 

is the thrust surface. The magnetite-bearing quartzites 

of the Yellowjacket may be the source of the magnetism.

STRATIGRAPHIC CHANGES ACROSS THE 

MEDICINE LODGE THRUST SYSTEM

The major changes in rock units across the Medicine 

Lodge thrust (table 1) suggest that the allochthonous 

rocks in the upper plate (fig. 3) have been technically 

transported far east of the Cordilleran miogeocline 

where they were deposited. These rocks now overlap 

other rocks that were deposited in a shallow shelf embayment or seaway. Paleogeographic reconstruction 

suggests that the miogeocline and the shelf embayment 

or seaway were separated by an intermittent uplift, an 

expanded Lemhi arch, during part of Precambrian and 

Paleozoic time. The changes in stratigraphic units 

across the thrust are summarized on table 1, and the 

rocks deposited in the miogeocline and in the seaway, 

on opposite sides of the Lemhi arch, are briefly discussed in the following paragraphs. The Lemhi arch is 

discussed more fully in a later section of this report.

PRECAMBRIAN X

Precambrian X crystalline metamorphic rocks are 

widespread in southwestern Montana east of the 

Medicine Lodge thrust, but are not known in the 

allochthonous block unless fault-bounded granitic 

rocks near Leadore, Idaho (Ruppel, 1968), similar to the 

Dillon Granite Gneiss of Scholten, Keenmon, and 

Kupsch (1955) farther east, represent slices of these 

basement rocks caught up in the thrust where it overrode the crystallite rocks. Problems posed by these 

granitic rocks in the allochthonous block are briefly discussed in a later section on the Beaverhead pluton.

PRECAMBRIAN Y

The Precambrian sedimentary rocks of the upper 

plate are the Lemhi Group and Swauger Formation. 

These rocks do not occur east of the Medicine Lodge 

thrust in the lower plate, and they show no features 

that suggest an approach to a depositional edge. In

southwestern Montana they are in thrust contact with 

Precambrian crystalline basement rocks. The association of very different rocks across the thrust strongly 

suggests major eastward transport of the Lemhi and 

Swauger rocks. In the Lemhi Range and part of the 

Beaverhead Mountains the micaceous, feldspathic 

quartzite and siltite of the Yellowjacket Formation are 

beneath the thrust, but Yellowjacket rocks have not 

been recognized farther east. Yellowjacket rocks are 

not known to occur any place in the upper plate.

PRECAMBRIAN Z

The Wilbert Formation, tentatively of Precambrian Z 

age (equivalent to part of the Brigham Quartzite south 

of the Snake River Plain), has been recognized in the 

upper plate in several areas in east-central Idaho (Ruppel and others, 1975, p. 27-29). The formation has not 

been definitely recognized east of the thrust trace.

CAMBRIAN

No rocks of Cambrian age are known in the Lost 

River and Lemhi Ranges in the eastern part of the 

allochthonous block, but they have been found about 50 

km farther west near Clayton, Idaho, in the western 

part of the block where the Lower or Middle Cambrian 

Cash Creek Quartzite has been described (Hobbs and 

others, 1968, p. J18^J19). The distribution of Cambrian rocks in the allochthonous block suggests that 

they must originally have thinned to the east.

In the autochthon the thicknesses and distribution of 

Cambrian rocks indicate that they thin to the south and 

west in southwestern Montana, partly as a result of 

Middle Cambrian deformation (Myers, 1952, p. 6). 

Cambrian rocks apparently were not deposited in the 

area now immediately east of the trace of the Medicine 

Lodge thrust.

ORDOVICIAN

The Kinnikinic Quartzite and dolomites of the Saturday Mountain Formation (of Middle and Late Ordovician age), are widespread in the central and southwestern Lemhi Range, and the Lower Ordovician Summerhouse Formation crops out locally in the same 

region. The Saturday Mountain Formation thins by 

onlap onto a high area at the south end of the range, 

where it includes thin sandstone interbeds.

In the Beaverhead Mountains the Kinnikinic 

Quartzite thins to the south by onlap onto the Skull 

Canyon uplift (Scholten, 1957, p. 166) in the southern 

part of the range. The Saturday Mountain Formation 

has been mapped in thrust plates near Leadore, Idaho, 

but probably is present there only because of tectonic 

transport within the allochthonous block. The Summerhouse Formation crops out on and near Maiden
10 MEDICINE LODGE THRUST SYSTEM, EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

114° 113°

FIGURE 3. — Sketch map of regions of autochthonous and allochthonous rocks, east-central Idaho and 

southwest Montana, indicating rocks overridden by Medicine Lodge thrust system. Heavy solid line, 

trace of Medicine Lodge thrust (teeth on overthrust plate), dashed where inferred.
REVISED DEFINITION OF THE MEDICINE LODGE THRUST SYSTEM 11

Peak, where M'Gonigle (1965, pi. 1) considered it to be 

in depositional contact with the Dillon Granite Gneiss. 

If the contact is depositional, the Summerhouse must 

have been deposited in shallow seas and tidal flats that 

covered all of east-central Idaho and adjacent southwestern Montana. The Maiden Peak outcrops of the 

Summerhouse Formation are complexly folded, thrust 

faulted, and broken by many younger faults, however, 

and are exposed only where the Precambrian Y Lemhi 

Group rocks have been thrust over the older crystalline 

rocks on the Medicine Lodge fault. I believe that it is 

most likely that the Summerhouse Formation near 

Maiden Peak is part of the upper plate of the Medicine 

Lodge thrust system, and so is in fault contact with the 

Dillon Granite Gneiss, rather than depositional contact 

as shown by M'Gonigle. If my interpretation is correct, 

there are no Ordovician rocks east of the Medicine 

Lodge fault trace.

The sedimentation patterns and distribution of Ordovician rocks suggest that they, like the Cambrian 

rocks, originally thinned to the east in the allochthonous block, and that they were not deposited on the 

autochthon east of the Medicine Lodge fault trace.

SILURIAN

In the allochthonous block Silurian rocks are represented by the upper part of the Saturday Mountain Formation, which in the Leadore quadrangle may be as 

young as Silurian, and by the Laketown Dolomite. The 

Laketown thins northward and eastward and disappears in the central part of the Lemhi Range, and Laketown rocks have not been recognized in the Beaverhead 

Mountains. The upper part of the Laketown was 

weathered and eroded before deposition of Devonian 

rocks in the Lemhi Range, and the formation thins both 

by onlap onto a lower Paleozoic high area and by later 

Silurian and Early Devonian erosion. No Silurian rocks 

are known east of the Medicine Lodge fault trace in 

southwestern Montana.

DEVONIAN

In the Lemhi Range the thick section of Devonian 

rocks in the allochthonous block includes: (1) a channel 

sandstone deposit that cuts deeply into the Saturday 

Mountain Formation at the head of Spring Mountain 

Canyon and which contains Middle Devonian freshwater fish remains; (2) a thick section of marine rocks 

of the Middle and Upper Devonian Jefferson Formation 

that is well exposed near Gilmore; and (3) a 100-mthick shale and limestone section of the Upper Devonian Three Forks(?) Formation. The lower part of the 

Jefferson contains sandstone lenses and sandy carbonate rocks; the upper part is a syndepositional limestone 

breccia that may represent an off-reef accumulation. In

the Beaverhead Mountains, the much thinner Jefferson Formation laps onto and finally overlaps a lower 

Paleozoic topographic high area called the Lemhi arch 

by Sloss (1954, p. 368) or the Tendoy dome by Scholten 

(1957, p. 167). The Three Forks Formation apparently 

is too thin to map separately in the Beaverhead Mountains. In the Tendoy Mountains autochthonous Devonian rocks include both the Jefferson and Three Forks 

Formations.

The pattern of eastward-thinning allochthonous 

rocks and westward-thinning autochthonous rocks in 

early and middle Paleozoic time is clearly repeated in 

Devonian rocks.

UPPER PALEOZOIC

The stratigraphic nomenclature of upper Paleozoic 

rocks in east-central Idaho remains in a state of flux, as 

continuing efforts are being made to apply the terminology developed by Huh (1967) and Mamet and 

others (1971) to this thick, structurally complicated, 

and tectonically telescoped group of rocks. But 

stratigraphic nomenclature is a secondary issue; the 

primary one, as first pointed out by Sloss and Moritz 

(1951, p. 2156-2160), is that the upper Paleozoic rocks 

of the allochthonous block in the Beaverhead Mountains, the Lemhi Range, and farther west are radically 

different from rocks of somewhat similar age in the 

autochthonous section immediately east of the 

Medicine^Lodge fault in the Tendoy Mountains of Montana.

The Mississippian rocks in the allochthon include 

shale and limestone of the McGowan Creek, Middle 

Canyon, Scott Peak, South Creek, Surrett Canyon, and 

Big Snowy Formations. East of the Medicine Lodge 

thrust autochthonous Mississippian rocks are dominated by limestones of the Lodgepole and Mission Canyon Limestones of the Madison Group. The Big Snowy 

Formation is also present but it differs in thickness and 

in lithofacies from place to place (Scholten and others, 

1955, p. 364).

Pennsylvanian rocks in the southern Lemhi Range 

and Beaverhead Mountains are mainly sandstone and 

sandy limestone and dolomite. The sandstone is most 

common at the base and top of the section and most of 

the Pennsylvanian rocks are carbonate (Shannon, 

1961, p. 1830). In contrast, the Pennsylvanian section 

in the Tendoy Mountains 10 -20 km to the east includes 

limestone and fine-grained clastic rocks of the Amsden 

Formation, overlain by the thick well-sorted quartzite 

of the Quadrant Quartzite.

Unnamed Permian rocks in the allochthonous block 

have been reported near the southern tip of the Lemhi 

Range, and although they have not been described in 

detail they are known to be mostly sandstone and
12 MEDICINE LODGE THRUST SYSTEM, EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

limestone (Shannon, 1961, p. 1830). Permian 

phosphatic limestones and shales of the Phosphoria 

Formation and equivalent units are known from the 

southern Beaverhead Mountains, and apparently are in 

normal stratigraphic sequence. East of the thrust the 

Permian section includes phosphatic carbonates and 

organic carbon-rich shales typical of the Permian shelf 

deposits in Idaho and Montana (Maughan, 1975).

In summary, upper Paleozoic rocks west of the 

Medicine Lodge thrust in the upper plate differ greatly 

from the lower plate rocks east of the Medicine Lodge 

thrust. For the most part, the differences in lithofacies 

and thickness are so profound that the two sequences of 

rocks cannot be correlated.

MESOXOIC

There are almost no allochthonous Mesozoic rocks in 

east-central Idaho or in southwestern Montana, except 

for a small area near the south end of the Beaverhead 

Mountains where the Triassic Din woody Formation has 

been reported. East of the fault, autochthonous rocks of 

Mesozoic age are widespread and relatively thick. In 

the Bannack area, the Mesozoic section includes the 

Triassic Dinwoody, Woodside, and Thaynes Formations 

overlain by the Jurassic Ellis Group and the Cretaceous 

Kootenai and Colorado Formations. Farther south, in 

the northern part of the Tendoy Mountains, only the 

Dinwoody and Thaynes are present. Jurassic rocks of 

the Ellis Group and the Morrison Formation are 

restricted to the central and southern parts of the Tendoy Mountains, as are Cretaceous rocks represented by 

the Kootenai Formation and a thick section of rocks 

equivalent to part of the Colorado Formation. This 

thick section of Mesozoic rocks abruptly disappears at 

the east edge of the Medicine Lodge fault, and probably 

is concealed beneath the allochthonous block.

THE LEMHI ARCH

Topographic highs described as small islands have 

been invoked to explain shoaling patterns of rocks 

deposited from Cambrian through Mississippian time 

(Sloss, 1954; Scholten, 1957), but the telescoping effect 

of the Medicine Lodge thrust on lithofacies distribution 

has never been considered. These small islands now appear to be displaced parts of what was once a major 

landmass that was overridden by the allochthonous 

block of the Medicine Lodge thrust. The first high area 

or island recognized was named the Lemhi arch (Sloss, 

1954). I retain the name, but redefine it as an intermittently emergent major landmass that separated the 

Cordilleran miogeocline in western Idaho from a shelf 

embayment or seaway in southwestern Montana during Precambrian and Paleozoic time (fig. 4). As 

redefined, the Lemhi arch may have been connected at

its northern end to Belt island (Harrison and others, 

1974, p. 2 -3, 5 -6, fig. 2). Its south end is unknown. Its 

maximum width probably was about 160 km, the inferred distance of movement on the Medicine Lodge thrust 

(Ruppel, 1975, p. 15-16). Lower and middle Paleozoic 

rocks in the upper plate thin eastward, and I interpret 

this thinning to reflect the former western shore of the 

Lemhi arch. Rocks of similar age thin westward in the 

lower plate, and I interpret this thinning to reflect the 

eastern shore of the arch. As a result of thrust faulting, 

the two shores are now placed nearly together, to create 

a false impression of a system of small islands.

The effect of tectonic fluctuations of the Lemhi arch 

on regional sedimentation patterns differed from Precambrian through Paleozoic time. The oldest rocks, 

those of the Yellowjacket Formation (Precambrian Y), 

are widely exposed beneath the Medicine Lodge thrust 

plate in east-central and central Idaho. Their distribution suggests that they were deposited across central 

and east-central Idaho and probably lapped against 

crystalline rocks someplace near the present IdahoMontana boundary. This distribution suggests that the 

Lemhi arch did not exist during deposition of the 

Yellowjacket sediments, and that the Precambrian geosynclinal shoreline was near southwest Montana, much 

farther east than at any later time.

The Lemhi Group and Swauger Formation, which 

everywhere are part of the allochthonous block above 

the Medicine Lodge thrust, are thrust across the older 

Yellowjacket Formation. The Lemhi and Swauger are 

thought to have been deposited in the Precambrian 

Cordilleran miogeocline (Ruppel, 1975, p. 15). The 

lithologic and stratigraphic differences between these 

rocks suggest that by later Precambrian Y time the 

Lemhi Arch had risen and had separated the Precambrian miogeocline in western Idaho from the cratonic region in east-central Idaho and southwest Montana. The arch may have separated depositional areas 

in Precambrian Z time, but almost nothing is known of 

rocks in southwest Montana that might be of this age, 

and so equivalent to the allochthonous Wilbert Formation.

Shoaling patterns of sedimentation against the opposite shores of the Lemhi arch are evident in 

Cambrian rocks, for rocks deposited in the seaway in 

Montana thin to the west against the eastern shore of 

the arch, and miogeoclinal rocks thin to the east in 

Idaho, against the western shore of the arch. By Ordovician time, the eastern side of the arch probably was 

continuous with the large landmass to the east, for Ordovician and Silurian rocks are absent in southwest 

Montana.

The earliest rocks of Devonian age are Middle Devonian channel sandstones containing freshwater fish remains (Denison, 1968) which are present on the block
THE LEMHI ARCH 13

120°

50

115

110"

CANADA 

UNTIED STATES

100

I

200

I

300 KILOMETERS

FIGURE 4. — Map showing approximate location of Lemhi arch. Queried where boundary unknown; dashed line outlines approximate maximum extent of Belt island-Lemhi arch; dotted line shows region where seaway may have reached south from Belt basin to miogeocline 

along east shore of Lemhi arch.
14 MEDICINE LODGE THRUST SYSTEM, EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

that represents the western shore of the arch. These 

sandstones are overlain by transgressive marine 

blanket sandstones which, in turn, are overlain by 

carbonate rocks. Marine carbonate rocks (the main 

part of the Jefferson Formation in Montana) were 

deposited against the eastern shore of the arch until 

later Devonian time, when for the first time the Lemhi 

arch was overlapped by marine deposits. Only then did 

the rocks on both sides of the arch acquire some 

lithofacies characteristics in common, for at least some 

of the western miogeoclinal Jefferson Formation rocks 

do resemble the type Jefferson rocks east of the arch.

The effect of the uplift seems to have persisted into 

later Paleozoic time, inasmuch as it influenced depositional patterns of Mississippian and Pennsylvanian 

rocks, as suggested by Sloss and Moritz (1951, p. 2161) 

and Scholten (1957, p. 167). During Mesozoic time the 

Lemhi arch probably was a persistent high area, supplying sediments rather than receiving them. In late 

Mesozoic and early Tertiary time the arch was overridden by the Medicine Lodge thrust, which shifted 

together or overlapped rocks from very different 

depositional environments from opposite sides of the 

arch. Various uplifts described in earlier reports, such 

as the Skull Canyon uplift, Tendoy dome, Beaverhead 

arch (Scholten, 1957, p. 167), and the Salmon River 

arch (Armstrong, 1975, p. 452), are all displaced parts 

of this single fluctuating but persistent pericratonic upwarp, the Lemhi arch.

The connecting of the Lemhi arch and Belt island, as 

shown in figure 4, is conjectural, particularly because so 

little is known — or perhaps knowable — of the 

Paleozoic history of Belt island (Harrison and others, 

1974, p. 6). In reviewing the Paleozoic and early 

Mesozoic history of the Belt basin, Harrison, Griggs, 

and Wells (1974) discussed an intermittently emergent 

sill, or mobile geanticline, described by Wheeler (1966) 

in the eastern tectonic belt of the southern Canadian 

Cordillera, and suggested that his "mobile geanticline" 

could have included Belt island. Perhaps, instead, Belt 

island is a northern part of the Lemhi arch, and 

together these features form a second mobile geanticline, south of the Belt basin. The intermittent but 

persistent uplift described by Wheeler is similar in 

many ways to the Lemhi arch. In a summarizing 

paragraph, Wheeler (1966, p. 37) stated:

The tectonic evolution indicates that until the mid-Paleozoic the 

eastern belt was essentially a locally fluctuating zone of transition 

between a eugeosyncline to the west and a miogeosyncline to the 

east, within which the Purcell region remained as a relatively elevated block. During late Paleozoic and early Mesozoic times, 

however, the eastern belt was the site of an intermittently emergent 

sill — essentially a mobile geanticline — which, except in its 

southernmost part, separated the two kinds of geosynclines. In late 

Mesozoic and Tertiary time the eastern belt was entirely land. At 

this time it was repeatedly the site of plutonic pulses, episodes of

deformation and uplift, and the principal source of Cretaceous sediments deposited to the east * * *

This paragraph serves equally well in most respects for 

describing the Lemhi arch. The eastern belt and the 

Lemhi arch occupied similar tectonic settings and have 

had largely parallel tectonic histories. Both features 

have left their imprint one way or another on rocks 

from Precambrian Y time to the Tertiary, and have 

been focal points of many major tectonic episodes 

affecting these regions. Two major intermittently positive regions along the margin of the Cordilleran 

miogeocline are defined by (1) the mobile geanticline of 

Wheeler and (2) the Lemhi arch joined to Belt island.

THE BEAVERHEAD FORMATION AND THE 

AGE OF THE MEDICINE LODGE 

THRUST SYSTEM

The Beaverhead Formation, named by Lowell and 

Klepper (1953), is recognized as a syntectonic deposit 

that resulted from regional overthrusting. Scholten 

(1973, p. 477^480) noted that "extensive remains of 

the subaerial Beaverhead conglomerate occur at the 

eastern margin of the overthrust belt. This conglomerate attains thicknesses of at least 5,000 m and a large 

part is composed of highly rounded pebbles and cobbles 

of lower Paleozoic and Belt quartzite, deposited continuously from Early Cretaceous (Albian) to Early Tertiary (Paleocene or early Eocene) time * * *."1 He suggested that eastward transport of the clastic debris that 

forms the Beaverhead Formation indicates the beginning of major regional uplift and tectonism to the west. 

The Beaverhead Formation is itself intensely folded 

and cut by thrust faults (Lowell, 1965), and must reflect 

several stages of tectonism extending from the initial 

uplifts in central Idaho to the eventual arrival of the 

Medicine Lodge thrust plate.

Although the age of the Beaverhead Formation is 

reasonably well known, the time of movement on the 

Medicine Lodge overthrust system is not. It is possible, 

however, to establish a minimum age of movement, for 

in the central part of the Lemhi Range folded and imbricate thrust-faulted rocks of the upper plate are cut 

by many undeformed monzonitic and granodioritic 

stocks; the stocks are obviously postthrusting in age, 

and were intruded and partly exposed by erosion before 

eruption of the Challis Volcanics. Potassium-argon age 

determinations on biotite from one of these stocks, the 

Blue Jay stock, indicate an age of 51.3 ±1.5 m.y., and

1 The Belt quartzites referred to by Scholten presumably are the Precambrian Y sedimentary rocks of east-central Idaho (Ruppel, 1975). but pebbles and cobbles of Precambrian 

rocks in the Beaverhead Formation have not been sufficiently studied to tie them to more 

specific source areas.
DEFORMATION IN THE ALLOCHTHON 15

biotite from a stock-related dike in Long Canyon near 

Gilmore provides a K-Ar age of 49.4 ±1.7 m.y. (John D. 

Obradovich, written commun., 1971). Therefore, the 

Medicine Lodge fault block was in its present position 

by early Eocene time. Fault movement occurred between Early Cretaceous (Albian) and early Eocene 

time; probably most movement occurred in Late Cretaceous and early Paleocene time.

The mechanism that caused the Medicine Lodge 

thrust remains uncertain. Scholten (1973) has suggested a model that requires gravitational gliding of 

allochthonous blocks from a rising region in central 

Idaho which is now occupied by the Idaho batholith. 

Scholten's model (1973, p. 485) requires that, "a pelitic, 

low viscosity potential decollement zone existed somewhere near the middle of the Belt at around 13 km 

below sea level prior to deformation * * * ." According to 

present interpretations of Precambrian stratigraphy 

(Ruppel, 1975), some of these conditions appear to be 

met in east-central Idaho where Precambrian rocks 

above the Yellowjacket Formation are 7 -10 km thick, 

and younger rocks have a total thickness of 4—5 km; 

the base of this part of the Medicine Lodge thrust zone 

presumably is at or near the contact of the Yellowjacket 

Formation and the younger Precambrian sedimentary 

rocks. North of the Snake River Plain, however, the 

southern part of the thrust brings miogeoclinal 

Paleozoic sedimentary rocks, at most 4—5 km thick, 

over shelf-deposited sedimentary rocks of Paleozoic and 

possible Mesozoic age. Perhaps it is fairest to conclude 

that the proposed requirements for gravitational gliding are only partly met by what is now known of the 

Medicine Lodge thrust system.

DEFORMATION IN THE ALLOCHTHON

Throughout east-central Idaho and adjacent Montana, rocks in the allochthonous block above the 

Medicine Lodge thrust are complexly folded and are cut 

by many imbricate thrusts of comparatively minor displacement. Scholten and Ramspott (1968, p. 25-37) 

concluded that the predominantly quartzitic lower 

Paleozoic and Precambrian rocks at lower stratigraphic 

and structural levels in the block are less intensely 

deformed than younger Paleozoic carbonate rocks higher in the block. In general this seems to be true 

throughout the Beaverhead Mountains, Lemhi Range, 

and Lost River Range, although the degree of deformation is relative and probably reflects the different 

response of different rocks to the stress field. However, 

the regional distribution of thrust-related deformation 

requires a broader explanation than that applied locally 

in the southern Beaverhead Mountains by Scholten and 

Ramspott (1968).

In the southern Beaverhead Mountains, Scholten and 

Ramspott (1968) described the lower layer structural 

zone as characterized by: (1) open north west-trending 

folds that are asymmetric to the northeast and superposed on a series of broad northwest-trending undulations; (2) short steeply dipping to vertical faults which 

trend northeast and northwest and which may not be 

related to thrusting; and (3) older medium- to highangle northwest-striking thrusts which appear to 

steepen with depth and which are broken by the steep 

faults. Scholten and Ramspott attributed the lower 

layer structure to compressional stresses and the rise of 

an anticlinorium in the Beaverhead Mountains. The 

upper layer structure is characterized by strongly overturned folds, recumbent folds or intense contortions, 

and by masses of rock displaced along nearly flat lying 

faults that displace rocks no older than Mississippian. 

The folds are asymmetric or overturned to the northeast, although some are locally asymmetric to the 

southeast. Near the Beaverhead pluton the folds are 

asymmetric away from the pluton. In some areas the 

structure is almost chaotic. The disharmonic zone between the upper and lower layers is in Mississippian 

shale and shaly limestone. Scholten and Ramspott considered upper layer deformation a result of radial gravitational movement away from the rising Beaverhead 

anticlinorium.

In the northern part of the Beaverhead Mountains 

and in the central part of the Lemhi Range, deformation in the allochthonous block is characterized by: (1) 

strongly overturned and nearly isoclinal northwesttrending folds, which are asymmetric to the northeast; 

and (2) closely spaced imbricate thrust faults that 

generally dip 20°-25° to the west or southwest and 

show no indication of steepening with increasing depth. 

The rocks involved in thrusting are of Precambrian or 

early Paleozoic age; younger Paleozoic rocks are present only in a few small areas.

In the southern part of the Lemhi Range (Ross, 1961; 

Beutner, 1968), rocks involved in the thrust are later 

Paleozoic carbonate rocks, and the type of deformation 

resembles that of the upper layer structural zone of the 

southern Beaverhead Mountains. Similar deformation 

is present in the Lost River Range where upper 

Paleozoic carbonate rocks are involved in overthrusting 

(Mapel and others, 1965; Mapel and Shropshire, 1973), 

and spectacular isoclinal folds are prominently exposed 

in many cliffs.

The differences noted by Scholten and Ramspott 

(1968) and described by them as structural "layers" 

probably result from the different physical responses of 

brittle quartzite and more plastic carbonate rocks in 

the same regional stress field. The brittle, quartzitic 

rocks are everywhere in asymmetric and overturned
16 MEDICINE LODGE THRUST SYSTEM, EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

folds cut by low-angle imbricate thrusts. The more 

plastic carbonate rocks are similarly folded and at least 

locally similarly faulted; but these structures, the primary response to the stress field, are complicated and 

partly masked by secondary features formed by internal gravitational sliding and cascading flow as these 

rocks piled up on successive imbricate thrusts. The apparent layered structure therefore reflects (1) the 

original sedimentary layering of quartzitic rocks in the 

Precambrian and lower Paleozoic section and carbonate rocks in the upper Paleozoic section, and (2) the 

different response of these different rocks to regional 

stress during thrusting.

SOME REGIONAL RELATIONS, 

IMPLICATIONS, AND PROBLEMS

Recognition of the great extent and of the structural 

and stratigraphic significance of the Medicine Lodge 

thrust system has lagged far behind studies of other 

tectonic elements of the North America Cordilleran 

fold and thrust belt. Perhaps one reason for this lag is 

that, even now, much of the region underlain by this 

large fault is either unmapped or inadequately mapped, 

and the pervasive consequences of Medicine Lodge 

thrusting have not been recognized. Most geologic 

problems in this region cannot be resolved without a 

clear understanding of the Medicine Lodge thrust 

system, and many of the regional geologic problems 

previously thought to be resolved now need reconsideration.

THE NORTHWARD EXTENSION OF 

THE MEDICINE LODGE THRUST SYSTEM

The Cordilleran fold and thrust belt has generally 

been assumed to pass through southwestern Montana, 

somehow linking the southeast Idaho segment with the 

"disturbed belt" farther north in Montana. The actual 

link has remained obscure, in the absence of detailed 

geologic mapping. What is now known about the 

Medicine Lodge thrust system indicates that it does not 

tie directly to the Montana disturbed belt farther north, 

but instead extends to the northwest along the west rim 

of the Big Hole Basin of western Montana (figs. 1, 5). 

The divergent traces of the thrust systems suggest that 

southwestern Montana is a region where different segments of the fold and thrust belt come together and 

overlap, and where the northern disturbed belt segment disappears.

The continuation of the Medicine Lodge thrust 

system north of the Big Hole Basin is not known. It may 

extend northward into the zone of thrust faults near 

Phillipsburg, Mont. (35 km northwest of Anaconda), 

but the rocks of the intervening region north of the Big

Hole Basin are not mapped; the thrust faults near 

Phillipsburg have recently been described as part of the 

eastern boundary of the Sapphire tectonic block (Hyndman and others, 1975, p. 401-402). If the fault continues northward beyond the Big Hole Basin, it crosses 

the Belt island that separated the Belt basin reentrant 

from the geosyncline to the south (Harrison and others, 

1974, p. 3), approaches the Idaho batholith, and enters 

a different structural province. The Medicine Lodge 

thrust may disappear between the Big Hole Basin and 

the Phillipsburg area, to be replaced by another structural system more compatible with the changed 

geologic framework. Scholten (1973, p. 487) has suggested that "different models are probably necessary 

for different segments (of the Cordilleran fold and 

thrust belt) in view of major differences in geosynclinal 

evolution, rock types, plutonism, and structural style 

between the segments," a view that helps explain the 

apparent overlapping of divergent thrust systems in 

southwest and western Montana. (See also Harrison 

and others, 1974, p. 7-9.)

THRUST SLICES OF LOWER PLATE 

ROCKS IN THE UPPER PLATE

The presence of rocks that do not fit the local 

stratigraphic framework in the Hawley Creek and 

Railroad Canyon area east of Leadore, Idaho (Ruppel, 

1968; Luchitta, 1966), has confused both local and 

regional stratigraphic studies. These anomalous rocks 

include parts of the Madison Group, Big Snowy Formation, Quadrant Quartzite, Phosphoria Formation and 

equivalent units, and Dinwoody Formation. All of them 

are very different from equivalent rocks in the 

allochthonous block even though they are part of that 

block, and are most like autochthonous rocks farther 

east. Because these rocks are in thrust plates, their 

location is best explained if they are considered as parts 

of the lower plate section that have been incorporated 

in the upper plate as thrust slices. Similar thrust slices 

almost certainly occur elsewhere, although no others 

have yet been mapped. The somewhat anomalous outcrops of Permian and Triassic rocks in the upper plate 

at the south end of the Beaverhead Mountains may be 

such thrust slices, although they might also be explained as resulting from very rapid facies changes.

THE BEAVERHEAD PLUTON AND ITS 

RELATION TO THRUST FAULTING

Ramspott (1962) and Scholten and Ramspott (1968) 

described the granitic Beaverhead pluton in the 

southern part of the Beaverhead Mountains, but did not 

discuss its relation to the Medicine Lodge thrust. The 

pluton is dated on the basis of a single K-Ar isotope age 

determination for biotite as being 441 (±15) m.y. old
SOME REGIONAL RELATIONS, IMPLICATIONS, AND PROBLEMS 17

118" 110

CANADA 

UNITED STATES

SAPPH IRE 

TECTONICj 8LOCK 7

0 , WASHINGTON . 

OREGON

100 KILOMETERS

FIGURE 5. — Schematic diagram of the Medicine Lodge thrust system and its relation to the Montana disturbed belt and the Sapphire tectonic

block. Solid lines, thrust faults; teeth on overthrust plate; queried where unknown.
18 MEDICINE LODGE THRUST SYSTEM, EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

(about at the Ordovician-Silurian boundary). This K-Ar 

determination is remarkably close to two Rb-Sr determinations (potassium feldspar, 435 ± 60 m.y. and whole 

rock, 430 ± 40 m.y. —recomputed with X = 1.39 x 10 u 

yr-1 ) for somewhat similar, but fault-bounded and 

strongly sheared granitic rocks farther north in the 

Beaverhead Mountains (Ruppel, 1968; Luchitta, 1966). 

These sheared granitic rocks north of the Beaverhead 

pluton closely resemble the Precambrian X Dillon 

Granite Gneiss, and I believe that they are thrust slices 

of Dillon Granite Gneiss in the allochthonous block. 

This interpretation is also conceivable for the 

Beaverhead pluton, particularly because aeromagnetic 

data (U.S. Geological Survey, 1971) in the vicinity of 

the pluton do not show any significant anomalous pattern, and so suggest that the pluton may be rootless and 

more probably part of a thrust slice than an intrusive 

mass. If these granitic rocks, including the Beaverhead 

pluton, are indeed thrust slices with isotope ages mixed 

as a result of tectonism, the similarity of the mixed ages 

is a problem. However, an equally formidable problem 

seems to exist in explaining how the Beaverhead 

pluton, if it is truly a lower Paleozoic intrusive body, 

came to its present position in the allochthonous block 

of the Medicine Lodge thrust, for it would then have to 

have been sheared off by the thrust and tectonically 

transported far eastward. The pluton is surrounded by 

sedimentary rocks that have been complexly deformed 

as a result of thrusting, but no similar deformation is 

reported in the granitic rocks. Perhaps the widespread 

alteration of the granite and the poorly exposed contacts (Scholten and Ramspott, 1968, p. 18 -21) are evidence of more extensive deformation than has been 

reported. In the absence of more definitive information, 

several interrelated problems remain unresolved: (1) 

the relation of the Beaverhead pluton to the Medicine 

Lodge thrust; (2) an explanation for the presence of 

that pluton in the allochthonous block; (3) the ages of 

granitic rocks in the Beaverhead Mountains, whether 

they are Precambrian X, Ordovician-Silurian, or even 

Tertiary and so neither deformed nor cut by the thrust; 

and (4) the structural and (or) intrusive relations of the 

Beaverhead pluton.

THE RELATION OF VOLCANIC ROCKS 

AND THE THRUST TRACE

Much of the trace of the Medicine Lodge thrust is 

paralleled by areas of Challis Volcanics of Tertiary age, 

and the approximately correlative Medicine Lodge 

Volcanics (Scholten and others, 1955, p. 375 -376) (fig. 

2). The allochthonous block had been deeply eroded and 

the fault zone exposed before eruption of the Challis 

Volcanics, and the westernmost known exposure of 

the fault along Twelvemile Creek in the Lemhi Range

borders on the eastern edge of the main Challis volcanic 

field. Locally the fault trace is concealed by these young 

volcanic rocks (for example south and west of Mill 

Mountain in the central Lemhi Range).

The close spatial relation of volcanic rocks and the 

fault trace suggests that the fault could have served as 

a conduit for local eruptive centers of Challis and 

Medicine Lodge Volcanics, satellitic to the main Challis 

volcanic field farther west. In areas where volcanism is 

not closely associated with the thrust trace, the 

volcanics might have broken through to the surface 

from the fault sole at relatively shallow depth, or the 

volcanics might have been erupted from the fault zone 

and concealed it. In both the Hayden Creek and 

Sawmill Canyon windows in the central Lemhi Range 

(fig. 2) the fault must be at shallow depth, partly concealed only by the blanket of Challis Volcanics.

THE RELATION OF INTRUSIVE IGNEOUS ROCKS 

AND ASSOCIATED MINERAL DEPOSITS TO 

THE MEDICINE LODGE THRUST SYSTEM

The distribution of exposed intrusive igneous rocks 

and known deposits of metallic minerals in and near 

the central part of the Lemhi Range indicates that their 

emplacement was to a large extent controlled and 

limited by structural features associated with the 

Medicine Lodge thrust system. The close relation of 

stocks, mineral deposits, and the thrust system in the 

Lemhi Range suggests that the mineral resource potential of the entire thrust belt region — from the Snake 

River Plain northward beyond the Big Hole Basin — 

should be reevaluated, and that a primary factor in this 

reevaluation should be the structural framework of the 

Medicine Lodge thrust system.

The relation of quartz monzonite-granodiorite stocks 

and their associated mineral deposits to the Medicine 

Lodge thrust system is quite clear in the mining districts of the central part of the Lemhi Range and the adjacent part of the Beaverhead Mountains; almost all of 

the known occurrences of metallic minerals in this area 

are in the lower part of the allochthonous block, in or 

near quartz monzonite-granodiorite stocks that were 

emplaced low in the block after thrusting. These 

deposits have been mined or prospected in the Little 

Eightmile district (antimony, lead, silver, copper), in the 

Beaverhead Mountains (figs. 1, 2), and in the Gilmore 

Gead, silver, gold), Spring Mountain Gead, silver, copper, iron), Blue Wing (or Patterson) (tungsten, silver, 

copper), and Blue Jay (copper, lead) districts in the 

Lemhi Range. The relation of metallic mineral deposits, 

granodiorite-quartz monzonite stocks, and the Medicine 

Lodge thrust system in these mining districts can be 

briefly summarized. The Blue Jay area is a short distance south and west of the fault exposures on Mill
SOME REGIONAL RELATIONS, IMPLICATIONS, AND PROBLEMS 19

Mountain. Copper and molybdenum minerals are disseminated in the Blue Jay stock, and lead and silver occur in small vein deposits in quartzite adjacent to the 

stock. The Gilmore and Spring Mountain districts are 

east of the Sawmill Canyon window into the fault, and 

lead and silver sulflde and secondary minerals occur in 

veins adjacent to the large, irregular Gilmore stock. The 

Ima mine, the principal mine in the Blue Wing district 

at Patterson, developed a network of tungsten-bearing 

veins and disseminated deposits in a complex thrust 

zone that formed the intrusive roof of the concealed Ima 

stock, which contains disseminated molybdenite. The 

prospects in the northern part of the Little Eightmile 

district explore veins near the Little Eightmile stock, a 

short distance south of the Peterson Creek segment of 

the Medicine Lodge fault.

Other granodiorite-quartz monzonite stocks, without 

known associated metallic mineral deposits or disseminated sulfide minerals, are exposed only in the central 

part of the Lemhi Range. These stocks, like those with 

associated mineral deposits, are roofed in or near imbricate thrust faults where geologic and geophysical 

studies indicate that the Medicine Lodge thrust fault is 

at relatively shallow depth, and thus it seems clear that 

no stocks penetrated far into the allochthonous block. 

Therefore, stocks and associated mineral deposits, if 

any, are likely to be found only in the lower part of the 

allochthonous block, and have been exposed only where 

faulting and erosion have exposed the lower part of that 

block.

Where the higher part of the allochthonous block is 

still preserved in the southern Lemhi Range, southern 

Beaverhead Mountains, and Lost River Range, still hidden stocks and associated mineral deposits, if they are 

present at all, may be suggested by deposits of secondary metallic minerals or by sulflde minerals in small 

veins. Small deposits of secondary copper carbonates 

are in imbricate thrusts south of the Little Eightmile 

stock in the Beaverhead Mountains north of Leadore 

(Ruppel, 1968; Staatz, 1973), and veins containing 

small amounts of silver-bearing galena are present in 

the southern part of the Little Eightmile district. 

Mineral deposits farther south in the Beaverhead 

Mountains — the large lead carbonate deposit at the 

Viola mine in the Nicholia district and small veins containing galena in the Birch Creek district (Shenon, 

1928), farther south — are not associated with exposed 

quartz monzonite-granodiorite stocks. The lead and 

silver sulfide bearing veins at the Wilbert mine, in the 

Dome district near the south end of the Lemhi Range, 

similarly are not near any exposed stock. Most of these 

deposits are in faults and they could reflect leakage 

from buried granodiorite stocks deeper in the 

allochthonous block, or deposition of secondary

minerals above a more deeply buried primary sulfide 

deposit.

The extent of metallic mineral deposits in the brecciated and mylonitized rocks immediately above the fault 

sole, or in rocks beneath the fault is largely untested 

and unknown in areas where the allochthonous block is 

preserved, for no mine workings or drill holes in the districts mentioned above have penetrated deeply enough 

to go through the Medicine Lodge fault zone or into 

rocks beneath the fault. North of the fault trace, 

however, in the lower block, there are many productive 

mines; for example, copper sulfides have been mined 

from vein deposits in the Yellowjacket Formation at the 

Pope-Shenon mine near Salmon, Idaho, and other 

mines and prospects have yielded cobalt, copper, gold, 

thorium, and iron.

There appear to be fairly consistent differences in 

mineral assemblages above and below the fault system 

at least in east-central Idaho. The mines and prospects 

in the upper plate yielded lead, zinc, silver, antimony, 

and tungsten; only a few mines, on the Andy and 

Martha claims at Gilmore, and at Gibbonsville, yielded 

a significant amount of gold. Small deposits of copper 

are widely scattered in the lower part of the upper 

plate, but few have yielded any quantity of copper ore. 

In contrast, cobalt, copper, gold, and thorium are the 

principal metals from mines in the lower plate. The 

reasons for the different mineral assemblages are not 

yet known — whether they reflect different periods of 

mineralization, different levels of mineralization above 

and beneath the Medicine Lodge fault system, different 

sources for the metals, or telescoping of unrelated 

mineral provinces above and beneath the fault system. 

The Medicine Lodge thrust system has localized both 

intrusive activity and mineralization, and this explains 

the distribution of scattered stocks and mineral 

deposits in east-central and central Idaho. It also suggests that more widespread mineral deposits, and 

enhanced prospecting possibilities, may exist in the 

region from the Snake River Plain northward beyond 

the Big Hole Basin, and perhaps even farther north into 

the thrust faulted rocks of the Sapphire tectonic block, 

east of the Bitteroot Valley, Mont. (C. A. Wallace, oral 

commun., 1975). Some specific possibilities for finding 

new mineral deposits include:

(1) Mineralized localities in the lower part of the upper 

plate near quartz monzonite-granodiorite stocks 

that are concealed but whose presence is suggested 

by geophysical or geochemical evidence. Metals likely 

to occur are copper and molybdenum as disseminated 

deposits in stocks, and lead, zinc, silver, and tungsten 

in veins near the stocks. Concealed metallic deposits 

might be found by geochemical methods or suggested 

by small sulfide veins and secondary deposits in im-
20 MEDICINE LODGE THRUST SYSTEM, EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

bricate thrusts or other faults.

(2) The crushed, brecciated, and mylonitized rocks of 

the Medicine Lodge thrust zone are untested and 

unknown, but could contain finely disseminated 

metallic deposits, or other deposits where penetrated 

by stocks. A specific question: Might the gold-bearing 

placer deposits at Chinatown, east of the head of 

Horse Prairie Creek in Montana, be derived from the 

Medicine Lodge thrust system, which is exposed a 

short distance farther south? The source of the gold 

in this placer deposit is not known, although a few 

small gold-bearing veins do occur at the head of Jeff 

Davis Creek, which contains the placer gravels.

(3) Mineral deposits in the lower plate, beneath the 

Medicine Lodge thrust system, perhaps especially 

near known mining districts and near concealed 

stocks found by geophysical means. Also, where the 

upper plate block has been removed by erosion, the 

Yellowjacket Formation (of the lower plate) contains 

deposits of cobalt, copper, gold, and thorium. These 

deposits suggest that similar deposits may underlie 

the upper plate where it is still preserved.

(4) Mineral deposits at greater depth around the 

known but seemingly barren stocks in the lower part 

of the upper plate in the central part of the Lemhi 

Range, perhaps particularly around those partly concealed by the younger Challis Volcanics, like the 

stock in Sawmill Canyon at the head of the Little Lost 

River.

PETROLEUM AND NATURAL GAS RESOURCES 

BENEATH THE MEDICINE LODGE 

THRUST SYSTEM

The search for petroleum and natural gas resources 

in southwest Montana has not been particularly fruitful (Klepper, 1950, p. 80^82; Scholten, 1967), despite 

the occurrence of feasible source rocks, suitable reservoir rocks of Paleozoic and Mesozoic age, and favorable 

structures. Recognition that these rocks disappear 

beneath the leading edge of the Medicine Lodge thrust 

system in the southern part of the Beaverhead Mountains suggests, however, that there is some potential for 

occurrences of petroleum and natural gas beneath the 

thrust system in eastern Idaho north of the Snake 

River Plain. The rocks overridden by the thrust system 

in this region range in age from Devonian through Cretaceous, and include the petroliferous rocks of the 

Jefferson Formation and the organic carbon-rich rocks 

of the Phosphoria Formation (Maughan, 1975). The 

Devonian rocks were the first to overlap the Lemhi 

arch, and suggest the possibility of stratigraphic traps 

on the flanks of the arch. The later, marine Paleozoic 

rocks apparently extended across the arch, from the 

shelf in Montana across a broad region of shallow seas

in eastern Idaho to the miogeocline in central and 

western Idaho. The shelf section is well known in southwest Montana, and the miogeocline section is probably 

partly represented by the allochthonous rocks of the 

southern Beaverhead Mountains and the Lemhi and 

Lost River Ranges.

The Paleozoic rocks beneath the thrust presumably 

include those interfingering facies that reflect the 

changes from shelf sedimentation to miogeoclinal sedimentation across the Lemhi arch. Marine and nonmarine rocks of Triassic, Jurassic, and Cretaceous age, 

overridden by the thrust, probably were mostly 

deposited as wedges of sediment against the eastern 

flank of an again emergent Lemhi arch, although some 

of the lower Mesozoic marine rocks could have extended across it. The northern boundary of Paleozoic 

and Mesozoic rocks beneath the thrust is unknown. The 

boundary must be south of Sawmill Canyon in the 

Lemhi Range (fig. 1) because only rocks of the Yellowjacket Formation are known beneath the thrust from 

there northward. Also, the nature of the boundary is 

unknown; it could be either a tectonic boundary, where 

the Paleozoic and younger rocks have been sheared off 

by the thrust, or a depositional boundary against the 

Lemhi arch, modified by later faulting.

The overthrust block of the Medicine Lodge thrust 

system thus may conceal both source and reservoir 

rocks in a stratigraphic, paleogeographic, and structural framework favorable for the accumulation of 

petroleum and natural gas. The stratigraphic and 

structural setting appears to be comparable in many 

respects to that of the Idaho-Wyoming thrust belt south 

of the Snake River Plain (Armstrong and Oriel, 1965; 

Monley, 1971; Royse and others, 1975), and the potential for accumulations of petroleum and natural gas 

also seems somewhat similar in the two regions.

SUMMARY

The Medicine Lodge thrust fault system, exposed in 

east-central Idaho and southwest Montana, is a major 

segment of the North America Cordilleran fold and 

thrust belt. The fault extends northward from the 

north flank of the Snake River Plain a distance of more 

than 200 km to the west flank of the Big Hole Basin, 

Montana. Its extension farther north, if any, is 

unknown. The fault thus does not merge with the Montana disturbed belt. Movement on the Medicine Lodge 

thrust system began in Early Cretaceous (Albian) time 

and had ended by early Eocene time; most of the movement probably occurred in Late Cretaceous and early 

Paleocene time.

As a result of movement on the Medicine Lodge fault, 

Precambrian and Paleozoic sedimentary rocks 

deposited in the Cordilleran miogeocline in western
REFERENCES CITED 21

Idaho have been telescoped and transported perhaps as 

much as 160 km eastward, to rest on rocks of similar 

age deposited in a marine embayment or seaway in 

southwest Montana. Sedimentation took place on opposite sides of the north-northwest-trending Lemhi 

arch, which from Precambrian Y to Late Devonian time 

was a landmass separating the miogeocline on the west 

from the marine embayment or seaway on the east. In 

Late Devonian time, the arch was overlapped by marine 

sediments, but as a still relatively high area it continued to influence sedimentation patterns through the 

rest of the Paleozoic. In the early Mesozoic, the Lemhi 

arch again was apparently emergent, supplying sediments rather than receiving them. In late Mesozoic and 

earliest Cenozoic time, the arch was overridden by the 

Medicine Lodge thrust block. Fragments of the arch, 

caught in the upper plate of the Medicine Lodge thrust, 

have been interpreted in the past as small islands, but 

instead they are displaced parts of a once major landmass. The arch probably was a southern extension of 

Belt island in the Precambrian and early Paleozoic, and 

the later history of the arch suggests that Belt island 

also could well have continued as an intermittent positive area through much of Paleozoic and Mesozoic time. 

Recognition that the Medicine Lodge thrust fault underlies much of east-central and central Idaho requires 

that many geological problems be reconsidered besides 

those relating to the stratigraphy of Precambrian and 

Paleozoic rocks. One of these problems, unresolved, 

concerns the granitic Beaverhead pluton, which has 

been interpreted as an Ordovician and Silurian intrusive body on the basis of isotope age determinations. 

If this age is correct, the pluton must have been sheared 

off by the Medicine Lodge thrust, and displaced eastward with the enclosing sedimentary rocks. If the 

isotope ages have been mixed as a result of tectonism, 

the pluton could instead be a thrust slice of Precambrian X Dillon Granite Gneiss, an interpretation 

more in keeping with relations of similar rocks farther 

north in the Beaverhead Mountains. Or, if the granite 

of the pluton is as little deformed as published descriptions suggest, it could be of Tertiary age, intruded into 

the allochthonous block.

The Medicine Lodge thrust was deeply eroded and 

widely exposed before eruption of the Challis Volcanics 

in Tertiary time, and the present thrust trace is closely 

paralleled and partly concealed by volcanic rocks from 

eruptive centers satellitic to the main Challis volcanic 

field. The close association of fault trace and volcanic 

rocks suggests that the fault served as a conduit for 

local eruptive centers.

The relations of post-thrusting granodiorite -quartz 

monzonite stocks and related mineral deposits to the 

Medicine Lodge fault, mainly in the central part of the

Lemhi Range, suggest that the stocks and related 

mineral deposits are largely confined to the lower part 

of the allochthonous block. Undiscovered mineral 

deposits possibly could be found by (1) examining areas 

where the Medicine Lodge fault and the lower part of 

the allochthonous block are near stocks but concealed 

either by the higher part of the allochthonous block or 

by later volcanic rocks; (2) searching for hidden stocks 

and associated mineral deposits by geophysical or 

geochemical techniques; and (3) using small sulfide 

veins and deposits of secondary metallic minerals, apparently not related to intrusive rocks, as guides to concealed stocks and mineral deposits that might be found 

at greater depth. The brecciated, crushed, and 

mylonitized rocks of the thrust zone itself have not 

been prospected, and their mineral potential is 

unknown. The rocks beneath the thrust have yielded 

substantial mineral deposits where they are exposed, 

and might be expected to contain similar deposits yet 

concealed.

Finally, the Paleozoic and Mesozoic sedimentary 

rocks beneath the Medicine Lodge thrust system north 

of the Snake River Plain include both source and reservoir rocks favorable for accumulation of petroleum and 

natural gas. The stratigraphic, paleogeographic, and 

structural setting suggests that the energy resource potential of this region should be reevaluated.

REFERENCES CITED

Anderson, A. L., 1959, Geology and mineral resources of the North 

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

Geology Pamph. 118, 92 p.

Anderson, A. L., and Wagner, W. R., 1944, Lead-zinc-copper deposits 

of the Birch Creek district, Clark and Lemhi Counties, Idaho: 

Idaho Bur. Mines and Geology Pamph. 70, 43 p.

Anderson, R. A., 1948, Reconnaissance survey of the geology and ore 

deposits of the southwestern portion of Lemhi Range, Idaho: 

Idaho Bur. Mines and Geology Pamph. 80, 18 p.

Armstrong, F. C., and Oriel, S. S., 1965, Tectonic development of 

Idaho-Wyoming thrust belt: Am. Assoc. Petroleum Geologists 

Bull. v. 49, no. 11, p. 1847-1866.

Armstrong, R. L., 1975, Precambrian (1500 m.y. old) rocks of central 

Idaho, the Salmon River Arch and its role in Cordilleran sedimentation and tectonics: Am. Jour. Sci., v. 275—A, p. 437-467.

Beutner, E. C., 1968, Structure and tectonics of the southern Lemhi 

Range, Idaho: Pennsylvania State Univ. Ph. D. thesis, University 

Park, Pa., 106 p.

Callaghan, Eugene, and Lemmon, D. M., 1941, Tungsten resources of 

the Blue Wing district, Lemhi County, Idaho: U.S. Geol. Survey 

Bull. 931-A, p. 1-21.

Coppinger, W., 1974, Stratigraphic and structural study of Belt 

Supergroup and associated rocks in a portion of the Beaverhead 

Mountains, southwest Montana, and east-central Idaho: Miami 

Univ. Ph.D. thesis, Oxford, Ohio, 224 p.

Denison, R. H., 1968, Middle Devonian fishes from the Lemhi Range 

of Idaho: Fieldiana—Geology, v. 16, no. 10, p. 269-287.
22 MEDICINE LODGE THRUST SYSTEM, EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

Embree, G. F., Hoggan, R. D., Skipp, B. A., and Williams, E. J., 1975, 

Stratigraphy of the southern Beaverhead Range, Clark and 

Lemhi Counties, Idaho [abs.]: Geol. Soc. America, Rocky Mtn. 

Section, Program with Abs., p. 607.

Hait, M. H., Jr., 1965, Structure of the Gilmore area, Lemhi Range, 

Idaho: Pennsylvanian State Univ. Ph. D. thesis, University Park, 

Pa., 134 p.

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.

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.

Huh, Oscar, 1967, the Mississippian System across the Wasatch line, 

east-central Idaho, extreme southwestern Montana, in Montana 

Geol. Soc. Guidebook 18th Ann. Field Conf., Centennial basin of 

southwest Montana, 1967: p. 31 -62.

Hyndman, D. W., Talbot, J. L., and Chase, R. B., 1975, Boulder 

batholith — A result of emplacement of a block detached from 

the Idaho batholith infrastructure?: Geology, v. 3, p. 401 —404.

Kirkham, V. R. D., 1927, A geologic reconnaissance of Clark and 

Jefferson and parts of Butte, Custer, Fremont, Lemhi, and 

Madison Counties, Idaho: Idaho Bur. Mines and Geology Pamph. 

19, 47 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.

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

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

Map I ^33.

Lowell, W. R., and Klepper, M. R., 1953, Beaverhead formation, a 

Laramide deposit in Beaverhead County, Montana: Geol. Soc. 

America Bull., v. 64, no. 2, p. 235-244.

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

Park, Pa., 204 p.

M'Gonigle, J. W., 1965, Structure of the Maiden Peak area, MontanaIdaho: Pennsylvania State Univ. Ph. D. thesis, University Park, 

Pa., 146 p.

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

Beaverhead Range east of Salmon, Idaho: Univ. Idaho M.S. 

thesis, Moscow, Idaho, 53 p.

Mamet, B. L., Skipp, Betty, Sando, W. J., and Mapel, W. J., 1971, 

Biostratigraphy of Upper Mississippian and associated Carboniferous rocks in south-central Idaho: Am. Assoc. Petroleum 

Geologists Bull., v. 55, no. 1, p. 20-33.

Mapel, W. J., Reed, W. H., and Smith, R. K., 1965, Geologic map and 

sections of the Doublespring Pass quadrangle, Custer and Lemhi 

Counties, Idaho: U.S. Geol. Survey Geol. Quad. Map GQ -464.

Mapel, W. J., and Shropshire, K. L., 1973, Preliminary geologic map 

and section of the Hawley Mountain quadrangle, Custer, Butte, 

and Lemhi Counties, Idaho: U.S. Geol. Survey Misc. Field Studies 

Map MF-546 [1974].

Maughan, E. K., 1975, Organic carbon in shale beds of the Permian 

Phosphoria Formation in eastern Idaho and adjacent states, in 

Wyoming Geol. Assoc. Guidebook 27th Ann. Field Conf.: p. 

107-115.

Monley, L. E., 1971, Petroleum potential of Idaho-Wyoming overthrust belt: Am. Assoc. Petroleum Geologists Mem. 15, p. 

509-530.

Myers, W. B., 1952, Geology and mineral deposits of the northwest 

quarter of Willis quadrangle and adjacent Browns Lake area, 

Beaverhead County, Montana: U.S. Geol. Survey open-file report, 

46 p.

Ramspott, L. D., 1962, Geology of the Eighteenmile Peak area, and

petrology of the Beaverhead Pluton: Pennsylvania State Univ.

Ph. D. thesis, University Park, Pa., 215 p. 

Ross, C. P., 1925, The copper deposits near Salmon, Idaho: U.S. Geol.

Survey Bull. 774, 44 p. 

____1933, The Dome mining district, Butte County, Idaho: Idaho

Bur. Mines and Geology Pamph. 39, 12 p. 

____1961, Geology of the southern part of the Lemhi Range, Idaho:

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

___1962, Paleozoic seas of central Idaho: Geol. Soc. America Bull.

v. 73, no. 6, p. 769-794.

Royse, F., Jr., Warner, M. A., and Reese, D. L., 1975, Thrust belt 

structural geometry and related stratigraphic problems, Wyoming-Idaho-northern Utah, in Deep drilling frontiers of the 

Central Rocky Mountains: Rocky Mountain Assoc. Geologists, p. 

41-54.

Ruppel, E. T., 1964, Strike-slip faulting and broken basin-ranges in 

east-central Idaho and adjacent Montana, in Geological Survey 

research 1964: U.S. Geol. Survey Prof. Paper 501-C, p. 

C14-C18.

____1968, Geologic map of the Leadore quadrangle, Lemhi County, 

Idaho: U.S. Geol. Survey Geol. Quad. Map GQ-733.

____1975, Precambrian Y sedimentary rocks in east-central

Idaho, Chap. A in Ruppel, E. T., and others, Precambrian and 

Lower Ordovician rocks in east-central Idaho: U.S. Geol. Survey 

Prof. Paper 889, p. 1 -23.

Ruppel, E. T., Watts, K. C., and Peterson, D. L., 1970, Geologic, 

geochemical, and geophysical investigations in the northern part 

of the Gilmore mining district, Lemhi County, Idaho: U.S. Geol. 

Survey open-file report, 56 p.

Ruppel, E. T., Ross, R. J., Jr., and Schleicher, David, 1975, Precambrian Z and Lower Ordovician rocks in east-central Idaho, 

Chap. B in Ruppel, E. T., and others, Precambrian and Lower Ordovician rocks in east-central Idaho: U.S. Geol. Survey Prof. 

Paper 889, p. 25-34.

Ryder, R. T., and Ames, H. T., 1970, Palynology and age of the 

Beaverhead Formation and their tectonic implications in the 

Lima region, Montana-Idaho: Am. Assoc. Petroleum Geologists 

Bull., v. 54, no. 7, p. 1155-1171.

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.

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.

____1967, Structural frame work and oil potential of extreme 

southwestern Montana in Montana Geol. Soc. Guidebook, 18th 

Ann. Field Conf., Centennial basin of southwest Montana: p. 

7-19.

____1968, Model for evolution of the 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, Inc., p. 473 -489.

Scholten, Robert, Keemon, K. A., and Kupsch, W. O., 1955, Geology of 

the Lima region, southwestern Montana and adjacent Idaho: 

Geol. Soc. America Bull., v. 66, no. 4, p. 345^04.

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.

Shannon, J. P., Jr., 1961, Upper Paleozoic stratigraphy of eastcentral Idaho: Geol. Soc. America Bull., v. 72, no. 12, p. 

1829-1836.
REFERENCES CITED 23

Shenon, P. J., 1928, Geology and ore deposits of the Birch Creek district, Idaho: Idaho Bur. Mines and Geology Pamph. 27, 25 p.

Sloss, L. L., 1954, 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.

Staatz, M. H., 1972, Geology and description of the thorium-bearing 

veins, Lemhi Pass quadrangle, Idaho and Montana: U.S. Geol. 

Survey Bull. 1351, 94 p.

____1973, Geologic map of the Goat Mountain quadrangle, Lemhi 

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

Survey Geol. Quad. Map GQ-1097 [1974].

Thune, H. W., 1941, Mineralogy of the ore deposits of the western portion of the Little Eightmile mining district, Lemhi County, 

Idaho: Univ. Idaho M.S. thesis, Moscow, Idaho.

Tucker, D. R., 1975, Stratigraphy and structure of Precambrian Y

(Belt?) metasedimentary and associated rocks, Goldstone Mountain quadrangle, Lemhi County, Idaho, and Beaverhead County, 

Montana: Miami Univ. Ph. D. thesis, Oxford, Ohio, 221 p.

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

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

____1917, Geology and ore deposits of the Mackay region, Idaho: 

U.S. Geol. Survey Prof. Paper 97, 129 p.

U.S. Geological Survey, 1971, Aeromagnetic map of part of eastcentral Idaho: U.S. Geol. Survey open-file map.

Wheeler, J. O., 1966, Eastern tectonic belt of western Cordillera in 

British Columbia, in A symposium on the tectonic history and 

mineral deposits of the western Cordillera, Vancouver, B. C., 

1964: Canadian Inst. Mining and Metallurgy Spec. Vol. 8, p. 

27^5.

Wilson, M. D., 1970, Upper Cretaceous-Paleocene synorogenic conglomerates of southwestern Montana: Am. Assoc. Petroleum 

Geologists Bull., v. 54, no. 10, p. 1843-1867.

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