Structural Geology of Western Part of 

Lemhi Range, East-Central Idaho 

U.S. Geological Survey Professional Paper 1659 

U.S. Department of the Interior 

U.S. Geological Survey 
Cover. Looking southwest toward Lem Peak, Lemhi Range. 
Structural Geology of Western Part of 

Lemhi Range, East-Central Idaho 

By Russell G. Tysdal 

U.S. Geological Survey Professional Paper 1659 

A reconstruction of the Poison Creek anticline, a very large amplitude 

fold now broken by normal faults; fold is in the hangingwall of a major 

thrust fault 

U.S. Department of the Interior 

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

Gale A. Norton, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

Version 1.0 2002


For sale by U.S. Geological Survey, Information Services


Box 25286, Federal Center


Denver, CO 80225


This publication is also available online at:


http://geology.cr.usgs.gov/pub/ppapers/p1659/ 

Any use of trade, product, or firm names in this publication


is for descriptive purposes only and


does not imply endorsement by the U.S. Government


ISBN: 0-607-98411-2

Contents 

Abstract ......................................................................................................................................... 1


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


Previous Work .................................................................................................................... 1


Arrangement of Report ..................................................................................................... 2


Structural Setting.......................................................................................................................... 2


Stratigraphic Setting .................................................................................................................... 3


Compressional Deformation ....................................................................................................... 3


Poison Creek Thrust Fault................................................................................................ 3


Hayden Creek and Bear Valley Faults ............................................................................ 7


Hayden Creek Fault .................................................................................................. 7


Bear Valley Fault....................................................................................................... 10


Restoration to Pre-Normal Fault Displacement........................................................... 14


Poison Creek Anticline..................................................................................................... 15


Relationship of Poison Creek Anticline and Basin Lake Syncline.................... 15


Intersection of Hayden Creek and Bear Valley Faults................................................. 16


Normal-Slip on Bear Valley Fault........................................................................... 16


Intersection of Structures in Northwestern Part of Map Area .................................. 17


Stereonet Data ................................................................................................................... 18


Bedding ...................................................................................................................... 18


Cleavage..................................................................................................................... 18


Cleavage in Diamictite .................................................................................... 22


Interpretation of Cleavage in Diamictite....................................................... 22


Cleavage in Carbonate Strata of Footwall McDevitt Creek Sheet ........... 23


Age of Cleavage................................................................................................ 24


Extensional Deformation ............................................................................................................. 24


Lem Peak Normal Fault ..................................................................................................... 24


Alder Creek Fault............................................................................................................... 25


Faults with Compound Histories of Compressional and Extensional Movement............... 25


Salmon River Fault ............................................................................................................. 25


Tear-Fault Movement............................................................................................... 26


Normal-Slip Movement Related to Emplacement of Lem Peak Block............. 26


Normal-Slip Movement Related to Regional-Scale Salmon River


Normal Fault ............................................................................................................. 27


Payne Creek Cross Fault ................................................................................................. 27


Apple Creek Fault .............................................................................................................. 28


Ages of Deformation ................................................................................................................... 28


Compressional Deformation............................................................................................. 28


Extensional Deformation................................................................................................... 29


Evaluation of Possible Proterozoic Compressional Deformation .............................. 29


Reevaluation of Medicine Lodge Thrust System and Hayden Creek Window .................. 29


Conclusions .................................................................................................................................. 31


References Cited ......................................................................................................................... 31


III

Figures


1. Index map of east-central Idaho, showing prominent geographic features............... 2


2. Index to mapping in study area ........................................................................................... 2


3. Tectonic map showing major faults, subsheets, and blocks in study area ................. 5


4. Geologic map of study area ................................................................................................. 6


5. Cross sections A–A′, B–B′, D–D′ ........................................................................................ 8


6. Cross sections restored to pre-normal-slip displacement ............................................. 10


7.	 Cross section showing large fold determined from reconstructed


cross sections ........................................................................................................................ 12


8.	 Map and cross section (C–C′) of Hayden Creek fault showing thrust and


normal displacement............................................................................................................. 13


9. Diagrammatic sketch showing fold pattern in northwestern part of study area........ 17


10. Stereonet diagrams of bedding........................................................................................... 19


11. Stereonet diagrams of cleavage......................................................................................... 20


Table 

1. 	Description and thickness of rock units within or directly adjacent


to study area........................................................................................................................... 4


IV

Structural Geology of Western Part of 

Lemhi Range, East-Central Idaho 

By Russell G. Tysdal 

Abstract 

The large Poison Creek anticline, now broken by normal 

faults, is the major structure in the western part of the Lemhi 

Range. The fold resulted from late Mesozoic compressional 

deformation in the hinterland of the Cordilleran thrust belt in 

central Idaho. Bedrock of the study area is mainly Mesoproterozoic strata that are part of a thrust sheet floored by the Poison 

Creek fault, a major thrust fault whose trace extends northwest 

across the northern part of the study area. The thrust fault displaced the Proterozoic rocks over lower Paleozoic formations 

composed of carbonate and quartzite. In the southern part of the 

study area, the easterly trending Lem Peak normal fault has 

vertical displacement of several thousand meters and may cut 

the Poison Creek thrust fault at depth. Between the trace of the 

Lem Peak normal fault and the Poison Creek thrust fault lie the 

Hayden Creek and Bear Valley faults, which in their present 

configuration show normal displacement. The Hayden Creek 

fault, however, has a demonstrable thrust displacement prior to 

normal displacement. Both it and the Bear Valley fault may have 

formed originally as subsidiary thrusts of the Poison Creek 

thrust fault, generated during rupture of tight folds within the 

Poison Creek thrust sheet. Removal of the normal component of 

slip on the Hayden Creek and Bear Valley faults, restoring rocks 

to about their pre-normal-slip positions, reveals the original 

geometry of the eastern part of the large Poison Creek anticline 

above the Poison Creek thrust fault. 

A single regionally extensive thrust sheet, the Medicine 

Lodge thrust system, previously was interpreted as the major 

structure of the region. Within the southeastern part of the 

present study area, a window was stated to exist in the vicinity of 

Hayden Creek and to expose rocks of the subthrust Mesoproterozoic Yellowjacket Formation. My mapping leads me to conclude that neither the Medicine Lodge thrust plate nor the 

window exists in this part of the range. 

The northwest-trending structural grain of the Lemhi Range 

is truncated by the north-striking Salmon River fault at the western margin of the study area. The fault is oblique to the structural grain of the range and is interpreted as a tear fault that 

formed during Mesozoic thrust faulting and folding of rocks of 

the Poison Creek thrust sheet. The fault served to compartmentalize the style of deformation that took place on opposite sides 

of it. Rocks east of the Salmon River fault, the main focus of 

this report, deformed into the major fold and associated subsidiary thrusts of the Poison Creek sheet. Rocks directly west of 

the Salmon River fault generally display low dips and locally 

define a shallow-dipping syncline oriented about parallel to the 

north-trending fault. A segment of the fault subsequently was 

reactivated as part of the Lem Peak normal fault during downdropping of the Lem Peak block. Later, the entire length of the 

present Salmon River fault was reactivated as a basin-and-range 

normal fault that dips steeply to the west. 

Introduction 

The study area is in the northern part of the Lemhi Range of 

central Idaho (fig. 1), a region entirely within the Cordilleran 

overthrust belt of the Rocky Mountains. Meso- and 

Neoproterozoic1 and Phanerozoic strata are prominent in the 

central and southern parts of the range, southeast of the study 

area. Bedrock of the northern part of the range is chiefly Mesoproterozoic rocks, with only local areas of Paleozoic strata preserved. Igneous rocks of the middle Eocene Challis Volcanic 

Group are widespread to the west and southwest. The volcanic 

rocks form locally extensive outcrops on the flanks of the northern part of the Lemhi Range, but within the higher parts of the 

range they are preserved only in small isolated patches. 

Previous Work 

The study area (fig. 2) contains some previously 

unmapped territory, whereas some parts are covered by previously published and unpublished mapping. Much of the area 

was covered by the thesis areas of Soregaroli (1961) and Tietbohl (1981). The eastern part includes the southwest corner of 

the Lemhi 15-minute quadrangle mapped by Anderson (1961) 

and the northwest corner of the Patterson 15-minute quadrangle mapped by Ruppel (1980). A discussion of the geology in 

the Patterson quadrangle was presented in Ruppel and Lopez 

(1988). In addition, the study area includes the southernmost 

part of the Goldbug Ridge 7.5-minute quadrangle (Evans, 1999; 

K.V. Evans, Falma Moye, and R.F. Hardyman, unpub. mapping, 1996) and the adjacent Poison Peak quadrangle (G.R. 

Winkler, unpub. mapping, 1996). This report is based on 

published geologic mapping studies of Tysdal (1996a, 1996b, 

1The terms Middle and Late Proterozoic, respectively, were used for these 

eras in previous publications by the author. The present usage follows that of the 

International Union of Geological Sciences (IUGS) subcommittee on Precambrian stratigraphy (Plumb, 1991). 

1

114°30' 114° 113°30' 114° 113°52'30" 113°45' 113°37'30" 113°30' 

45°30' 

45° 

44°30' 

0 

0 

MONTANA 

Area of 

this figure 

IDAHO 

IDAHO

Study area 

and area of 

figures 3, 4 

93 

93 

28 

LOST 

SALMON 

RIVER 

MOUNTAINS 

Gunsight 

Peak 

Poison 

Peak 

LEMH I RANGE

MOU NT AINS 

BEAVERHEAD 

LEMHI 

VALLEY 

RIVER 

CLEARWATER 

MOUNTAINS 

River 

Panth er

Creek 

Pahsimer oi River

Lemhi

H 

River 

a ydenCr eek 

DeepCr 

S a lmon Fk 

N 

Salmon 

Yellow 

Lake 

Salmon 

River 

Challis 

Salmon 

North Fork 

Yellowjacket 

Leadore 

Cobalt 

Lemhi 

5 10 15 MILES 

10 20 KILOMETERS 

RANGE 

RIVER 

River 

Figure 1. Index map of east-central Idaho, showing prominent 

geographic features. 

1996c) and Tysdal and Moye (1996), as well as local areas of 

unpublished mapping of Tysdal. 

Arrangement of Report 

This report generally discusses compressionally formed 

structures first, then extensional features. However, removal of 

later normal-slip movement from several (originally) thrust 

faults is necessary before discussion of the compressional 

features is feasible. Hence, the two types of deformation are 

considered together for several faults within the range. Interpretation of compressional deformation is presented in a section 

that makes use of stereonet compilations of bedding and 

cleavage. Extensional faults for which no restoration to pre-slip 

positions was undertaken are then described. A final part of the 

report discusses the previously described Medicine Lodge thrust 

plate and the Hayden Creek window. 

Structural Setting 

Central Idaho is part of the hinterland of the thrust belt in 

the eastern part of the Cordillera of the western United States. 

2 Structural Geology, Western Lemhi Range, Idaho 

0 

0 

Study area 

and area of 

figures 3,4 

C D E 

F 

G 

H 

I 

A 1 

2 

GOLDBUG 

RIDGE 

PATTERSON 

HA

LEM 

YDEN 

ALLISON 

MOGG 

MOUNTAIN 

MAY 

NTAIN 

POISON 

LEMHI 

45° 

44° 

44° 

37' 

30" 

44° 

30' 

5 10 MILES 

5 10 KILOMETERS 

B 

CREEK 

PEAK

CREEK 

MOU

PEAK 

Figure 2. Index to geologic mapping studies in western part of 

Lemhi Range. Names of 7.5- and 15-minute topographic quadrangles 

are labeled in figure. 

The study area (fig. 1) lies within this region and generally displays lower structural levels and older stratigraphic units than 

are exposed to the east and south. Along the extent of the Lemhi 

and Lost River Ranges and the Beaverhead Mountains, preserved Paleozoic strata generally become progressively older 

from southeast to northwest. In the northern part of the Lemhi 

Range and the Salmon River Mountains to the west, Paleozoic 

rocks crop out only in isolated erosional remnants. Upper units 

of the Mesoproterozoic sequence also have been eroded from 

broad areas in the northern parts of these mountain ranges. 

Proterozoic and Paleozoic strata in the general region of 

the study area underwent compressional deformation, probably 

in the Late Cretaceous, creating a series of thrust faults and 

related folds. Subsequent extension was accompanied by formation of normal faults, some with major displacement. The 

compressional deformation and at least some of the extensional 

movement predate deposition of the Eocene Challis Volcanic 

Group. This probable Cretaceous to Tertiary deformation is in 

addition to (1) Mesoproterozoic compressional deformation 

52' 

30" 

44° 

45' 

1:24,000 and other geologic maps 

A K.V. Evans, Falma Moye, and R.F. 

Hardyman (unpub. mapping, 1996) 

B G.R. Winkler (unpub. mapping, 1996) 

C Tysdal and Moye (1996) 

D Tysdal (1996b) 

E Tysdal (unpub. mapping, 1996-2000) 

F Soregaroli (1961) 

G Tietbohl (1981) 

H Tysdal (1996a) 

I Tysdal (1996c) 

1:62,500 geologic maps 

1 Anderson (1961) 

2 Ruppel (1980) 
reported for local areas of the general region (for example, 

Evans, 1986; Evans and Zartman, 1990), and (2) Neoproterozoic 

to early Paleozoic extension, both of which may have affected 

rocks of the area. Normal displacement on several major faults 

in the Lemhi Range took place prior to uplift of the mountain 

range. The present topographic expression of the Lemhi Range 

and the adjacent mountain ranges is largely the effect of Cenozoic basin-and-range normal faulting (Ruppel, 1993). 

Interpretations presented in this report differ significantly 

from those published previously. Rocks of the study area previously were interpreted as part of a single, regionally extensive 

thrust plate that crossed from central Idaho into southwestern 

Montana (Ruppel, 1978). Called the Medicine Lodge thrust 

system, it was perceived to include the thrust faults in the Beaverhead Mountains and adjacent areas of Montana, and the concept was later extended to include faults farther west in the 

Lemhi Range. Ruppel (1978) and Ruppel and Lopez (1984, 

1988) envisioned the Medicine Lodge thrust as separating 

allochthonous rocks of the Mesoproterozoic Lemhi Group and 

overlying strata from subjacent, autochthonous, older Mesoproterozoic rocks of the Yellowjacket Formation. These workers 

considered the Yellowjacket to be autochthonous because they 

believed that, in most places, the rocks were only slightly 

deformed or exhibited a style of deformation different from that 

of strata of the Lemhi Group and younger rocks. 

Within the Lemhi Range, Ruppel (1978, p. 8) stated that 

one of the best exposures of the Medicine Lodge fault system is 

in the vicinity of Mill Mountain, in the southeastern part of the 

present study area, where a window (the Hayden Creek window, 

fig. 4) resulted from erosion through the Medicine Lodge thrust 

plate. My mapping in this area and farther west in the northern 

part of the Lemhi Range (Tysdal, 1996a, 1996b, 1996c; Tysdal 

and Moye, 1996) leads me to conclude that the Medicine Lodge 

thrust plate does not exist in this part of the range, that no window was eroded through such a thrust plate, and that rocks 

assigned (by Ruppel and Lopez, 1988) to the Yellowjacket Formation in the northern part of the Lemhi Range are as intensely 

deformed as the rocks of other Mesoproterozoic formations. 

Stratigraphic Setting 

The stratigraphic succession, description, and thickness of 

rock units exposed in the study area are shown in table 1. Paleozoic strata crop out only locally, in the southern and northern 

parts of the study area. Volcanic rocks of the Eocene Challis 

Volcanic Group are extensively exposed on the flanks of the 

Lemhi Range, and locally atop the range. The sequence of 

Proterozoic formations shown in table 1 is in agreement with 

that determined by Ruppel (1968, 1975, 1980; Ruppel and 

Lopez, 1981), who set the stage for subsequent workers on Proterozoic stratigraphy in central Idaho during the course of their 

geologic mapping in the Lemhi Range. All Proterozoic rocks 

are metamorphosed to the lower greenschist facies, except for a 

local area of biotite-grade greenschist facies in the western part 

of the study area. 

Discussion of the stratigraphy and interpretation of 

depositional environments represented in the Mesoproterozoic 

formations of the study area are presented in Tysdal (2000a). 

No strata of the Proterozoic Yellowjacket Formation (Ross, 

1934) occur in the study area. The Yellowjacket was redefined 

by Tysdal (2000b) and restricted to strata like those originally 

assigned to the unit by Ross (1934). Rocks that Ruppel (1980), 

Lopez (1981), and Ruppel and Lopez (1988) designated as Yellowjacket within the Lemhi Range were reassigned to the Apple 

Creek Formation, and locally the Big Creek Formation, by 

Tysdal (1996a, 1996b, 1996c; Tysdal and Moye, 1996). Hence, 

in this report, strata that Ruppel (1978, 1980) and Ruppel and 

Lopez (1984, 1988) mapped or interpreted as Yellowjacket in 

the Lemhi Range here are designated “Yellowjacket” to 

distinguish the previous usage. This usage occurs mainly in the 

subsection on the Medicine Lodge thrust system. 

Compressional Deformation 

Structural terminology use in this report is summarized in 

the illustration of figure 3. The Poison Creek thrust fault, which 

trends northwest across the study area, is the major fault of the 

western part of the Lemhi Range. Hangingwall rocks of the 

Poison Creek thrust fault are named the Poison Creek thrust 

sheet, and those in the footwall are assigned to the McDevitt 

Creek thrust sheet. Subsidiary thrust and (or) normal faults of 

the Poison Creek sheet are the Hayden Creek and Bear Valley 

faults, which in their present configuration show normal displacement. The Hayden Creek fault, however, has a demonstrable thrust displacement prior to normal displacement. Both it 

and the Bear Valley fault may have formed originally as subsidiary thrusts of the Poison Creek thrust fault, generated during 

rupture of tight folds within the Poison Creek sheet, but attained 

only minor displacement. Rocks downdropped southwest of the 

Lem Peak normal fault are part of the Lem Peak block. 

Major thrust faults are described first, followed by faults 

that have undergone multiple sense of movements. Then, the 

more recent movements are removed from these faults so as to 

look at early fault movements and to reconstruct the associated 

folds. A detailed evaluation of cleavage then is undertaken. 

Normal faults are described last. 

Poison Creek Thrust Fault 

The Poison Creek name was introduced by Soregaroli 

(1961) for a fault that trends northwest along Poison Creek in 

the northwestern part of the study area (fig. 4). An unnamed, 

concealed thrust fault was shown by Anderson (1961) in a structural section across the southwest 1/4 of the Lemhi 15-minute 

quadrangle (fig. 2). The name Poison Creek thrust fault is here 

applied to both previously recognized faults, interpreted as segments of one continuous feature. The name Hawley Creek 

thrust fault was applied to part of the Poison Creek thrust fault in 

the Lemhi Range by Skipp (1987) in a regional reconnaissance 

paper and by Skipp (1988). Janecke and others (2000) continued this usage in the Lemhi Range in a recent regional reconnaissance paper. The Hawley Creek name is not used for the 

fault because of reasons discussed in the last two paragraphs of 

this section of the present report. 

Compressional Deformation 3 
4


Table 1. Description and thickness of rock units within or directly adjacent to study area. 

Structural Geology, Western Lemhi Range, Idaho 

Eocene 

Paleozoic carbonate rocks, undivided 

Mesoproterozoic 

Lenses of orthoquartzite in poorly exposed siltite. 

Siltite is argillaceous, poorly exposed. 

to north of quadrangle is 1,500+ m, where formation is mainly metasandston

350-400 

600-1,500 

STRATIGRAPHIC UNIT THICKNESS (m) AGE DESCRIPTION

Alternating 10–20-cm thick beds of siltite and fine-grained metasandstone. Poison Creek fault, northwestern part of map area. 

Mainly flows of dacite, less abundant andesite, basalt, and rhyolitic ashflow autoclastic breccia, agglomerate. es, plugs, small intrusions of gran

Mississippian Middle Canyon Formation—limestone, thin-bedded; locally si

local small area near Hayden Creek—sucrosic dolomite. vician and Sildolomite, thin- to medium-bedded, locally massive, finely crystalline; minor 

local chert nodules. All carbonate rocks form ledges and cliffs. 

Orthoquartzite, fine- to medium-grained. 

Tightly cemented, hard, resistant to 

Sills, dikes, and one plug-shaped body. 

cleaved and altered that classification is uncertain. 

Argillaceous siltite, siltite, and metasandstone. 10–150 cm thick. wer part of formation present in study area. 

Orthoquartzite and quartzite; minor siltite in upper part. 

Medium- to coarse-grained siltite and fine-grained metasandstone. wer paquartz-rich fine-grained metasandstone. inlocally contains soft-sediment deformation structures. 

Argillite, argillaceous siltite, and metasandstone. with non-pebbly argillite and siltite. Beds generally are only a few centimeters thick and are finely laminated. 

Siltite and argillaceous siltite. undant in gravel-size argillite clasts form horizons one to two clast diameters thick loca

Coarse-grained siltite and fine- to medium-grained metasandstone, composed 

Heavy-mineral laminae widespread. sparse zones, as thick as 10 m, of dark-gray argillaceous siltite. 

Mississippian, 

Devonian, 

Silurian, and 

Ordovician 

Ord. 

Neoproterozoic 

Swauger Formation 

Challis Volcanic Group 0-1,000 

300-400 

0-100 

Kinnikinic Quartzite 

Lawson Creek Formation 300+ 

3,100 

Lemhi 

Apple Creek Formation 

Gunsight Formation 

Banded siltite unit 

Coarse siltite unit 

Diamictite unit 

Fine siltite unit 

Big Creek Formation 2,700 

2,500+ 

1,000 100+ 

100+ 

Altered intrusive rocks 

Lenses as thick as 10 m, e

Crops out only in northeastern part of 

E

Local dik

Ordo

Some rocks are diabase, others gabb

Clots of calcite present w

Metasandstone is generally q

Only lo

Beds about 1 m thic

Lo

Upper strata of unit generally are f

Upper part of unit is e

Contains pebbly strata in u

Pebbles are dispersed, matrix-supported

Fine silt and clay content is most ab 

Carbonate laminae common. Dunes co

Beds commo

(part) Group 
114° 113°52'30" 113°45' 113°37'30" 

0 

0 LEM 

PEAK 

FAULT 

HAYDEN 

CREEK 

FAULT 

BEAR 

VALLEY 

FAULT 

POISON 

CREEK 

FAULT 

LEM 

PEAK 

BLOCK 

POISON 

M c

A A' 

POISON 

CREEK 

BEAR 

FAULT

VALLEY 

FAULT 

HAYDEN 

CREEK 

FAULT 

LEM 

PEAK 

FAULT 

PAYNE CREEK 

FAULT 

LEM PEAK FAULT 

SALMON 

RIVER FAULT 

LEM

AULT 

A 

A' 

44°52'30" 

44°45' 

HAYDEN 

BEAR 

ALLEY 

SUBSHEET 

BASIN 

LAKE 

SUBSHEET 

CREEK 

SUBSHEET

LEM 

PEAK 

BLOCK 

POISON CREEK SHEET 

CREEK

 SHEET

DEVITT

MC 

Normal fault—Ball and bar on 

Thrust fault—Sawteeth on upper plate 

Thrust fault with later normal 

downthrown side 

displacement 

Normal fault—Arrow shows 

downward displacement 

Thrust fault—Arrow shows 

thrust displacement 

Thrust fault with later 

normal displacement— 

Double arrow shows thrust 

displacement and later 

1 2 3 MILES 

1 2 3 KILOMETERS

CREEK SHEET 

DEVITT CREEK SHEET 

F PEAK

V

normal displacement DIAGRAMMATIC CROSS SECTION 

Figure 3. Tectonic map showing major faults, thrust sheets and subsheets, and diagrammatic 

section across these structures of study area. 

Soregaroli (1961) interpreted the Poison Creek fault as a 

high-angle reverse fault, which he described as dipping 68° SW. 

For an area directly south of Poison Peak, I used a three point 

construction to determine a dip of 30°–35° SW. (The profile of 

the present erosion surface from Poison Peak to Poison Creek is 

30°, indicating a minimum dip necessary for the fault.) Measurement on a 25 m long exposure of the fault surface, parallel 

to the dip of the fault, yielded an inclination of 40° SW. The 

concealed segment of the fault mapped by Anderson (1961) in 

the Lemhi 15-minute quadrangle (fig. 2) displays the same 

structural relationships as in the northwestern part of the map 

area of figure 4. 

The exposed part of the Poison Creek thrust fault, and the 

directly down-dip part of the fault illustrated in the cross sections (figs. 5, 6, and 7), shows the 30°–35° SW. dip of the fault. 

This inclined fault surface is interpreted to be only the ramp of 

a major thrust fault that transported hangingwall strata to the 

northeast. Beneath the depth of the cross sections, the fault 

surface likely becomes nearly horizontal, as shown diagrammatically in cross section A–A’ of figure 3. Similarly, the fault 

surface is interpreted to have been nearly horizontal in nearly 

flat lying rock that has been eroded from above the nearly horizontal strata that underlie the present land surface of the 

exposed part of the McDevitt Creek sheet footwall. The rocks 

of the footwall McDevitt Creek sheet directly beneath the 

inclined part of the Poison Creek thrust fault thus constitute a 

footwall ramp; those beneath the (interpreted) nearly horizontal 

part of the fault constitute a footwall flat. The amount of horizontal transport on the fault is unknown, although it could be 

several tens of kilometers. 

The Poison Creek fault displaced Mesoproterozoic strata 

against lower Paleozoic formations composed of carbonate and 

quartzite (fig. 4) in the western part of the Lemhi Range. Areas 

of lower Paleozoic units occur discontinuously in the footwall 

from the Poison Peak area on the northwest to east of Hayden 

Creek on the east. Anderson (1961, pl. 1) showed the fault only 

in his cross section C–C’, not on his geologic map of the Lemhi 

quadrangle (fig. 2) where the fault trace lies beneath Quaternary 

sediments, but he showed the same relationship of Mesoproterozoic rocks displaced over lower Paleozoic strata as exists along 

the Poison Creek fault west of the Lemhi quadrangle. Stratigraphic displacement across the Poison Creek thrust fault is 

10,000–12,000 m. 

The Hawley Creek thrust fault was originally mapped by 

Lucchitta (1966) along the southwest flank of the Beaverhead 

Mountains east of Leadore (fig. 1). Skipp (1987) recognized 

that the Hawley Creek fault is a zone of thrust faulting that 

extends into the Lemhi Range. She applied the Hawley Creek 

Compressional Deformation 5 
6


44°52'30" 

Structural Geology, Western Lemhi Range, Idaho 

HAYDEN CREEK "WINDOW" 

SALMON 

RIVER 

FAULT 

Yaf 

SALMON RIVER 

LEM 

LEM 

PEAK 

FAULT 

POISON 

CREEK FAULT 

BEAR 

VALLEY 

FAULT 

LEM 

PA Y N E 

CREEK FA U LT 

HAYDEN 

CREEK 

FAULT 

Yb 

Qu 

PEAK 

LEM 

FAULT 

BASIN 

LAKE 

SYNCLINE 

A 

A' 

B 

B' 

C 

C' 

D 

Cooper Cree

POISON PEAK 

Wade Creek 

Poison 

Creek 

Allison 

Creek 

Cow 

Creek 

N 

Fork Cow 

Creek 

Hayden 

Creek 

N 

Fork 

McKim

Creek 

McKim 

Creek 

Creek 

Wright 

Bear Valley 

Creek 

X" 

X'" 

X 

B'' 

Basin Lake 

Ylc 

Ylc 

Tc 

Tc 

Yac 

Tc 

Yaf 

Yad 

Yaf 

Tc 

Qu 

Ys 

Yaf 

Yaf 

Yad 

Yb 

Yb 

Yac 

Yad 

Tc Ylc 

Ylc 

Ys 

Ys 

Ys 

Yb 

Yaf 

Yad 

Tc 

Tc 

Yac 

Tc 

Yac 

Ys Y

Ys Ok

Qu 

Yb 

Yad 

Yac 

Yaf 

Yaf 

Yad 

Yac 

Yac 

Qu 

Tc 

 

Yab Yg 

Ys 

Qu 

Qu 

Tc 

Tc 

Tc 

Tc 

Phantom 

Creek 

X' 

PEAK 

FAUL Tc 

T

114° 113°52'30" 113°45' 

 

0 

0 

41 

85 

57 

33 

65 

25 

60 

59 

57 

85 

56 

62 

60 

64 

40 

50 

43 

27 

12 

38 

40 

51 

54 

44 

40 

80 

42 

77 

30 

40 

56 

Figure 4 (above and facing page). Geologic map of study area in western part of (unpub. mapping, 1996–2000). f Ruppel and in figures 5–8. 

1 2 3 MILES 

1 2 3 4 KILOMETERS 

Lemhi Range, compiled from maps of Tysdal (1996a; 1996b)

Bold gold line shows outline of Hayden Creek “window” (dashed) and edge of “autochthon” (solid) o 

44°45' 
 

Qu 

Tc 

Ok 

Ylc 

Ys 

Yg 

Yab 

Yac 

Yad 

Yaf 

Yb 

x x' 

B B' 

64 

40 

EXPLANATION 

Quaternary sediments, undivided 

Eocene Challis Volcanic Group 

Paleozoic carbonate rocks, undivided 

Ordovician Kinnikinic Quartzite 

Mesoproterozoic units 

Lawson Creek Formation 

Swauger Formation 

Gunsight Formation 

Apple Creek Formation 

Banded siltite unit 

Coarse siltite unit 

Diamictite unit 

Fine siltite unit 

Big Creek Formation 

Contact 

Normal fault—Dotted where concealed; 

ball and bar on downthrown side; 

arrows show relative lateral movement 

directions; X, X', X", X"' denote fault 

segments discussed in text 

Tear fault—Dotted where concealed; 

arrows show relative lateral movement 

directions 

Thrust fault—Dotted where concealed; 

sawteeth on upper plate 

Thrust fault with later normal 

displacement—Dotted where 

concealed; sawteeth on upper plate; 

ball and bar on downthrown side 

Syncline—Showing trace of axial plane; 

dotted where concealed 

Overturned syncline—Showing trace 

of axial plane 

Anticline—Showing trace of axial plane 

Overturned anticline—Showing trace 

of axial plane 

Cross section line 

Strike and dip of beds 

Inclined 

Vertical 

Overturned 

name to the zone in the Lemhi Range. I do not follow this usage 

in the Lemhi Range for the following two reasons: 

1. The Poison Creek thrust fault displaced Mesoproterozoic rocks over Paleozoic strata; Paleozoic strata are not part of 

the hangingwall sheet of the Poison Creek thrust fault. The geologic map of Lucchitta (1966, pls. 1, 2) shows that the Hawley 

Creek fault has placed uppermost Mesoproterozoic rocks and 

depositionally overlying lower Paleozoic strata (and crosscutting Ordovician-Silurian granite) over younger Paleozoic rocks. 

In the Lemhi Range, this structural relationship is akin to one of 

the subsidiary imbricate thrust faults mapped by K.V. Evans, 

Falma Moye, and R.F. Hardyman (unpub. mapping, 1996) and 

Evans, K.I. Lund, and Tysdal (work in progress) in the McDevitt 

Creek sheet that is footwall to the Poison Creek thrust fault: latest Mesoproterozoic and depositionally overlying lower Paleozoic strata are thrust northeastward over lower Paleozoic and 

depositionally underlying Mesoproterozoic strata. 

2. Stratigraphic separation on the Hawley Creek thrust fault 

of the Beaverhead Mountains, and that on the imbricate thrust 

faults in the McDevitt Creek sheet (footwall) mapped by K.V. 

Evans, Falma Moye, and R.F. Hardyman (unpub. mapping, 

1996) and Evans, K.I. Lund, and Tysdal (work in progress) in the 

Lemhi Range, are minor relative to that of the Poison Creek 

thrust fault. Therefore, the Hawley Creek sheet might be the 

appropriate name for the footwall sheet in the Lemhi Range. 

However, because the Hawley Creek thrust sheet probably contains several footwall imbricate thrusts of a more major thrust 

fault that is equivalent to the Poison Creek thrust fault and that is 

concealed beneath the Leadore Valley, the Hawley Creek name is 

not used in the Lemhi Range. The position of the Poison Creek 

thrust fault concealed beneath the Leadore Valley is shown by 

Evans, K.I. Lund, and Tysdal (work in progress). 

Hayden Creek and Bear Valley Faults 

The Hayden Creek and Bear Valley faults in their present 

structural relationships are normal faults. The present offset is 

normal. However, I interpret the Hayden Creek fault to have 

formed initially as a splay from the Poison Creek thrust fault 

(fig. 3). The thrust displacement formed to relieve stress in 

tightly folded strata. The splay may have acted to dissipate slip 

on the Poison Creek thrust fault, a role for secondary thrusts 

suggested by Mitra (1990, p. 934). The Bear Valley fault also 

may have formed this way, but the present relationship is 

entirely normal displacement. The two faults merge in the 

northwestern part of the study area. Interpretation of the complex pattern in the area of intersection, shown in cross section 

A–A’ (fig. 5), requires that description and interpretation of the 

individual faults be presented first. 

Hayden Creek Fault 

The Hayden Creek fault trends northwest across the study 

area (figs. 3, 4). It is truncated by the Lem Peak normal fault on 

the southeast and merges with the Bear Valley fault on the northwest. The fault dips moderately to the southwest in its south 

half, but in the northwest, the fault steepens to vertical, indicated 

by a fairly straight trend across terrain of considerable differential relief. Farther northwest, in the vicinity of the intersection 

with the Bear Valley fault, the dip of the Hayden Creek fault 

changes to steep northeast. The northeast dip is related to normal faulting of folded strata, as explained later. The Hayden 

Creek fault zone locally is marked by a several-meter-thick 

white fault gouge; in other places it contains veins, pods, and 

stringers of white vein quartz. These characteristics are indicative of extension. In contrast, rocks on the northeast side of the 

fault indicate compressional deformation because of tight folding and intensely developed cleavage and shearing. 

In the vicinity of Wright Creek (fig. 4), rocks on the southwest side of the Hayden Creek fault form a northeast-facing 

salient, or “flap,” over rocks east of the fault. The “flap” forms a 

hangingwall composed of the Mesoproterozoic Big Creek Formation and overlying lower strata of the fine siltite unit of the 

Compressional Deformation 7 
8


A 

Structural Geology, Western Lemhi Range, Idaho FEET BA

10,000 

9,000 8,000 

7,000 6,000 

Tc Yaf 

Yb 

Ys 

Ylc 

Yb 

Qu 

Qu 

Yaf 

Yad Yac 

Yaf 

Yad 

Ya

LEM PEAK 

FAULT 

SALMON RIVER 

FAULT 

HAYDEN CREEK 

FAULT 

BEAR VALLEY 

FAULT 

S

BEND 

SECTION 

IN 

5,000 

4,000 

3,000 

2,000 

1,000 

SEA LEVEL 

NO VERTICAL EXAGGERATION SECTION IS NOT SAME SCALE AS MAP 

B 

FEET 

Ylc 

Ys Yb 

Yb Yaf Yad 

Yac 

Yb 

Yaf 

Yad 

Yac 

 Ys 

Tc 

LEM PEAK 

FYaf AULT 

HAYDEN 

CREEK 

FAULT 

BEAR VALLEY 

FAULT 

POISON CREEK 

FAULT BASIN LAKE SYNCLINE 

? ? 

? 

11,000 

10,000 

9,000 

8,000 

7,000 

6,000 

5,000 

4,000 

3,000 

2,000 

1,000 

SEA LEVEL 

NO VERTICAL EXAGGERATION SECTION IS NOT SAME SCALE AS MAP 

D 

FEET 

10,000 

9,000 

8,000 7,000 6,000 

5,000 

4,000 

3,000 

2,000 

Tc 

Yaf Yb 

Ys 

Yb 

Yad 

Yad 

Yac 

Yb 

Yac 

Yaf 

LEM PEAK 

FAULT 

HAYDEN CREEK 

FAULT 

BEAR VALLEY 

FAULT BASIN LAKE SYNCLINE

BEND 

SECTION 

IN 

1,000 

SEA LEVEL 

NO VERTICAL EXAGGERATION SECTION IS NOT SAME SCALE AS MAP 

0 1 2 MILES 

0 1 2 3 KILOMETERS 

Figure 5 (above and facing page). Cross sections A–A’, B–B’, and D–D’. Cross section C–C’ is shown in figure 8. Lines of cross sections are 
 

Qu 

Tc 

Ok 

Ylc 

Ys 

Yg 

Yac 

Yad 

Yaf 

Yb 

EXPLANATION 

Quaternary sediments, undivided 

Eocene Challis Volcanic Group 

Paleozoic carbonate rocks, undivided 

Ordovician Kinnikinic Quartzite 

Mesoproterozoic units 

Lawson Creek Formation 

Swauger Formation 

Gunsight Formation 

Apple Creek Formation 

Coarse siltite unit 

Diamictite unit 

Fine siltite unit 

Big Creek Formation 

Contact


Bedding


Normal fault—Arrow shows downward movement


Thrust fault—Arrow shows thrust movement


Thrust fault with later normal displacement— 

Double-barbed arrow shows both directions 

of movement 

Apple Creek Formation (fig. 8). There, footwall strata are composed of the fine siltite unit and the diamictite unit of the Apple 

Creek Formation. Directly beneath the flap and to the northeast, 

footwall strata contain tight folds that display steep southwestdipping axial planes. Northwest of the flap, this pattern of tight 

folds, some with overturned limbs, continues along strike for 

several kilometers. The folds are compressional structures, but 

the map pattern shows Apple Creek strata displaced over Gunsight Formation, reflecting normal displacement on the Hayden 

Creek fault. Removal of the normal displacement, and calculations of thickness and structural orientation of strata of the flap 

relative to concealed strata of the fine siltite unit, indicate that a 

thrust relationship exists in the vicinity of the flap, as shown in 

cross section C–C’ (fig. 8). Normal displacement was not sufficient to reverse the thrust relationship. The following paragraphs explain the analysis2. 

On the map of Tysdal (1996b), the Yb/Yaf contact at the 

western margin of the flap (vicinity of Y–Y′) was shown northeast of where it is shown in figure 8, due to an error made in the 

original mapping. The Yb/Yaf contact near point Y is the correct location. The nearly vertical fault about 0.25 km northeast 

of the newly placed contact is of unknown magnitude. The following calculations make use of the newly placed contact. 

Using the contact as previously placed would only reinforce the 

conclusion concerning early thrust fault displacement. 

1. Removal of the offset that now exists between the Yb/ 

Yaf contact of the flap and the Yb/Yaf contact directly north of 

the flap (fig. 8, points Y and Y′) is accomplished by moving the 

hangingwall northeastward up the plane of the fault, which dips 

2The rock symbols of the map in figure 8 are used in the following 

explanation; otherwise, the unit terminology becomes too cumbersome: Yb, Big 

Creek Formation; Yaf, Apple Creek Formation, fine siltite unit. 

about 45° SW. in the area of the flap, and for about 4 km to the 

northwest (fig. 8, section C–C’). This reconstruction results in 

the Yb/Yaf contact of the two subsheets becoming one contiguous contact. The vertical distance from the Yb/Yaf contact of 

the footwall to the Yb/Yaf contact of the hangingwall is about 

700 m. Because the two legs of a 45° right-angle triangle are 

equal, the existing horizontal distance between the Yb/Yaf contact of the hangingwall and the footwall also is about 700 m. 

2. At the north edge of the flap (that is, north of the eastwest part of the Hayden Creek fault), the northwest-striking Yaf 

unit disappears southward beneath (“dives under”) the flap. Its 

upper strata and upper contact emerge about 2.5 km to the southeast (fig. 8). The approximate location of the concealed Yab/ 

Yaf contact, traced on the map, must change trend from a northwest strike north of the flap to an almost south strike beneath the 

flap to permit Yaf strata north of the flap to connect with Yaf 

strata southeast of the flap. (This change of direction cannot be 

due to topography because the steep to moderate northeast dips 

of unit Yaf beds would require that the concealed Yaf unit 

change direction to an east trend, and thus would have to crop 

out northeast of the flap.) The thickness of unit Yaf is about 900 

m, but because of internal folding and moderate dips, the unit 

appears somewhat thicker. 

3. Considering the 45° SW.-dipping section of the Hayden 

Creek fault, removal of the offset that now exists between the 

Yb/Yaf contact at the south edge of the flap (fig. 8, point Z′) and 

the concealed contact directly to the southwest (fig. 8, point Z) 

requires a horizontal component of movement of 1,050–1,200 

m. (And, because the two legs of a 45° right-angle triangle are 

equal, the vertical component of offset of the Yb/Yaf contact 

also is 1,050–1,200 m.) This measurement is considerably 

greater than the 700 m determined for the northern part of the 

flap. 

4. The trace of the Yb/Yaf contact of the hangingwall flap 

does not trend south in concert with the Yb/Yaf contact in the 

concealed footwall, but continues its southeast orientation in 

agreement with the recorded strikes of the flap. No strikes 

observed anywhere within the hangingwall display the southerly 

trends required for agreement with the strikes required in the 

concealed Yaf unit of the footwall. No tight folding in the rocks 

of the flap is recorded that would permit the contact in the hangingwall and footwall to accommodate the 45° difference in trend 

indicated in figure 8. But the two trends could be accommodated if a fold, now cut out by the normal fault movement, had 

existed previously between the hangingwall flap and the footwall rocks. Overturned Yaf and Yb beds in the northwestern 

part of the footwall suggest that this possibility is real. The difference in strike orientation of beds in the hangingwall flap and 

the concealed footwall strata then could be accommodated by 

kinking of the fold. Such a structure would be compressional, in 

concert with observed minor structures of the footwall. 

In any case, removal of 700 m of normal-slip displacement 

on Y–Y′ is insufficient to account for all of the 1,050–1,200 m of 

reverse-slip offset along Z–Z′. Restoration of Y–Y′ to a pre-normal-slip position yields a larger thrust fault relationship in the 

area of Z–Z′. This indicates that early thrust faulting took place 

and that later normal displacement was not sufficient to entirely 

reverse the thrust displacement. 

Compressional Deformation 9 
 

B B' 

FEET 

METERS

40,000 

12,000 

38,000 

? 

? 

? 

BASIN 

SYNCLINE 

POISON CREEK FAULT 

BEA R 

LEM 

FAU L T 

HA YDEN 

CREEK 

FA ULT 

FAUL T 

VALLEY 

PEAK 

POISON

CREEK 

Yaf 

Yad 

Yac 

 Ys 

Tc 

Yb 

Yaf 

Yad 

Yac 

Ylc 

Ys Yb 

Yad 

Yac 

Yab 

Yg 

LAKE 

ANTICLINE 

11,500 

36,000 11,000 

34,000 10,500 

10,000 

32,000 

9,500 

30,000 

9,000 

28,000 8,500 

26,000 8,000 

7,500 

24,000 

7,000 

22,000 

6,500 

20,000 6,000 

18,000 5,500 

16,000 5,000 

4,500 

14,000 

4,000 

12,000 3,500 

10,000 3,000 

8,000 2,500 

2,000 6,000 

1,500 

4,000 

1,000 

2,000 500 

SEA LEVEL SEA LEVEL NO VERTICAL EXAGGERATION SECTION IS NOT SAME SCALE AS MAP 

0 1 2 MILES Tc 

0 1 2 3 KILOMETERS	 Ylc 

Ys 

Yg 

Yab 

Yac 

Yad 

Yaf 

Yb 

EXPLANATION 

Eocene Challis Volcanic Group 

Paleozoic carbonate rocks, undivided 

Mesoproterozoic units 

Lawson Creek Formation 

Swauger Formation 

Gunsight Formation 

Apple Creek Formation 

Banded siltite unit 

Coarse siltite unit 

Diamictite unit 

Fine siltite unit 

Big Creek Formation 

Contact—Dashed where inferred 

Bedding 

Incipient fault—Dotted where inferred; 

thrust and (or) normal fault restored to 

predisplacement position 

Normal fault—Dotted where inferred; 

arrow shows downward movement 

Thrust fault with later normal 

displacement—Double-barbed arrow 

shows both directions of movement 

Figure 6 (above and facing page). Cross sections B–B ’ and D–D ’ as reconstructed after normal-slip displacement 

removed from faults. Figure 9 shows the reconstructed segment of cross section A–A’ for which normal-slip has been 

removed. No reconstruction is shown for cross section C–C ’. 

Bear Valley Fault 

The Bear Valley fault trends northwest, dips to the southwest, and is flanked on the northeast by the Basin Lake syncline 

(fig. 4). Features suggestive of extension are common along the 

fault. Intensely developed southwest-dipping cleavage is cut by 

10 Structural Geology, Western Lemhi Range, Idaho 

fractures that are filled with quartz veins as thick as 10 cm; 

quartz masses as much as 0.5 m across were observed in some 

areas. Chloritic alteration occurs in some areas and silicification 

was observed locally in the northern extent of the fault. Cobblesize pieces of undeformed specular hematite characterize float 

in both the northern and the southern extent of the fault. 
40,000 

D 

FEET 

Yg Yg 

Ys 

Yb 

Yb 

Yaf 

Yb 

Yaf 

Yad 

Yad 

Tc 

Yad LEM PEAK FAULT 

POISON 

CREEK 

HAYDEN CREEK FAULT 

Tc Ys Yg Yab Yac Yad Yaf Yb 

Eocene Challis Volcanic Group Mesoproterozoic units 

Swauger Formation Gunsight Formation Apple Creek Formation 

Banded siltite unit Coarse siltite unit Diamictite unit Fine siltite unit 

Big Creek Formation 

EXPLANATION 

Normal fault—Dotted where inferred; 

arrow shows downward movement 

Incipient fault—Dotted where inferred; 

thrust and (or) normal fault restored to predisplacement position 

Contact—Dashed where inferred 

Bedding—Dotted where inferred 

0 

0 

1 2 MILES 

1 2 3 KILOMETERS 

38,000 

36,000 

34,000 

32,000 

30,000 

28,000 

26,000 

24,000 

22,000 

20,000 

18,000 

16,000 

14,000 

12,000 

10,000 

8,000 

6,000 

4,000 

2,000 

Compressional Deformation 

SEA LEVEL 

NO VERTICAL EXAGGERATION SECTION IS NOT SAME SCALE AS MAP 

11

12


FEET 

40,000 

? 

? 

? 

0 

0 

Tc  Ylc Ys Yg Yab Yac Yad Yaf Yb 

Eocene CPaleozoiMesopro

LawsSwauGunsAppl

BCDF

Big C

EX

Thrust fau

displac

shows b

Normal fa

arrow sh

Bedding—

Contact—

queried 

Incipient f

thrust an

predispl

B B' 

BASIN 

SYNCLINE 

POISON CREEK FAULT 

BEAR 

LEM 

FAULT 

HAYDENCREEK 

FAULT 

FAULT 

VALLEY 

PEAK 

POISON 

CREEK 

AN

TICLINE 

POISON CREEK THRUST SHEET 

Yaf 

Yad 

Yac 

 

Tc 

Yb 

Yaf 

Yad 

Yac 

Ylc 

Ys Yb 

Yaf 

Yad 

Yac 

Yab 

Yg 

Yac 

Ys 

Ys 

Yg 

? 

? 

Figure 7. Cross section showing relationship of Poison Creek anticline of Poison Creek sheet and the McDevitt Creek footwall sheet. anticline is 

shown in figure 6. 

1 

LAKE 

MCDEVITT CREEK THRUST SHEET 

The 

See text for explanation and interpretations. 

Structural Geology, Western Lemhi Range, Idaho 38,000 36,000 34,000 32,000 

30,000 

28,000 

26,000 

24,000 

22,000 

20,000 

18,000 

16,000 

14,000 

12,000 

10,000 

8,000 

6,000 

4,000 

2,000 

SEA LEVEL 

NO VERTICAL EXAGGERATION SECTION IS NOT SAME SCALE AS MAP 
Yb 

Yb 

Yb 

Qu 

Qu 

Yad Yaf 

Yaf 

Wright 

Creek 

Y 

Z' 

? 

Qu 

Z 

Y' 

HAYDEN 

CREEK 

FAULT 

footwall 

contact

C 

C' 

concealed 

SEA LEVEL 

9,000 

8,000 

7,000 

6,000 

5,000 

4,000 

3,000 

2,000 

1,000 

10,000 

FEET 

11,000 

NO VERTICAL EXAGGERATION 

HAYDEN 

CREEK 

FAULT 

C 

Yb 

Yb 

Yaf 

Yad 

C' 

Yaf—concealed 

2,500 

1,500 

500 

3,000 

2,000 

1,000 

SEA LEVEL 

METERS 

44°47' 

44°46' 

44°45' 

0 

0 

75 

70 

75 

76 

80 

78 

57 73 

64 

48 

74 

80 

68 

83 

74 

60 

47 

77 

52 

42 

C C' 

48 

80 

Contact—Dotted where concealed 

Normal fault—Dotted where concealed; 

ball and bar on downthrown side; 

queried where uncertain 

Thrust fault with later normal 

displacement—Dotted where 

concealed; sawteeth on upper plate; 

ball and bar on downthrown side 

Syncline—Showing trace of axial plane 

Anticline—Showing trace of axial plane 

Cross section line 

Strike and dip of beds 

Inclined 

Overturned 

Vertical 

Quaternary sediments, undivided 

Mesoproterozoic units 

Apple Creek Formation 

Diamictite unit 

Fine siltite unit 

Big Creek Formation 

Qu 

Yad 

Yaf 

Yb 

EXPLANATION 

113°50' 113°47'30" 

Figure 8. Geologic map and cross section of area where Hayden Creek fault shows both thrust and normal displacement. 

Vicinity of Wright Creek, south-central part of study area (fig. 4). C–C ’ is the same as 

that shown by Tysdal (1996a), except that here the northeast end is truncated at the limit of the area shown in the map view. Y, 

Y’, Z, and Z’ are points of reference used in the interpretation of fault movements as discussed in the text. solidated sediments are not shown on cross section. 

along fault indicates thrust movement followed by later normal-slip displacement; light dashed lines, bedding. 

0.5 1 MILE 

0.5 1 KILOMETER 

See text for interpretation. Cross section 

Quaternary uncon

Concealed rocks of lower sheet shown. In cross section, double-barbed arrow 

The Bear Valley fault shows normal displacement in its 

present structural relationship (Tysdal, 1996a, 1996b). However, the fault may have formed originally as a minor thrust and 

later underwent normal-slip. Metasandstone within 100–300 m 

of the fault is intensely cleaved locally, whereas farther away the 

cleavage is less intense. The intensely developed cleavage dips 

30°–40° SW., similar to the regional cleavage. In its southeastern part, the fault is a zone that is entirely within the Big Creek 

Formation; footwall strata are not overturned, and I cannot 

determine the existence of compressional features. In the 

Compressional Deformation 13 
northwest part of the study area, overturned footwall strata of the 

Bear Valley fault host intensely developed cleavage that dips 

southward at moderate angles. 

Abundant closely spaced folds (wavelengths 100–300 m) 

characterize deformation in the Basin Lake syncline along Hayden Creek (fig. 4; fig. 5, cross section D–D’). The folds form 

pairs in the diamicite and coarse siltite units of the Apple Creek 

Formation in the lower part of the Bear Valley subsheet along 

Hayden Creek: the repeated pattern is of a closely spaced anticline-syncline pair separated by several hundred meters from 

another closely spaced fold pair. Folds in the coarse siltite unit 

of the hangingwall in the area directly east of Wright Creek are 

more open than those in the coarse siltite unit of the Apple Creek 

Formation in the footwall of the Bear Valley fault (Basin Lake 

subsheet) in the Hayden Creek area and southeastward. This 

contrast likely reflects the higher percentage of coarse siltite to 

fine-grained metasandstone beds in the Wright Creek area. 

Flowage is characteristic of the clay-rich fine-grained siltite 

between the coarser interbeds of the Wright Creek area and is 

characteristic of the diamictite of the Hayden–Alder Creek area. 

Along the central extent of the Bear Valley fault, beds at the 

leading edge of the hangingwall steepen progressively (from 

southwest to northeast) toward the fault, in the process of rolling 

over to form the steep limb of an anticline that is the counterpart 

of the Basin Lake syncline (fig. 5, cross section B–B’). Only part 

of the anticlinal limb is now preserved, and it is displaced to the 

southwest due to normal movement on the Bear Valley fault. 

The two folds form an anticline/syncline couple, sharing one 

fold limb that is referred to as the common limb. 

Overturning of beds in the anticlinal limb of the fold-couple 

became so great locally that a minor thrust fault developed 

within the Bear Valley subsheet directly west of Wright Creek 

(fig. 4). This minor thrust fault, directly northeast of the northeast end of section C–C’ (fig. 4), developed at the upper contact 

of the diamictite unit and displaced the diamictite unit onto overturned strata of the coarse siltite unit, both of the Apple Creek 

Formation. The minor thrust fault gives way northwestward to 

an overturned anticline, then an upright anticline. The actual 

connection of the thrust segment with the anticline is an interpretation in an area of poor exposures, but the strata are consistently 

overturned for the full 3 km of the trend. Southeast from Wright 

Creek, the minor thrust fault is covered; directly west of Hayden 

Creek, the contact is depositional. 

The Bear Valley fault formed along the axial area of the 

anticline/syncline fold-couple that is seen along the central area 

of the Bear Valley fault. The cross sections of figure 6 are reconstructions across this central area after removal of the normalslip shown in the faults of figure 5, and with the anticline of the 

fold-couple placed above the syncline. This depicts the fold at 

an initiation point of thrusting to the northeast or normal-slip 

displacement to the southwest. The Bear Valley fault, northeast 

of cross section C–C’, is covered by volcanic rocks and the dip 

there is uncertain, but it probably is similar to sections B–B’ and 

D–D’. These sections show that the fault developed along the 

common, stretched limb of the fold-couple. (Its dip has been 

reinterpreted—decreased—from that shown on the map of 

Tysdal (1996b) to be similar to that in cross sections B–B’ and 

D–D’ of fig. 6.) 

Normal-fault movement of the hangingwall placed strata 

from a higher structural level of the fold-couple (southwest of 

the fault) adjacent to rocks of the lower structural level on the 

footwall Basin Lake subsheet (northeast of the fault) (fig. 5, 

cross section B–B’). The lowest structural level exposed along 

the Bear Valley fault is in the southeast, the highest level in the 

northwest. Similarly, older strata are most extensively exposed 

in the southeast and younger strata in the northwest. From 

southeast to northwest, the reconstructed cross sections of figure 

6 permit an along-structure (as opposed to down-structure) view 

of the Bear Valley fault and the now-separated limbs of the foldcouple from deeper to shallower structural levels. The fault cuts 

upward from the Big Creek Formation, along the extended common limb of the anticline/syncline fold-couple (fig. 6, cross section D–D’), and farther upward through the diamictite unit of 

the Apple Creek Formation and into younger strata of the coarse 

siltite unit of the formation (fig. 6, cross section B–B’). 

Restoration to Pre-Normal Fault Displacement 

The Bear Valley and Hayden Creek faults have been 

restored to pre-normal-slip geometries (1) to give an indication 

of the scale of structures in the area and in the larger region (fig. 

6); (2) to give an indication of the type and scale of folding 

present; (3) to show the relationships of faults to the major fold 

of the study area, placing them in a perspective that permits an 

interpretation of their origin; and (4) to provide a way to interpret stereonet data—which reflect compressional deformation— 

from fold components that are in their approximate relative 

positions at the end of compression and just prior to extensional 

deformation. The pre-restored cross sections (fig. 5) were constructed at a scale of 1:24,000 and accompanied the published 

geologic maps (Tysdal, 1996a, 1996b, 1996c; Tysdal and Moye, 

1996) of the study area. For this study, the northeast end of section A–A′, which appeared only on the map of Tysdal and Moye 

(1996), is extended about 2 km farther northeast into the Lem 

Peak quadrangle (Tysdal, 1996b) than the limit originally published. Slight modifications have been made to the deeper part 

of some faults in the cross sections. 

In an attempt to reconstruct the structural situation prior to 

extensional faulting, normal displacement on faults has been 

removed and the sections reconstructed as shown in figures 6 

and 7. Normal faults in the thrust belt of western Montana and 

central Idaho are known to be listric in shape, as indicated by 

seismic data, and those of the study area are interpreted to be 

listric at depth, as shown schematically in figure 3. But within 

the depths depicted in the cross sections (fig. 5), only a slight 

curvature is shown for most of the normal faults. The scale of 

structures in the hinterland of the Cordilleran thrust belt is large, 

considerably more so than to the east in the leading part of the 

thrust belt. The normal faults have a large radius of curvature 

and cut deep into the crust before flattening out. (See for 

example the many cross sections of Harrison and Cressman 

(1993) in the hinterland of the thrust belt in northwestern 

Montana.) Hence, in removing the normal displacement from 

faults, fault attitudes determined during mapping and construction of the cross sections were maintained. For the amount of 

14 Structural Geology, Western Lemhi Range, Idaho 
displacement removed, fault curvature is small and was not 

incorporated into the reconstructions. 

The normally displaced strata were moved up the fault 

plane until contacts of the hangingwall aligned with those of the 

footwall. This procedure is not as satisfactory as I would like 

because erosion occurred after normal displacement had taken 

place; hence, the restored cross sections contain a “gap” above 

the present ground surface of the footwall of each fault. This 

gap was minimized as much as possible and still allow a reasonable reconstruction. I recognize, however, that the amount of 

eroded rock (thus, the gap) could be considerably greater than 

interpreted. Downsection viewing, and projection of structure 

into each restored cross section, afforded some measure of control. In some cases, extensive, thick sequences of the Challis 

Volcanic Group were deposited on the erosion surfaces in the 

Eocene, thus arresting erosion of the Proterozoic bedrock. 

Some of the bedrock has only recently been exhumed, lessening 

the amount of interpreted erosion. Despite the uncertainties, the 

general style and magnitude of the fold are quite apparent and 

would not change even assuming three to four times the amount 

of erosion depicted in the reconstructed sections. 

The diamictite unit of the Apple Creek Formation also 

adds uncertainty to the reconstruction of the cross sections, 

because it deformed plastically during folding, owing to its 

high content of clay and fine silt. It is isoclinally folded at 

least locally and overthickened in the core of the fold depicted 

in the restored sections, particularly in cross section D–D’ (fig. 

6). Further, the unit thins rapidly to the northwest, due to its 

depositional pattern. A northwest-southeast cross section, 

which would be normal to the interpreted tectonic transport 

direction of the unit, would show a lens shape, the unit thinning to both the northwest and the southeast. These characteristics are reflected in the cross sections. 

Poison Creek Anticline 

The reconstructed cross sections indicate that the map area 

in the northern part of the Lemhi Range is the hangingwall of 

the Poison Creek thrust fault and that the strata originally 

formed a very large anticline. The fold may have had a radius of 

as much as 10,000–12,000 m (fig. 7). Such a large fold is of a 

reasonable scale in the hinterland of the Cordilleran overthrust 

belt. For example, the cross sections of Harrison and Cressman 

(1993) show folds of this size in the Mesoproterozoic strata of 

northwestern Montana. 

The now-disrupted megafold, named the Poison Creek 

anticline, lies above the footwall ramp of the Poison Creek 

thrust fault and is interpreted as a fault-propagation fold. 

Folds so classified are steep to overturned structures of the 

hangingwall of a propagating thrust fault with which they 

grow contemporaneously, initiating from the fault tip (Suppe, 

1983, 1985). The Poison Creek fault and fold have not been 

traced to or near their point of zero displacement, thus this 

fault-fold classification is inferred. However, the fault does 

lack a footwall syncline, a feature that characteristically is 

lacking from models of “traditional” fault-propagation folds 

(McNaught and Mitra, 1993). 

Figure 7 shows the relationship of the Poison Creek hangingwall thrust sheet to the nearly flatlying footwall strata of the 

McDevitt Creek sheet. In figure 7, the reconstructed hangingwall anticline of cross section B–B’ shown in figure 6 is 

extended northeast from B’ to B”. Strata of the Gunsight Formation exposed at the surface of the B’–B” extension are 

thought to be in the uppermost part of the formation; hence, the 

inferred Gunsight-Swauger contact is shown above the ground 

surface of figure 7’s cross section B’–B”. Concealed Swauger 

strata shown in the cross section are of approximately the correct thickness for the formation. Their placement in the cross 

section also is in agreement with their projection into the plane 

of section B–B’–B” from the footwall strata northeast from point 

A’ of cross section A–A’, in the McDevitt Creek sheet north of the 

study area. North of the study area, in the Poison Peak 7.5-

minute quadrangle (fig. 2), mapping by G.R. Winkler showed 

that footwall Gunsight strata dip 6°–10° southward for more 

than 10 km into the southern part of the quadrangle, where the 

Swauger-Gunsight contact is preserved and displays a similar 

inclination. Exposed Gunsight strata of the cross section B’–B” 

extension show the low dips of the Gunsight, but no Swauger 

strata are exposed. Within the study area, the Poison Creek 

sheet overlies a footwall ramp; northeastward, the footwall ramp 

probably gave way to a footwall-flat (now eroded), based on the 

gentle dips of the Gunsight strata in the footwall, as described 

previously. 

The stratigraphic height of a ramp is approximately equal 

to the height of the associated fold, as measured from anticlinal 

crest to synclinal trough, for both fault-bend and fault-propagation folds (Jamison, 1992, p. 156). The reconstructed Poison 

Creek anticline has an amplitude of at least 10,500 m. Calculation of the fold height assumes that the entire thicknesses of the 

Swauger (3,100 m) plus the banded siltite unit of the Apple 

Creek Formation (1,500 m) (table 1) were present at the time of 

thrusting but now are eroded from the Poison Creek sheet within 

the Lemhi Range. Along the trace of the Poison Creek thrust 

fault in the Lemhi Range, upper strata of the coarse siltite unit of 

the Apple Creek are in thrust contact above Paleozoic strata as 

young as the Middle Ordovician to Lower Silurian Saturday 

Mountain Formation. Consequently, the Poison Creek thrust 

fault has cut upsection about 7,500 m through the hangingwall 

rocks. This makes for a discrepancy of about 3,000 m (7,500 m 

versus 10,500 m) between the fold estimate and the demonstrable stratigraphic displacement across the Poison Creek fault. 

The difference, or a large part of it, may be reconciled by addition of the stratigraphic displacement on the subsidiary thrust 

faults to the 7,500 m. Further, the 10,500 m figure is not an 

anticlinal-crest-to-synclinal-trough measurement—that distance 

must be considerably greater than 10,500 m. 

Relationship of Poison Creek Anticline and 

Basin Lake Syncline 

The Basin Lake syncline occurs between the Poison 

Creek thrust fault and the Bear Valley fault. The syncline, discussed previously, was interpreted as part of a compressional 

Compressional Deformation 15 
fold-couple related to the Bear Valley fault, formed prior to 

normal displacement on that fault. Three explanations for the 

occurrence of the syncline adjacent to the Poison Creek fault 

are considered. 

1. The syncline formed in the hangingwall of the thrust 

sheet during regional compression. In discussing fault-propagation folding, Suppe and Medwedeff (1990, p. 422) reported 

that it is particularly common for a thrust fault to break through 

strata at an angle that is less than the dips of the synclinal and 

anticlinal axial surfaces, illustrating the observation with a syncline in the leading part of the hangingwall of the Absaroka 

thrust fault of southeastern Idaho. Similarly, Mitra (1990, p. 

933) stated that the occurrence of a syncline in the hangingwall or the footwall of a thrust fault depends on whether the 

fault propagated through the footwall or the anticlinal forelimb 

(Mitra, 1990, p. 933), and that a syncline may be transported as 

part of the hangingwall of the thrust sheet—a translated faultpropagation fold. Building on the observations of these 

authors, I suspect that the Basin Lake syncline may have 

formed early in the developing compressional deformation of 

the area. Once the anticline and syncline of the fold-couple 

associated with the Bear Valley fault had formed, minor compressional displacement ceased on the Bear Valley fault. The 

Poison Creek thrust fault then splayed from near the base of the 

Bear Valley fault as a thrust imbricate, which proceeded to 

develop into a major thrust fault that propagated northeastward 

along beds of the southwest-dipping limb of the Basin Lake 

syncline. With this scenario, the syncline would be preserved 

within the hangingwall of the Poison Creek thrust fault. 

2. Subsequent to thrusting, normal displacement on the 

Poison Creek fault occurred, which accentuated the Basin Lake 

syncline. This possibility can be evaluated only in the northwestern part of the study area, where bedrock is widely exposed 

in both the hangingwall and footwall of the Poison Creek thrust 

fault. No direct evidence of back-slip (normal displacement) 

was found during fieldwork, although some amount of back-slip 

cannot be ruled out. This explanation requires that the northeast 

limb of the syncline steepened its southwest inclination during 

rotation, and that mesoscopic folds and axial-plane cleavage of 

the limb also were rotated. Axial planes of mesoscopic folds and 

the related cleavage in the Apple Creek Formation of the hangingwall adjacent to the thrust fault dip southwest and are about 

coplanar with the plane of the fault; this suggests that rotation of 

the hangingwall limb of the syncline is not significant. 

3. An unrecognized normal fault lies between the Poison 

Creek thrust fault and the axis of the Basin Lake syncline. Bedrock widely exposed between the Poison Creek thrust fault and 

the axis of the Basin Lake syncline (fig. 4), entirely of the coarse 

siltite unit of the Apple Creek Formation, contains areas of limited exposure south of Phantom Creek in which a normal fault 

could be present but unrecognized. (The fault, if it exists, is not 

shown in fig. 4 or in cross section A–A’ of fig. 5.) Such a normal 

fault likely would have curved at depth and merged with the Poison Creek thrust fault. This possibility would permit accentuation of the syncline and normal displacement on much of the 

Poison Creek thrust fault. 

Possibility (1), a hangingwall fold, is favored here. Possibility (2), back-slip on at least the exposed part of the Poison 

Creek thrust fault, appears to be least likely. Possibility (3), an 

unrecognized normal fault south of Phantom Creek, could exist. 

Two directions of movement on the Poison Creek fault are indicated in the cross sections, to accommodate at least some normal displacement on the Poison Creek thrust fault. The double 

symbol also illustrates the widespread occurrence of extensional 

movement on thrust faults subsequent to compressional deformation in the study area and regionally. 

Intersection of Hayden Creek and Bear Valley Faults 

The Hayden Creek and Bear Valley faults join in the 

northwestern part of the map area (fig. 4), vicinity of cross 

section A–A’ (fig. 5). The fault pattern in the cross section is 

much simplified from that shown by Tysdal and Moye (1996). 

A possible configuration of faults prior to normal displacement, along with a related fold, is shown in figure 9 and is 

interpreted in this section. 

North of the Bear Valley fault, and west of cross section 

line A–A’ (fig. 4), bedding and folds, including the Basin Lake 

syncline, trend westerly in concert with the westerly strike of 

the Bear Valley fault. These footwall structures related to the 

Bear Valley fault are in structurally low strata of the anticline/ 

syncline fold-couple. South of the Bear Valley fault (and west 

of cross section line A–A’, fig. 4), beds strike northwest and are 

cut off by the Bear Valley fault. These hangingwall structures 

related to the Bear Valley fault are a structurally high part of the 

fold-couple, even though they may be topographically lower, 

and trend northwest in contrast to the westerly trends of the 

footwall strata. Figure 9 shows an interpreted fold-couple with 

progressively increasing deformation toward the west. Hangingwall strata have been moved up the Bear Valley fault to their 

interpreted pre-normal-slip position. Down-to-the-south 

normal-slip on an incipient Bear Valley fault (dashed in fig. 9), 

which trends about parallel to the synclinal axis, would leave 

northwest-striking rocks on the south juxtaposed against westerly-striking rocks (including overturned beds) on the north, 

much as shown in figure 4. An incipient Hayden Creek normal 

fault is shown on the west side of the Bear Valley fault. 

Interpretation of the fault pattern shown in cross section A– 

A’ (fig. 5) assumes that the Hayden Creek and Bear Valley faults 

formed originally as compressional features, but that offset of 

the fold-couple was minor. Figure 9 shows how these faults 

may have appeared just prior to normal-slip during extensional 

failure of the nose area of the anticline of the fold-couple. 

Minor thrust displacement that is suspected to have taken place 

on these faults prior to normal-slip is not shown in the illustration—any such thrust displacement has been removed by backslip to the positions depicted in the reconstruction illustrated. 

The interpretive cross section is speculative, of course, because 

the pre-faulting rocks have been eroded; down-structure relationships are not available for projection into a cross-sectional 

plane to aid the reconstruction. 

Normal-Slip on Bear Valley Fault 

Normal-slip on the Bear Valley fault developed along the 

overturned segment of the anticlinal limb of the fold-couple. 

16 Structural Geology, Western Lemhi Range, Idaho 
SW 

SE 

NE 

NW 

BEAR VALLEY FAULT 

HAYDEN CREEK FAULT 

Figure 9. Diagrammatic sketch of fold and incipient faults interpreted to have existed just prior to normal displacement on the Hayden Creek and Bear Valley faults, cross section A–A’ in northwestern part of study area. Strike and 

dip symbols show areas of right-side-up and overturned beds. Dotted lines are used to emphasize shape of fold. See 

text for explanation. 

The overturned segment is present (or was present) in the northwestern part of the study area (west of Basin Lake, fig. 4). 

Along the central extent of the fault (southeastern part of Lem 

Peak quadrangle and northern part of Mogg Mountain quadrangle, fig. 2), the anticlinal limb becomes vertical or dips steeply 

to the northeast. But the fault maintained the southwestward 

inclination and permited down-to-the-south(west) displacement 

of hangingwall strata of the Bear Valley subsheet. 

When rock of the Bear Valley subsheet east of the Payne 

Creek fault (figs. 3, 4) is moved northeast, into restored fold 

position (just before the fold was broken along the Bear Valley fault), normal-slip on the fault appears to have initiated 

along overturned diamictite strata of the Apple Creek Formation in the steepest part of the common limb of the foldcouple. In the area of Hayden Creek, the estimated minimum 

normal-slip displacement of the Bear Valley subsheet is 4 km 

east of the Payne Creek fault and about 5.5 km west of it. 

My impression is that displacement on the Bear Valley fault in 

the Hayden Creek area may be greater than that in the Basin 

Lake area; thus, the Basin Lake area is a pivot area for the 

Bear Valley subsheet. 

Intersection of Structures in Northwestern Part 

of Map Area 

The overall pattern of compressional faults and folds shows 

a convergence toward the northwest, toward the Poison Creek 

thrust fault (figs. 3, 4). The Bear Valley and Hayden Creek 

faults, and the Basin Lake syncline, all strike at a small angle to 

the Poison Creek thrust. This is the case even after normal displacement has been removed from the Bear Valley and Hayden 

Creek faults and the strike of the structures is adjusted for the 

Hayden Creek fault cutting downsection to the northwest. (1) 

One possible cause of the convergence is slightly oblique transport of the Poison Creek sheet over the footwall rocks, resulting 

in formation of the Bear Valley and Hayden Creek faults. (2) 

Another possibility is a northwestward steepening of the Poison 

Creek thrust fault toward the northwest due to the existence of 

resistant Ordovician dolomite and quartzite in the footwall 

(McDevitt Creek sheet). In either case, the oldest rocks of the 

Poison Peak sheet are exposed closer to the fault in the northwestern part of the map area because they moved farther up the 

thrust plane there than strata to the southeast. 

Compressional Deformation 17 
Stereonet Data 

Stereonet data were plotted to aid interpretation of the compressional history of the study area. The central Idaho region 

underwent compression during Cretaceous orogeny, but Mesoproterozoic compressional deformation also has been reported 

(for example, Evans, 1986; Evans and Zartman, 1990). A goal 

of the geologic mapping and structural studies was to collect sufficient data to determine if rocks of the study area had undergone 

more than one compressional deformation. Bedding and cleavage elements were compiled for each of the structural sheets, the 

subsheets, and the block shown in figure 3. 

Structural data from all subsheets of the Poison Creek 

thrust sheet were compiled into one data set each for bedding 

and for cleavage, permitting comparison of data from an individual subsheet with that of other subsheets and with that for the 

entire thrust sheet (figs. 10, 11). Cleavage data from the part of 

the McDevitt Creek thrust sheet directly north of the trace of the 

Poison Creek thrust fault also are shown in a stereoplot and compared with data from the Poison Creek thrust sheet. Data from 

the Lem Peak block are considered only in a general way 

because (1) the block has only sparse observations, (2) data exist 

mainly for the leading area of the block, and (3) the block underwent a large amount of normal-slip displacement that may have 

been accompanied by a significant component of rotation of the 

block or parts of it. 

In the initial stages of data compilation and analysis, stereoplots of bedding and cleavage were constructed for each individual formation and formational unit within each sheet and 

subsheet. The compiled data showed that structural orientations 

for these individual formations were virtually identical with 

those for the aggregate of all formations and rock units within 

each sheet or subsheet. Hence, only stereoplots of the aggregated data are presented (except for fig. 11E, F). Bedding for 

the Poison Creek sheet and subsheets is shown in figure 10A–D, 

and cleavage for the sheet and subsheets is shown in figure 11A– 

D. An exception is made for cleavage data from the diamictite 

unit of the Apple Creek Formation, which is shown in the stereoplots of figure 11E–H. These data are illustrated because the 

diamictite recorded cleavage more readily than did the other 

Proterozoic rock units, probably because of its generally finer 

grain size, and would be more likely to preserve structural elements of more than one compressional event, should more than 

one have taken place. Cleavage data for lower Paleozoic carbonate strata in the rearward part of the McDevitt Creek sheet are 

shown in the stereoplot of figure 11J. This data set is compared 

with the cleavage measured in the Mesoproterozoic rocks in the 

northeastern part of the directly overlying Poison Creek thrust 

sheet (that is, the northeastern part of the Basin Lake subsheet), 

shown in figure 11K. 

Bedding 

Bedding data for each lithology and each thrust sheet show 

a high degree of consistency of orientation properties among the 

different rock types within each subsheet, and among the different subsheets. Poles to bedding for the Poison Creek thrust sheet 

(fig. 10A) depict a girdle distribution oriented northeast-southwest. This pattern reflects the broad, overall shape of the forward part of the large anticline reconstructed by removing the 

normal displacement from the Hayden Creek and Bear Valley 

faults (fig. 7). The concentration of data points in the southwest 

quadrant reflects the generally steep-to-the-northeast dips characteristic of all three subsheets that are a part of the Poison 

Creek anticline. Beds of the Hayden Creek subsheet (fig. 10B) 

all dip steeply. Beds of the Bear Valley subsheet (fig. 10C) show 

a girdle distribution that records the many small anticlines and 

synclines formed in the argillite and argillaceous siltite of the 

diamictite unit of the Apple Creek Formation. These small folds 

formed in the footwall of the Hayden Creek fault and formed in 

the same stress field that formed the larger Poison Creek anticline. Beds of the Basin Lake subsheet (fig. 10D) show a range 

of dips from vertical to nearly horizontal, a distribution reflecting the low to moderate dips of the Basin Lake syncline as well 

as the steep dips in the footwall of the Bear Valley fault. 

The plot of poles to bedding for the Bear Valley and Basin 

Lake subsheets displays a fairly narrow belt (girdle) of poles to 

bedding (fig. 10C, D). The concentration of data points in the 

Hayden Creek subsheet does not show a girdle distribution, but 

the concentration of data points lies within the girdles defined in 

the other two subsheets. This is well shown in the girdle distribution for the Poison Creek thrust sheet as a whole (fig. 10A). 

None of the diagrams of bedding is suggestive of more than one 

orientation of stress field. Some individual plots have somewhat 

more scatter of poles than composite diagrams for each subsheet 

or for the three subsheets combined, but this is only for a few 

poles at the 1 percent contour interval used. 

Cleavage 

Cleavage data for the Poison Creek thrust sheet and for 

each lithology within the sheet show a high degree of consistency of orientation properties among the different rock types 

within each subsheet, and among the different subsheets. Poles 

to cleavage for the Poison Creek thrust sheet (fig. 11A) depict a 

point maxima in the northeast quadrant, which becomes diffuse 

toward the northeast limit of the quadrant. This pattern reflects 

the consistent northwest strike of cleavage and abundance of 

30°–60° dips to the southwest. The cleavage of each subsheet 

differs somewhat in its steepness of dip. This likely reflects (1) 

that the subsheets constitute different parts of the megafold. 

The subsheets are now juxtaposed along faults that display significant normal displacement, but the structures within the subsheets were formed when the subsheets were in structural 

positions different from those where they now reside. (2) The 

differing steepness of dip of cleavage among the subsheets also 

probably reflects the tendency of cleavage to become asymptotic to thrust faults. This is particularly true for rocks in the 

vicinity of cross section B–B’ (fig. 5), where the cleavage 

becomes shallower at depth as the Bear Valley fault is 

appoached. In general, the shallowest dipping cleavage is in the 

footwall of the Bear Valley fault (Basin Lake subsheet) and in 

the overturned strata of the anticline/syncline fold-couple of the 

Bear Valley and Basin Lake subsheets. The dips are mostly in 

18 Structural Geology, Western Lemhi Range, Idaho 
19

 A

n = 719

5

1

1

 B

n = 210

5

1

10

1

5

C

n = 288

2

2

2

6

6

D

n = 221

2

2

4

2

2

Poison Creek sheet—bedding (all rock units)

Hayden Creek subsheet—bedding (all rock units)

Bear Valley subsheet—bedding (all rock units)

Basin Lake subsheet—bedding (all rock units)

the 25°–35° range. 

of the anticline of the fold-couple, throughout most of the Bear 

Valley subsheet and the remainder of the study area. These 

observations are in agreement with findings of Mitra and Elliott 

(1980) that cleavage within a thrust sheet can be followed downward and is asymptotic to the thrust plane, suggesting that 

thrusts approximate planes of maximum shearing strain.

Poles to cleavage of the Hayden Creek subsheet (fig. 11B) 

are highly focused about a point maxima in the northeasternmost 

part of the northeast quadrant. The poles record steep dips (70°–

90°) of cleavage to the southwest. The cleavage is mostly 

recorded from the Big Creek Formation (Yb, fig. 4). 

cleavage of the Bear Valley subsheet (fig. 11C) show two point 

maxima, in part a combination of the point maxima depicted for 

the Hayden Creek and Basin Lake subsheets. vage 

of the Big Creek Formation and the fine siltite of the Apple 

Creek Formation are most common in the point maxima nearest 

the center of the diagram. vage of the diamictite 

Figure 10. Stereonet plots of bedding. A–D are discussed in text and identified in figure for visual clarity.

Compressional Deformation

Steeper dips are common in the upright part 

Poles to 

Poles to clea

Poles to clea

Views 
n = 575 

Coarse siltite unit of Apple Creek Formation 

Diamictite unit of Apple Creek Formation 

Fine siltite unit of Apple Creek Formation, and Big Creek Formation 

5 

n = 233 

Coarse siltite unit of Apple Creek Formation 

Diamictite unit of Apple Creek Formation 

Fine siltite unit of Apple Creek Formation, and Big Creek Formation 

2 

2 

4 

4 

A Poison Creek sheet—cleavage (all rock units) C Bear Valley subsheet—cleavage (all rock units) 

n = 102 

5 

10 

5 

Coarse siltite unit of Apple Creek Formation 

Diamictite unit of Apple Creek Formation 

Fine siltite unit of Apple Creek Formation, and Big Creek Formation 

n = 240 

5 

Coarse siltite unit of Apple Creek Formation 

Diamictite unit of Apple Creek Formation 

Fine siltite unit of Apple Creek Formation, and Big Creek Formation 

5 

10 

B Hayden Creek subsheet—cleavage (all rock units) D Basin Lake subsheet—cleavage (all rock units) 

Figure 11 (above, facing page, and overleaf). Stereonet plots of cleavage. Views A–K are discussed in text and identified in figure for visual clarity. 

There is no stereonet plot for the letter I. 

unit of the Apple Creek are most common in the point maxima coarse siltite unit of the Apple Creek are approximately of the 

near the outer part of the quadrant, reflecting steeper dips of same concentration in the area of both maximas. Poles to 

cleavage. However, poles of the diamictite also range into the cleavage in the Basin Lake subsheet (fig. 11D) show a bullseyearea of the more central point maxima. Poles to cleavage of the pattern point maxima in the northeast quadrant. Southwest dips 

20 Structural Geology, Western Lemhi Range, Idaho 
E 

n = 204 

Bear Valley subsheet 

Basin Lake subsheet 

5 

5 

Bear Valley and Basin Lake subsheets—unrotated 

cleavage (for all stations in diamictite unit of Apple 

Creek Formation) 

F 

n = 53 

Bear Valley subsheet 

Basin Lake subsheet 

5 

5

10 

Bear Valley and Basin Lake subsheets—unrotated 

cleavage (for 53 selected stations in diamictite 

unit of Apple Creek Formation) 

are in the range of 30°–60°. Strata from which these data were 

obtained include all units of the Apple Creek Formation, but 

none from the Big Creek Formation. Poles to cleavage for each 

of the three units of the Apple Creek Formation are intimately 

mixed within the point maxima, such that the poles to cleavage 

G 

n = 53 

5 

10 

Bear Valley subsheet 

Basin Lake subsheet 

5 

Bear Valley and Basin Lake subsheets—rotated 

cleavage (for 53 selected stations in diamictite unit 

of Apple Creek Formation) 

H 

n = 9 

Unrotated cleavage 

Rotated cleavage 

Bear Valley subsheet—rotated and unrotated 

6 

4 

4 

1 5 8 

7 

2 

3 

3 

6 

7 8 

9 2 

1 

5 

9 

cleavage (for 9 of 53 selected stations in diamictite unit of Apple Creek Formation); numbers 

show individual data points before and after 

rotation 

of any one of the rock units would define very nearly the same 

point maxima. 

The foregoing data, however, suggest that lithology exerts 

some control on the dip of cleavage in the different thrust sheets. 

Diamictite, with its finer grain size of argillite to fine silt, 

Compressional Deformation 21 
J 

n = 84 

5 

10 

K 

n = 133 

5

10 

5 

Limestone 

Dolomite 

McDevitt Creek sheet—cleavage (Paleozoic 

carbonate rocks) 

Basin Lake subsheet, leading part—cleavage 

(coarse siltite unit of Apple Creek Formation) 

Figure 11—Continued. Stereonet plots of cleavage. 

deformed more readily than the coarser grained rocks. This is 

also indicated by the intensely developed cleavage within the 

fine strata of the diamictite unit, whereas interbeds of coarsegrained siltite to fine-grained metasandstone are not so well 

cleaved. The argillite and fine siltite tend to flow to relieve 

stress, facilitating development of steeply inclined cleavage. 

None of the diagrams of poles to cleavage is suggestive of more 

than one general orientation of stress field of deformation. 

22 Structural Geology, Western Lemhi Range, Idaho 

Cleavage in Diamictite 

The diamictite unit of the Apple Creek Formation recorded 

cleavage more readily than did the other Proterozoic rock units, 

probably because of its generally finer grain size. For this reason, the diamictite unit was selected for further study because it 

would be more likely to preserve structural elements of more 

than one compressional event, should more than one have taken 

place. Nearly all the diamictite of the study area occurs within 

the Bear Valley and Basin Lake subsheets, which are separated 

by the Bear Valley fault (fig. 4). With respect to the Bear Valley 

fault, cleavage of the Bear Valley subsheet primarily is in the 

hangingwall anticline, that of the Basin Lake subsheet in the 

footwall syncline. Diamictite crops out mainly in the southeastern part of the mapped limits of the Basin Lake subsheet, 

directly beneath the Bear Valley fault (fig. 4). (Diamictite is 

concealed beneath Quaternary sediments and Tertiary volcanic 

rocks in much of the subsheet.) The data reflect a concentration 

of observations from the Hayden Creek area, owing to the good 

exposures of tightly folded rocks and to the interbeds from 

which bedding attitudes could be measured. Interbeds were 

more difficult to find in other outcrop areas of the diamictite unit 

of the subsheet. In contrast, cleavage in diamictite of the Bear 

Valley subsheet is representative of much of the length of the 

hangingwall anticline owing to the extensive exposures of the 

subsheet. 

The distribution of poles to cleavage for the diamictite (fig. 

11E) is a pattern expected for progressive southwest-to-northeast rollover. A more uniform northeast-southwest spread of 

cleavage dips might be expected, but the strata of the Bear Valley subsheet represent mainly the steeply dipping to overturned 

forelimb of the anticline and those of the Basin Lake subsheet 

represent mainly the steep southwestern part of the syncline. 

The remainder of the anticline has been eroded, and the moderately dipping part of the syncline is covered by volcanic rocks 

and Quaternary sediments. 

Interpretation of Cleavage in Diamictite 

Cleavage in the diamictite unit shows no readily apparent 

consistent geometric relationship to bedding orientation, as 

would be expected if cleavage formed prior to folding. The 

general approach leading to this conclusion is presented in the 

following paragraph, with a more detailed presentation and 

interpretation following. 

An examination was made of stereoplots of poles to cleavage for which both cleavage and bedding orientations were 

obtained from the same outcrop of the diamictite unit. The 

poles to cleavage were plotted for the 53 stations that yielded 

both bedding and cleavage data. Poles to cleavage as recorded 

in the field for these stations are displayed in figure 11F. The 

poles to cleavage for each of the 53 beds were then plotted after 

the corresponding bed was rotated to horizontal (and upward 

facing), shown in figure 11G. The rotated data show two 

domains, representing the anticlinal and synclinal limbs present 

in the two subsheets. In both domains, poles to cleavage are 

broadly distributed along a northeast-southwest trend, 
indicating the general northwest strike of cleavage, and are most 

abundant in the southwest quadrant (fig. 11G). Cleavage dips 

range across a spectrum from nearly vertical to nearly horizontal, a pattern that is not consistent with uniform layer-parallel 

shear developed prior to folding. This unfolding of beds and 

cleavage to determine if cleavage revealed a prior deformation 

does not adjust for fold plunge, which is 0°–10° in the map area. 

The unfolded cleavage would not reveal deformations other than 

the prefolding that it was designed to determine. Because the 

cleavage strikes northwest, both in outcrops and in each bed 

after being rotated to horizontal, the strata are unlikely to have 

undergone a deformation wherein an axis of folding deviated 

significantly from a northwest trend. 

The comparison of stereoplots of poles to cleavage as measured in the field (that is, unrotated) was made for the 53 stations 

(fig. 11F) with the same beds after they were individually rotated 

(about their respective strikes) to horizontal (fig. 11G). The distribution pattern of poles is similar in the two figures, except that 

the concentration of poles occurs in opposing quadrants. 

The unrotated poles to cleavage (fig. 11F) clearly show two 

point maxima: one is for the domain of the anticlinal limb on the 

Bear Valley subsheet, the other for the synclinal limb on the 

Basin Lake subsheet. Figure 11F shows much the same pattern 

as the 204 poles of figure 11E, which contains all the cleavage 

observations recorded from the diamictite of both subsheets. 

(The 53 poles to cleavage shown in fig. 11F and G are a subset 

of these 204 poles.) 

The plots of unrotated poles (fig. 11F) and rotated poles 

(fig. 11G) are quite similar, but only in appearance, not in substance. The plot of cleavage as recorded in the field (fig. 11F) 

shows that the 21 stations of the Basin Lake subsheet define a 

point maxima of highly concentrated poles in the northeast 

quadrant. Poles to cleavage of the 32 stations of the Bear Valley 

subsheet form a diffuse point maxima, also in the northeast 

quadrant but with a few poles in the southwesternmost part of 

the southwest quadrant. The diffuse point maxima is northeast 

of the point maxima of the Basin Lake subsheet. The diffuse 

maxima is defined by about 3/4 of the stations of the Bear Lake 

subsheet; poles to the other 1/4 of the stations are distributed 

across the diagram. 

The same 53 rotated poles to cleavage (fig. 11G) show a 

similar contour pattern. But the 21 data points of the Basin Lake 

subsheet are now spread fully across the diagram. The distribution indicates cleavage planes that have dips ranging from 0° to 

90°, and the full range of dips inclined to both the northeast and 

the southwest. This clearly is not indicative of uniform layerparallel shear. The 32 poles to cleavage of the Bear Valley subsheet (that is, after rotation) (fig. 11G) are mostly present in the 

southwest quadrant. They are broadly and fairly uniformly 

spread across the quadrant, indicating dips from 5°–10° to 90°. 

This pattern also is not indicative of uniform layer-parallel 

shear. 

The foregoing analysis raises a question about the 1/4 of the 

poles to cleavage (actually, 9 of the 32 poles) that are spread 

about the central part of the stereogram of the 53 unrotated beds 

(fig. 11F). The pattern indicates cleavage with 10°–45° dips. 

To determine if the nine poles spread about the central part of 

figure 11F represent a previous compressional deformation 

event, I plotted the nine poles to cleavage in a separate diagram 

(fig. 11H). The diagram shows the nine poles, individually 

numbered, in both their unrotated (as recorded in the field) and 

rotated positions. It reveals that the nine poles are more widely 

dispersed after rotation than before rotation. Hence, they do not 

indicate a previous deformation. 

The plot of poles to the rotated cleavage (fig. 11G) is interesting by comparison. The contour pattern for the poles to 

cleavage of the 53 unrotated beds (fig. 11F) is similar to the pattern for these same beds after rotation (fig. 11G), although 

clearly the poles to the rotated beds (fig. 11G) are more widely 

scattered. The rotated beds (fig. 11G) show only one fairly 

broad point maxima. But, unlike the unrotated poles to cleavage 

for the 53 poles (fig. 11F), which show two fairly close point 

maxima (one each for the anticline of the Bear Valley subsheet 

and the syncline of Basin Lake subsheet), the poles to cleavage 

of the rotated beds (fig. 11G) are not separated, but rather the 

poles of the two domains are mixed together! This is a second 

indication that the cleavage was not formed prior to folding. 

The poles to cleavage for the two domains in diamictite, 

represented in figure 11E, are fairly narrowly confined—that is, 

they are not scattered far to the northwest or southeast beyond a 

narrow northeast-southwest zone. This indicates that the 

domains are not complicated by significant variations such as 

lateral ramps or tear faults. A few poles are for axial areas of 

folds where cleavage strike is about normal to the general northwest trend, but fold plunges are shallow; thus, poles fall within 

the narrow girdle shown in figure 11E. The belt possibly could 

have been even narrower; normal-slip along the Bear Valley 

fault may have caused the Bear Valley subsheet to have had a 

slight clockwise rotation, contributing to a broader distribution 

of poles than otherwise might have been the case. Still, the great 

circle azimuth and plunge for the 53-sample set in diamictite are 

very nearly the same as those determined for all cleavages 

recorded for the entire Poison Creek sheet (fig. 11A). 

Cleavage in Carbonate Strata of Footwall 

McDevitt Creek Sheet 

Cleavage in lower Paleozoic limestone and dolomite was 

recorded in the McDevitt Creek sheet, footwall of the Poison 

Creek thrust fault (fig. 11J). The data are from southern areas of 

the Poison Peak and Hayden Creek 7.5-minute quadrangles (fig. 

2). The cleavage orientations are consistent with those of the 

Poison Creek sheet as a whole (compare fig. 11A and J) and of 

the Basin Lake subsheet (fig. 11D). Figure 11K, a subset of data 

from the Basin Lake subsheet, shows only poles for the coarse 

siltite unit of the Apple Creek Formation. Except for a small 

thrust slice of banded siltite unit of the Apple Creek, the coarse 

siltite unit makes up the northernmost exposures of Mesoproterozoic rocks of the subsheet (and of the Poison Creek sheet; fig. 

4) and contains rocks that are in direct contact with carbonate 

strata of the McDevitt Creek sheet. 

Carbonate rocks of the McDevitt Creek sheet are separately 

distinguished as limestone and dolomite in figure 11J. Poles to 

Compressional Deformation 23 
cleavage developed in limestone are more tightly confined than 

those formed in dolomite. In addition, poles just of the dolomite 

would depict a point maxima slightly to the north of that for 

limestone only, indicating somewhat steeper dips for cleavage in 

dolomite (fig. 11J). These observations may reflect several factors. (1) The two kinds of carbonate rocks behaved differently 

under the same stress conditions. The limestone tended to 

deform readily, forming a pervasive, continuous cleavage. The 

dolomite behaved in a brittle manner, forming a closely spaced 

cleavage (Tysdal, unpub. data, 1999). (2) Limestone occurs 

mainly in the eastern part of the McDevitt Creek sheet shown in 

figure 4, dolomite in the western part. The two areas may have 

experienced slightly different stress orientations during thrusting. The Poison Creek thrust fault makes a small change in orientation to a more westerly trend in the western part of the area 

of figure 4, which may reflect a slightly different stress orientation. This change in orientation is shown, for example, by the 

change in the axis of the Basin Lake syncline from a northwest 

trend in the eastern part of the Basin Lake subsheet (fig. 4) to a 

more westerly trend in the western part of the subsheet. However, the westerly orientation also is partly due to normal faulting in the western part of the study area. 

Age of Cleavage 

Coincident cleavages of the Proterozoic and Paleozoic 

strata of the study area are younger than the youngest affected 

Mississippian rocks. The orogenic compressional event that 

generated the cleavage is interpreted as Mesozoic, probably Cretaceous, and is based on regional data presented in a subsequent 

section, “Ages of Deformation.” Data supporting a post-Mississippian age for development of cleavage are as follows. (1) 

Cleavage is directly related to the Poison Creek thrust fault and 

to the Hayden Creek and Basin Lake faults, which are interpreted as subsidiary thrusts that splay upward from the Poison 

Creek thrust fault. (2) Cleavage is most intensely developed 

along the faults and in the directly adjacent footwall of the subsidiary thrust faults. (3) Overturned beds along the Hayden 

Creek and Basin Lake faults consistently display cleavage dips 

that are less than bedding dips. This is unlikely to be the case if 

the cleavage is unrelated to the overturning. (4) Cleavage is 

axial planar to folds, which are related to the subsidiary faults. 

(5) I found only one continuous (pervasive) cleavage in the Proterozoic rocks of the Poison Creek sheet within the study area. In 

addition, cleavage measured in the carbonate strata of the footwall McDevitt Creek sheet displays an orientation pattern that is 

consistent with patterns in the Poison Creek sheet. The coarse 

siltite unit of the Mesoproterozoic Apple Creek Formation 

makes up a widespread unit of the Basin Lake subsheet, and it is 

in direct contact with the footwall Paleozoic carbonate rocks in 

the western part of the study area (fig. 4). The cleavage pattern 

for the coarse siltite unit is nearly identical to that of the subthrust carbonate strata. This clearly places the age of these coincident cleavages in the Phanerozoic. Some Proterozoic-age 

cleavage possibly may have formed (1) if the Proterozoic stress 

regime was about parallel to that of the Phanerozoic cleavage, or 

(2) if a Proterozoic cleavage formed during burial metamorphism, which would be parallel to bedding, or nearly so, and 

would have generated micas about parallel to bedding. In contrast to this study area, two cleavages, at high angles to one 

another, occur in Proterozoic rocks of the Poison Creek thrust 

sheet in the eastern part of the Salmon River Mountains, west of 

the study area (Tysdal, unpub. data, 1999). Reconnaissance 

study suggests that both cleavages there are of Phanerozoic age 

and formed during thrusting of the Poison Creek sheet over an 

oblique ramp in a section of the footwall. 

Extensional Deformation 

Normal faults that did not demonstrably develop along preexisting compressional faults are described and discussed in this 

section. No restoration to a pre-normal-slip position was undertaken for these faults. 

Lem Peak Normal Fault 

The Lem Peak fault trends northwest, similar to the other 

major structures of the study area. It commonly formed in the 

less competent siltite and metasandstone of the Lawson Creek 

Formation near the contact with the underlying competent 

orthoquartzite of the Swauger Formation. The fault dips south 

and underwent a large amount of displacement: about 12,000 m 

of slip is indicated by the stratigraphic separation shown in 

cross section D–D’ of figure 6. The dip of the fault is steep at 

the present depths of erosion, as shown in exposures of the juxtaposed rock units and by the three-dimensional patterns 

revealed across the differentially eroded mountainous terrain of 

the Lemhi Range. It is my interpretation that the Lem Peak normal fault curves and displays a lower inclination at a greater 

depth than that of the present exposures. 

In plan view the Lem Peak fault displays a very irregular, 

jagged pattern marked by abrupt changes in direction of as 

much as 90° (fig. 4). The pattern is reflected by juxtaposition of 

unrelated rock units and structures. For example, along the 

uppermost part of Hayden Creek a segment of the Lem Peak 

fault trends northeast, about parallel to the creek. Steeply dipping beds of the Big Creek Formation of the Hayden Creek 

sheet southeast of the Lem Peak fault display the same strike as 

steeply dipping beds of the Swauger Formation of the Lem Peak 

block northwest of the fault segment. A similar relationship 

exists between the headwaters of North Fork Cow Creek and 

Allison Creek. These corner areas display complex patterns of 

normal faults of limited extent. (See Tysdal and Moye, 1996; 

and Tysdal, 1996c, for map details that could not be shown in 

fig. 4.) In the Allison Creek–North Fork Cow Creek area, tight 

folds in the Swauger and overlying Lawson Creek Formations 

are not present in the Big Creek Formation on the opposite side 

of the fault. Further, axes of these folds have a westerly trend as 

opposed to the northwest trend common for most of the folds in 

strata north of the Lem Peak fault. 

24 Structural Geology, Western Lemhi Range, Idaho 
Compressional structures occur in several areas of the Lem 

Peak block. The compressional structures probably are related 

to thrust faulting, but not related to juxtaposition of younger 

Swauger strata over older Big Creek rocks in the same area. 

Such an inferred fault may have been a higher structural level 

subsidiary thrust, similar in origin to the Hayden Creek or Bear 

Valley faults that formed within the Poison Creek anticline. In 

the Cow Creek–Allison Creek area (fig. 4), folds that formed 

prior to normal displacement on the Lem Peak fault lack corresponding folds in the adjacent rocks of the footwall. This lack 

of corresponding footwall folds suggests that the Lem Peak normal fault developed within a single subsheet. 

One of the keys to normal-slip movement on the Lem Peak 

fault is the omission of strata along its trend. In the Hayden 

Creek subsheet, between the south half of the line of cross 

section C–C’ and the Payne Creek fault (fig. 4), the preserved 

thickness of the Big Creek Formation increased markedly in 

both directions along strike. This is evident directly north of 

the head of Hayden Creek by the closeness of the Swauger 

Formation to the base of the fine siltite unit of the Apple Creek 

Formation in the southern part of the Bear Valley subsheet. 

Strata omitted from the thinned sequence of the Big Creek 

Formation are missing mainly due to normal-slip on both the 

Lem Peak and Hayden Creek faults. In the central segment of 

the Lem Peak fault, the dip of the Swauger of the Lem Peak 

block gradually increases from southwest to northeast and 

locally is overturned near the Lem Peak fault. These steeply 

dipping to overturned beds likely defined an anticline that was 

eroded or displaced during normal-slip. 

When lateral offset is removed from the Payne Creek fault 

east of Wade Creek, the Lem Peak fault on the east side of the 

Payne Creek fault aligns with the segment farther west. Removal 

of the offset along the Payne Creek fault also facilitates interpretation of the Lem Peak fault from the junction of Hayden and 

Wade Creeks eastward to north of Mill Mountain, an area of 

sparse outcrops of Proterozoic strata. The Lem Peak fault must 

exist in the largely covered area because steeply dipping 

Swauger beds in the Mill Mountain area are on strike with 

steeply dipping Big Creek strata in the Cooper Creek drainage— 

a structural discontinuity is needed to accommodate the lithologic change along trend of the strikes. 

The arcuate-southward trace of the Lem Peak fault eastward from Wade Creek to the north side of Mill Mountain 

reflects the southward dip of the fault and the lesser elevation of 

the topography in this area. If the area were of near constant 

elevation, the steeply dipping Lem Peak normal fault would 

trend in a nearly straight line across the area (after removal of 

offset along the Payne Creek fault). 

Alder Creek Fault 

On the Basin Lake subsheet, north of the Bear Valley fault 

and southeast from Hayden Creek (fig. 4), the contact of the 

diamictite unit with the fine siltite unit of the Apple Creek 

Formation is interpreted as a fault along part of its length. From 

the east edge of the map area to near the Apple Creek fault, the 

contact is covered. Sheared strata of the fine siltite unit occur a 

short distance southeast from the Apple Creek fault, indicating 

movement along the steeply dipping contact for at least part of 

its southeastern extent, but there is no apparent angularity 

between strata of the two units. From the vicinity of Apple 

Creek westward to near Hayden Creek, the contact cuts across 

bedding. 

Faults with Compound Histories of 

Compressional and Extensional Movement 

Two faults discussed in this section are interpreted to have 

originated as compressional faults but to be expressed presently 

as normal faults. No restoration to a pre-normal-slip position 

was undertaken for these faults. (1) The Salmon River fault, in 

the western part of the map area, is interpreted as a tear fault that 

formed during differential movement within the Poison Creek 

sheet as it was transported to the northeast. During subsequent 

extensional movement, a segment of the fault was reactivated as 

the Lem Peak block was downdropped southwest of the Lem 

Peak normal fault. Later, the tear fault was reactivated again as 

part of the present through-going Salmon River normal fault. 

(2) The Payne Creek fault trends north along Hayden Creek in 

the eastern part of the map area. It is interpreted as a tear fault 

that formed during compressional emplacement of the Poison 

Creek thrust sheet, similar to the Salmon River fault, but probably has undergone only one stage of subsequent extensional 

movement. 

Both the Salmon River fault and the Payne Creek fault may 

have formed prior to thrusting, or may have exploited previously 

existing structural weaknesses. This possibility is suggested 

because of the contrasting styles and (or) the amount of fault 

displacement along different segments of these northerly trending faults or fault zones. No study was undertaken to evaluate 

this possibility. Data in hand do not suggest two episodes of 

compressional structures along the faults or fault zones. An 

extensional setting existed in the Cordilleran region during the 

Neoproterozoic and Early Cambrian, and evidence for it in eastcentral Idaho was summarized by Skipp and Link (1992) and 

Link and others (1993). Skipp and Link (1992) suggested that 

strata of these ages preserved in the central and southern parts of 

the Lemhi Range and Beaverhead Mountains were deposited in 

a north-trending rift basin. Neither these authors nor those cited 

by them presented evidence for northerly striking syn-rift faults 

that may have trended at a high angle to such a rift basin. 

If the Payne Creek fault does have a Proterozoic history, 

then a third fault described in this section, the Apple Creek fault, 

possibly may share this history, because both faults lie along the 

same general trend. Hence, the Apple Creek fault is described in 

this section rather than in the section on extensional faulting. 

Salmon River Fault 

The Salmon River fault trends north across the western part 

of the study area and cuts the northwest-trending structural grain 

of the Lemhi Range at an angle of 60°–90° (fig. 4). It is 

Faults with Compound Histories of Compressional and Extensional Movement 25 
interpreted as a tear fault formed during Mesozoic thrust faulting 

and folding of rocks of the Poison Creek sheet. West of the 

Salmon River fault, and north of the concealed segment of the 

Lem Peak normal fault in the southwest corner of the area of figure 4, Mesoproterozoic strata of the Apple Creek Formation are 

exposed in limited areas of outcrop. No Paleozoic or Mesozoic 

rocks are present. Rocks of the Eocene Challis Volcanic Group 

are widespread. All these strata are downdropped relative to 

rocks east of the Salmon River fault. Mesoproterozoic strata 

west of the Salmon River fault display north-trending strikes, 

contrasting with the northwest-trending strikes present in the 

Mesoproterozoic rocks east of the fault (fig. 4). 

Tear-Fault Movement 

The Mesozoic compressional development of the Salmon 

River fault of the study area fits Dahlstrom’s (1970, p. 374–375) 

definition of a tear fault. He described a tear as a type of strikeslip fault that terminates both upwards and downwards against 

movement planes that may be detachments of thrust faults or 

low-angle normal faults. In thrusted strata, tears wholly within 

one thrust sheet or packet of thrust sheets can be either transverse or oblique to the regional trend. The Salmon River fault is 

oblique to the regional trend. Dahlstrom (1970) further noted 

that a tear fault wholly within a deformed sheet permits abrupt 

changes in the pattern of deformation on opposite sides of the 

tear through differential movement of adjacent component parts 

of the sheet. This is the case with the Salmon River fault, which 

served to compartmentalize the style of deformation that took 

place on opposite sides of it. Rocks east of the Salmon River 

fault, the main focus of this report, deformed into the major Poison Creek anticline and associated subsidiary thrusts of the Poison Creek sheet. Rocks directly west of the Salmon River fault 

in the map area were not so intensely deformed (fig. 4). 

Structures in the Mesoproterozoic rocks east of the Salmon 

River fault display northwest strikes, in marked contrast to the 

north strike of the structures in the Mesoproterozoic rocks west 

of the fault (fig. 4). North strikes are to be expected for 

downthrown strata close to the Salmon River fault because they 

would have been reoriented toward parallelism with the fault as 

they were downdropped. For example, the north-striking syncline between Allison and McKim Creeks (fig. 4; fig. 5, west end 

of cross section A–A’) is about parallel to the Salmon River fault 

and likely formed during downdropping of the rocks west of the 

fault. Dip of the fold limbs is moderate except for steep inclination of the east limb where it is upturned adjacent to the Salmon 

River fault. But north strikes are characteristic of all rocks west 

of the fault, not just those close to the fault. Further, the Apple 

Creek strata west of the fault generally display low dips, contrasting with the generally steep dips of strata east of the fault. 

These data suggest that the north strikes of Mesoproterozoic 

strata west of the fault were extant prior to being downthrown 

during extensional faulting. They may have formed during 

(compressional) tear faulting. Alternatively, the north-striking 

structures of the Salmon River fault zone may have an older origin, a possibility not evaluated here. 

Normal-Slip Movement Related to Emplacement of 

Lem Peak Block 

The Lem Peak block, delimited on its northeast side by the 

Lem Peak normal fault, is present on both sides of the Salmon 

River fault (figs. 3, 4). However, part of the block shows apparent left-lateral offset along the Salmon River fault. Understanding of this apparent lateral offset requires examination of 

segments of the Lem Peak fault. Mesoproterozoic strata west of 

the Salmon River fault are chiefly units of the Apple Creek Formation, except for the Mesoproterozoic Swauger Formation that 

crops out in the southwest corner of the map area (fig. 4). About 

1 km directly west of the Swauger shown on the map, strata of 

the Big Creek Formation abut the Swauger on the south along a 

fault. This fault segment extends into the study area along the 

trace of X–X′ (fig. 4), where it is concealed beneath volcanic 

rocks. X–X′ is interpreted to be part of the Lem Peak normal 

fault because the rocks on opposite sides of this fault have the 

same stratigraphic-structural succession and relationships as in 

the Lemhi Range (Tysdal, 2000b). 

Along the Salmon River fault, the Lem Peak fault and the 

associated stratigraphic units of the Lem Peak block display an 

apparent left-slip offset of as much as 8–10 km (fig. 4, offset is 

X′–X′′). (The apparent offset is not more precisely determined 

because, within the study area, the trace of the Lem Peak fault 

west of the Salmon River fault is entirely concealed by volcanic 

rocks.) Two possible interpretations are considered to accommodate the apparent left-slip offset on the Salmon River fault 

seen south of the Poison Creek fault. (1) The Swauger Formation east of the Salmon River fault and west of the Salmon River 

fault, in the southwest corner of the map area, behaved as one 

contiguous block. The block is delimited by the full length of 

the Lem Peak fault in the Lemhi Range, the concealed segment 

of the Lem Peak fault in the southwest corner of the map area, 

and by the Salmon River fault segment that connects these two 

parts of the Lem Peak fault (fig. 4, X′′′ [and fault farther to 

east]–X′′–X′–X). In effect, the connecting part of the Salmon 

River fault is a segment of the Salmon River tear fault that was 

reactivated as a north-trending segment of the Lem Peak normal 

fault. This interpretation is favored here and is explained first 

(next paragraph). (2) A less likely interpretation attributes the 

apparent slip to mainly vertical movement along the Salmon 

River fault, and thus apparent left-lateral offset would be the 

result of dips of the flanking strata and of the Lem Peak fault. 

Such nearly vertical movement would have to be very large. 

1. All Swauger strata south of the Lem Peak fault, on both 

sides of the Salmon River fault, were downdropped as one contiguous block. In this interpretation, the X′–X′′ segment of the 

Salmon River fault is a reactivated segment of the compressionally formed Salmon River tear fault. The tear fault zone of 

weakness was reactivated during downdropping of the Lem 

Peak block, and the block was broken into east and west parts 

along the X′–X′′ segment of the tear fault. The previously continuous nature of the block is shown by the sequence of strata 

and structural relationships that are preserved in both parts of 

the Lem Peak block. East of the X′–X′′ fault segment, in the 

eastern part of the Lem Peak block (that is, in the western part 

26 Structural Geology, Western Lemhi Range, Idaho 
of the Lemhi Range), the south to north stratigraphic-structural 

succession is Swauger Formation–Lawson Creek Formation– 

Lem Peak normal fault (south-side-down)–Big Creek Formation–Apple Creek Formation. West of the X′–X′′ fault segment, 

in the western part of the Lem Peak block (that is, in the eastern 

part of the Salmon River Mountains), the similar stratigraphicstructural succession is Gunsight Formation–Swauger Formation–Lem Peak normal fault (south-side-down)–Big Creek 

Formation–Apple Creek Formation. Thus, in the Salmon River 

Mountains, the Lawson Creek Formation is cut out by the Lem 

Peak normal fault; only the lower part of the Swauger 

Formation is present and it is underlain by upper strata of the 

Gunsight Formation. 

2. For vertical movement to account for the apparent leftslip offset of the Lem Peak fault and associated strata, the 

amount of apparent offset (fig. 4, X′–X′′) is dependent on the 

magnitude of vertical displacement, the component of horizontal 

displacement (if any), and the dip of the Lem Peak fault. It is 

unlikely that, by itself, downdropping of strata and the contained 

Lem Peak fault west of the Salmon River fault could account for 

significant lateral displacement of the Lem Peak fault. First, 

assume no displacement on the Salmon River fault, and place a 

piercing point in the Lem Peak fault on opposite sides of the 

incipient Salmon River fault. Relative to the fixed piercing point 

on the east side of the Salmon River fault, the corresponding 

piercing point west of the fault would move south as strata west 

of the fault were downdropped. At the present depth of erosion, 

the Lem Peak fault is vertical or dips steeply southward. 

Downdropping of strata west of the Salmon River fault as 

the sole explanation of the apparent lateral offset along the fault 

has another problem, related to the triangular wedge of Swauger 

of the Lemhi Range. The wedge lies east of the X′–X′′ segment 

of the Salmon River fault and southwest of the X′′–X′′′ segment 

of the Lem Peak fault (fig. 4). Middle Eocene volcanic rocks 

locally overlie the Lem Peak fault in the Lemhi Range, and the 

fault is assumed to have ceased movement before deposition of 

the volcanic rocks (Ruppel, 1980; Tysdal, 1996a). In contrast, 

the Salmon River fault offsets Eocene volcanic rocks and may 

still be active. Therefore, ignoring the triangular wedge of 

Swauger, the Apple Creek strata west of the Salmon River fault 

are downthrown relative to the Big Creek–Apple Creek 

sequence of the Lemhi Range northeast of the wedge of 

Swauger (fig. 4, X′–X′′–X′′′). And, assuming the wedge of 

Swauger was faulted into place before down-on-the-west movement along the full length of the Salmon River fault (that is, 

along the segments north of X′′ and south of X′, as well as along 

the X′–X′′ segment), the Apple Creek west of the Salmon River 

fault actually is downthrown against the Swauger Formation 

east of the Salmon River fault—even though the apparent relative displacement of the two formations is just the opposite. 

Thus, the map of figure 4 shows west-side-down along the full 

length of the fault. The relative up-down relationships change 

along the length of the Salmon River fault. In the northern part 

of the study area, the (actual) relationship is west-side-down. 

South of X′′, the (apparent) relationship is east-side-down for 

the Proterozoic Apple Creek units west of the fault relative to the 

Swauger Formation east of the fault, but the (actual) relationship 

is west-side-down for the Tertiary Challis Volcanic Group rocks 

relative to the Swauger east of the fault (and the volcanic rocks 

are adjacent to and overlie the Apple Creek strata west of the 

fault). 

Normal-Slip Movement Related to Regional-Scale 

Salmon River Normal Fault 

The Salmon River normal fault extends across the entire 

map area (fig. 4). North of the map area, the fault extends along 

the western margin of the Lemhi Range (K.V. Evans, Falma 

Moye, and R.F. Hardyman, unpub. mapping, 1996) and likely 

continues farther north where it is concealed beneath the Lemhi 

River Valley east of Salmon (fig. 1), as shown on the mapping of 

Evans, K.I. Lund, and Tysdal (work in progress). South of the 

map area, the fault cuts the Challis Volcanic Group directly 

north of the Pahsimeroi River Valley (fig. 1), according to a general map of Janecke (1993, fig. 6; the name Allison Creek fault 

was used by Janecke for the fault). 

The regional-scale normal displacement of the Salmon 

River normal fault developed subsequent to downdropping of 

the Lem Peak block. The apparent strike-slip displacement of 

the east and west parts of the Lem Peak block along the X′–X′′ 

segment of the Salmon River fault is confined to the area south 

of the Poison Creek thrust fault. This is shown by the lack of (or 

very minor) strike-slip offset of the footwall McDevitt Creek 

sheet along the Salmon River normal fault. Directly north of the 

map area (figs. 3, 4), the Poison Creek thrust fault and the subjacent Paleozoic carbonate rocks of the footwall McDevitt Creek 

sheet extend to the northwest beyond the map area. This continuation is concealed beneath unconsolidated sediments and volcanic rocks in the valley of the Salmon River, but the thrust fault 

and carbonate rocks crop out west of the valley (Ekren, 1988, pl. 

2). The north-striking Salmon River fault crosses the northweststriking Poison Creek fault and the carbonate footwall rocks, but 

if these features are offset by lateral movement on the Salmon 

River fault, such offset is no more than about 1 km. Any such 

offset would be right-slip, a direction opposite that of the apparent offset of rocks south of the Poison Creek fault. 

Payne Creek Cross Fault 

The Payne Creek fault trends north across the eastern part 

of the study area and cuts the Bear Valley subsheet and the Lem 

Peak block (figs. 3, 4). It dips steeply to the east, indicated by its 

eastward curving trace as the flanking topography decreases in 

elevation along the northern third of the fault (fig. 4). Proterozoic strata of the Bear Valley subsheet west of the Payne Creek 

fault show about 1.5 km of left-slip relative to strata east of the 

fault. The sequence east of the fault is from a greater depth in 

the hangingwall sequence than that west of the fault. 

The origin of the Payne Creek fault is unclear, but it may 

have originated as a compressionally formed tear fault. The 

Payne Creek fault displays the following features in common 

with the Salmon River fault. (1) The north end of the Payne 

Creek fault does not offset footwall strata, but terminates at the 

Faults with Compound Histories of Compressional and Extensional Movement 27 
subsidiary Bear Valley fault that floors the Bear Valley subsheet. 

(2) The fault is oriented at about 45° to the transport direction of 

the host subsheet, thus is an oblique fault. (3) The strike is about 

north, subparallel to the Salmon River fault. (4) The southern 

extent of the Payne Creek fault, as far south as it has been 

mapped, offsets Swauger strata of the downdropped Lem Peak 

block. In contrast with the Salmon River fault, the Payne Creek 

fault does not serve to compartmentalize contrasting styles of 

deformation on opposite sides of it. 

The following relationships are evident in the vicinity of 

the intersection of Wade and Hayden Creeks, where the Lem 

Peak block shows left-lateral offset along the Payne Creek 

fault. (1) In the vicinity of Wade Creek, the Payne Creek fault 

is marked by sheared orthoquartzite of the Mesoproterozoic 

Swauger Formation along its trend, although Ruppel (1980) did 

not map the fault in the Wade Creek area. (2) Beds directly 

adjacent to the fault have been rotated such that their strike is 

oriented at a small angle to the fault. A similar reorientation of 

bedding occurs about 5 km to the north along the fault. (3) Isolated outcrop areas of the Ordovician Kinnikinic Quartzite west 

of the fault (just south of the area of fig. 4) are much more 

closely aligned with Kinnikinic rocks east of the Payne Creek 

fault after removal of the offset of about 1.5 km. Similarly, the 

segments of the Lem Peak fault (described previously) on 

opposite sides of the Payne Creek fault become aligned and 

form a continuous Lem Peak fault. But alignment of the two 

segments does not accommodate all of the offset of stratigraphic units farther north along the Payne Creek fault. For 

example, the Big Creek–Apple Creek contact west of the Payne 

Creek fault does not align with the same contact east of the 

fault. The misalignment remains unexplained; I suspect that 

some structural complications lie concealed beneath the large 

area of unconsolidated sediments between Cooper Creek and 

the Lem Peak fault (fig. 4). 

The Payne Creek fault may have originated as a compressional tear within the Bear Valley subsheet during thrusting. 

Rocks east of the Payne Creek fault moved farther north relative 

to those west of the fault and exposed deeper levels of the stratigraphic succession. Subsequent to, or during, normal-fault 

emplacement of the Lem Peak block, the Payne Creek fault was 

reactivated and movement along it offset Swauger strata of the 

Lem Peak block. This interpretation assumes that the Payne 

Creek fault formed in a manner similar to the Salmon River normal fault. Alternatively, (1) the Payne Creek fault could predate 

normal faulting along both the Bear Valley and Lem Peak faults, 

with the Payne Creek fault reactivated south of the Lem Peak 

fault when the Lem Peak block was initially downdropped into 

its present position and (or) during later basin-and-range extensional activity. (2) The Payne Creek fault may predate thrusting, 

thus formed a structural weakness that subsequently was reactivated during both thrusting and extensional faulting. 

Apple Creek Fault 

The Apple Creek fault (east-central part of area, fig. 4) is 

concealed by landslide deposits and volcanic rocks. Its existence is necessary, however, to accommodate the contrast in 

thickness of the diamictite unit on opposite sides of the covered 

area. Strata northeast of the Alder Creek fault were downdropped during normal movement on the Alder Creek fault. The 

Apple Creek fault may be a tear fault, with normal displacement 

on its east side, that formed as a tear during the downdropping. 

An alternative interpretation could consider the Apple 

Creek fault in the footwall of the Bear Valley fault to be the 

same structure as the Payne Creek fault of the hangingwall. 

Both faults display apparent left-slip offset. This connection is 

unlikely, however. (1) Directly south of the south end of the 

Apple Creek fault as shown in figure 4, the mutual contact of the 

Big Creek and Apple Creek Formations does not appear to be 

offset. The actual contact is not exposed, but no offset of the 1+ 

km magnitude evident directly to the north along the Apple 

Creek fault has taken place. (2) The footwall rocks of the Bear 

Valley fault would need to be shifted about 3 km to the northwest for the faults to be aligned. I have no evidence to suggest 3 

km of right-slip along the Bear Valley fault. (3) West of the 

Apple Creek fault, the Alder Creek fault at the base of the diamictite unit truncates the fine siltite unit of the Apple Creek 

Formation with an angular relationship (Tietbohl, 1981; Tysdal, 

1996a). If the Payne Creek and Apple Creek faults originally 

were one structure (a tear fault), the angular truncation of the 

fine siltite unit of the Apple Creek Formation also would have 

lain to the west of the Payne Creek fault. No counterpart of this 

cross-cutting fault segment occurs in the corresponding strata of 

the hangingwall. Further, west of the Apple Creek fault and 

north of the Bear Valley fault, the diamictite unit of the footwall 

contains a distinctive sequence of dark-gray magnetite-bearing 

beds. Directly south on the hangingwall, these distinctive beds 

were observed only near Hayden Creek and atop the ridge 

directly west of Hayden Creek. These data suggest that the 

hangingwall and the footwall have not undergone lateral shift 

relative to each other. 

If the Payne Creek cross fault developed along a preexisting zone of structural weakness, a possibility raised at the beginning of this section, then it is appropriate to speculate that the 

Apple Creek fault could have developed along the same general 

preexisting zone. No direct evidence was obtained to evaluate 

this idea, however. 

Ages of Deformation 

Compressional Deformation 

The maximum age of thrusting in the study area is indicated by the Paleozoic rocks of the footwall McDevitt Creek 

sheet. As discussed in the section on the age of cleavage, only 

one continuous (pervasive) cleavage was found in the hangingwall Mesoproterozoic rocks of the Poison Creek thrust sheet. 

Cleavage measured in the carbonate strata of the footwall 

McDevitt Creek sheet displays an orientation pattern that is consistent with patterns in the Poison Creek sheet. Nearly identical 

cleavage patterns are displayed by the footwall Paleozoic carbonate strata and the hangingwall Mesoproterozoic rocks near 

the trace of the Poison Creek thrust fault (fig. 11J and K). This 

28 Structural Geology, Western Lemhi Range, Idaho 
clearly places the age of these coincident cleavages in the Phanerozoic. The youngest Paleozoic strata known in the footwall of 

the Poison Creek thrust fault in the Lemhi Range are of the 

Upper Devonian Jefferson Formation (Anderson, 1961). East of 

the map area, in the Beaverhead Mountains northeast of Leadore, Idaho (fig. 1), strata as young as Triassic age underwent 

compressional deformation during thrusting associated with the 

Hawley Creek fault (Lucchitta, 1966; K.I. Lund, oral commun., 

2000), which I interpret as corresponding to subsidiary thrust 

faults in the footwall of the Poison Creek thrust fault. 

The minimum age of thrusting in the Lemhi Range is provided by igneous rocks. Several undeformed granitic stocks in 

the central part of the Lemhi Range clearly are younger than the 

thrust faulting and were intruded and partly exposed by erosion 

before eruption of the Challis Volcanic Group (Ruppel, 1978, p. 

14; Tysdal, 1996b). The Big Eight Mile pluton, mapped by 

Ruppel (1980) about 50 km southeast of this study area, postdates thrust faulting (Ruppel, 1980). The Big Eight Mile pluton 

contains biotite that yielded a potassium-argon date of 49.4+1.7 

Ma (Ruppel and Lopez, 1988, p. 72) (50.6+1.7 Ma, adjusted for 

currently accepted decay constants; J.D. Obradovich, oral commun., 1993). [Apparently, Ruppel (1978, p. 14) mistakenly 

applied this date to a dike related to another pluton, but correctly 

attributed the date in Ruppel and Lopez (1988).] 

A granitic pluton is inferred to lie beneath a magnetic 

anomaly in the headwaters area of the North Fork McKim Creek 

(fig. 4). The anomaly is one of several that lie along a northwest 

trend within the general region (Zietz and others, 1978; Webring 

and Mabey, 1995; pl. 2 in Worl and others, 1989); it is similar to 

the anomaly over the exposed Big Eight Mile pluton. A metamorphic aureole and associated silicified rock characterize the 

northern part of the McKim Creek anomaly. The metamorphosed and silicified rocks extend across structures in the map 

area. Silicified and tourmaline-bearing rocks replace sheared 

rock along parts of the Hayden Creek and Bear Valley faults, 

postdating movement on these faults. 

In the Beaverhead Mountains, about 20 km north-northeast 

of Salmon (fig. 1), compressionally deformed Proterozoic strata 

and a compressional fault are intruded by the Carmen Creek pluton (map of Ruppel and others, 1993; K.V. Evans, oral commun., 1999; personal observation). Using 40Ar/39Ar dating, 

Kilroy and Sutter (1984) determined that a quartz diorite phase 

of the pluton yielded a Late Cretaceous date of 80.9+1.9 Ma; a 

more mafic phase of the intrusive is considered to be equivalent 

in age, or older. A granodiorite phase yielded middle Eocene 

dates. Although the pluton is not in the Lemhi Range, it lies farther northeast than the Lemhi Range, and, if deformation generally progressed eastward with time, as is the general situation in 

the Rocky Mountain thrust belt, then thrusting in the Lemhi 

Range may have ceased prior to intrusion of the Carmen Creek 

pluton. 

Extensional Deformation 

Challis volcanic rocks postdate normal-slip movement on 

faults at least locally. South and west of Mill Mountain, in the 

vicinity of Wade Creek (fig. 4), a segment of the Lem Peak fault 

is locally concealed by rocks of an unknown unit of the Eocene 

Challis Volcanic Group. Other rocks of the Challis Volcanic 

Group were deposited across the Bear Valley fault west of Hayden Creek after normal displacement took place on the fault, as 

shown north of the head of Bear Valley Creek in figure 4 and 

cross section B–B’ (fig. 5). 

The tuff of Ellis Creek (McIntyre and others, 1982; Fisher 

and others, 1992), a unit of the Challis Volcanic Group, overlies 

the Poison Creek thrust fault at the head of Poison Creek (fig. 4) 

and locally in the western part of the Allison Creek quadrangle 

(fig. 2; Tysdal and Moye, 1996). Biotite from tuff of this unit 

about 45 km directly west of the Poison Creek occurrence 

yielded a middle Eocene potassium-argon date of 48.4+1.6 Ma 

(Fisher and Johnson, 1995; Fisher and others, 1992). Movement 

on the Poison Creek thrust fault had ceased by the time of deposition of the tuff. 

Evaluation of Possible Proterozoic Compressional 

Deformation 

Compressional deformation of Mesoproterozoic age has 

been reported for local areas of the central Idaho region. Radiometric dating of coarse-grained granitic plutons in the Salmon 

River Mountains yielded Mesoproterozoic dates (Evans, 1986; 

Evans and Zartman, 1990). One of the plutons, about 8 km 

northwest of Salmon, crosscuts folds formed in the Mesoproterozoic host strata (Brown, 1973; Evans, 1981, 1986; Lopez, 

1981). Further, regional biotite-grade metamorphism of the host 

strata occurred prior to intrusion of the pluton. Contact metamorphic aureoles around the Proterozoic plutons overprint the 

regional metamorphic assemblage of the country rocks (Evans 

and Zartman, 1990). 

Only one compressional deformation was recognized in 

the study area of the Lemhi Range, and it probably is Late 

Cretaceous. If an unrecognized Proterozoic compressional 

deformation took place as well, cleavage that may have been 

generated during the deformation could have been coincident, 

or nearly so, with that of the Cretaceous event, thus was not 

distinguished from the Cretaceous cleavage. 

Reevaluation of Medicine Lodge Thrust 

System and Hayden Creek Window 

The foregoing presentation and interpretation of structural 

features set the stage for a reevaluation of the concept of the 

Medicine Lodge thrust fault and its associated features in the 

western part of the Lemhi Range. 

A Medicine Lodge thrust fault was first mapped in the Beaverhead Mountains east of Leadore, Idaho (fig. 1) by Kirkham 

(1927). Other workers later mapped more thrust faults northwestward in the Beaverhead Mountains and in nearby southwestern Montana. Ruppel (1978) introduced the name 

“Medicine Lodge thrust system” for the assemblage of these 

faults and extended the system to include faults that he had 

mapped in the Lemhi Range. 

Reevaluation of Medicine Lodge Thrust System and Hayden Creek Window 29 
Within the Lemhi Range, Ruppel (1978) initially treated the 

“thrust system” as a single megathrust fault. Later, Ruppel and 

Lopez (1984, p. 1) characterized the Medicine Lodge thrust plate 

as a unique assemblage of rocks hosting a “network of interrelated flat thrust faults that both bound and cut the thrust 

plate***and dip into a basal decollement zone that forms the 

lowermost part of the plate.” Thrust faults of the plate also were 

described as “interlaced” imbricates. The basal zone was characterized as 50–300 m of intensely sheared, crushed, brecciated, 

and mylonitized rocks grading upward into less internally 

sheared but complexly folded and faulted rocks of the plate 

(Ruppel, 1978, p. 8; Ruppel and Lopez, 1984). Ruppel (1978) 

and Ruppel and Lopez (1984, 1988) envisioned the Medicine 

Lodge thrust as separating allochthonous rocks of the Mesoproterozoic Lemhi Group and younger strata from subjacent, autochthonous, older Mesoproterozoic rocks of the Yellowjacket 

Formation. These workers interpreted the Yellowjacket as 

autochthonous because they considered its strata generally to be 

only slightly deformed or to exhibit a style of deformation different from strata of the Lemhi Group and younger rocks designated as allochthonous. These interpreted differences in 

deformation led them to “suggest that the Yellowjacket has not 

been tectonically transported by thrusting like the overriding, 

intricately folded and faulted rocks in the Medicine Lodge plate” 

(Ruppel and Lopez, 1984, p. 23). Ruppel (1978, p. 8) stated that 

one of the best exposures of the Medicine Lodge fault system is 

in the Lemhi Range north of Mill Mountain, within the southeastern part of the study area of this report, where a window (the 

Hayden Creek window) resulted from erosion through the Medicine Lodge thrust plate. 

My mapping in this area in the northern part of the Lemhi 

Range (Tysdal, 1996a, 1996b, 1996c) leads me to conclude that 

the Medicine Lodge thrust plate does not exist in this part of the 

range, that no window was eroded through such a thrust plate, 

and that rocks assigned (by Ruppel and Lopez, 1988) to the Yellowjacket Formation in the northern part of the Lemhi Range (1) 

belong to another formation and (2) are as intensely deformed as 

rocks of other Mesoproterozoic formations. My mapping 

extends for 8 km east and 18 km west of their Hayden Creek 

window (hereafter called “window”; location of “window” 

shown in fig. 4) and reveals three northwest-trending faults that 

cross it: from north to south, the Poison Peak, Bear Valley, and 

Hayden Creek faults. Stratigraphic units present between any 

pair of these northwest-trending faults also occur on strike on 

both sides of the “window.” Where not covered by Cenozoic 

strata, the Proterozoic units are contiguous across the “window” 

as well. 

The Hayden Creek “window” measures approximately 

6×20 km (fig. 4). The outline is taken mainly from plate 1 of 

Ruppel and Lopez (1984), modified slightly to accommodate 

the distribution of “Yellowjacket” rocks shown on the geologic map of the Patterson 15-minute quadrangle mapped by 

Ruppel (1980). Strata east of Hayden Creek, and north of the 

Alder Creek fault, were assigned to the “Yellowjacket” Formation by Ruppel and Lopez (1984) and considered to be part of 

the autochthon (fig. 4). Other than the Hayden Creek “window,” the remainder of the pre-thrusting rocks (that is, older 

than the Eocene Challis Volcanic Group) of the map area were 

assigned to the allochthonous Medicine Lodge thrust plate by 

these authors. 

Directly west of Wade Creek (fig. 4), at the western margin 

of the Patterson quadrangle mapped by Ruppel (1980), a 200 m 

long fault within an island of rock surrounded by glacial deposits was interpreted as the southern limit of the “window.” Ruppel and Lopez (1988, p. 76) concluded that this fault delimited 

the sole of the Medicine Lodge thrust plate. Ruppel and Lopez 

(1984) showed this island of rock as steeply inclined, slightly 

overturned Swauger Formation on the south abutting “Yellowjacket” strata on the north; the entire Lemhi Group is absent. In 

figure 4, the island of rock is shown as a small lens-shaped area 

adjacent to the Lem Peak normal fault and less than 1 km west 

of the Payne Creek fault. The entire Lemhi Group (table 1) is 

not missing from this area as believed. Strata of the lens-shaped 

area shown as “Yellowjacket” on the map of Ruppel (1980) are 

Big Creek Formation. Nevertheless, most of the Lemhi Group 

is missing, not due to the existence of a “window,” but due to a 

large amount of normal displacement (down to the southwest) 

on the Lem Peak normal fault and a comparatively small 

amount on the Hayden Creek fault as well (Tysdal, 1996a, 

1996b, 1996c). 

Rocks in the area of the southeast edge of the Hayden 

Creek “window,” on the ridge directly east of the southern 

reaches of Cooper Creek, were described as sheared and brecciated Big Creek Formation in thrust contact above the “Yellowjacket” (Ruppel, 1980; Ruppel and Lopez, 1988, p. 76). I found 

no evidence of a (sole) thrust fault in this area. Proterozoic 

rocks of the Big Creek Formation lie above and below the 

depicted sole-fault, although good outcrops are scarce. No 

rocks in outcrop or float are sheared; outcrops of the finegrained rock exhibit cleavage that dips about 30o SW., a common inclination for rocks in the general region. This southern 

part of the “window” lies between the Bear Valley and Hayden 

Creek faults (fig. 4). The Big Creek and Apple Creek strata 

between these faults are continuous from west of the “window,” 

through it, and on east beyond the limit of figure 4. These data 

show that the “window” does not exist. 

About 6 km north of Mill Mountain, near the Alder Creek 

fault (fig. 4), the Big Creek Formation was interpreted as thrust 

over turbidites and debris flows of the “Yellowjacket” (Ruppel, 

1980; Ruppel and Lopez, 1984, pl. 1; 1988, p. 76). The solethrust fault depicted on the map of Ruppel (1980) is at the horizon that I have locally mapped as the contact of the fine siltite 

unit and the diamictite unit of the Apple Creek Formation. 

Their fault extended northward on plate 1 of Ruppel and Lopez 

(1984) and clearly trends across the northwest structural grain 

of the rocks of the region and cuts across the rock units that 

Tietbohl (1981, 1986) and Tysdal (1996b) mapped in the area 

(fig. 4). The Medicine Lodge thrust fault of Ruppel (1980) and 

Ruppel and Lopez (1984) clearly would have to cut across beds 

that are steep to vertical and are continuous above and below 

their flat “fault.” No horizontal or shallowly dipping structural 

surface cuts across their “autochthonous Yellowjacket” rock 

units north of the Alder Creek fault (“autochthon” boundary 

shown in fig. 4). Further, a 3–4 km wide area of upper plate 

rocks of the Medicine Lodge plate was mapped previously 

between the “autochthonous” strata to the east and the Hayden 

30 Structural Geology, Western Lemhi Range, Idaho 
Creek “window” (Ruppel and Lopez, 1984, pl. 1). However, the 

same Apple Creek strata (fig. 4, units Yad and Yac) continue 

along strike from the east edge of the area to the west side of 

Hayden Creek where they are shown by Ruppel and Lopez 

(1984, pl. 1) as part of the “authochthon” and as allochthonous 

rocks of the Medicine Lodge thrust plate that lie east of the Hayden Creek “window.” 

The Alder Creek fault is interpreted to exist along the contact of the fine siltite unit with the overlying diamictite unit of 

the Apple Creek Formation (Tysdal, 1996a). The amount of displacement on this fault, which is largely covered, is unknown 

but is considered to be minor. Along the fault, at the very east 

edge of the area covered by figure 4, Ruppel (1980) reported an 

area of breccia in rocks mapped as “Yellowjacket” (that is, 

within the fine siltite unit of the Apple Creek Formation) in the 

Alder Creek drainage. Ruppel and Lopez (1984) interpreted the 

breccia to be tectonic in origin and to have developed in the sole 

zone of the Medicine Lodge thrust plate. I do not believe the 

breccia to be of tectonic origin. It is hydraulically fractured (and 

hydrothermally altered), but little to no offset is evident across 

fractures that define the breccia fragments. In any case, the 

Alder Creek fault is steep to vertical and is not a flat thrust as 

Ruppel and Lopez (1984, 1988) described it. 

Stretched, intensely brecciated, and mylonitized rocks were 

reported to constitute the sole zone of the Medicine Lodge thrust 

sheet in the Hayden Creek area (Ruppel, 1978, p. 8; and Ruppel 

and Lopez, 1984, 1988). No mylonite or intensely brecciated 

rocks were found in this area by me or reported by Tietbohl 

(1981, 1986). The diamictite unit is intensely cleaved, however. 

The cleavage reflects a high ductility that is due to a high content of clay and silt, and strong cleavage near Hayden Creek is 

due to location of the diamictite in the footwall of the Bear Valley fault, which dips about 30° SW. The fault is not a horizontal 

(flat) thrust of the sole zone of a decollement at the base of a 

(Medicine Lodge) thrust sheet of Ruppel (1978). 

Conclusions 

The following are major conclusions resulting from mapping and structural studies in the western part of the Lemhi 

Range. 

1. Most of the study area lies within the hangingwall of the 

Poison Creek thrust sheet. 

2. The thrust has a stratigraphic separation of 10,500+ m 

and an unknown amount of transport to the northeast. 

3. The Poison Creek thrust sheet comprises three subsheets 

separated, from northeast to southwest, by the Bear Valley and 

Hayden Creek faults, which presently display normal displacement. The Hayden Creek fault also displays thrust displacement, and both faults likely originated as minor thrusts. 

4. Removal of normal-slip displacement on the Bear 

Valley, Hayden Creek, and lesser faults within the subsheets to 

pre-normal-slip positions revealed that the thrust sheet makes up 

the leading part of a large anticline. Amplitude of the anticline 

exceeds 12,000 m. 

5. Cleavage and bedding in the hangingwall and footwall 

of the thrust record only one compressional deformation event. 

The deformation is younger than cleaved Mississippian rocks of 

the footwall in the study area, and based on regional data, it is 

younger than deformed Triassic strata: deformation likely is of 

Late Cretaceous age. 

6. Compressional deformation ceased prior to extrusion of 

the tuff of Ellis Creek, a unit of the middle Eocene Challis Volcanic Group, and prior to intrusion of Eocene granitic plutons. 

The deformation likely occurred prior to emplacement of the 

Late Cretaceous Carmen Creek pluton, present in the Beaverhead Mountains to the northeast of the Lemhi Range. 

7. Significant extensional deformation took place prior to 

extrusion of the Eocene Challis Volcanic Group. 

8. The Medicine Lodge thrust system, a single regionally 

extensive thrust sheet previously interpreted to extend across 

east-central Idaho and into southwestern Montana, does not 

exist within the study area, nor does the Hayden Creek window 

that was interpreted to have been eroded through it. 

References Cited 

Anderson, A.L., 1961, Geology and mineral resources of the Lemhi 

quadrangle, Lemhi County: Idaho Bureau of Mines and Geology 

Pamphlet 124, 111 p. 

Brown, D.C., 1973, Fold geometry in the Belt sequence, Salmon, 

Idaho: Lancaster, Pa., Franklin and Marshall College M.S. thesis, 

71 p. 

Dahlstrom, C.D.A., 1970, Structural geology in the eastern margin of the 

Canadian Rocky Mountains: Bulletin of Canadian Petroleum 

Geology, v. 18, p. 332–406. 

Ekren, E.B., 1988, Stratigraphic and structural relations of the Hoodoo 

Quartzite and Yellowjacket Formation of Middle Proterozoic age 

from Hoodoo Creek eastward to Mount Taylor, central Idaho: U.S. 

Geological Survey Bulletin 1570, 17 p. 

Evans, K.V., 1981, Geology and geochronology of the eastern Salmon 

River Mountains, Idaho, and implications for regional Precambrian 

tectonics: University Park, Pa., Pennsylvania State University Ph. D. 

dissertation, 222 p. 

Evans, K.V., 1986, Middle Proterozoic deformation and plutonism in 

Idaho, Montana, and British Columbia, in Roberts, S.M., ed., Belt 

Supergroup—A guide to Proterozoic rocks of western Montana and 

adjacent areas: Montana Bureau of Mines and Geology Special 

Publication 94, p. 237–244. 

Evans, K.V., 1999, The Yellowjacket Formation of east-central Idaho, in 

Berg, R.B., ed., Proceedings—Belt symposium III: Montana Bureau 

of Mines and Geology Special Publication 112, p. 17–30. 

Evans, K.V., and Zartman, R.E., 1990, U-Th-Pb and Rb-Sr geochronology 

of Middle Proterozoic granite and augen gneiss, Salmon River 

Mountains, east-central Idaho: Geological Society of America 

Bulletin, v. 102, p. 63–73. 

Fisher, F.S., and Johnson, K.M., 1995, Challis volcanic terrane, in Fisher, 

F.S., and Johnson, K.M., eds., Geology and mineral-resource 

assessment of the Challis 1°×2° quadrangle, Idaho: U.S. Geological 

Survey Professional Paper 1525, p. 41–43. 

Fisher, F.S., McIntyre, D.H., and Johnson, K.M., 1992, Geologic map of the 

Challis 1°×2° quadrangle, Idaho: U.S. Geological Survey 

Miscellaneous Investigations Series Map I–1819, 39 p., 1 sheet, 

scale 1:250,000. 

References Cited 31 
Harrison, J.E., and Cressman, E.R., 1993, Geology of the Libby thrust belt 

of northwestern Montana and its implications to regional tectonics: 

U.S. Geological Survey Professional Paper 1524, 42 p. 

Jamison, W.R., 1992, Stress controls on fold thrust style, in McClay, 

K.R., ed., Thrust tectonics: New York, Chapman and Hall, Inc., p. 

155–164. 

Janecke, S.U., 1993, Structures in segment boundary zones of the Lost 

River and Lemhi faults, east central Idaho: Journal of Geophysical 

Research, v. 98, no. B9, p. 16223–16238. 

Janecke, S.U., VanDenburg, C.J., Blankenau, H.J., and M’Gonigle, 

J.W., 2000, Long-distance longitudinal transport of gravel across 

the Cordilleran thrust belt of Montana and Idaho: Geology, v. 28, 

p. 439–442. 

Kilroy, K.C., and Sutter, J.F., 1984, 40Ar/39Ar geochronology and structural 

relationships of some intermediate intrusions in the northern Beaverhead Mountains, Idaho-Montana: Geological Society of America 

Abstracts with Programs, v. 16, no. 4, p. 226. 

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

and parts of Butte, Custer, Fremont, Lemhi, and Madison Counties, 

Idaho: Idaho Bureau of Mines and Geology Pamphlet 19, 47 p. 

Link, P.K.,Christie-Blick, Nicholas, Stewart, J.H., Miller, J.M.G., Devlin, 

W.J., and Levy, M., 1993, Late Proterozoic strata of the United 

States Cordillera, in Link, P.K., and others, Middle and Late Proterozoic stratified rocks of the western U.S. Cordillera, Colorado Plateau, and Basin and Range province: Boulder, Colo., Geological 

Society of America, The Geology of North America, v. C-2, Chapter 

6, p. 536–558. 

Lopez, D.A., 1981, Stratigraphy of the Yellowjacket Formation of eastcentral Idaho: U.S. Geological Survey Open-File Report 81-1088, 

218 p. 

Lucchitta, B.K., 1966, Structure of the Hawley Creek area, IdahoMontana: University Park, Pa., Pennsylvanian State University Ph. 

D. dissertation, 235 p. 

McIntyre, D.H., Ekren, E.B., and Hardyman, R.F., 1982, Stratigraphic and 

structural framework of the Challis volcanics in the eastern half of 

the Challis 1°×2° quadrangle, Idaho, in Bonnichsen, Bill, and Breckenridge, R.M., eds., Cenozoic geology of Idaho: Idaho Bureau of 

Mines and Geology Bulletin 26, p. 3–22. 

McNaught, M.A., and Mitra, Gautam, 1993, A kinematic model for 

the origin of synclines: Journal of Structural Geology, v. 15, p. 

805–808. 

Mitra, Gautam, and Elliott, David, 1980, Deformation of basement in the 

Blue Ridge and the development of the South Mountain window, 

in Wones, D.R., ed., Caledonides in the U.S.A., proceedings of 

International Geological Congress project 27: Blacksburg, Va., 

Virginia Polytechnic Institute and State University Memoir 2, p. 

307–311. 

Mitra, Shankar, 1990, Fault-propagation folds—Geometry, kinematic 

evolution, and hydrocarbon traps: American Association of Petroleum Geologists Bulletin, v. 74, p. 921–945. 

Plumb, K.A., 1991, New Precambrian time scale: Episodes, v. 14, no. 2, p. 

139–140. 

Ross, C.P., 1934, Geology and ore deposits of the Casto quadrangle, 

Idaho: U.S. Geological Survey Bulletin 854, 135 p. 

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

County, Idaho: U.S. Geological Survey Geologic Quadrangle Map 

GQ–733, scale 1:62,500. 

Ruppel, E.T., 1975, Precambrian Y sedimentary rocks in east-central 

Idaho: U.S. Geological Survey Professional Paper 889–A, 23 p. 

Ruppel, E.T., 1978, Medicine Lodge thrust system, east-central Idaho and 

southwest Montana: U.S. Geological Survey Professional Paper 

1031, 23 p. 

Ruppel, E.T.,1980, Geologic map of the Patterson quadrangle, Lemhi 

County, Idaho: U.S. Geological Survey Geologic Quadrangle Map 

GQ–1529, scale 1:62,500. 

Ruppel, E.T.,1993, Cenozoic tectonic evolution of southwest Montana 

and east-central Idaho: Montana Bureau of Mines and Geology 

Memoir 65, 62 p. 

Ruppel, E.T., and Lopez, D.A., 1981, Geologic map of the Gilmore quadrangle, Lemhi County, Idaho: U.S. Geological Survey Geologic 

Quadrangle Map GQ–1543, scale 1:62,500. 

Ruppel, E.T., and Lopez, D.A., 1984, The thrust belt in southwest Montana 

and east-central Idaho: U.S. Geological Survey Professional Paper 

1278, 41 p. 

Ruppel, E.T., and Lopez, D.A., 1988, Regional geology and mineral 

deposits in and near the central part of the Lemhi Range, Lemhi 

County, Idaho: U.S. Geological Survey Professional Paper 1480, 

122 p. 

Ruppel, E.T., O’Neill, J.M., and Lopez, D.A., 1993, Geologic map of the 

Dillon 1°×2° quadrangle, Idaho and Montana: U.S. Geological 

Survey Miscellaneous Investigations Series Map I–1803–H, scale 

1:250,000. 

Skipp, Betty, 1987, Basement thrust sheets in the Clearwater orogenic 

zone, central Idaho and western Montana: Geology, v. 15, p. 220– 

224. 

Skipp, Betty, 1988, Cordilleran thrust belt and faulted foreland in the Beaverhead Mountains, Idaho and Montana, in Schmidt, C.J., and Perry, W.J., Jr., eds., Interaction of the Rocky Mountain foreland and 

the Cordilleran thrust belt: Geological Society of America Memoir 

171, p. 237–266. 

Skipp, Betty, and Link, P.K., 1992, Middle and Late Proterozoic rocks 

and Late Proterozoic tectonics in the southern Beaverhead Mountains, Idaho and Montana—A preliminary report, in Link, P.K., 

Kuntz, M.A., and Platt, L.B., eds., Regional geology of eastern Idaho and western Wyoming: Geological Society of America Memoir 

179, p. 141–154. 

Soregaroli, A.E., 1961, Geology of the McKim Creek area, Lemhi County, 

Idaho: Moscow, Idaho, University of Idaho M.S. thesis, 53 p. 

Suppe, John, 1983, Some theory of parallel folding useful in predicting subsurface structure: Princeton, N.J., Princeton University 

Department of Geological and Geophysical Sciences, and Taiwan 

Petroleum Exploration Division of Chinese Petroleum Corporation, p. 1–11. 

Suppe, John, 1985, Principles of structural geology: Englewood Cliffs, 

N.J., Prentice-Hall, 537 p. 

Suppe, John, and Medwedeff, D.A., 1990, Geometry and kinematics of 

fault-propagation folding: Eclogae Geologicae Helvetiae, v. 83, p. 

409–454. 

Tietbohl, D.R., 1981, Structure and stratigraphy of the Hayden Creek 

area, Lemhi Range, east-central Idaho: University Park, Pa., Pennsylvania State University M.S. thesis, 121 p. 

Tietbohl, D.R., 1986, Middle Proterozoic diamictite beds in the Lemhi 

Range, east-central Idaho, in Roberts, S.M., ed., Belt Supergroup— 

A guide to Proterozoic rocks of western Montana and adjacent 

areas: Montana Bureau of Mines and Geology Special Publication 

94, p. 197–207. 

Tysdal, R.G., 1996a, Geologic map of adjacent areas of the Hayden Creek 

and Mogg Mountain quadrangles, Lemhi County, Idaho: U.S. 

32 Structural Geology, Western Lemhi Range, Idaho 
Geological Survey Miscellaneous Investigations Series Map I–2563, 

scale 1:24,000. 

Tysdal, R.G., 1996b, Geologic map of the Lem Peak quadrangle, Lemhi 

County, Idaho: U.S. Geological Survey Geologic Quadrangle GQ– 

1777, scale 1:24,000. 

Tysdal, R.G., 1996c, Geologic map of part of the May Mountain quadrangle, Lemhi County, Idaho: U.S. Geological Survey Open-File 

Report 96-537, scale 1:24,000. 

Tysdal, R.G., 2000a, Stratigraphy and sedimentology of Middle 

Proterozoic rocks in northern part of Lemhi Range, Lemhi County, 

Idaho: U.S. Geological Survey Professional Paper 1600, 40 p. 

Tysdal, R.G., 2000b, Revision of Middle Proterozoic Yellowjacket 

Formation of central Idaho: U.S. Geological Survey Professional 

Paper 1601–A, 13 p. 

Published in the Central Region, Denver, Colorado


Manuscript approved for publication November 28, 2001


Graphics by author and Gayle M. Dumonceaux


Photocomposition by Gayle M. Dumonceaux


Edited by L.M. Carter


Tysdal, R.G., and Moye, Falma, 1996, Geologic map of the Allison Creek 

quadrangle, Lemhi County, Idaho: U.S. Geological Survey Geologic 

Quadrangle Map GQ–1778, scale 1:24,000. 

Webring, M.W., and Mabey, D.R., 1995, Geophysical anomalies, in Fisher, F.S., and Johnson, K.M., Geology and mineral-resource assessment of the Challis 1°×2° quadrangle, Idaho: U.S. Geological 

Survey Professional Paper 1525, p. 53–62. 

Worl, R.G., Wilson, A.B., Smith, C.L., Kleinkopf, M.D., and Sykes, 

R.C., 1989, Mineral resource potential and geology of the Challis 

National Forest, Idaho: U.S. Geological Survey Bulletin 1873, 

101 p. 

Zietz, Isidore, Gilbert, F.P., and Kirby, J.R., Jr., 1978, Aeromagnetic map 

of Idaho; color coded intensities: U.S. Geological Survey Geophysical Investigations Map GP–920, scale 1:1,000,000. 

References Cited 33