Geologic Map of the Central Beaverhead Mountains, 

Lemhi County, Idaho, and Beaverhead County, Montana

U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3413
Cover. Top left—Three-dimensional draped image showing the east-directed Cow Creek thrust fault, Lower Cow Creek-Agency Creek tear 

fault segment, and folds associated with the thrust fault, as well as the normal-fault system that overprinted the compressional structures, 

defined the Ramsay Mountain ridge and formed the Yearian-Cow-Agency basin(s) into which Cenozoic volcanic and sedimentary rocks were 

deposited. See figure 3 for geologic features. View looks south across Agency Creek and up the Cow Creek drainage. Geologic polygons, fold 

axes, and fault lines are draped over the shaded relief map. Image by Stuart A. Giles.

Top right—Three-dimensional draped image of the map area, looking north. Image by Stuart A. Giles.

Botttom—Photograph of angular unconformity between Mesoproterozoic Gunsight Formation and overlying Mississippian Middle Canyon and 

Scott Peak Formations. View looks southwest from Goat Mountain toward lower Little Eightmile Creek. Mountain goats are along the ridgeline, 

above the unconformity. Photograph by K. Lund.
Geologic Map of the Central Beaverhead 

Mountains, Lemhi County, Idaho, and 

Beaverhead County, Montana

By Karen Lund

Pamphlet to accompany

Scientific Investigations Map 3413

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Lund, K., 2018, Geologic map of the central Beaverhead Mountains, Lemhi County, Idaho, and Beaverhead County, 

Montana: U.S. Geological Survey Scientific Investigations Map 3413, pamphlet 27 p., scale 1:50,000, 

https://doi.org/10.3133/sim3413.

ScienceBase data release GIS and other files that support this report are available at 

https://doi.org/10.5066/P905PTI4

ISSN 2329-132XX (online)
iii

Contents

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

Setting.....................................................................................................................................................1

Present Study ........................................................................................................................................1

Depositional Settings of Mesoproterozoic and Paleozoic Rocks..........................................................4

Settings of Cenozoic Deposits.....................................................................................................................8

Deformation History.......................................................................................................................................9

Pre-Cretaceous Deformation..............................................................................................................9

Cretaceous Compressional Structures...........................................................................................10

Predominantly Proterozoic Domains......................................................................................10

Maiden Peak Domain.......................................................................................................10

Deadhorse Thrust Fault ..........................................................................................10

Shenon Creek Thrust Fault.....................................................................................10

Cabin Thrust Plate ............................................................................................................10

Cow Creek Thrust Plate ...................................................................................................10

Southern, Predominantly Paleozoic Domains.......................................................................11

Goat Mountain Domain....................................................................................................11

Grizzly Hill Domain ............................................................................................................12

Thompson Gulch Domain ................................................................................................13

Baby Joe Gulch Domain ..................................................................................................13

Thrust-Fault Geometries...........................................................................................................13

Cenozoic Extensional Structures .....................................................................................................14

Description of Map Units............................................................................................................................16

Surficial Deposits................................................................................................................................16

Cenozoic Sedimentary Rocks ...........................................................................................................16

Eocene Challis Volcanic-Plutonic Complex....................................................................................17

Challis Volcanic Group..............................................................................................................17

Intrusive Rocks...........................................................................................................................18

Mesozoic To Neoproterozoic? Sedimentary and Metasedimentary Rocks..............................19

Paleozoic Intrusive Rocks .................................................................................................................22

Mesoproterozoic Metasedimentary Rocks....................................................................................22

Lemhi Group................................................................................................................................23

Paleoproterozoic Intrusive Rocks....................................................................................................24

References Cited..........................................................................................................................................24

Figures

1. Index map showing the study area and nearby geographic features.................................2

2. Index map showing areas of previous mapping in the study area ......................................3

3. Index map showing locations of named structures, structural domains, and

topographic features....................................................................................................................5

4. Stratigraphic correlation chart for units in individual structural domains and

subdomains....................................................................................................................................6
iv

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

International System of Units to U.S. customary units

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
Geologic Map of the Central Beaverhead Mountains, 

Lemhi County, Idaho, and Beaverhead County, Montana

By Karen Lund

Introduction

Setting

The Beaverhead Mountains (fig. 1) have a uniquely longlived, complex geologic setting. The mountain range includes 

orthogneiss in the western margin of the Paleoproterozoic and 

Archean Wyoming craton. This circa (ca.) 2.5 giga-annum 

(Ga) orthogneiss (Kellogg and others, 2003) intruded metasedimentary rocks of the Montana metasedimentary province 

(possibly older than 3 Ga; M'Gonigle and others, 1991; 

Mueller and Frost, 2006). After its formation, the basement 

block was structurally reactivated and uplifted multiple times. 

In the Mesoproterozoic, ca. 1.4 Ga extension resulted in a 

basement high on the east, paired with a depositional depression on the west. Debris shed from the basement high was 

transported westward into a large Mesoproterozoic basin to 

form the Lemhi Group and related strata; the coastline for this 

basin is preserved about 10 kilometers (km; 6 miles [mi]) east 

of the map area (Tysdal and others, 2005) but is structurally 

disrupted and concealed along the eastern part of the map area. 

The very thick, fine-grained, nonfossiliferous arkosic strata of 

the Lemhi Group and overlying units were tilted and eroded 

prior to deposition of Paleozoic strata.

In the late Neoproterozoic to early Paleozoic, the map 

area was part of the intracontinental rift zone along the 

western portion of the paleocontinent Laurentia (the Precambrian core of North America) that was caused by breakup of 

the supercontinent Rodinia. Both Precambrian basement and 

Mesoproterozoic Lemhi basin rocks were upthrown in forming 

the Lemhi arch (Sloss, 1954; Scholten, 1957; “note that Lemhi 

Arch” was used in those initial publications and in Lund, 

2008; Lund and others, 2010), which formed along the rifted 

shoulder of western Laurentia (Lund, 2008). The Late Cambrian to Early Ordovician syenite pluton of the Beaverhead 

Mountains (commonly called “Beaverhead pluton” and dated 

in Lund and others, 2010) and stock of Lemhi Pass (Gillerman 

and others, 2008) were emplaced as part of these extensional 

events; the intrusive rocks help provide timing for and location 

of a significant segment of this rift event. Paleozoic strata preserved in the map area were deposited on parts of the Lemhi 

arch, including some deposited on the inboard edge of the 

passive margin to the west and some in the paired intracratonal 

basin to the east. Formation of the arch resulted in a major 

Paleoproterozic-Paleozoic and Mesoproterozoic-Paleozoic 

angular unconformity across which Paleozoic strata were 

deposited. Paleozoic rock packages vary significantly across 

the map area in (1) thicknesses, (2) presence and position of 

erosional horizons, and (3) local and transitional facies—all 

related to punctuated rift activity and the resultant structurally 

unstable margin.

The map area lies in the hinterland of the Cretaceous 

thrust belt; thus, the Mesoproterozoic and Paleozoic rocks 

were complexly involved in east-directed, deep-seated 

compression. Mesoproterozoic rocks, with Paleozoic rocks 

piggybacking on top of them, caused basin inversion as the 

strata were stacked over the original basin margins on the 

eastern side of the mapped area. In this deformation, slivers 

of basement were also transported (Scholten and others, 1955; 

Scholten and Ramspott, 1968; M'Gonigle, 1994; M'Gonigle 

and Hait, 1997) and Mesoproterozoic and Paleozoic facies, 

deposited in a variety of settings, were juxtaposed.

Several phases of Tertiary extensional faulting also 

encompassed the range and reactivated or obliquely cut the preexisting structural geometries. Normal faults of this generation 

formed basins that localized and preserved Eocene volcanic 

rocks and Eocene through Miocene basin-fill deposits. The 

Beaverhead Mountains lie within the northern Basin and Range 

Province and the present topography formed during Miocene to 

Pleistocene(?) basin-and-range faulting.

Present Study

This map portrays detailed geology of the central 

Beaverhead Mountains at the 1:24,000 scale. Although the 

map is appropriate for use at 1:24,000 scale, it is prepared to be 

printed at 1:50,000 scale because of the large size of the study 

area. The initial geologic mapping for this study was carried 

out primarily between 1996 and 2002 and was generalized for 

incorporation in the geologic map of the Salmon National Forest (Evans and Green, 2003). Limited topical work was undertaken in subsequent years and resulted in minor stratigraphic 

reinterpretation but significant structural reinterpretation.

Previous studies have covered most of the study area 

(fig. 2). Those in the northern part of this map area were published in conjunction with studies of thorite deposits in the 
2 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

Figure 1. Index map showing the study area and nearby geographic features.

IDAHO

MONTANA

Study area

SALMON

NATIONAL

FOREST

SALMON

NATIONAL

FOREST

BEAVERHEAD

NATIONAL

FOREST

BEAVERHEAD

 NATIONAL

 FOREST

SALMON

 NATIONAL

 FOREST

CHALLIS

 NATIONAL

 FOREST

BEAVERHEAD NATIONAL

FOREST

Lemhi

Salmon

Leadore L e m h i R a n g e

Central Beaverhead Mountains

Maiden Peak spur

Salmon River

Tendoy Mountains

River

Lemhi

SALMON

NATIONAL

FOREST

BEAVERHEAD

NATIONAL

FOREST

CHALLIS

NATIONAL

FOREST

15

93

93

278

28

Base from U.S. Geological Survey digital data,

The National Map, 2017

Universal Transverse Mercator projection, zone 12N

North American Datum of 1983

LEMHI COUNTY

CUSTER COUNTY

0 4 8 12 16 20 KILOMETERS

0 4 8 12 16 20 MILES

114° 113°30' 113°

45°

44°30'

Lemhi

Pass

Bannock

Pass

BEAVERHEAD

COUNTY

LEMHI

COUNTY

Map area

MONTANA

IDAHO

Lemhi Pass district (Staatz, 1972, 1973, 1979). Maps of the 

southern corner are presented in Lucchitta (1966) and Ruppel 

(1968). Quadrangle-scale geologic maps are available for 

the Maiden Peak spur (eastern margin of the map area and 

20 km east, fig. 1) of the Beaverhead Mountains, overlapping the eastern edge of the present map area (M'Gonigle 

and others, 1991; M'Gonigle, 1994). Where the present map 

overlaps that of M'Gonigle (1994) and Lucchitta (1966), 

geologic contacts are modified and compiled; selected bedding and cleavage measurements from those authors are 

also shown on the map. Generalized geologic maps and 

attitudinal data for Cenozoic basin deposits east and west 
Introduction 3

Figure 2. Index map showing areas of previous mapping in the study area. (1, Blankenau [1999]; 2, Lucchitta 

[1966]; 3, Lund and others [2003b]; 4, M'Gonigle [1994]; 5, M'Gonigle and Hait [1997]; 6, Ruppel [1968]; 7, Staatz [1972]; 

8, Staatz [1973]; 9, Staatz [1979]; 10, VanDenburg [1997]; 11, Hansen [1983]).

Extent of geologic

mapping, this

publication

IDAHO

MONTANA

CUSTER

COUNTY

6

8

2

4

10 5

1

3

7

9

113°40' 113°30' 113°20' 113°10'

45°

44°50'

44°40'

11

BEAVERHEAD CO

LEMHI CO

of the range (VanDenburg, 1997; VanDenburg and others, 

1998; Blankenau, 1999) are compiled with the new bedrock 

mapping to better illustrate the geologic development of the 

range in comparison to adjacent basins.

In comparison to the previous studies, the present map 

benefits from significant improvements in our understanding 

of regional Mesoproterozoic stratigraphic nomenclature and 

sedimentologic interpretation as well as of regional structural 

geometry (Tysdal 2000a, b, 2002, 2003; Evans and Green, 

2003). The resultant, regionally integrated stratigraphic 

nomenclature was applied to this area by Lund and others 

(2003b). The present study provides specific details for 

these units in the Beaverhead Mountains and helps develop 

a regional picture of Mesoproterozoic basin geometries. The 

present map also incorporates modern, regional stratigraphic 

terminology and depositional interpretations for the Paleozoic 

rocks (Skipp and others, 1979a; Isaacson and others, 1983; 

Grader and Dehler, 1998).
4 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

The collection of stratigraphic and structural data for the 

map area results in new interpretations of the stratigraphic 

relations, structural geometries, and structural history. To illustrate the stratigraphic settings and structural history, identified 

geographic areas containing a set of similar stratigraphic and 

structural characteristics are designated as structural domains. 

Locations and stratigraphy of domains are illustrated in figures 

3 and 4.

Depositional Settings of 

Mesoproterozoic and Paleozoic Rocks

The Mesoproterozoic rocks in the map area are subdivided 

in this study into the Apple Creek (Yac) and Gunsight Formations (Yg) (upper two formations in the Lemhi Group) plus the 

overlying Mesoproterozoic Swauger Formation (Ys) (Lund 

and others, 2003b). A total of about 3,500 meters (m) of these 

nonfossiliferous, fine-grained arkosic metasandstone and siltite 

rocks are in the map area. The general depositional settings for 

these three formations were below-wavebase-marine, fluvial, and 

tidal-marine environments, respectively (Tysdal, 2000b, 2003). 

The eastern margin of their depositional basin is exposed on the 

east side of the Beaverhead Mountains, 18 km east-southeast of 

Bannock Pass, where arkosic conglomerate and sandstone of the 

Gunsight Formation unconformably overlie Paleoproterozoic 

orthogneiss (Xg) of the Wyoming basement province and the 

contact is undeformed (Tysdal and others, 2005).

This unconformity is also present in the southeast corner 

of the map area where it is characterized by minor shearing and chloritization due to decoupling during movement 

on the subjacent Cabin thrust fault. The PaleoproterozoicMesoproterozoic unconformity was previously mapped as 

a younger-on-older, possibly reactivated thrust “fault D” in 

the hanging wall of the Cabin thrust fault (M'Gonigle, 1994). 

Sediment transport was generally west directed, away from 

that basin margin, and the strata are finer grained and exhibit 

deeper-water depositional characteristics westward into the 

main basin (Tysdal and others, 2005). In the map area, this 

trend is most apparent in the Gunsight Formation, which 

contains finer grained and more biotite-rich rocks in the thrust 

plate in the western part of the range compared to rocks in 

the eastern thrust plate. In addition to the stratigraphic and 

sedimentologic data, detrital zircon dating of the sedimentary 

rocks (Lund and others, 2004; Link and others, 2007; Aleinikoff and others, 2012) shows that a significant portion of the 

zircon was sourced locally from Paleoproterozoic basement 

rocks generally east of the map area.

A particular characteristic of Mesoproterozoic units in the 

northern part of the study area is angular relations between the 

Gunsight Formation and the underlying Apple Creek Formation. This is exhibited in each of the several thrust plates of the 

northern and central map area. The discordance is manifested 

by a shallowly dipping, planar basal contact of the Gunsight 

Formation that truncates moderately dipping to overturned 

bedding as well as folds in the underlying Apple Creek Formation. In these cases, neither upright nor overturned folds, 

which deformed the Apple Creek rocks beneath the contact, 

are expressed in the overlying Gunsight and Swauger rocks. 

Additionally, preserved thicknesses of the Gunsight Formation 

between Apple Creek and Swauger Formations vary across the 

area. The Gunsight Formation bears extra-basinal conglomerate 

in nearby areas north and east of the map (Tysdal and others, 

2005) as well as northwest of the map (Tysdal, 2003). These 

observations suggest that the Gunsight rocks were deposited 

across an angular unconformity. Although many aspects of the 

Apple Creek and Gunsight Formations remain little studied on 

a regional basis, detailed studies in the Lemhi Range indicates 

that contacts between units are generally gradational in that area 

(Tysdal, 2000b, 2003). Additionally, there are other indications 

that the top of the Apple Creek Formation is erosional in the 

northern Beaverhead Mountains and may be an unconformity 

on a regional scale (Tysdal and others, 2005). If that is the case, 

the amount of angularity between these two units increases eastward from the Lemhi Range to the Beaverhead Mountains.

Prior to deposition of Paleozoic rocks, Mesoproterozoic 

formations were tilted and eroded as a package, as demonstrated by the major angular unconformity at the base of 

the Paleozoic rocks. A likely cause of the deformation was 

formation of the Lemhi arch. Above the unconformity, Cambrian through Triassic strata were deposited on the flanks and 

across the top of the arch, and thus, in various depositional 

environments and water depths. Important characteristics 

of the local Paleozoic strata include the abrupt deepening 

of depositional environments from rocks of the Beaverhead 

Mountains to those of the Lemhi Range that is the next range 

to the west (Beutner and Scholten, 1967; Isaacson and others, 

1983; Grader and Dehler, 1998; Lund and others, 2010) and 

discontinuous preservation and multiple unconformities in 

the lower Paleozoic section, both of which were caused by 

down-to-basin (down-to-west) fault activity related to the 

rifting of Rodinia, on the miogeoclinal (western) flank of 

the Lemhi arch (Grader and Dehler, 1998). Other evidence 

for the proximity of such structures to the map area includes 

the rip-up clasts in the unnamed Cambrian to Ordovician 

dolostone (O_d) and the quartzite-clast conglomerate of the 

Neoproterozoic?-Lower Cambrian? quartz-pebble conglomerate to sandstone, herein referred to as the Neoproterozoic?-

Lower Cambrian? conglomerate (_Zc) as well as the limited 

preservation of these two units.

In the Goat Mountain domain (figs. 3 and 4), the unconformity is overlain by Lower Mississippian carbonate rocks 

from deeper water environments than most of the Paleozoic 

packages. Elsewhere in the map area, the MesoproterozoicPaleozoic unconformity is primarily overlain by Pennsylvanian to Permian strata. Across the Rocky Canyon fault in 

the Grizzly Hill domain, the unit above the unconformity 

changes relatively abruptly. West of the Rocky Canyon fault, 

the unconformity is overlain by Permian strata, but east of the 

fault, the unconformity is overlain by Upper Mississippian 

strata. The area where the change in units is manifested is 
Depositional Settings of Mesoproterozoic and Paleozoic Rocks 5

Figure 3. Index map showing locations of named structures, structural domains, and topographic features. Salmon basin

Horse

Prairie

basin

Lower Cow Creek–

Agency Creek segment

K e n n e y C r e e k — L e m h i P a s s

b a s i n

COW CREEK THRUST FAULT

RAILROAD

CANYON

FAULT

DEADHORSE

THRUST FAULT

Tend

oy

b

asin

segment

Yearian—

Cow

—Ag

e

n

cy

basin

C O W C R E E K T H R U S T F A U L T

L e m h i

b a s i n

Leadore basin segment

JAKES CANYON

THRUST FAULT

CEDAR

CANYON FAULT

BABY JOE

GULCH THRUST

FAULT

ROCKY CANYON FAULT CABIN

THRUST

FAULT

GOAT

MOUNTAIN — BLACK CANYON FAULT SYSTEM

COW CREE

TH

K

RUST FAU LT

L E M H I PA S S N O R M A L F A U L T

BULL MOOSE FAULT

DAN PATCH FAULT

LITTLE EIGHTMILE FAULT SYSTEM

HORSE PRAIRIE BASIN FAULT

MAIDEN PEAK FAULT

BIG HOLLOW

THRUST FAULT

SHENON CREEK

THRUST

FAULT

Little

Eightmile

basin

THOMPSON

GULCH

THRUST

FAULT 

THOMPSON GULCH

THRUST FAULT 

BABY JOE GULCH

THRUST FAULT

45°

44°50'

44°40'

113°40' 113°30' 113°20' 113°10'

Base from U.S. Geological Survey digital data,

The National Map, 2017

Universal Transverse Mercator projection, zone 12N

North American Datum of 1983

0 2 4 6 8 KILOMETERS

0 2 4 6 8 MILES

Basin boundary

Grizzly Hill

domain

Transcurrent fault

Normal fault

Thrust fault

Map boundary

EXPLANATION

Goat Mountain domain

Northwest Grizzly Hill subdomain

Stroud subdomain

Thrust duplex subdomain

Rocky Canyon subdomain

Lemhi Pass domain

Shenon Creek thrust plate

Cow Creek thrust plate

Pattee Creek domain

Basins

Cabin thrust plate

Baby Joe Gulch domain

Thompson Gulch domain

Maiden Peak domain

Map area

MONTANA

IDAHO
6 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

Figure 4. Stratigraphic correlation chart for units in individual structural domains and subdomains. Location of domains, subdomains, 

and thrust plates are shown in figure 3. Column labeled “Thrust plates north of Goat Mountain domain” includes the Cow Creek thrust 

plate and Lemhi Pass and Pattee Creek domains plus Shenon Creek thrust plate. Wavy red lines indicate angular unconformities.

Ms

Mr

^d

*Ms

Pp

Mb

O_g

Ok

SOs

Dj

Mm

O_d

_Zc

Ys

_f

Xg

Yac

Yg

MESOZOIC TO NEOPROTEROZOIC? SEDIMENTARY 

AND METASEDIMENTARY ROCKS

Angular unconformity

Dinwoody Formation (Lower Triassic)

Phosphoria Formation (Permian)

Snaky Canyon Formation, undivided (Lower

Permian to Upper Mississippian)

Bluebird Mountain Formation (Upper Mississippian)

Railroad Canyon Formation (Upper Mississippian)

Scott Peak Formation (Middle Mississippian)

Middle Canyon Formation (Middle and Lower

Mississippian)

Jefferson Formation (Upper Devonian)

MESOPROTEROZOIC METASEDIMENTARY ROCKS

Swauger Formation (Mesoproterozoic)

LEMHI GROUP

Gunsight Formation (Mesoproterozoic)

Coarse siltite unit of Apple Creek

Formation (Mesoproterozoic)

PALEOPROTEROZOIC INTRUSIVE ROCKS

Orthogneiss (Paleoproterozoic)

Saturday Mountain Formation (Lower Silurian

to Middle Ordovician) Kinnikinic Quartzite (Middle Ordovician)

Dolostone (Middle Ordovician? to Upper

Cambrian?)

Flathead Quartzite (Middle Cambrian)

Conglomerate (Lower Cambrian? to Neoproterozoic?)

PALEOZOIC INTRUSIVE ROCKS

Syenogranite (Early Ordovician to Late Cambrian)

EXPLANATION

Xg

O_g

_f

Pp

Xg

Yac

Yg

Ys

Yac

Yg

Ys

Ok

SOs

Dj Dj

Mm Mm

Ms

Mr

Mm

Ms

Mr

Ms

Mr

^d

Yac

Yg

Ys

Yac

Yg

Ys

Yg

Ys Ys

PMs

PMs

Pp

PMs

Pp

PMs

^d

Pp

PMs

Ms

Mr

Ms

Mb

?

?

O_d

?

?

?

_Zc

?

?

_Zc

Goat

Mountain

domain

Thrust

plates

north of

Goat

Mountain

domain

Stroud

subdomain

Thrust

duplex

subdomain

Thompson

Gulch

domain

Grizzly Hill domain

Baby Joe

Gulch

domain Rocky

Canyon

subdomain

Cabin

thrust

plate

Maiden

Peak

domain Northwest

Grizzly Hill

subdomain
Depositional Settings of Mesoproterozoic and Paleozoic Rocks 7

covered, but the stratigraphic relations indicate that the Rocky 

Canyon fault reactivated a Paleozoic (and possibly also older) 

basin-margin fault.

Cambrian and Devonian units overlying Paleoproterozoic 

gneiss on the eastern margin of the map (Maiden Peak spur on 

fig. 1) were deposited in the cratonal basin setting. The stratigraphic section in the Maiden Peak spur (M'Gonigle, 1994) is 

significantly different from that of sections in the Beaverhead 

Mountains. Devonian strata are both in the Maiden Peak spur, 

where they overlie older cratonal basin strata, and in the Baby 

Joe Gulch domain (fig. 3), where they are in stratigraphic association with miogeoclinal units. The Upper Devonian Jefferson 

Formation overtopped the Lemhi arch (Sloss, 1954; Scholten, 

1957) and was deposited over rocks of both settings. The Pennsylvanian and younger units were deposited over older units in 

transitional settings between those of the continental shelf and 

those of the cratonal basin (Skipp and others, 1979a).

The Paleozoic rocks of different depositional settings 

and, with different units overlying the unconformity, compose 

characteristic stratigraphic packages. In addition to discrete 

stratigraphy, the individual stratigraphic packages are generally delimited by faults and characterized by similar structural 

styles. The combined stratigraphic and structural characteristics 

are used to define discrete structural domains and subdomains.

The Goat Mountain domain contains Mississippian units 

that directly overlie Mesoproterozoic units along a sheared 

angular unconformity. The Mississippian section includes 

basal carbonate phyllite of the Lower to Middle Mississippian 

Middle Canyon Formation, massive limestone of the Middle 

Mississippian Scott Peak Formation, and carbonaceous shale 

of the Upper Mississippian Railroad Canyon Formation (Mm, 

Ms, and Mr, respectively). This Mississippian package formed 

on the outer carbonate banks of the continental shelf (Skipp 

and others, 1979b). The lack of Early Mississippian Antler 

orogenic sediments between them and the underlying Mesoproterozoic units demonstrates that this Mississippian section 

was deposited on a local pre-Mississippian high between the 

foreland basin to the west and the cratonal basin to the east 

(see model in Skipp and others, 1979b).

Distinctly different, thrust-bounded stratigraphic sections 

are in the Grizzly Hill domain. The structurally lowest thrust 

plate is the northwest Grizzly Hill subdomain (fig. 3), which 

contains the broadest age range for rocks at the base of the 

Paleozoic section. In this subdomain, Neoproterozoic?–Lower 

Cambrian? conglomerate (_Zc) or Cambrian(?) dolostone 

(_d) locally overlie the unconformity. The most widespread 

unit at or near the base of the Paleozoic section is the Upper 

Mississippian–Lower Permian Snaky Canyon Formation 

(PMs), an outer-carbonate-bank, miogeoclinal deposit (Skipp 

and others, 1979b). The overlapping Permian Phosphoria Formation exposed in the map area was deposited at the extreme 

western edge of the basin. The Phosphoria depositional basin 

was a sub-basin that developed late in the history of the 

Paleozoic miogeocline and was characterized by restricted 

sediment influx and nutrient-rich upwelling seawater (Wardlaw 

and Collinson, 1986). Structurally higher in the Grizzly Hill 

domain, the thrust duplex subdomain (fig. 3) carried Middle 

Mississippian Scott Peak to Upper Mississippian–Permian 

Snaky Canyon (Ms, Mr, Mb, and PMs). These units originated 

in a miogeoclinal shelf setting. The most distinctive characteristic about this stratigraphic section is the presence of the most 

complete exposure of the Railroad Canyon Formation (Mr type 

section, Wardlaw and Pecora, 1985). The Railroad Canyon Formation is a siltstone, shale, and calcisiltstone succession. It is 

interpreted to have been deposited in an intraramp extensional 

basin along the central shelf ramp. These intraramp rocks are 

flanked to both east and west by carbonate-rich deposits (Batt 

and others, 2007), and the structural setting presented herein 

changes the interpretation from a paleogeographic explanation 

to a structural juxtaposition of facies.

The composition of Upper Mississippian to Permian 

Snaky Canyon rocks changes across the Railroad Canyon fault. 

East of the fault, the Snaky Canyon Formation is relatively 

more quartz rich than equivalent rocks west of the fault. The 

three members recognized west of the fault are difficult to recognize east of the fault using the same criteria, suggesting the 

presence of different facies. In the eastern part of the area, these 

rocks were originally mapped by Lucchitta (1966) as the Upper 

Mississippian to Permian Quadrant Formation. The present 

mapping suggests that these more quartz-rich rocks are probably a transitional facies between the Snaky Canyon Formation 

of the central miogeoclinal ramp and the Quadrant Formation 

of the eastern (upper) ramp (Skipp and others, 1979b).

Three distinctly different stratigraphic packages structurally overlap the subdomains of the Grizzly Hill domain 

(fig. 3). The Thompson Gulch domain contains the structurally 

lowest of these stratigraphic packages and is characterized by 

phyllitic rocks of the Lower to Middle Mississippian Middle 

Canyon Formation (Mm). The Middle Canyon Formation in 

this domain is a greenschist facies, isoclinally folded tectonite 

in which stratigraphic details are obscured. This metamorphic 

fabric contrasts with the general lack of tectonic fabric in the 

same unit in the Goat Mountain domain.

The Baby Joe Gulch domain structurally overlies the 

Thompson Gulch domain and contains unmetamorphosed 

Middle Ordovician Kinnikinic Quartzite (Ok), Middle Ordovician to Lower Silurian Saturday Mountain Formation (SOs), 

and Upper Devonian Jefferson Formation (Dj). This is the 

only domain containing the Jefferson Formation in the main 

part of the central Beaverhead Mountains and its presence 

indicates that the Jefferson was deposited unconformably over 

Ordovician strata in this setting. This package of rocks was 

transported eastward, telescoping deeper water shelf sections 

onto shallower water (upper-shelf) sections. The highest thrust 

fault of the Baby Joe Gulch domain transported a large thrust 

sliver of Late Cambrian to Early Ordovician Beaverhead 

pluton, which is nonconformably overlain by Middle Ordovician Kinnikinic Quartzite and Middle Ordovician to Lower 

Silurian Saturday Mountain Formation (this study; Lund and 

others, 2010). The units in these upper two domains originated 

in distinctly different settings in comparison to other Paleozoic 

rocks in the map area.
8 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

The depositional settings of Paleozoic rocks in the central 

Beaverhead Mountains were markedly different from the 

setting of equivalent units in the Maiden Peak spur (fig. 1), 

where lower Paleozoic strata were deposited unconformably 

on Archean and Paleoproterozoic rocks (M'Gonigle and others, 1991; M'Gonigle, 1994). In the Maiden Peak spur, basal 

Paleozoic rocks locally include the Middle Cambrian Flathead 

Quartzite (_f) and (or) Wolsey Shale (not present in the map 

area) of the cratonal basin or the Upper Devonian Jefferson 

Formation that is found in both miogeoclinal and cratonal 

basin settings; the Upper Cambrian-Silurian section is missing 

between Middle Cambrian and Devonian rocks.

The present mapping determined that different amounts 

of time are represented by the gap in the rock record along the 

Mesoproterozoic-Paleozoic unconformity. On the east side of 

Horse Prairie basin, the unconformity represents a gap in time 

of about 2 billion years (Paleoproterozoic gneiss at 2.45 Ga 

overlain by Middle Cambrian rocks at about 510 mega-annum 

[Ma] or Devonian rocks at about 385 Ma). In the five thrust 

plates carrying Paleozoic strata in the map area, the stratigraphic gaps beneath the unconformity include missing rocks 

between the Mesoproterozoic (about 1.4 Ga) formations and 

locally the Neoproterozoic–Lower Cambrian conglomerate 

(_Zc), the Lower to Middle Mississippian Middle Canyon Formation (Mm), the Middle Mississippian Scott Peak 

Formation (Ms), or the Upper Mississippian–Lower Permian 

(359–326 Ma) Snaky Canyon Formation (PMs) in different 

domains (fig. 4). Prior to the present mapping, these basal 

angular unconformities were interpreted as Paleocene to early 

Eocene normal faults excising different parts of the Paleozoic 

section (Janecke and others, 2001).

Settings of Cenozoic Deposits

The Challis Volcanic Group is the manifestation of 

widespread, Eocene (about 52–45 Ma) crustal extension in 

the area north of the Snake River Plain, Idaho, into Canada. 

These volcanic rocks and related orogenic sediments were 

deposited in synextensional fault-bounded basins (fig. 3). On 

the west side of the Beaverhead Mountains, the main southern 

depocenters included the Tendoy basin segment and the Lemhi 

basin, both part of the Lemhi River drainage that empties into 

the complex, north-striking Salmon basin (Harrison, 1985); 

on the east, the main depocenter was the Horse Prairie basin 

(fig. 3). Lesser fault graben and depocenters include the narrower Kenney Creek-Lemhi Pass, Yearian-Cow-Agency, and 

Little Eightmile basins that obliquely transect the range in 

different orientations. Successive changes in basin geometries 

caused differences in the age and volume of deposits among 

the basins (VanDenburg and others, 1998; Janecke and others, 

2001). The latter part of the progression reflects initiation of 

the present basin-and-range topography.

The timing of onset and early history of basin development are constrained by detailed studies of the volcanic 

successions (Staatz, 1979; M'Gonigle, 1994; M'Gonigle 

and Hait, 1997; VanDenburg, 1997; Blankenau, 1999). The 

Kenney Creek-Lemhi Pass basin, a structural paleovalley 

that obliquely transects the range in a southeasterly direction 

(fig. 3), hosts the oldest part of the volcanic succession. The 

deposits in this early graben system include basal conglomerate, tuffs with conglomerate zones, and thick dacite flows.

A younger succession of tuffs, a thick andesite flow unit, 

and overlying sandstone were deposited in both the previously 

active northwest-striking Kenney Creek-Lemhi Pass basin and 

the younger, north-striking Tendoy basin segment; equivalent 

tuffs and conglomerates were deposited in the north-striking 

Horse Prairie basin. 

The youngest part of the volcanic succession is predominantly tuffaceous rocks, including the regionally recognized tuff of Curtis Ranch at the base. This young package 

is restricted to the north-striking Tendoy basin segment and 

Horse Prairie basin but is thicker in the Horse Prairie basin 

where several dacite flow units and interlayered tuffs and conglomerates are at the top of the section.

Samples of volcanic rocks from the Horse Prairie basin 

have been dated. Near the base of the section there, ages are 

about 48.6 Ma; near the upper part of the section, an age of 

46.0 Ma was determined (M'Gonigle and Dalrymple, 1993; 

VanDenburg and others, 1998).

Challis Volcanic Group is also preserved in the 

northeast-striking Little Eightmile basin and in local faultbounded exposures along the range crest nearby. These 

volcanic rocks remain undated so the relations between them 

and the larger exposures are unclear. However, the perched 

deposits in southeast-striking paleobasins indicate that the 

volcanic rocks originally were more widely distributed above 

the older rocks and were deposited more widely across older 

bedrock and not restricted to the basin settings where they 

are best preserved at present.

In addition to the Challis Volcanic Group, the structural basins of the Beaverhead Mountains also contain as 

many as three unconformity-bounded sequences of orogenic 

sedimentary deposits. These range in age from late Eocene 

to Miocene. The sequences have different characteristics in 

the different basins and through time, reflecting deposition 

punctuated by coeval normal faulting and tilting (VanDenburg 

and others, 1998).

The oldest unconformity-bounded sequences are Eoceneto Oligocene-age deposits in the large basins on both sides of 

the range (Tsl; called sediments of Bear Creek in the Horse 

Prairie basin, VanDenburg and others, 1998). The sediments 

range in grain size from cobble conglomerate to tuffaceous 

shale and contain interlayered gastropod-bearing, lacustrine 

limestones. The early Kenney Creek-Lemhi Pass and YearianCow-Agency basins, which transect the range, locally contain 

thin deposits of the same unconformity-bounded sequence 

(Staatz, 1979; Blankenau, 1999).

The middle unconformity-bounded sequences are composed of middle Oligocene- to Miocene-age deposits. On the 

west side of the Beaverhead Mountains between Sandy and 

Kenney Creeks, these are cobble to boulder conglomerates 
Deformation History 9

(Blankenau, 1999), which were deposited along the active 

range-front fault at the southern end of the Salmon-basin 

detachment fault of Janecke and others (1998). On the east 

side of the mountain range, in the Horse Prairie basin, faulting was less active and the middle unconformity-bounded 

sequence (dated at 30 to 19 Ma by VanDenburg and others, 

1998) was deposited in a more quiescent setting. Here, it is 

composed of clastic deposits ranging in grain size from pebble 

conglomerate through shale and contains thin limestone 

lenses. This unit is called “the sediments of Everson Creek” 

by M'Gonigle (1994), VanDenburg and others (1998), and 

Blankenau (1999).

The youngest unconformity-bounded basin-fill deposits are those called mudstone deposits (Tm) and formed in a 

paleobasin near Bannock Pass in the southern Horse Prairie 

basin. This young sequence contains a 16–12 Ma, relatively 

fine-grained succession (M'Gonigle 1994; VanDenburg and 

others, 1998). Approximate age-equivalent deposits are in the 

Lemhi basin as well (VanDenburg and others, 1998).

Within Cenozoic deposits in the southern part of the 

Yearian-Cow-Agency basin are linear outcrops of brecciated 

Mesoproterozoic rocks belonging to either the Apple Creek 

or Gunsight Formation. They form east-dipping ribs about 

200–400 m wide, several of which directly abut or overlie the 

volcanic rocks; other ribs of the brecciated Mesoproterozoic 

rocks are surrounded by valley-fill deposits. These exposures 

of brecciated Mesoproterozoic rocks are interpreted to be 

Eocene to Miocene slide blocks (megabreccia) transported 

into the southern segment of the Yearian-Cow-Agency basin 

during basin development.

Deformation History

The rocks were affected by multiple, diverse deformation events. Late Cretaceous compressional deformation and 

Cenozoic extensional events are recognized in the broader 

region, but pre-Cretaceous structural events are not well documented. Faults related to Mesoproterozoic extension, which 

downdropped the Mesoproterozoic Lemhi basin relative to the 

uplifted Paleoproterozoic basement rocks, have not been identified in the field. Similarly, although Neoproterozoic–early 

Paleozoic rifting and formation of the Lemhi arch controlled 

miogeocline geometries and deposition in and adjacent to the 

map area, related structures also remain unidentified. Either or 

both of these extensional events might have caused the tilting 

of Mesoproterozoic strata beneath the Paleozoic unconformity 

in the study area.

Potential Mesoproterozoic deformation is represented 

by moderately dipping to overturned bedding and folds in the 

Apple Creek Formation, features which are not expressed in 

the overlying Gunsight and Swauger rocks. These characteristics of the contact are widespread in each of the several 

Cretaceous-age thrust plates in the northern and central parts 

of the study area. These examples of discordant bedding 

suggest that a deformation event interrupted Mesoproterozoic 

sedimentation. However, corollary evidence for geometry and 

age of a pre-Gunsight Formation Mesoproterozoic fold event, 

manifested primarily in the Beaverhead Mountains exposures, 

needs further study.

The entire Mesoproterozoic section was tilted, possibly 

gently folded, and exposed to erosion prior to deposition of 

the Paleozoic rocks. These relations are evidence of deformation before Cambrian or later Paleozoic sedimentation, 

depending on the location. One likely cause for deformation 

of Mesoproterozoic rocks prior to Paleozoic sedimentation is 

Neoproterozoic to Late Cambrian–Early Ordovician extension during breakup of the supercontinent Rodinia (Lund and 

others, 2010). Across the region, early Paleozoic units show 

significant differences in thickness between the Beaverhead 

Mountains near the axis of the Lemhi arch and deeper water 

units in the Lemhi Range, as recognized from stratigraphic 

studies (Beutner and Scholten, 1967; James and Oaks, 1977; 

Isaacson and others, 1983; Ruppel and Lopez, 1988). The differences in stratigraphy and depositional water depth indicate 

that major down-to-the-west, basin-bounding normal faults 

existed between those two depositional settings (Isaacson and 

others, 1983; Grader and Dehler, 1998). One structure of this 

type is inferred in the southern part of the map, where there is 

a significant change in which unit overlies the Mesoproterozoic-Paleozoic unconformity across the Rocky Canyon fault. 

Permian rocks overlie the unconformity west of the fault but 

Mississippian rocks overlie the unconformity east of the fault 

(across an interval of about 7.4 km [4.6 mi] between these 

Paleozoic exposures near the southern edge of Horse Prairie 

basin). This change indicates the location of a generally northstriking, down-to-the-west, Mississippian–Pennsylvanian 

synsedimentary growth fault.

Pre-Cretaceous Deformation

The contact between Apple Creek and Gunsight Formations is interpreted as an angular unconformity on this map 

because, in several places, the Apple Creek Formation is 

folded at outcrop to kilometer scales but overlain by unfolded 

Gunsight Formation. This indicates Mesoproterozoic-aged 

deformation. The Mesoproterozoic units are also broadly 

arched beneath the Paleozoic unconformity, providing further 

indication of Mesoproterozoic or Neoproterozoic deformation. 

The contact between Apple Creek and Gunsight Formations 

in the structural domain that lies in the footwall to the Cow 

Creek thrust fault is characterized by pervasive cleavage and 

is cut by many small quartz veinlets. The shearing may have 

been caused by decoupling related to broad arching of the 

Mesoproterozoic formations because evidence of tectonic 

transport along the contact is lacking. Both the arching of 

Mesoproterozoic units and the sheared Apple Creek-Gunsight 

contact are found in several different Cretaceous structural 

domains, suggesting that the decoupling predated Cretaceous 

thrust faulting.
10 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

Cretaceous Compressional Structures

Predominantly Proterozoic Domains

The Apple Creek-Gunsight contact mapped in this study 

closely matches the location of a thrust fault mapped by Staatz 

(1973) and Ruppel (1978) and of the “Medicine Lodge thrust 

fault” of Ruppel (1978), Ruppel and Lopez (1984), Ruppel and 

others (1993). The present mapping indicates that the contact 

in the central Beaverhead Mountains is a sheared unconformity, but evidence was not found to substantiate the presence 

of the Medicine Lodge thrust fault. In the Maiden Peak spur, 

the Paleoproterozoic-Mesoproterozoic contact (“fault D” of 

M'Gonigle, 1993, 1994) is shown as part of the Medicine Lodge 

thrust fault (Ruppel, 1978), but this contact is reinterpreted 

as an unconformity (Tysdal and others, 2005). The Medicine 

Lodge thrust fault was also shown in the Lemhi Range and 

northern Beaverhead Mountains by Ruppel (1978) and Ruppel 

and Lopez (1984), but more recent mapping reveals that a thrust 

fault of this type does not exist in those areas either (Tysdal, 

2002; Tysdal and others, 2005, respectively). Thus, a youngeron-older Medicine Lodge thrust fault has not been substantiated 

in any of the areas where it was originally depicted, including in 

the central Beaverhead Mountains.

Maiden Peak Domain

Deadhorse Thrust Fault

The structurally lowest thrust fault is exposed east of 

the Horse Prairie basin on the western flank of the Maiden 

Peak spur (Maiden Peak domain on figure 3). There, Paleoproterozoic basement orthogneiss was faulted over the Upper 

Devonian Jefferson Formation along a north-striking thrust 

fault (M'Gonigle, 1994). A number of thrust slivers with 

similar relations are mapped across the Maiden Peak spur 

directly east of the present map (M'Gonigle, 1994; M'Gonigle 

and Hait, 1997). Another exposure of these is in a structural 

window along Deadhorse Canyon at the eastern map boundary. There, a small window of Mississippian limestone is juxtaposed beneath Paleoproterozoic granite gneiss (reinterpreted 

from M'Gonigle, 1994). These east-directed thrust slivers of 

basement rocks are oblique to, structurally overlapped by, and 

therefore probably older than the rest of the thrust faults in the 

study area.

Shenon Creek Thrust Fault

South of the Shenon Creek drainage on the eastern edge 

of the Horse Prairie basin, small exposures of older rocks 

beneath Cenozoic units reveal a thrust fault that placed Mesoproterozoic Gunsight over probable Mississippian limestone 

and Cambrian(?) quartzite-pebble conglomerate. This Shenon 

Creek thrust fault (fig. 3) is the northeasternmost thrust fault 

that transported rocks of the Lemhi Group. Because of the 

amount of normal-fault activity that formed the Horse Prairie 

basin, direct connections between this thrust fault and others 

in the map area cannot be determined.

Cabin Thrust Plate

Adjacent to and structurally above the Deadhorse 

Canyon window is the regionally persistent, east-striking 

Cabin thrust fault (fig. 3). It crosses the southern end of the 

Maiden Peak spur and is named for exposures directly east of 

the map area (Scholten and others, 1955; Tysdal and others, 

2005; reinterpretation of fault C of M'Gonigle, 1993, 1994; 

M'Gonigle and Hait, 1997; note that use of “Cabin thrust 

fault” and “Cabin thrust plate” herein is more restricted 

than that of Skipp, 1987). The hanging wall contains Mesoproterozoic Gunsight Formation unconformably overlying 

Paleoproterozoic granite basement gneiss and the footwall 

contains small remnants of Cambrian quartzite to Devonian 

carbonate that had been deposited directly on Paleoproterozoic gneiss (Scholten and others, 1955; M'Gonigle, 1994). 

The Cabin thrust fault is north- or northeast-vergent, and it 

is important for several reasons: (1) it lies at the geographic 

junction between the Maiden Peak spur and the Beaverhead 

Mountains, where it bounds very different stratigraphic 

and structural domains; (2) it is nearly perpendicular to and 

overlying a set of directly subjacent, north-striking thrust 

faults of the Maiden Peak spur (M'Gonigle, 1993, 1994); and 

(3) it transported thick sections of Mesoproterozoic arkosic 

metasandstone and quartzite of the Gunsight and Swauger 

Formations with a thin sliver of their basal, unconformable 

contact with Paleoproterozoic gneiss (Tysdal and others, 

2005) over a footwall that contains Paleozoic rocks lying on 

gneiss with no Mesoproterozoic strata preserved (M'Gonigle, 

1994; M'Gonigle and Hait, 1997). As such, this is a key 

thrust fault that transported strata from the Mesoproterozoic 

Lemhi basin onto the Mesoproterozoic-Paleozoic basement 

high of southwestern Montana.

Because of the significant Cenozoic normal-fault displacement related to formation of the Horse Prairie basin, 

the Big Hollow thrust fault (fig. 3) mapped adjacent to the 

northeastern corner of the map area by Hansen (1983) could 

connect with the Cabin thrust fault or, because of changes in 

basement or basin geometries within the map area, footwall 

and hanging wall relations may vary significantly along the 

fault such that the fault may have stepped up section to the 

northeast in a manner described by Skipp (1985) from the 

adjacent area to the south.

Cow Creek Thrust Plate

The large, relatively well-exposed Cow Creek thrust fault 

crosses much of the map area, from the northwestern corner 

to the east-central area (fig. 3). The main segment is relatively 

well exposed and is manifested by Apple Creek Formation 

juxtaposed over Gunsight Formation strikes southeast-northwest for about 17 km (11 mi) across the range from upper 

Cow Creek in Idaho to Nip and Tuck Creek in Montana. For 

several kilometers away from the fault in both the hanging 

wall and footwall, rocks are characterized by complex ductile 

deformation fabrics. In footwall rocks (Lemhi Pass domain, 

fig. 3), phyllitic foliation or pervasive cleavage are most well 
Deformation History 11

developed for about 1 km from the fault trace. Bedding in 

footwall rocks was deformed into northwest-striking open 

folds, and cleavage is primarily southwest dipping, both 

generally parallel to the Cow Creek thrust fault. In hangingwall rocks (Cow Creek thrust plate, fig. 3), phyllitic foliation 

or pervasive cleavage are superimposed on bedding for about 

5 km from the fault trace. Although upright bedding generally dips east and overturned bedding generally dips west, the 

foliation consistently strikes northwest and dips moderately 

southwest, parallel to the trace of the Cow Creek thrust fault.

A northwest-striking, northeast-vergent overturned 

anticline (traced for about 3.7 km [2.2 mi]) parallels the Cow 

Creek thrust fault in its hanging wall along much of the main 

range-crest segment. The fold is characterized by overturned 

bedding and by a broad zone of pervasive, northwest-striking, 

southwest-dipping, axial-planar cleavage and phyllitic foliation. The overturned limb of the fold is partly truncated by an 

unnamed, minor thrust fault (traced for about 6.2 km [3.8 mi]) 

within the Apple Creek Formation section. Elsewhere in the 

hanging wall, well-developed foliation and the spaced cleavage 

that overprinted the foliation both consistently strike north and 

dip west. The hanging wall is also characterized by numerous 

east-overturned, minor folds that deformed bedding.

Whereas the main segment of the Cow Creek thrust fault 

strikes northwest across the range to Cow Creek, the fault 

changes direction and strikes north along lower Cow Creek 

and northeast along the central reaches of Agency Creek for 

6.5 km (4 mi). Although the amount of fault separation along 

the lower Cow Creek-Agency Creek segment (fig. 3) was 

reduced by several normal faults and this segment is largely 

buried beneath rocks of the Challis Volcanic Group, two lines 

of evidence indicate the location of this cryptic segment of 

the Cow Creek thrust fault: (1) older rocks of the Apple Creek 

Formation remain on the western (hanging wall) side of the 

fault relative to the upper Apple Creek and Gunsight Formations on the east (footwall) side, and (2) the fault system is 

paralleled by map-scale folds in footwall rocks.

Another northwest-striking segment of the Cow Creek 

thrust fault, also largely concealed beneath Cenozoic volcanic 

and basin deposits, is present along Gould Basin and as far 

northwest as Kenney Creek (more than 12 km [7.5 mi] long). 

The fault is exposed in and near central Pattee Creek, where 

it is defined by older-over-younger relations of Apple Creek 

Formation juxtaposed over Gunsight Formation. In another 

small exposure of hanging-wall rocks of the Cow Creek thrust 

fault, complexly deformed Apple Creek Formation rocks and 

older-on-younger relations can be interpreted by projecting 

units across lower Kenney Creek. Much of the offset on this 

Pattee Creek segment was reduced by Cenozoic normal faulting, particularly along the western extent of the Lemhi Pass 

normal fault.

Geometries of the segments of the Cow Creek thrust 

fault indicate that, relative to the main segment and the Pattee 

Creek segment, the lower Cow Creek-Agency Creek segment 

is a down-to-the-north step that was a lateral ramp or tear fault 

during compression.

Several megascopic, open folds in bedding are in the 

Apple Creek and Gunsight Formations of both the hanging 

wall and footwall of the Cow Creek thrust fault. Fold axes 

are parallel to the local adjacent strike of the thrust fault. One 

large fold east of Agency and Cow Creeks has a curvilinear 

trace that describes a north- through northwest-striking arc. 

The arcing fold lies in the footwall of the Cow Creek thrust 

fault, and the fold arc parallels the trace of the main segment 

of the Cow Creek thrust fault and its lower Cow CreekAgency Creek ramp or tear segment. 40Ar/39Ar analysis of metamorphic white mica and biotite 

in the thrust-fault-related fabrics of Mesoproterozoic strata 

produced complex age data (Lund and others, 2004). The age 

data showed that the micas were not detrital (they are younger 

than the about 1.4 Ga age of the rocks) rather, that the micas 

formed during younger regional metamorphic events. The age 

data are complex; they do not reveal a consistent age for the 

mica formation or for the structural events in which they grew.

Southern, Predominantly Paleozoic Domains

In the southeastern part of the map area, where Paleozoic 

strata were involved in Cretaceous deformation, transport was 

also northeast directed but structural domains had complicated 

rotational motions relative to each other. However, the details 

of style and geometry of compressional structures are different 

from those developed in the domains encompassing Mesoproterozoic strata in the northern part of the map area. Southeastern structural domains are demarcated by steeply dipping, 

curvilinear compressional structures.

Goat Mountain Domain

The Goat Mountain domain is defined by a curvilinear set 

of compressional faults, which control the drainages of Black 

Canyon and Peterson Creek, and by a related set of faults 

bracketing Goat Mountain (generalized as “Goat MountainBlack Canyon fault system” in figure 3). These related strands 

have southeast-dipping, listric geometries. The western extents 

of these faults exhibit geometries that are consistent with 

sinistral, transpressional movement, whereas the northeastern 

extents of the faults exhibit components of older-on-younger 

(reverse fault) offset. Several fault strands were reactivated by 

Eocene or younger, down-to-the-southeast, normal displacement. The Mississippian strata, which are part of the Goat 

Mountain domain, were unconformably deposited on Mesoproterozoic strata. These Mississippian rocks originated from 

an outer-shelf miogeoclinal setting in contrast to the strata that 

primarily overlie the unconformity in the domain to the east 

(Grizzly Hill domain, fig. 3), which represent shelf transitional 

to cratonal settings of the Upper Mississippian to Permian 

system. Thus, the Mississippian strata as well as the underlying Mesoproterozoic strata in the Goat Mountain domain 

originated in deeper water settings (farther west) than most 

other stratigraphic sections in the map area. This structural 

interpretation is significantly different from published maps 
12 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

and reconnaissance studies because, in one interpretation, 

the Mesoproterozoic unconformity was depicted as a thrust 

fault (Staatz, 1973; Ruppel, 1978) and, in another interpretation, this angular unconformity was depicted as a normal fault 

(called “Goat Mountain fault” in Janecke and others, 2001).

Grizzly Hill Domain

Another large curvilinear compressional fault system 

bounds a complex thrust domain herein named the “Grizzly 

Hill domain” and extending from Little Eightmile fault 

system to the Rocky Canyon fault (fig. 3). The curvilinear 

fault system, parallel to and southeast of Little Eightmile 

Creek, juxtaposed Apple Creek and Gunsight Formations of 

the Grizzly Hill domain against the Mississippian rocks of 

the Goat Mountain domain. Although largely obscured by 

the Cenozoic deposits that fill Little Eightmile Creek, the 

bounding fault system underwent sinistral oblique reverse 

offset. The Grizzly Hill domain is underlain by the Mesoproterozoic formations and Paleozoic units of a broad range of 

ages atop the Mesoproterozoic-Paleozoic unconformity. The 

Mesoproterozoic-Paleozoic unconformity in the Grizzly Hill 

domain was previously interpreted as a normal fault (named 

the “Grizzly Hill fault” in Janecke and others, 2001).

Between Little Eightmile Creek and Mollie Gulch, the 

Apple Creek Formation was deformed into a large, northeastvergent, overturned, megascopic anticline expressed by 

progressive eastward overturning of bedding and associated 

southwest-dipping, axial-planar foliation in the Apple Creek 

Formation (cross section B–B′ ). The overlying Gunsight 

Formation was not involved in the folding of the Apple Creek 

Formation; this provides evidence from the southern part of 

the map area as well as from the northern for possible Mesoproterozoic deformation before deposition of the Gunsight 

Formation. Additionally, the depositional contact of Paleozoic 

units over all three Mesoproterozoic units is widely preserved 

in the Grizzly Hill domain, indicating Precambrian-age tilting 

of the older rocks before Paleozoic deposition. On previous 

maps, several thrust and normal faults were interpreted within 

the Mesoproterozoic section (Staatz, 1973; Ruppel, 1978; and 

Janecke and others, 2001); additionally, the MesoproterozoicPaleozoic unconformity in the Grizzly Hill domain was previously named the Grizzly Hill fault (Janecke and others, 2001). 

The interpretations herein are significantly different from 

those previous.

The major compressional structure of the Grizzly Hill 

domain is the Jakes Canyon thrust fault. The Grizzly Hill domain 

is subdivided into three subdomains by (1) structural complications related to lateral ramps and tear faults in the hanging wall 

of the Jakes Canyon thrust fault and (2) a structural geometry 

whereby the units above the Mesoproterozoic-Paleozoic unconformity change across the ramp or tear faults. Each subdomain is 

identified by significantly different structural geometries and by 

disparate Paleozoic sections.

The Stroud subdomain, at the western extent of the 

Jakes Canyon thrust fault, is a compound thrust sliver that 

double stacked the Mesoproterozoic-Permian unconformity. 

This subdomain is bounded on the northwest by an oblique 

reverse fault and contains two associated minor thrust slivers 

(fig. 3). In the structurally lowest rocks of the subdomain, 

the Mesoproterozoic-Permian contact zone was anticlinally 

folded. The Stroud subdomain terminates on the east along 

a lateral ramp in Jakes Canyon. The upper thrust fault of the 

Stroud subdomain (carrying Middle Canyon Formation and 

Mesoproterozoic strata) and the Thompson Gulch thrust fault 

(fig. 3) may be kinematically linked.

The other two subdomains of the Grizzly Hill domain 

are composed of the same Mississippian to Lower Permian 

units but have different structural characteristics on either side 

of the Railroad Canyon fault. The larger of the subdomains, 

herein called “thrust duplex subdomain,” is bounded by the 

Jakes Canyon thrust fault on the west and north and by the 

Railroad Canyon fault, a lateral ramp. The main characteristic 

of this subdomain is a complex stack of imbricate-thrust 

slivers forming a hinterland (generally west)-dipping thrust 

duplex (cross section C–C′ ) that is confined to this subdomain. 

The Rocky Canyon subdomain, east of the Railroad Canyon 

fault, is overlapped by Cenozoic deposits of the southern 

Horse Prairie basin east of Hood Gulch, so the Jakes Canyon 

thrust fault and the unconformity in its footwall are exposed 

only at the northeastern corner of the subdomain. Although 

both the thrust duplex and Rocky Canyon subdomains contain 

the same basic stratigraphy, the Rocky Canyon subdomain 

does not contain the hinterland-dipping thrust duplex, so it 

is simpler and thinner than the thrust duplex subdomain. The 

structural differences in the subdomains on either side of the 

Railroad Canyon fault document that the geometry of the fault 

is a lateral ramp in the Jakes Canyon thrust fault.

The Rocky Canyon fault is a down-to-the-west normal 

fault that forms the eastern boundary of the Rocky Canyon 

subdomain. Although the offset on the Rocky Canyon fault 

is relatively minor in the southern part of the fault, offset is 

significantly greater across the northern part, where Pennsylvanian strata of the Rocky Canyon subdomain are juxtaposed 

against Mesoproterozoic strata of the Cabin thrust plate. A 

small exposure of Phosphoria Formation on the east side of the 

Rocky Canyon fault (originally called a klippe by Lucchitta, 

1966) is probably a remnant of the Mesoproterozoic-Paleozoic 

unconformity as present throughout the Grizzly Hill domain. 

South of the Phosphoria exposure, Middle Mississippian Scott 

Peak Formation directly overlies Mesoproterozoic strata along 

a faulted contact. Based on those stratigraphic and structural 

relations, the Jakes Canyon thrust fault either wedged out 

eastward or changed stratigraphic levels across the Rocky 

Canyon fault. This change of thrust fault character is at the 

edge of the Precambrian high as documented throughout the 

Cabin thrust plate (Scholten and others, 1955; Lucchitta, 1966; 

Skipp, 1985), and the presence of the basin margin may have 

caused the northeast-vergent Jakes Canyon thrust fault to cut 

structurally down from Paleozoic rocks in the Grizzly Hill 

domain into Proterozoic rocks to the east. Similar structural 

geometries of stepped-up-to-the-east Paleozoic basin margin, 

which were cut and transported by the Cabin thrust fault, 
Deformation History 13

were used to explain changes in ages of hanging wall and 

footwall rocks along the Cabin thrust fault for a nearby area 

to the south (Skipp, 1985). The fault carrying the Mississippian and younger units may also have been reactivated by a 

low-angle normal fault, as shown by Lund and others (2003b), 

and may continue to the south as part of the Divide Creek 

fault (Lucchitta, 1966; note that the Divide Creek fault does 

not continue to the north of the Wildcat fault as depicted in 

Janecke and others, 2001).

Thompson Gulch Domain

The southern margin of the thrust-duplex and Rocky 

Canyon subdomains, as well as the Railroad Canyon lateralramp fault, are overlapped by the west-striking Thompson 

Gulch thrust fault. This fault superimposed older and more 

distal Middle Canyon Formation over Upper Mississippian 

to Permian strata. The Middle Canyon strata were metamorphosed to marble phyllite and most bedding features 

were transposed. This is the only Paleozoic unit in the map 

area with regional metamorphic mineral growth and fabrics. 

Although the Thompson Gulch thrust fault was not offset 

across the underlying Railroad Canyon fault, the thrust fault is 

in a laterally down-ramped position east of the fault compared 

to its position on the thrust-duplex subdomain on the west side 

of the fault. Short thrust-fault segments in the southern Stroud 

subdomain and Goat Mountain domain are kinematically 

related to the Thompson Gulch thrust fault. These segments 

transported Mesoproterozoic as well as Paleozoic strata due to 

lateral-ramp faults, especially in Jakes Canyon.

Baby Joe Gulch Domain

Along the west-striking range-front segment near the 

town of Leadore, Idaho, west-striking thrust faults, herein 

called the Baby Joe Gulch thrust fault system (fig. 3), overlapped the Thompson Gulch domain as well as the subdomains and oblique thrust and lateral ramp faults of the Grizzly 

Hill domain and the Cabin thrust plate. The lower imbricate 

contains Kinnikinic Quartzite (Ok) and Saturday Mountain 

(SOs) and Jefferson Formations (Dj); locally, small slivers 

of Late Cambrian to Early Ordovician syenite gneiss of the 

Beaverhead pluton (O_g) are at the base of the thrust plate. 

The upper imbricate primarily contains the Late Cambrian 

to Early Ordovician syenite gneiss of the Beaverhead pluton 

and the unconformably overlying Middle Ordovician Kinnikinic Quartzite. None of the units in the Baby Joe Gulch 

domain are in other thrust plates of the central Beaverhead 

Mountains (except for the Jefferson Formation, which is also 

in the Maiden Peak spur). They were transported from deeper 

water settings and match settings of rocks in the Lemhi Range 

to the west. The rocks in the imbricate faults were folded and 

broken by high-angle faults, coincident with and in part prior 

to thrust emplacement.

The Baby Joe Gulch thrust fault system extends to the 

mouth of Hawley Creek at the southeast corner of the present 

map, where the basal fault was named “Hawley Creek thrust 

fault” (Lucchitta, 1966; Skipp, 1985). The semi-continuous 

thrust-fault segments from west of Jakes Canyon to the mouth 

of Hawley Creek (and south) confirm that the Baby Joe Gulch 

thrust fault system along the range front is a more complicated 

version of the Hawley Creek thrust fault and the overlying 

Hawley Creek thrust plate. Despite earlier interpretations 

that the Hawley Creek fault is a major thrust system crossing 

central Idaho (Skipp, 1987), the Hawley Creek thrust fault is 

more recently interpreted as an imbricate fault in the footwall 

of the Poison Creek thrust fault (see Lund and others, 2003b). 

This fault geometry is based on the apparently much greater 

offset across the Poison Creek thrust fault (Apple Creek 

Formation over imbricated footwall Ordovician units, Lund 

and others, 2003b) than the units across the Hawley Creek 

fault (Ordovician over Mississippian). The southeasternmost 

exposed extent of the Poison Creek thrust fault is in the Lemhi 

Range (fig. 1) and is interpreted to be buried in the Lemhi 

Valley west of the southern map area. This interpretation 

remains the best geometric interpretation based on offset on 

the Poison Creek thrust fault relative to that on the Baby Joe 

Gulch thrust fault system and the related Hawley Creek thrust 

fault as well as based on the units involved in footwall imbricate thrust faults in exposures in both ranges.

Thrust-Fault Geometries

The main thrust faults in the region are northeast vergent and northwest striking (Skipp, 1987; Evans and Green, 

2003); these include the thrust faults mapped in the central 

Beaverhead Mountains. This transport direction resulted in 

oblique impingement of the thrust faults against generally 

north-striking basin margins of both Mesoproterozoic and 

Paleozoic ages, underlain by the Archean–Paleoproterozoic 

rocks now exposed in the Maiden Peak spur. The northern thrust plates, which were lower in the thrust stack, had 

relatively more room for northeastward transport than those 

farther to the south because of the orientation of northeastdirected thrust movement compared to the north-striking 

edge of the basin margins and basement high. The southern 

thrust faults in the map area, which were structurally higher 

in the thrust stack, had a space problem during impingement 

against the basement high. The space problem is manifested 

by the structural domains demarcated by compressional 

structures that are curvilinear in trace and listric in three 

dimensions. These bounding faults exhibit complex motions: 

(1) relative sinistral rotations between upper and lower thrust 

plates along the western transpressional sides of the domains 

(and subdomains), (2) reverse motion on northern sides, and 

(3) lateral thrust ramp faults along eastern sides. The steep 

bounding faults and the lateral-ramp structures are restricted 

to specific structural domains and (or) subdomains, such that 

they are closely tied to specific thrust plates and structural 

levels. In addition to structural interpretations, paleogeographic information gained from comparison of Paleozoic 

stratigraphic sections among domains is consistent with relative sinistral rotation between structural domains.
14 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

The Baby Joe Gulch thrust fault system, the thrust fault 

at the southern edge of the Stroud subdomain, the thrust fault 

on the west flank of the southern Goat Mountain domain, and 

the Cabin thrust fault are all west striking. These faults overlap 

and (or) are oblique to many of the lesser structures of the 

Grizzly Hill domain, the major northwest-striking Cow Creek 

thrust fault system, and the north-striking thrust faults in the 

Maiden Peak spur. Thus, the west-striking thrust fault system 

is a second, younger thrust-fault phase.

Both the Thompson Gulch thrust fault and the Baby 

Joe Gulch thrust fault system and their associated domains 

change character over the Railroad Canyon lateral-ramp fault. 

Whereas the Thompson Gulch thrust fault stepped down over 

the Railroad Canyon fault, the Baby Joe Gulch thrust fault 

system was involved in either a horst or anticline over the 

Railroad Canyon fault. These geometries add to evidence that 

the Railroad Canyon lateral ramp formed during a thrust fault 

event that preceded emplacement of the Thompson Gulch 

thrust fault and Baby Joe Gulch thrust fault system. The 

eastern extent of the main Baby Joe Gulch thrust fault system 

was truncated at another steep fault along Cedar Gulch, which 

originated as another lateral ramp in the Baby Joe Gulch 

thrust fault system but was reactivated such that its offset was 

reduced by younger range-front faults.

Cenozoic Extensional Structures

The sequence of extensional deformation events is 

revised by the present mapping in comparison to that previously presented by Janecke and others (2001). The most 

significant change is in interpretations of their “fault set one.” 

Features in “fault set one” are named and considered to be the 

largest low-angle normal faults of the region (Grizzly Hill, 

Wildcat, and Phosphoria faults of Janecke and others, 2001; 

also Divide Creek fault of Janecke and others, 1998, and 

VanDenburg and others, 1998, renamed Goat Mountain fault 

by Janecke and others, 2001). However, all are unconformable contacts of Mississippian, Pennsylvanian, or Permian 

strata deposited on Mesoproterozoic strata. Based on detailed 

mapping of the stratigraphy and structure, these features are 

reinterpreted as angular unconformities between tilted Mesoproterozoic strata and disparate overlying Paleozoic sections 

(this is also a reinterpretation of Lund and others, 2003b). 

Specifically, the proximity of different stratigraphic successions and different units overlying the unconformity was the 

result of juxtaposition in thrust plates. In many localities, the 

unconformable surface displays shearing or brecciation, which 

probably originated primarily during Cretaceous compressional deformation but also during Tertiary extension.

Many northwest-striking, moderate-angle normal faults 

cut the Mesoproterozoic strata and bound restricted Eocene 

Challis Volcanic Group and Eocene to Oligocene depocenters 

in the northern part of the study area. The largest faults of 

this type are the west-northwest-striking Lemhi Pass normal 

fault (Staatz, 1973, 1979; Janecke and others, 2001) and 

several subparallel faults in the Lemhi Pass thorite district (for 

example, Bull Moose and Dan Patch faults of Staatz, 1973, 

1979). These relatively long, west-northwest-striking, normalfault segments, within and south of the Lemhi Pass district, 

are generally the oldest of the Cenozoic extensional faults 

and depocenters. Although, at its southeast end, the Lemhi 

Pass normal fault is claimed to dip as shallowly as 24–27° 

(VanDenburg and others, 1998), its straight map trace suggests 

its average dip is moderate to steep. These west-northweststriking normal faults tend to parallel older Cretaceous compressional structures, which were reactivated under extension 

and tend to cause down dropping of the hanging wall of thrust 

faults and the upper limb of large, thrust-fault-parallel folds. 

The clearest example is the northwest half of the Lemhi Pass 

normal fault, where it reactivated the Cow Creek thrust fault.

Thorite-vein deposits of the Lemhi Pass district were 

previously studied in detail (Staatz, 1972, 1979). However, the 

wholesale revisions to stratigraphy of Mesoproterozoic rocks 

and to structures presented in generalized form in Lund and 

others (2003) and on this map affect interpretations in relation 

to the Lemhi Pass district. The thorite veins of Lemhi Pass district are restricted to less than 9 km (5.6 mi) into the hanging 

wall of the Lemhi Pass normal fault and scattered veins extend 

0.8 km (0.5 mi) into the footwall. Most veins are in the footwall of the Cow Creek thrust fault but a few are in its hanging wall. The deposits are primarily hosted by the Gunsight 

Formation, above and closely associated with the enigmatic 

contact between Apple Creek and Gunsight Formations; a few 

outlier deposits are hosted by rocks of the Apple Creek Formation. These new details suggest post-Cretaceous formation of 

the veins, corroborating interpretations of Tertiary origins as 

advocated by Staatz (1979).

Arcuate, east-northeast-striking faults, which are also 

transverse to the range, controlled deposition and preservation 

of Eocene Challis Volcanic Group and Oligocene–Miocene 

sedimentary deposits on the northwest side of the Little 

Eightmile fault system and south of the Peterson Creek-Black 

Canyon fault. These east-northeast Cenozoic normal faults 

reactivated Cretaceous arcuate transpressional faults and were 

active prior to and coeval with deposition of the Challis Volcanic Group as well as afterwards.

The north-northwest orientation of the Beaverhead 

Mountains is controlled by north-northwest-striking, active 

range-front fault segments (Crone and Haller, 1991; Janecke 

and others, 2001). The range-front faults are orthogonal to 

the regional extension direction (Stickney and Bartholomew, 

1987). Because of that, the main north-northwest-striking 

segments are interspersed with shorter west-northweststriking, range-front segments where the range-front deviates 

to nearly west for 10–20 km stretches. The most significant 

west-northwest-striking segments in the map area are the 

prominent segment at the southwest margin of the Goat 

Mountain domain and the Leadore basin segment (fig. 3). 

The present mapping, which details stratigraphy and structures in the range itself, suggests that the range-front faults 

that caused these jogs were controlled by, and reactivated, 

both the northwest- and the west-striking thrust faults. 
Deformation History 15

Cenozoic reactivation of steep transpressional faults bounding domains and subdomains also controlled locations and 

orientations of these short range-front segments.

Both the north-northwest- and west-northwest-striking 

range-front fault systems are associated with parallel sets of 

moderate- and low-angle normal faults in footwalls of rangefront faults. Several of the more westerly low-angle normal 

faults were identified previously and tentatively suggested 

to have controlled the west-striking jogs in the range front 

(Janecke and others, 2001). Large, shallow-dipping zones of 

brecciated Mesoproterozoic rocks are associated with several 

low-angle normal faults. The largest of these are on Ramsey 

Mountain, the ridgeline to the north, and the low ridge on 

the west side of the Lemhi River. Formation of these breccia 

bodies and nearby belts of breccia associated with Eocene 

volcanic rocks and basin-fill deposits in Yearian Creek (probable slide blocks) is probably related to the formation of the 

north-striking, extensional Yearian-Cow-Agency basin and 

Tendoy basin segment. Observations from the present study 

suggest that both north- and west-striking low-angle normal 

faults formed as part of the range-front system. The low-angle 

faults in the range may have originated either as integral parts 

of the extension associated with range uplift or as earlier steep 

normal faults that rotated to shallower dips during subsequent 

footwall uplift along presently active range-front faults.

A variety of normal faults along the western side of the 

range reactivated Cretaceous thrust faults and extended the 

older thrust plates. This is particularly evident for the normal 

fault that controlled the Leadore basin segment and caused 

extension in the Baby Joe Gulch domain. This range-front 

segment is spatially associated with a large number of leadzinc-silver deposits that are restricted to the Baby Joe Gulch 

domain near the range front. Similar structural remnants of 

upper Paleozoic units are preserved near the range-front faults 

all along the southern half of the map area.

In addition to Eocene volcanic rocks, which are widely 

preserved within fault-bounded basins adjacent to and 

transecting the range, Eocene granites of the Challis volcanicplutonic complex intruded Mesoproterozoic rocks on the south 

side of the fault-controlled Little Eightmile basin and along 

the northwest-striking range-front fault segment between 

Little Eightmile fault system and Mollie Gulch. The presence 

of both the volcanic and plutonic rocks conveys evidence for 

deep-seated Eocene extension in the Beaverhead Mountains, 

as part of the extension and magmatism that occurred in much 

greater volumes across central Idaho. Additionally, the late 

Eocene and younger basin deposits compose a complex set 

of unconformity-bounded sequences that, together with the 

extensional igneous rocks, indicate active Eocene through 

post-Miocene normal-fault displacements along basinbounding faults. The Miocene to Holocene basin-fill sedimentary deposits are not present near the range crest where local 

fault-bounded Challis Volcanic Group outcrops are preserved. 

The absence of Miocene to Holocene deposits in those higher 

elevation settings indicates active uplift closely followed 

deposition of the volcanic rocks (after about 45 Ma).

The Eocene to Miocene deposits are commonly tilted. 

A number of these dipping packages are interpreted to show 

Eocene-age extension-related folds, which also deformed preEocene rocks (VanDenburg, 1997; Janecke and others, 1998, 

2001). Some folds were directly observed due to continuous 

exposure, and others are delineated by attitudes that define 

limbs and hinge, but some of the largest folds are interpreted 

from opposing attitudes in fault-disrupted or noncontinuous 

exposures. These extensional folds are commonly orthogonal 

to and less commonly parallel to bounding normal faults and 

interpreted as fault-bend type folds (Janecke and others, 1998). 

In a number of cases, where attitudes in pre-Cenozoic rocks 

beneath the interpreted fold are available due to discontinuous cover by the younger rocks, evidence for geometrically 

related folding in pre-Cenozoic rocks is absent (for example, 

large “Tendoy anticline,” of Blankenau, 1999, extending from 

Ramsay Mountain to Gould Basin, is not corroborated by the 

present study). The opposing attitudes in Cenozoic rocks in 

these cases are better explained as localized preservation of 

tilted sections and (or) perhaps as original deposition in different sub-basins. A number of cross sections, drawn across 

these interpreted Cenozoic folds, display significant changes 

in stratigraphic units, both in thickness and in unit preservation, that also do not support folds in the Cenozoic rocks. In 

those cases, attitudes more clearly represent syndepositional 

rotations along faults.
16 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

Qa Alluvium (Holocene)—Composite unit of unconsolidated cobble, gravel, sand, silt, and 

clay. Formed as alluvial deposits in valleys of active streams and in alluvial fans. 

Includes reworked moraine and outwash deposits. Thickness variable, generally less 

than 5 m (16 feet [ft]) thick

Qls Landslide deposits (Holocene and Pleistocene)—Debris from landslides. Most common in 

areas underlain by Eocene volcanic rocks (Tc) and Eocene to Miocene sedimentary deposits 

(Tsl, Tsu, and Tm). Thickness variable, some deposits as much as 20 m (66 ft) thick

Qg Glacial deposits (Holocene and Pleistocene)—Unconsolidated boulder, cobble, and sand 

deposits in glacial moraines and in extensive glacial outwash deposits. Locally includes 

glacial deposits reworked by active streams. Thickness variable, generally 5–20 m 

(16–66 ft) thick

Qt Terrace deposits (Holocene and Pleistocene)—Unconsolidated deposits perched in 

terraces above modern streams and forming ridge-capping gravels. Unconsolidated 

boulder to pebble conglomerate interbedded with sand and mud. Accumulated as 

stream, alluvial fan, and colluvial debris. Locally includes lesser landslide and glacial 

outwash deposits. Thickness variable, generally less than 20 m (66 ft) thick

CENOZOIC SEDIMENTARY ROCKS

Tm Mudstone deposits (middle Miocene)—Thin- to thick-bedded, brownish mudstone, 

siltstone, and sandstone containing thin pebble-conglomerate lenses (M'Gonigle, 

1994; VanDenburg, 1997). Contains beds with distinctive horse, camel, and rhinoceros 

fossil assemblages (M'Gonigle, 1994). Dated at about 16–12 Ma (40Ar/39Ar dating, 

VanDenburg and others, 1998). Preserved near Bannock Pass in southern Horse Prairie 

basin, a feature that extends across the Montana-Idaho border. Named “mudstone 

unit” by M'Gonigle (1994) and referred to as “sedimentary rocks of Bannock Pass” by 

VanDenburg and others (1998). Commonly overlain by ridge-capping gravels (Qt). 

Angular unconformity at base, above the older basin-fill units (Tsu, Tsl, and Tc). 

Probably age-equivalent to strata along west side of central Beaverhead Mountains 

south of Yearian Creek. As much as 230 m (755 ft) thick but poorly exposed

Tsu Basin-fill deposits, upper (lower Miocene to Oligocene)—Basin-fill deposits exhibiting 

distinctly diverse characteristics dependent on structural setting. Unit present in Horse 

Prairie basin on east side of central Beaverhead Mountains. Named “sedimentary rocks 

of Everson Creek” and correlated with units on west side of Horse Prairie basin by 

VanDenburg and others (1998).

 In Horse Prairie basin, unit consists of interbedded shale, siltstone, sandstone, 

and metasandstone- and siltite-pebble conglomerate. Conglomerate localized in paleovalleys. Local thin lacustrine limestone lenses (VanDenburg, 1997). Sediments are 

locally tuffaceous. Angular unconformities at top and base of unit.

 Along west side of the Beaverhead Mountains between Sandy and Kenney

Creeks, unit is coarser than equivalent rocks on east side of central Beaverhead 

Mountains. Unit composed of cobble to boulder conglomerates (Blankenau, 1999). 

Deposited in active range-front-fault environment.

 During deposition, Horse Prairie basin was relatively quiescent, and these 

deposits exhibit less influence of syndepositional faulting than age-equivalent deposit 

on western flank of range. As much as 2,000 m (6,562 ft) thick in Horse Prairie basin 

(M'Gonigle, 1994; VanDenburg, 1997)

Tsl Basin-fill deposits, lower (Oligocene to upper Eocene)—Composite unit of sandstone, 

conglomerate, limestone, and shale. Boulders (less than about 0.5 m in diameter), 

cobbles, and pebbles in conglomerate mainly from Mesoproterozoic metasedimentary rocks of the locally exposed Apple Creek (Yac), Gunsight (Yg), and Swauger 

(Ys) Formations; deposits in upper Pattee Creek and Gould Basin contain cobbles to 
Description of Map Units 17

boulders of Mesoproterozoic granite (as much as 2 m in diameter). Most conglomeratic 

deposits contain minor volcanic clasts; some sandstones and shales are tuffaceous.

 Limestones are fine grained to aphanitic, generally laminated, and platy. Small 

gastropod fossils are common (Flores and M'Gonigle, 1991). Formed in freshwater 

lakes and hot springs.

 Top and base of unit are angular unconformities. Within the section, angular 

unconformities and lenticular layers are also common (Lucchitta, 1966; VanDenburg

and others, 1998). These are the “Medicine Lodge beds” of Scholten and others 

(1955) and M'Gonigle and Hait (1997) and also called “sediments of Bear Creek” by 

VanDenburg and others (1998).

 Deposited during early phases of extensional faulting that led to local structurally controlled paleovalleys and to present basin-and-range topography (VanDenburg 

and others, 1998; Janecke and others, 2000). The very large boulders of Mesoproterozoic augen gneiss in upper Pattee Creek and Gould Basin were interpreted as evidence 

of major river drainages in which the boulders traveled about 65 km from locations 

where Mesoproterozoic granite is presently exposed (Janecke and others, 2000). 

However, the present mapping determined that the Cow Creek thrust fault is concealed 

beneath the Gould Basin boulder occurrence and, because this fault carries the granite 

(Lund and others, 2003a, b), it is possible that granite is also in the buried upper thrust 

plate. Thus, there may have been a more proximal source for the granite boulders prior 

to development of the present Gould Basin topography. As thick as 1,900 m (6,234 ft) 

(M'Gonigle and Hait, 1997; VanDenburg and others, 1998)

EOCENE CHALLIS VOLCANIC-PLUTONIC COMPLEX

CHALLIS VOLCANIC GROUP

Tc Challis Volcanic Group, undivided (Eocene)—Composite acidic- to intermediate-composition

tuff and flow units of the Challis Volcanic Group. Regionally, extrusive units mostly erupted 

about 51–45 Ma, although minor activity persisted to about 39 Ma (Fisher and others, 1992; 

Fisher and Johnson, 1995). There are four informal members in the map area, which are not 

mapped separately in this study.

 The youngest part of the volcanic succession is tannish to greenish, 

quartz-sanidine welded tuffs with interlayered dark dacite lava flows and conglomerate. These overlie the regionally recognized greenish-white, crystal-vitric, 

biotite-bearing, “rhyolite tuff of Curtis Ranch” of Staatz (1979: VanDenburg, 1997; 

Blankenau, 1999), in which crystal fragments are predominantly quartz and plagioclase. Some exposures of the rhyolite tuff of Curtis Ranch contain abundant volcanic lithic fragments and upper part contains flow-breccia layers. Tuffs throughout 

upper part of the volcanic succession are compacted and locally welded. Upper part 

of the volcanic succession is as much as 400 m (1,312 ft) thick where most completely preserved.

 The middle part of the volcanic succession consists of thick andesitic, 

basaltic, and rhyodacitic flow and flow-breccia units with several thin interlayered 

rhyolite tuffs (Staatz, 1979; VanDenburg, 1997; Blankenau, 1999). The most widespread unit in the middle part of the volcanic section contains dark olive-green, to 

gray, to reddish-brown, aphanitic to fine-grained, compound basalt to rhyodacite 

flows. These flows are commonly porphyryitic. Augite phenocrysts are common and 

biotite phenocrysts are also present. Matrix contains mostly plagioclase but potassium feldspar is in some flows and some flow units have relatively high potassium 

contents (Staatz, 1979). Flows have basal flow breccias and vesicular-textured tops. 

These basalt-rhyodacite flows are the most widespread volcanic rocks in the map 

area. These middle-unit flows form the base of the Challis Volcanic Group in the fault 

block south of the Lemhi Pass normal fault. The thin rhyolites interlayered in the 

middle unit are composed of light-colored, crystal, vitric, or lithic tuffs. Middle unit is 

as much as 1,100 m (3,608 ft) thick.
18 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

 The oldest part of the volcanic succession is composed of interlayered tuff and 

conglomerate units, including basal conglomerate. Greenish-white, unwelded rhyolite 

vitric tuff (vitric tuff of Lemhi Pass in Staatz, 1979) locally forms the top of the basal 

part of the volcanic succession in the study area. This vitric tuff bears minor tuffaceous 

and crystal fragments and is as much as 125 m (410 ft) thick. The bulk of the basal unit is 

light pinkish-gray, poorly welded to unwelded rhyolite tuff. The distinctive feature of the 

basal tuff is abundant lithic fragments, especially clasts of the fine-grained metasandstone 

and siltite country-rock, but also vitric, crystal, and tuffaceous clasts. The fragments are 

generally less than 1 centimeter (cm) in diameter and subangular to subrounded. Conglomerate, predominantly bearing metasedimentary country-rock pebbles to cobbles is 

locally interbedded between the two rhyolitic units (“conglomerate of Flume Creek” in 

Staatz, 1979). Basal unit of the volcanic succession is as much as 500–650 m thick.

 Coarse conglomerate is locally preserved at or near base of Challis Volcanic 

Group, where base is best exposed in narrow, early tilt blocks that cross the northern 

part of the map area. Subrounded to rounded clasts are pebbles to cobbles of Mesoproterozoic quartzite, fine-grained feldspathic metasandstone, and fine-grained micaceous 

siltite. Cobbles are derived from rocks that underlie the Eocene volcanic rocks. Sandy 

matrix is locally tuffaceous and moderately lithified. Basal conglomerate is in wedgeshaped deposits that are as much as 45 m (148 ft) thick.

 Challis Volcanic Group is preserved as thick deposits in the fault-bounded valleys 

on both sides of the range, although not widely exposed there. Best exposures within the 

tilted fault block of the range were preserved in local early fault basins. General stratigraphic 

sections are described from exposures in the following localities: the Lemhi basin on the 

west side of the range (Blankenau, 1999), the Horse Prairie basin east of the range (M'Gonigle, 1994; M'Gonigle and Hait, 1997; VanDenburg, 1997), and the Kenney Creek-Lemhi 

Pass basin that straddles the range (Staatz, 1979; Blankenau, 1999). There is considerable 

apparent stratigraphic diversity among those isolated exposures, but the three general units 

of the volcanic succession are recognized in each basin. The stratigraphic succession of the 

Beaverhead Mountains generally matches the more complete succession described in the 

Lemhi Range about 24 km (15 mi) southwest of the map area where middle potassium-rich 

andesite-dacite-basalt flows are underlain by massive rhyolitic tuffs and overlain by a variety 

of thinner welded ash-fall-tuff units (Tysdal and Moye, 1996). Both Lemhi Range (proximal) 

and Beaverhead Mountains (distal) volcanic rocks were probably outflow deposits from one 

of the calderas of the Van Horn Peak cauldron complex (Tysdal and Moye, 1996), the eastern 

margin of which lies about 70 km (43 mi) west of the Beaverhead Mountains.

 Stratigraphic variations between exposures of volcanic rocks in the map area 

resulted from differential erosion and different distances from sources. The variation 

may also have resulted from deposition in middle Eocene fault-controlled paleovalleys, 

including in the Kenney Creek-Lemhi Pass basin (Janecke and others, 2000). Additionally, the local, interbedded, conglomeratic wedges indicate synvolcanic faulting that also 

caused disrupted landscapes and local differences in time of basin formation

INTRUSIVE ROCKS

Tr Rhyolite dike (Eocene)—Medium-gray, aphanitic matrix, fine-grained, 

feldspar-phenocrystic rhyolite dikes

Tg Granite (Eocene)—Light gray, hornblende-biotite granite and porphyritic granite. Also, finegrained, hornblende-biotite granodiorite in margin of granite south of Little Eightmile 

Creek and in other small exposures. Intruded into reactivated, steep, transpressional parts 

of fault zones and ductilely deformed rocks along and near Little Eightmile Creek. The 

largest exposure, south of the mouth of Little Eightmile Creek, is surrounded by an about 

200-m (656 ft)-wide contact metamorphic hornfels zone in the Apple Creek Formation

Td Mafic dike (Eocene)—Black to dark-gray, fine-grained, mafic intrusive rock in thin dikes. 

Reported compositions include diorite and basalt. Commonly near the faulted contact 

between Apple Creek and Gunsight Formations and in fold hinges and fracture zones 

(for example, Copper Queen mine area). Also, several small bodies in exposures of 

Challis Volcanic Group
Description of Map Units 19

MESOZOIC TO NEOPROTEROZOIC? SEDIMENTARY AND 

METASEDIMENTARY ROCKS

^d Dinwoody Formation (Lower Triassic)—Yellow-tan, fissile, silty shale and interbedded 

yellowish-weathering, dark-gray limestone. Poorly exposed. Base unconformable. 

About 300 m (984 ft) thick

Pp Phosphoria Formation (Permian)—Black chert, gray carbonate rocks, and bluish siltstone preserved in the southern range crest in the map area. Upper part of preserved 

Phosphoria Formation is bedded black chert with thin beds of dolostone and limestone. 

Chert beds exhibit characteristic bluish-white phosphatic spots on surface. Chert is 

about 50 m (164 ft) thick and equivalent to “Tosi Chert Member” of McKelvey and 

others (1959). Lower part of formation is dark-gray dolostone and minor limestone 

containing dark-gray-to-black, ropy chert nodules, mapped as “Phosphoria and Park 

City Formations, undivided” by Lucchitta (1966). About 280 m (919 ft) thick. Lowest 

exposed beds are bluish siltstone with millimeter-scale, orange-weathering fragments. 

Lowest beds less than 30 m (98 ft) thick.

 These rocks were deposited in the western extent of the Phosphoria epicontinental basin (Wardlaw and Collinson, 1986). Lower contact with the Snaky Canyon 

Formation (PMs) is gradational. Where structural complexity makes it difficult 

to assign these beds to appropriate formation because lowest beds are not present, some exposures of Phosphoria Formation may be combined with the upper 

Juniper Gulch Member of the Snaky Canyon Formation. Upper contact is erosional. 

Locally, unit is unconformably overlain by Lower Triassic Dinwoody Formation 

(^d) or Challis Volcanic Group (Tc). 

 Rocks of the Phosphoria epicontinental basin have potential for phosphate 

resources as well as byproduct uranium (U), fluorine (F), cadmium (Cd), chromium 

(Cr), nickel (Ni), molybdenum (Mo), silver (Ag), rare earth elements (REE), and 

black-shale vanadium (V) resources (Oberlindacher and Hovland, 1979). Base of the 

unit is exposed in many bulldozer-cut prospects made during past uranium exploration. Phosphate in the Phosphoria Formation forms pelletal phosphorite interbedded 

with organic-matter-enriched mudstone and siltstone, limestone, dolomite, and chert. 

Potential economic interest is restricted to shales in the Phosphoria Formation, as 

described by Oberlindacher and Hovland (1979). Total thickness is about 260 m 

(853 ft) (Lucchitta, 1966)

PMs Snaky Canyon Formation, undivided (Lower Permian to Upper Mississippian)—Upper 

and lower carbonate members and a middle sandstone member comprise the Snaky 

Canyon Formation. The Juniper Gulch Member, upper member of the Snaky Canyon, is 

medium-bluish-gray, sandy dolostone with interbedded quartz sandstone layers. Amount 

of interbedded sandstone decreases upward, grading to predominantly light- to medium-gray, cherty dolostone. The underlying Gallagher Peak Sandstone Member, the middle 

member, is similar to the older Upper Mississippian Bluebird Mountain Formation (Mb) 

but with larger proportion of calcareous cement. The basal Bloom Member is medium- to 

dark-gray, cherty limestone with common chert nodules and interbedded thin quartz sandstone similar to sandstone of Bluebird Mountain Formation.

 Lower contact is abrupt, at base of lowest carbonate beds; upper contact is 

gradational.

 Different facies in thrust duplex and Rocky Canyon subdomains of southeastern part of map. Rocks in the Rocky Canyon subdomain contain more sandstone and 

quartzite, in comparison to rocks in the thrust duplex subdomain. They may be more 

properly mapped as the Quadrant Formation, as used by Lucchitta (1966), now considered an eastern, cratonal equivalent facies (Skipp and others, 1979a). 

 Type section for formation is in southern Beaverhead Mountains, where total thickness is 1,350 m (4,429 ft) (Skipp and others, 1979a). In the study area, the Snaky Canyon 

Formation totals about 560 m (1,837 ft) thick (reinterpreted from Lucchitta, 1966)

Mb Bluebird Mountain Formation (Upper Mississippian)—Light-brown weathering, very 

fine grained, light-gray, quartzitic sandstone and lesser quartzite, calcareous sandstone, 

and sandy limestone. Unfossiliferous. Lower contact is gradational, at base of first 
20 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

carbonate-cemented quartz sandstone; upper contact is abrupt. Quartz sands derived 

from inner craton (Skipp and others, 1979a). Only recognized in small area on west 

side of Railroad Canyon, directly overlying type section of the Railroad Canyon Formation. 105 m (344 ft) thick (Skipp and others, 1979a)

Mr Railroad Canyon Formation (Upper Mississippian)—Pink-, yellow-, orange-weathering, 

dark-gray shale, mudstone, silty limestone, and calcareous siltstone. Strata are thin bedded 

to fissile and flaggy. Dark shale and siltstone have a musty odor. Lower and upper contacts 

are conformable and relatively abrupt. Unit is poorly exposed and the surface expression is 

as swales and valleys, in which springs and slumps are preferentially located. Basal strata 

are siltstone and mudstone of shallow-water origin, overlain by deep-subtidal shale and by 

cycles of deep- to intermediate-subtidal calcareous siltstones (Batt and others, 2007).

 Formation mapped previously as Big Snowy Group (Lucchitta, 1966) and 

Big Snowy Formation (Skipp, 1984). Unit occupies a narrow facies belt formed in an 

intraramp basin known from the Beaverhead Mountains study area (Skipp and others, 

1979b; Batt and others, 2008). Considered facies equivalent to the South Creek, Surrett 

Canyon, and Arco Hills Formations of the foredeep carbonate bank sequence of Idaho 

and the Big Snowy Formation of the cratonal section of Montana (Skipp and others, 

1979b; Wardlaw and Pecora, 1985). 

 Shale in unit commonly acts as locus for faults that in many places excise all 

or part of formation. Slivers of Railroad Canyon Formation are preferentially silicified 

along fault zones; narrow zones of silicified siltstone are common indicators of faults 

in southern part of map area. Railroad Canyon Formation is 205 m (673 ft) thick at 

type section on west side of upper Railroad Canyon (Wardlaw and Pecora, 1985)

Ms Scott Peak Formation (Middle Mississippian)—Medium-gray, medium- to thick-bedded, 

mostly pure limestone. Much of unit contains orange-weathering, wispy and bedded, 

dark-gray chert. Beds are fossiliferous, with corals and brachiopods predominant; 

encrinite layers are common but not diagnostic. Thin-bedded strata were described 

in the upper part by Lucchitta (1966). Typically forms cliffy exposures. Intensely 

cleaved as the result of deformation associated with thrust faulting. Brecciated and 

recemented by calcite near low-angle normal faults. Previously mapped as Madison 

Limestone following cratonal basin terminology (Lucchitta, 1966) prior to more recent 

facies interpretations that the unit in the central Beaverhead Mountains was deposited 

in a miogeoclinal setting (Skipp and others, 1979b). Although commonly thinned by 

low-angle normal faults, unit may be as much as 700 m (2,296 ft) thick (Skipp and 

others, 1984)

Mm Middle Canyon Formation (Middle and Lower Mississippian)—Silty limestone with 

shale partings and thinly interbedded shales. Limestones are in thin to medium beds 

that originated as carbonate mudstone (Skipp and others, 1979b). Base is exposed 

on the south side of Goat Mountain and, there, determined to be an erosional unconformity. Unit formed in lower portions of prograding carbonate-bank complex (Huh, 

1967). Also preserved in Thompson Gulch domain where unit was transformed into 

thinly laminated, fine-grained, dark-gray phyllitic marble and calcareous phyllite. 

Previously mapped as Lodgepole Formation following cratonal basin terminology 

(Lucchitta, 1966) but herein included in shelf (miogeoclinal) succession (Skipp and 

others, 1979b). About 250 m (820 ft) thick but complicated by isoclinal folding and 

thrust faulting

Dj Jefferson Formation (Upper Devonian)—Primarily dark-gray dolostone with lesser 

amounts of limestone, limestone evaporite-solution breccia, and sandy dolostone. 

Tectonic brecciation of unit in most locations makes identification of bedding difficult. 

Base of unit is in angular unconformity with underlying Mesoproterozoic, Ordovician, 

and Silurian rocks. Ruppel and Lopez (1988) reported a maximum thickness of about 

825 m (2,707 ft) in Lemhi Range to the west where formation includes Middle and 

Upper Devonian members older than rocks deposited in this map area; unit abruptly 

thins from Lemhi Range toward Beaverhead Mountains but thickens eastward into 

southwestern Montana (Sloss, 1954; Scholten, 1957; Scholten and Hait, 1962; Grader 

and Dehler, 1998). Jefferson Formation in Lemhi Range and Beaverhead Mountains 
Description of Map Units 21

was deposited in miogeoclinal setting but was deposited in the cratonal basin east of the 

study area. Preserved in lower thrust slivers of Baby Joe Gulch thrust fault system along 

western range front. About 60 m (197 ft) thick in study area where only upper part of 

formation is present.

SOs Saturday Mountain Formation (Lower Silurian to Middle Ordovician)—Upper part 

is medium-dark-gray to medium-gray, finely crystalline dolostone that weathers to 

distinctly mottled lighter gray colors. Black chert nodules are also common as well as 

numerous beds with abundant fossils and fossil fragments. This grades downward into 

yellowish-gray to medium-gray, finely crystalline dolostone characterized by a network of irregular, interlaced, hairlike wisps and veinlets of white dolostone and locally 

by abundant black chert nodules. Most exposures in Beaverhead Mountains are the 

yellowish-gray lower dolostone. Subdivided into several members to the west in the 

Lemhi Range (Ruppel and Lopez, 1988). The two rock types that are in the Beaverhead 

Mountains are probably equivalent to lower part of formation as mapped in the Lemhi 

Range. Top of formation is an erosional unconformity. About 20 m (66 ft) thick

Ok Kinnikinic Quartzite (Middle Ordovician)—White to light-gray, fine- to medium-grained, vitreous orthoquartzite. Unit is compositionally supermature and well 

sorted. Subrounded quartz grains are tightly cemented and, in many cases, are difficult 

to distinguish from the quartz cement. Where lower contact is not structural, unit rests 

unconformably on Upper Cambrian-Lower Ordovician syenogranite (O_g). Although 

determined to have been deposited on the Beaverhead pluton in the restricted exposures 

of the present map area, detrital zircon analyses do not indicate grains of Late Cambrian 

age either in the Beaverhead Mountains (one sample from Italian Gulch in present study 

area) or regionally (Baar, 2009). Deposited in foreshore to shoreface environments 

(James and Oaks, 1977). Preserved only in Baby Joe Gulch domain along western range 

margin. As much as 30 m (98 ft) thick

O_d Dolostone (Middle Ordovician? to Upper Cambrian?)—Fine-grained, greenish-gray, 

fossiliferous dolostone and dolomitic limestone. Thin, light-orange wisps common. 

Distinctive 1- to 2-cm (<1 inch [in]) rip-up clasts, indicating possible tidal reworking of 

mud-cracked deposits. Unit preserved between underlying quartzite-pebble conglomerate (_Zc) or quartzite of Mesoproterozoic Swauger Formation (Ys) and overlying 

Upper Mississippian to Lower Permian Snaky Canyon Formation (PMs). Both upper 

and lower contacts are unconformable. Locally exposed in far eastern extent of Grizzly 

Hill domain, in bulldozer cuts in upper Hood Gulch and about 1 km (0.6 mi) east of 

there. These rocks are unlike other carbonate units to the west; they are most similar 

to thin Middle Cambrian shale and carbonate adjacent to the eastern edge of the map 

where they were probably included in Summerhouse Formation by M'Gonigle (1994). 

May represent a remnant of the western edge of cratonal-shelf sedimentary rocks, 

possibly equivalent to the Middle Cambrian Meagher Limestone in southwest Montana. 

About 15 m (50 ft) thickness preserved

_Zc Conglomerate (Lower Cambrian? to Neoproterozoic?)—Predominantly quartzite-pebble 

to -cobble conglomerate and quartzite. Most recognized exposures are quartzite with 

interbedded conglomerate. Where conglomerate is lacking, unit is difficult to distinguish from and probably included with underlying Swauger Formation (Ys). Quartzite 

contains bimodal grain-size distribution and is cemented by silica in lower part of unit. 

Conglomerate layers are as thick as 2 m (6.6 ft); cobbles derived primarily from subjacent quartzite of the Mesoproterozoic Swauger (Ys) and Gunsight (Yg) Formations; 

minor clasts of vein quartz and rare chips of light-colored siltite. Amount of carbonate 

cement increases upward. Base of formation is angular unconformity over Apple Creek, 

Gunsight, and Swauger Formations; top of formation unconformable. Unit may be correlative to Lower Cambrian to Neoproterozoic Wilbert Formation. Maximum thickness 

about 20 m (66 ft)

_f Flathead Quartzite (Middle Cambrian)—Light gray quartz sandstone to quartzite. Quartz 

grains rounded. Some layers contain minor iron-oxide cement, causing reddish-tan 

weathering. Lower beds are arkosic and contain grit (very coarse sand to fine pebble) 

layers. Upper beds bear scolithos tubes. Base deposited unconformably on Paleop-
22 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

roterozoic gneiss (Xg). Unit only in Maiden Peak spur at eastern edge of study area 

where Mesoproterozoic strata are absent and Paleozoic rocks were deposited directly 

on Paleoproterozoic basement. About 7 km (4.3 mi) northwest of the northernmost 

exposure of Flathead Quartzite (along the eastern map margin) upper beds of quartzite 

are overlain by greenish, micaceous (glauconitic) shales typical of the Middle Cambrian Wolsey Shale. These characteristics led to reinterpretation of the quartzite from 

Lower Ordovician Summerhouse Formation of the Idaho miogeoclinal succession as 

interpreted by M'Gonigle (1994) to the Middle Cambrian Flathead Quartzite and Wolsey Shale of the cratonal basin succession. Thickness variable, as much as 70 m (230 ft) 

thick

PALEOZOIC INTRUSIVE ROCKS

O_g Syenogranite (Early Ordovician to Late Cambrian)—Reddish-brown weathering, pink 

to red syenogranite of Beaverhead pluton and tan-weathering leucosyenite in a small 

body about 4 km (2.5 mi) south of Lemhi Pass. Syenogranite of Beaverhead pluton is 

medium to coarse grained, equigranular to subporphyritic. It is composed of quartz, 

alkali feldspar, biotite, and opaque minerals and bears accessory zircon and apatite. 

Widespread alteration produced sericite, hematite, leucoxene, limonite, magnetite, 

chlorite, and quartz. Age of Beaverhead pluton is 488±5 Ma (sensitive high resolution ion microprobe [SHRIMP] U-Pb zircon dating, Lund and others, 2010). Previously, relations from exposures about 20 km (12 mi) south of the present study area 

were interpreted as intrusive contacts between the Beaverhead pluton and the Middle 

Ordovician Kinnikinic Quartzite (Scholten and Ramspott, 1968; Skipp, 1984). 

However, based on dates obtained from the Beaverhead pluton, the contact must be 

depositional because the pluton is significantly older than the Middle Ordovician 

Kinnikinic Quartzite (Lund and others, 2010). Thus, this pluton was exhumed by the 

Middle Ordovician, before deposition of the Kinnikinic Quartzite (Lund and others, 

2010).

 The Beaverhead pluton is preserved in Baby Joe Gulch thrust fault system 

(Hawley Creek thrust sheet as shown by Lucchitta, 1966), a lower imbricate thrust 

sliver related to the Poison Creek thrust fault (Lund and others, 2003b) at low elevations on the western flank of the range in the southern part of the map.

 Leucosyenite of Lemhi Pass is fine- to medium-grained, buff-colored 

alkali-feldspar syenite. It consists of microcline, albite microperthite, cryptoperthite, 

quartz, and lesser biotite, opaque minerals, zircon, and apatite. The leucosyenite crops 

out in an area about 250 m long and about 75 m (820×246 ft) wide. It intruded Mesoproterozoic Gunsight Formation. Age of the Lemhi Pass syenite stock is about 530 Ma 

(unpublished SHRIMP U-Pb date, Gillerman and others, 2008)

MESOPROTEROZOIC METASEDIMENTARY ROCKS

Ys Swauger Formation (Mesoproterozoic)—Tan-weathering, light-gray, medium- to coarsegrained quartzite. Quartz content typically ranges from 90 to 95 percent in well-rounded, 

well-sorted, glassy grains. Quartz grains are quartz cemented but detrital quartz grains 

remain distinguishable. Feldspar content seldom exceeds 5–10 percent. Beds are 

0.5–2 m (1.6–6.6 ft) thick and commonly display trough cross laminations and lesser 

herringbone cross laminae. Two-dimensional ripples and subaqueous dunes are common in unit (Tysdal, 2000b). Some beds capped by thin (<0.5 cm [<0.2 in]), reddish-tan 

or greenish-tan siltite or argillite layers. Deposited in a tidal environment based on 

compositional maturity of quartzites, presence of herringbone crossbeds, subordinate 

current-cap deposits on some dunes, rare antidunes, and local flaser and lenticular 

bedding (Tysdal, 2000b). Base gradational; top eroded. Regional thickness as much as 

3,100 m (10,171 ft), but only about 500 m (1,640 ft) are exposed in the map area
Description of Map Units 23

LEMHI GROUP

Yg Gunsight Formation (Mesoproterozoic)—Light- to dark-gray, very fine to medium grained, 

arkosic metasandstone; about 35 percent feldspar, 50 percent quartz, and as much as 15 

percent metamorphic micas. Bed thicknesses from 5 to 100 cm (2–39 in). Most common 

sedimentary features are parallel laminations, including common heavy-mineral lamination. Decimeter-thick trough and planar crossbeds are typical; hummocky cross-stratification locally. Mud chips locally. Unit contains finer grained sand and relatively more 

biotite on west side of range (Cow Creek thrust plate) and is lighter colored, containing 

less biotite but more distinct heavy-mineral lamination in eastern exposures. Strata in the 

footwall to the Cow Creek thrust fault (Lemhi Pass and Pattee Creek domains) contain 

two generations of metamorphic muscovite (early oriented and younger random).

 Deposited in fluvial environment (Tysdal, 2003). Sediment derived in part from 

Archean and Paleoproterozoic basement on eastern edge of map (Tysdal and others, 

2005). Through most of area, lower contact is with coarse siltite unit of Apple Creek 

Formation (Yac) and contact is commonly angular but also contains quartz veins and 

local tectonic fabrics, suggesting it is a sheared unconformity. In Cabin thrust plate, 

lower contact is sheared and brecciated but interpreted to be an unconformable depositional contact above Paleoproterozoic gneiss (Tysdal and others, 2005) rather than a fault 

(contact is labeled “fault D” by M'Gonigle, 1994). Unit grades upward into Swauger 

Formation (Ys). Total unit thickness regionally is 4,000 m (13,123 ft; (Tysdal, 2003) but 

only about 1,000 m (3,280 ft) are present in central Beaverhead Mountains exposures

Yac Coarse siltite unit of Apple Creek Formation (Mesoproterozoic)—Medium- to 

coarse-grained siltite and fine-grained metasandstone. Color varies with metamorphic 

grade and structural position; light- to medium-grayish-green in lowest metamorphic-grade exposures mostly extending from range crest to Montana side of range 

and light to dark gray to west on Idaho side of range. Weathers dark rusty-gray to 

dark maroon-gray. Contains 40–60 percent quartz, 25–30 percent potassium feldspar, 

5–15 percent plagioclase, about 3 percent clay matrix, and minor fine-grained, metamorphic chlorite, biotite, and (or) muscovite localized near ductile structures. Much of 

unit is massive, coarse siltite containing rare magnetite-bearing, heavy-mineral layers. 

Other common bedding type is parallel lamination, most commonly displaying erosional bases and grading upward from fine-grained, light-green or light-gray metasandstone to medium-grained darker greenish or gray siltite. Graded beds range from 1 to 

100 cm thick (0.4–39 in); most about 10–25 cm (3.9–9.8 in) thick. Other sedimentary 

structures common in the laminated and graded beds include intrabasinal mud-chip 

clasts in the graded-bedding layers and more rarely climbing ripples, convolute lamination, and dish structures.

 Rocks metamorphosed to greenschist facies, mostly containing sparse finegrained chlorite, muscovite, or biotite. Chlorite, where present, is generally randomly 

oriented. In contrast, biotite and (or) muscovite formed phyllitic fabrics in ductile 

deformation zones—generally either along fold hinges or near thrust faults. Additionally, second generations of fine- to medium-grained muscovite formed in secondary 

cleavage fabrics in the most deformed rocks.

 In Lemhi Pass and Pattee Creek domains (footwall to Cow Creek thrust fault and 

footwall to Lemhi Pass normal fault), upper contact with Gunsight Formation (Yg) is 

characterized by angular bedding relations—several locations demonstrate overturned 

Apple Creek Formation (Yac) below upright Gunsight Formation (Yg). In these areas, 

the contact zone is quartz veined and characterized by phyllitic fabrics. In Cow Creek 

thrust plate (hanging wall of Cow Creek thrust fault), the contact is more difficult to 

identify, less tectonized, and possibly gradational. Regionally, the contact is not overtly 

angular, but upper contact may have been truncated by erosion before deposition of 

Gunsight Formation (Tysdal and others, 2005). In the map area, the contact may represent 

an important angular unconformity as well as a zone of structural decoupling of undetermined origin.

 Apple Creek Formation is defined following studies of Tysdal (2000a, b). These 

rocks formed as below-wave-base deposits, primarily as turbidite deposits with minor 
24 Geologic Map of the Central Beaverhead Mountains, Idaho and Montana

debris flows (Tysdal, 2000b). In this central Beaverhead Mountains map area, coarse 

siltite unit of Apple Creek Formation is uniformly thicker and coarser than described 

for Lemhi Range, directly to the west (Tysdal, 2000b). Paleoproterozoic basement 

rocks on the east side of the Beaverhead Mountains were a significant source terrain 

as described for the Gunsight Formation (Tysdal and others, 2005). Total thickness 

exposed in map area about 2,000–2,500 m (6,560–8,200 ft). Base not exposed

PALEOPROTEROZOIC INTRUSIVE ROCKS

Xg Orthogneiss (Paleoproterozoic)—Reddish-orange-weathering, light-gray to brown, medium-grained porphyroblastic granite gneiss. Medium-grained groundmass is formed of 

quartz and subequal amounts of plagioclase, potassium feldspar, and combined hornblende and biotite (M'Gonigle, 1994). Porphyroblasts are coarse-grained microcline 

that originally were phenocrysts. Exhibits pervasive planar fabric and poorly formed 

compositional layering defined by oriented mafic minerals, elongated porphyroblasts, 

and groundmass quartz and feldspar. Some local mafic masses occur in the gneiss, as 

do crosscutting quartz veins and pegmatite dikes. Paleoproterozoic emplacement age is 

about 2.46 Ga (thermal ionization mass spectrometer [TIMS] U-Pb zircon, Kellogg and 

others, 2003)

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For more information concerning the research in this report, contact the

Center Director, USGS Geology, Geophysics, and Geochemistry 

Science Center

Box 25046, Mail Stop 973

Denver, CO 80225

(303) 236-1800

Or visit Geology, Geophysics, and Geochemistry Science Center 

website at https://www.usgs.gov/centers/gggsc
Lund—Geologic Map of the Central Beaverhead Mountains, Lemhi County, Idaho, and Beaverhead County, Montana—Scientific Investigations Map 3413

ISSN 2329-132XX (online)

https://doi.org/10.3133/sim3413