BLEED BLEED
Cover. Hummocky crossbedded metasandstone, east side of Lake Mountain, east-central part 

of Salmon River Mountains.
Correlation, Sedimentology, Structural Setting, Chemical 

Composition, and Provenance of Selected Formations in 

Mesoproterozoic Lemhi Group, Central Idaho 

By Russell G. Tysdal, David A. Lindsey, and Joseph E. Taggart, Jr. 

A. Correlation, Sedimentology, and Structural Setting, 

 Upper Strata of Mesoproterozoic Apple Creek Formation

 and Lower Strata of Gunsight Formation, Lemhi Range to 

 Salmon River Mountains, East-Central Idaho

 By Russell G. Tysdal

B. Chemical Composition and Provenance of the 

 Mesoproterozoic Big Creek, Apple Creek, and 

 Gunsight Formations, Lemhi Group, Central 

 Idaho

 By David A. Lindsey, Russell G. Tysdal, and

 Joseph E. Taggart, Jr.

 

U.S. Geological Survey Professional Paper 1668–A–B

Chapters A and B are issued as a single volume

and are not available separately

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

Reston, Virginia, First Printing 2003

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

Box 25286, Federal Center

Denver, CO 80225

Published in the Central Region, Denver, Colorado

Manuscript approved for publication August 28, 2002

Graphics by authors 

Photocomposition by Norma J. Maes

Edited by L.M. Carter

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

Library of Congress Cataloging-in-Publication Data

Tysdal, Russell G.

 Correlation, sedimentology, structural setting, chemical composition, and provenance of 

selected formations in Mesoproterozoic Lemhi Group, central Idaho / by R.G. Tysdal, David A. 

Lindsey, and Joseph E. Taggart, Jr.

p. cm.—(U.S. Geological Survey professional paper ; 1668 A-B)

Includes bibliographical references.

1. Geology, Stratigraphic—Proterozoic. 2. Formations (Geology)—Idaho. 3. Lemhi 

Group (Idaho). I. Lindsey, David A. II. Taggart, Joseph E., Jr. III. Title. IV. Series.

QE653.5 .T964 2003

551.7′15′09796—dc21 

ISBN: 0-607-99680-3

2002190815
Correlation, Sedimentology, and Structural Setting, 

Upper Strata of Mesoproterozoic Apple Creek Formation

and Lower Strata of Gunsight Formation, Lemhi Range to 

Salmon River Mountains, East-Central Idaho

By Russell G. Tysdal

Chapter A of

Correlation, Sedimentology, Structural Setting, Chemical Composition, 

and Provenance of Selected Formations in Mesoproterozoic Lemhi Group,

Central Idaho 

By Russell G. Tysdal, David A. Lindsey, and Joseph E. Taggart, Jr.

U.S. Geological Survey Professional Paper 1668–A

U.S. Department of the Interior

U.S. Geological Survey
III

Contents

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

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

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

 General Stratigraphy.................................................................................................................... 3

 Terminology ................................................................................................................................... 5

Apple Creek Formation........................................................................................................................ 5

 Lemhi Range .................................................................................................................................. 5

 Fine Siltite Unit....................................................................................................................... 6

 Diamictite Unit....................................................................................................................... 6

 Coarse Siltite Unit ................................................................................................................. 6

 Salmon River Mountains ............................................................................................................. 6

 General Setting of Banded Siltite Unit of Apple Creek Formation................................ 6

 Banded Siltite Unit................................................................................................................ 6

 Lower Strata of the Banded Siltite Unit..................................................................... 6

 Finely Laminated Strata ........................................................................................ 7

 Soft-Sediment Deformation Features ................................................................ 8

 Upper Strata of the Banded Siltite Unit ..................................................................... 8

 Scapolite-Rich Beds ............................................................................................. 10

Gunsight Formation.............................................................................................................................. 12

 Lemhi Range .................................................................................................................................. 12

 Salmon River Mountains ............................................................................................................. 12

 Stratigraphic Relationships................................................................................................. 13

 Hummocky Crossbedded Strata................................................................................. 13

 Shoreface Strata........................................................................................................... 14

 Intraformational Conglomerate .......................................................................... 14

 Origin ............................................................................................................................... 15

 Erosion Surface ............................................................................................................. 17

 Fluvial Strata .................................................................................................................. 17

 Map Pattern........................................................................................................................... 17

 Structure ................................................................................................................................ 18

 Poison Creek Thrust Fault ............................................................................................ 18

 Juxtaposed Strata ................................................................................................ 18

 Tectonic Breccia........................................................................................................... 19

 Thickness of Gunsight Formation............................................................................... 19

 Other Implications of Existence of Poison Creek Thrust Sheet in 

 Salmon River Mountains.......................................................................................... 19

Conclusions........................................................................................................................................... 20

References Cited ................................................................................................................................. 20

Figures

 1. Index map of east-central Idaho .......................................................................................... 2

 2. Map showing geology of central part of Salmon River Mountains and western 

 part of Lemhi Range................................................................................................................ 3

 3. Map showing geology of Lake Mountain–Deep Creek area, and location of 

 Poison Creek thrust fault........................................................................................................ 4

 4. Diagram showing facies relationships for strata within banded siltite unit of 

 Apple Creek and Gunsight Formations in Salmon River Mountains .............................. 7

V
IV

5–14. Photographs showing:

 5. Finely laminated strata in banded siltite unit of Apple Creek Formation in 

 Salmon River Mountains................................................................................................... 8

 6. Dikelets in banded siltite unit of Apple Creek Formation ............................................ 9

 7. Typical outcrop of upper strata of banded siltite unit of Apple Creek Formation, 

 showing interlayered beds of light-gray metasandstone and dark-gray siltite...... 10

 8. Detailed view of upper strata of banded siltite unit of Apple Creek Formation ...... 11

 9. Light-gray metasandstone bed cut by dark-gray siltite during soft-sediment 

 deformation ........................................................................................................................ 12

 10. Dikelets associated with light-gray metasandstone bed cut by dark-gray 

 siltite during soft-sediment deformation ...................................................................... 13

 11. Hummocky crossbeds at Lake Mountain in Salmon River Mountains ..................... 14

 12. Slump structure overlain by non-slumped hummocky crossbedded meta-

 sandstone and other metasandstone beds .................................................................. 15

 13. Light-gray hummocky crossbedded metasandstone bed deposited on 

 dark-gray siltite, interpreted as water-saturated at time of deposition................... 16

 14. Flow-rolls in siltite, interpreted as channel deposit associated with the 

 hummocky crossbedded metasandstone unit.............................................................. 16

 

Table

1. Correlation diagram for Mesoproterozoic formations in hangingwall and 

 footwall of Poison Creek thrust sheet in western part of Lemhi Range and 

 eastern part of Salmon River Mountains .................................................................................. 5

 

VI
1

 Abstract

A unit of the Mesoproterozoic Apple Creek Formation of 

the Lemhi Range previously was correlated with part of the 

lower subunit of the Mesoproterozoic Yellowjacket Formation in 

the Salmon River Mountains. Strata currently assigned to the 

middle subunit of the Yellowjacket Formation lie conformably 

above the Apple Creek unit in the Salmon River Mountains, and 

are here renamed the banded siltite unit and reassigned to the 

Apple Creek Formation. Almost all of the banded siltite unit is 

preserved within the Salmon River Mountains, where it grades 

upward into clastic rocks that currently are assigned to the upper 

subunit of the Yellowjacket Formation and that here are reassigned to the Gunsight Formation. 

The banded siltite unit of the Apple Creek Formation is 

composed of a turbidite sequence, as recognized by previous 

workers. Uppermost strata of the unit were reworked by currents, possibly storm generated, and adjusted to a high water 

content by developing abundant soft-sediment deformation features. Basal strata of the overlying Gunsight Formation in the 

Salmon River Mountains display abundant hummocky crossbeds, storm-generated features deposited below fair-weather 

wave base, that are conformable above the storm-reworked 

deposits. The hummocky crossbedded strata grade upward into 

marine shoreface strata deposited above fair-weather wave base, 

which in turn are succeeded by fluvial strata. Hummocky and 

shoreface strata are absent from the Gunsight Formation in the 

Lemhi Range. The major thickness of the Gunsight Formation 

in both the Salmon River Mountains and the Lemhi Range is 

composed of fluvial rocks, transitional in the upper part into 

marine rocks of the Swauger Formation. The fluvial strata are 

mainly characterized by stacked sheets of metasandstone and 

coarse siltite; they are interpreted as deposits of braided rivers.

The Poison Creek thrust fault of the Lemhi Range extends 

northwestward through the study area in the east-central part of 

the Salmon River Mountains. The Apple Creek and Gunsight 

Formations on the southwest side of the thrust fault were transported to the northeast as part of the Poison Creek thrust sheet. 

A segment of the thrust fault within the Gunsight Formation in 

the Salmon River Mountains subsequently underwent normal 

displacement. Along this segment, lower Gunsight strata on the 

Correlation, Sedimentology, and Structural Setting, Upper 

Strata of Mesoproterozoic Apple Creek Formation and 

Lower Strata of Gunsight Formation, Lemhi Range to 

Salmon River Mountains, East-Central Idaho

By Russell G. Tysdal

southwest were juxtaposed against upper Gunsight and Swauger 

strata on the northeast.

Introduction

The general goal of this report is to correlate selected stratigraphic units in the Salmon River Mountains with those established in the Lemhi Range (fig. 1). New stratigraphic 

correlations and reconciliation of conflicting correlations and 

nomenclature for the Mesoproterozoic1Yellowjacket Formation 

and most units of the Mesoproterozoic Apple Creek Formation 

in central Idaho (fig. 1) were presented in Tysdal (2000a). The 

purpose of the present report is (1) to demonstrate that a unit previously assigned to the Yellowjacket Formation is actually the 

uppermost unit of the Apple Creek Formation, (2) to show that 

strata previously assigned to the upper subunit of the Yellowjacket Formation or to other formations or units clearly correlate 

with the Gunsight Formation, and (3) to reconcile the differing 

nomenclature presently used for these formations in the Salmon 

River Mountains. The effort focused on working out the geology of selected local areas in the east-central part of the Salmon 

River Mountains critical to determining the correlations. Interpretations of depositional environments are employed in determining stratigraphic relationships for the Apple Creek 

Formation. The interpetations generally differ from those presented in Winston and others (1999), although their work is of a 

reconnaissance nature.

This report builds on earlier work of Connor and Evans 

(1986), Connor (1990), Sobel (1982), and Tysdal (2000a). The 

latter report correlated rocks of the Lemhi Range with rocks in 

the Salmon River Mountains of central Idaho (fig. 1) and reconciled conflicting usage of the Yellowjacket Formation name and 

rocks assigned to it. The present report is mainly concerned 

with the upper part of the Apple Creek Formation and with the 

directly overlying Gunsight Formation. The stratigraphic chart 

(table 1) shows the correlation of rock units. The chart is simplified from that in Tysdal (2000a), which predates the scheme of 

Winston and others (1999), even though its actual publication 

date followed that of the latter report.

The present report is an outgrowth of studies related to 

regional stratigraphic correlation designed to determine the 

stratigraphic setting and limits of stratabound deposits of cobaltcopper-gold in the Blackbird mining district, in the vicinity of 

the townsite of Cobalt (fig. 1). The original work subsequently 

1

The 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).
2 Mesoproterozoic Lemhi Group, Central Idaho

 Challis

Salmon

North

Fork

0 10 20 MILES

 

0 10 20 KILOMETERS 

 River

 

Salmon

River

Salmon

River 

Fork

Middle

Panther

Cr

Pahsimeroi

River

Lemhi

River

Yellowjacket

Yellowjacket

Creek

 

Hayden

Creek

Leadore

DeepCr

Camas Creek

POISON

PEAK

Yellow

 Lake

93

Cobalt

Salmon

Area of

this figure

28

MONTANA

IDAHO

CLEARWATER MOUNTAINS

MOUNTAINS

RIVER

SALMON

BEAVERHEAD 

MOUNTAINS

LEMHI

RANGE

LOST RIVER RANGE

93

Moyer Cr

Woodtick Cr

Basin

Creek

Challis National Forest

Challis

 National

 Forest

 Beaverhead

 National Forest

Salmon

 National

 Forest

Payette

 National

 Forest

Bitterroot National Forest

IDAHO

43

GUNSIGHT

PEAK

Area of

figure 2

Area of

figure 3

114°30' 114° 

45°

30'

44°

30'

45°

113°30'

was augmented to provide a stratigraphic basis for mineral 

resource and geoenvironmental studies of the Salmon National 

Forest, and eventually for Federal and other lands in central and 

northern Idaho and western Montana. Forest areas in the study 

region are shown in figure 1.

Structural Setting

The general structural setting of the study area is briefly 

described to place the Apple Creek and Gunsight Formations 

into a broad context that shows the structural continuity of the 

formations from the Lemhi Range to the Salmon River Mountains. This general structural context aids the interpretation of 

the correlations of strata of the two ranges.

East-central Idaho lies in the hinterland of the Cordilleran 

thrust belt. The region consists of a series of thrust sheets that 

were transported to the northeast, probably during the Late Cretaceous (Tysdal, 2002). Differing lithofacies of the Mesoproterozoic rocks discussed in this report have been telescoped—

placed closer to one another than when originally deposited—by 

the thrust faulting. The Poison Creek thrust fault, a major fault 

of the region, trends northwest across the western part of the 

Lemhi Range and into the eastern part of the Salmon River 

Mountains (fig. 2). Mapping of the fault and its related features 

in the Salmon River Mountains (figs. 2, 3) proved critical to 

unraveling the juxtaposed lithofacies discussed in this report. 

The hangingwall of this thrust fault is the Poison Creek thrust 

sheet and the footwall is the McDevitt Creek thrust sheet (fig. 2, 

inset map) (Tysdal, 2002). The Mesoproterozoic Apple Creek 

Formation areally is the most extensively exposed formation of 

the hangingwall Poison Creek thrust sheet in the map area (fig. 

2). The Mesoproterozoic Gunsight Formation is the most extensively exposed formation of the footwall McDevitt Creek thrust 

sheet. The Gunsight Formation is conformably overlain by the 

Swauger Formation. Lower Paleozoic formations, preserved 

only in small areas of the footwall McDevitt Creek thrust sheet 

in the Lemhi Range, lie unconformably on the Mesoproterozoic 

strata. These formations are composed of limestone, dolostone, 

and quartzite.

Normal faults formed during Cenozoic and (?)Cretaceous 

extension, subsequent to the thrust faulting. The Lem Peak normal fault (fig. 2, inset map) trends west to northwest across the 

Lemhi Range and into the eastern part of the Salmon River 

Mountains. It downdropped rocks directly southwest of the 

Poison Creek thrust sheet. Stratigraphic units southwest of the 

Figure 1. East-central Idaho and adjacent part of southwestern Montana, showing geographic features and areas of figures 2 and 3. Light shade, mountains; dark shade, areas of 

Salmon National Forest within mountains. Dash-dot lines, boundaries of national forests.
Correlation, Sedimentology, Structural Setting 3

Lem Peak fault differ from those on the Poison Creek sheet, as 

discussed in the next section.

General Stratigraphy

The Apple Creek Formation of the Lemhi Range was 

divided into four units by Tysdal (2000b). Three units lie on the 

Poison Creek thrust sheet (fig. 2, inset map) and, in ascending 

order, are the fine siltite unit, the diamictite unit, and the coarse 

siltite unit (table 1). The fourth unit, named the Yellow Lake 

unit, lies within another thrust sheet (fig. 2, inset map), now 

downdropped southwest of the Lem Peak normal fault, and is 

not considered in any detail in this report. The Yellow Lake unit 

is thinner than any of the other units and of somewhat different 

character. It is considered to be of a different facies and to be the 

time equivalent of part or all of the other four lithofacies of this 

report (table 1) (Tysdal, 2000b). The diamictite unit thins westward in the Lemhi Range; it apparently pinches out at the east 

edge of the Salmon River Mountains (Connor, 1990; Tysdal and 

Moye, 1996). In the Salmon River Mountains, the coarse siltite 

unit correlates with part of the sequence of strata that Connor 

Tc

Yg

Tc

Yg

Yg

Yab

Ogr

Ygr

Yg

Ogr Yg

Ys

Qu

Yg

Od

Yg

Qu

Yg

Tc

Qu Tc

Yg

Ys

Qu

Tc

Qu & Tc

Yy

Yy?

Tc

Tc

Tc Tc

Tc

Yb

Yg

Yaf

Yh

Yh

Yh & Yaq Yy

Yac

Tc

Qu Tc

Tc

Tc

Yaf

Tc

Tc

Tc

Tc

Tc

Yaf

Yad

Qu

Tc

Qu

Qu

Yac

Tc

Yb

Ys

Ys

Tc

Tc

Yad

Yac

Tc

Tc

Qu

Yaf

Ys

Ys

Ylc

Ylc Qu

Yb

Ylc

Yb

Qu

Qu

Tc

Tc

Yg

Yab

Yac

Yac

Od

Yad

Yad

Yaf

Yb

IRON

LAKE

FAULT

44 o

52'

30"

00'

45 o

07'

30"

 Sobel

(1982)

 study

 area

Panther

Creek

Road

Cobalt

Townsite

CREEK

POISON

FAULT

 114 o 15' 114

1 2

3

4

LEM

PEAK

FAULT

River

SALMON

RIVER

FAULT

LEM

PEAK

FAULT

 Iron

Lake

Williams

Cr Road

Deep

Creek

Road

Moccasin

Cr

Road

Pass

Salmon

o

 00'

?

?

?

44 o

45'

45 o

EXPLANATION

Quaternary sediments, undivided

Eocene Challis Volcanic Group

Tertiary granitic rocks

Ordovician dolomite, undivided

Ordovician granitic rocks

Mesoproterozoic units

 Granitic rocks

 Lawson Creek Formation

 Swauger Formation

 Gunsight Formation

 Apple Creek Formation

 Banded siltite unit

 Coarse siltite unit

 Diamictite unit

 Fine siltite unit

 Big Creek Formation

 Argillaceous quartzite unit

 Hoodoo Quartzite

 Yellowjacket Formation

Twin Peaks mine

Degan Mountain

Lake Mountain

Blackbird mine

1

2

3

4

Contact

Normal fault--Dotted where concealed;

bar and ball on downthrown side

Thrust fault--Dotted where concealed;

 sawteeth on upper plate; queried

where uncertain

Thrust fault, with later normal

displacement--Dotted where

 concealed; sawteeth on upper plate;

bar and ball on downthrown side

Breccia

Syncline--Showing trace of axial plane

Qu

Tc

Tg

Od

Ogr

Ygr

Ylc

Ys

Yg

Yab

Yac

Yad

Yaf

Yb

Yaq

Yh

Yy

Mine

POISON

CREEK

THRUST SHEET

LEM

PEAK

T FAUL

LEM

EAK P FAULT Area of

figure 2 Yellow

Lake

POISON

FAULT

CREEK

 SALMON RIVER

 FAULT

cDEVITT McD

CR THRUST SHEET

IRON L F

0 1 2 3 4 5 MILES

0 1 2 3 4 5 6 KILOMETERS

Figure 2. Geology of central part of Salmon River Mountains and adjacent western part of Lemhi Range. Lemhi Range lies east 

of Salmon River fault shown in inset map, lower left corner of figure.
4 Mesoproterozoic Lemhi Group, Central Idaho

35

40

39

20

45

29

54

23

63

28

40

66

56

40

63 62

36

25

15

75

45

40

60

Yg

Qu

Qu

Qu

Qu

Tc

Yg

Yg

Tc

Yg

Ys

Ygr

Ogr

Yab

Tc

Yac

Yac

Yac

Tc Tc

Qu

Yg

Od

Qu

Yg

Tc

Yg

Tc

Tc

TWIN

PEAKS

MINE

DEGAN

MTN

LAKE

MTN

CREEK

FAULT

LAKE 

MOUNTAIN

FAULT

POISON

DEEP

CREEK

FAULT

?

?

?

Pepper

Creek

Deep Creek

South Fork Williams Cr

Williams Creek

Williams

Creek

Road

 PHELAN

MOUNTAIN

DEEP

CREEK

RIDGE

Moccasin

Creek

Road

Deep

Creek

Deep Creek

Road

Panther

Creek 

Road 

Cobalt

Townsite

Napias

Creek

Panther

Creek 

 114 15' 114 07' 30" 114 o o

45 o

00'

o

Pass

?

?

SWAN

PEAK

45 o

07'

30"

0 1 2 3 MILES

0 1 2 3 4 KILOMETERS

and Evans (1986) and Connor (1990) called the “lower subunit 

of the Yellowjacket Formation,” as Connor (1990, 1991) recognized. In my revision of the Yellowjacket, I (Tysdal, 2000a) 

reassigned these Yellowjacket strata to the Apple Creek Formation. Rocks assigned to the Yellowjacket Formation previously 

were restricted to terrain south of the Iron Lake fault (Tysdal, 

2000a). These changes in nomenclature are based on mapping 

and stratigraphic studies (Tysdal, 1996a, 1996b, 1996c, 2000a, 

2000b; Tysdal and Moye, 1996) conducted in the Lemhi Range 

subsequent to the mapping of Connor and Evans (1986), Connor 

(1990, 1991), and Evans and Connor (1993) in the Salmon River 

Mountains.

Figure 3 (above and facing page). Lake Mountain to Deep Creek area, Salmon River Mountains, showing Poison Creek thrust fault.
Correlation, Sedimentology, Structural Setting 5

Qu

Tc

Od

Ogr

Ygr

Ys

Yg

Yab

Yac

Quaternary sediments, undivided

Eocene Challis Volcanic Group

Ordovician dolomite

Ordovician granitic rocks

Mesoproterozoic units

 Granitic intrusive rocks

 Swauger Formation

 Gunsight Formation

 Apple Creek Formation

 Banded siltite unit

 Coarse siltite unit 

EXPLANATION

Contact

Normal fault--Dotted where concealed;

 bar and ball on downthrown side

64

40

Thrust fault--Dotted where concealed,

 queried where uncertain; sawteeth on

 upper plate

Syncline--Showing trace of axial plane

Anticline--Showing trace of axial plane

Overturned anticline--Showing trace

 of axial plane

Strike and dip of beds 

Inclined 

Overturned 

Vertical 

Mine

Breccia

Table 1. Correlation diagram for Mesoproterozoic formations in 

hangingwall and footwall of Poison Creek thrust sheet (fig. 2) in 

western part of Lemhi Range and eastern part of Salmon River 

Mountains. The Big Creek Formation in the Salmon River 

Mountains is reassigned to the Swauger Formation, and the 

directly underlying rocks are assigned to the Gunsight Formation. 

The contact of the two formations is conformable, as originally 

interpreted by Shockey (1957), and is not a thrust fault as later 

interpreted by Connor and Evans (1986).

 **Lower subunit (remaining part)—Included conformable sequence 

of Yellowjacket Formation as restricted by Tysdal (2000a), Hoodoo 

Formation, and unnamed argillaceous quartzite unit. These units were 

not correlated with strata in Lemhi Range (left column).

[No thickness of formations is implied. Height of boxes is greater for some

units to accommodate different nomenclature and (or) subdivisions of formations]

LEMHI RANGE

 AND

SALMON RIVER

MOUNTAINS

SALMON RIVER

MOUNTAINS

Tysdal (2000a, 2000b)

THIS REPORT

Connor and Evans (1986),

Connor (1990), Evans and 

Connor (1993), Evans (1999)

Swauger Formation Big Creek Formation

(thrust fault at base)

Gunsight Formation Apple Creek Formation

Yellowjacket Formation (part)

Banded siltite unit

Coarse siltite unit

Diamictite unit

Fine siltite unit

Upper subunit, upper part

Upper subunit, lower part

Middle subunit

Lower subunit (part)

Big Creek Formation

(**)

Orthoquartzite. Metamorphosed sandstone composed of at 

least 95 percent quartz grains.

Apple Creek Formation

Lemhi Range

The fine siltite unit, the diamictite unit, and the coarse 

siltite unit constitute the Apple Creek Formation of the Poison 

Creek thrust sheet. The thickness of the three units totals 

4,000–5,000 m. These three units are described briefly here 

because they set a context for the stratigraphic and sedimentologic interpretation of the banded siltite unit of the Salmon 

River Mountains. The coarse siltite unit, the upper unit in the 

In the Salmon River Mountains, a stratigraphic unit 

assigned to the “middle subunit of the Yellowjacket Formation” 

by Connor and Evans (1986) lies conformably above the coarse 

siltite unit of the Apple Creek Formation. This stratigraphic 

unit, almost entirely absent from the Lemhi Range, is recognized as a facies of the Apple Creek Formation and is here 

renamed the “banded siltite unit,” an informal name that was 

used originally by Connor and Evans (1986). Strata assigned to 

the “upper subunit” of the Yellowjacket Formation by Connor 

and Evans (1986) are correlated with the Gunsight Formation, to 

which they are reassigned. Most of this unit lies on the footwall 

McDevitt Creek thrust sheet. 

Terminology

Proterozoic rocks throughout most of the region discussed 

here have been metamorphosed to the greenschist facies. Chlorite-grade metamorphism is typical of most of the rocks in the 

Lemhi Range, although biotite-grade rocks are common in the 

northwesternmost part of the range (north of Poison Peak, fig. 

1). Biotite-grade metamorphism characterizes most of the rocks 

in the east-central part of the Salmon River Mountains. In this 

report, the following terminology is used for the clastic rocks.

Argillite. Metamorphosed claystone.

Siltite. Metamorphosed siltstone or mudstone.

Mudstone. Rock composed of a mixture of silt- and clay-size 

grains; may contain a small percent of sand-size grains.

Metasandstone. Metamorphosed sandstone; generally applied 

only to rocks composed of less than 90 percent quartz 

grains.

Quartzite. Metamorphosed sandstone composed of at least 90 

percent quartz grains.
6 Mesoproterozoic Lemhi Group, Central Idaho

Lemhi Range, is widespread in the Salmon River Mountains as 

well, where it conformably underlies the banded siltite unit.

Fine Siltite Unit

The fine siltite unit of the Apple Creek Formation of the 

Poison Creek thrust sheet is planar-laminated siltite, ripple crosslaminated siltite, and argillaceous siltite. Many beds are graded, 

with 1–2 cm thick layers of siltite grading upward into argillite. 

The unit locally contains matrix-supported granule- to pebblesize argillite clasts. Rippled beds within the fine siltite unit are 

attributed to reworking by bottom currents. Graded-stratified siltite is common, forming bands of coarse-grained siltite to finegrained metasandstone that are of finest grain size in the upper 

part; uppermost parts of some bands grade to fine-grained siltite, 

but argillite is absent. These strata fit the Bouma (1962) 

sequence of graded turbidite beds (Tysdal, 2000b).

Diamictite Unit

Poorly sorted, matrix-supported conglomerate—diamictite—characterizes a succession of otherwise fine-grained strata 

within the Apple Creek Formation of the Poison Creek thrust 

sheet. Most of the diamictite beds are composed of disorganized 

matrix-supported clasts; they are interpreted as turbidites or 

debris flows. Argillite, argillaceous siltite, siltite, and lesser 

metasandstone form successions as much as 10 m or more thick 

between the diamictite beds. Grading is obvious where deformation is not intense, and Bouma sequences are most prominent 

in the upper part of the unit. Paleocurrent directions determined 

from current ripples reveal transport from both the northeast and 

southwest. These fine-grained (non-conglomerate) strata are 

attributed to a turbidite origin (Tysdal, 2000b).

Coarse Siltite Unit

The coarse siltite unit is composed of gray-green mediumto coarse-grained siltite and fine-grained metasandstone. Siltite 

beds observed are normally graded. Soft-sediment deformation 

structures are widespread: ball-and-pillow structures are fairly 

common; flute casts and tool marks were observed only locally. 

The unit derives its name from distinctive graded beds of sharpbased light-gray, quartz-rich, fine-grained metasandstone interspersed within the sequence. The graded bedding of the siltite 

and the sharp-based light-gray metasandstone beds suggest deposition from turbidity currents (Tysdal, 2000b).

Salmon River Mountains

General Setting of Banded Siltite Unit of Apple Creek Formation

Figure 4 shows the general southwest-northeast interpreted 

facies relationships for the banded siltite unit of the Apple Creek 

Formation for the area of the Salmon River Mountains shown in 

figure 3. The banded siltite unit includes turbidites (lower strata 

of unit) and current-reworked turbidites (upper strata of unit). 

The directly overlying Gunsight Formation consists of hummocky crossbedded storm deposits, shoreface strata, and fluvial 

rocks. The fluvial strata are thought to be separated from the 

marine strata by an erosion surface that cuts downward stratigraphically toward the west. Fluvial rocks lie on the shoreface 

strata in the eastern part of the Salmon River Mountains and on 

storm-reworked turbidites on the west (fig. 3, Deep Creek road 

area).

The lithofacies of the banded siltite unit of the Apple Creek 

Formation are described and interpreted in this section. Hummocky, shoreface, and fluvial lithofacies are described and interpreted in the section on the Gunsight Formation.

Banded Siltite Unit

The banded siltite unit of the Apple Creek Formation was 

examined in some detail by Sobel (1982) during a thesis study of 

a 10 km2 area in the vicinity of the Blackbird mine (fig. 2), which 

hosts most of the stratabound cobalt- and gold-bearing deposits 

of the district. Sobel’s (1982) study of the banded siltite unit was 

part of extensive work by NORANDA, Inc., during mineral 

exploration and mining of cobalt. The mineral deposits are not a 

part of the present report, but they are mentioned because my 

findings herein bear on the stratigraphic and structural setting of 

the deposits. 

Sobel (1982) recognized three distinct facies, in ascending 

order: graded siltite, rippled “quartzite,” and laminated “quartzite” for the banded siltite unit. Upper strata of his sequence 

include rocks that Evans and Connor (1993) placed in the upper 

part of the banded siltite unit and lower part of the overlying 

Gunsight Formation (upper part of the “middle subunit of the 

Yellowjacket Formation” and lower strata of the “upper Yellowjacket,” respectively, of their nomenclature). Sobel’s (1982) 

lower two lithofacies are discussed next, in “Lower Strata of the 

Banded Siltite Unit.” Strata of his uppermost unit, the laminated 

quartzite facies, are similar to rocks that I describe in the section 

entitled, “Upper Strata of the Banded Siltite Unit.”

Lower Strata of the Banded Siltite Unit

In his study of rocks in the vicinity of Blackbird Creek, 

Sobel (1982, p. 112, 117) described a stratigraphic succession 

that displays an upward increase in grain size, bed thickness, and 

abundance of sandy beds. His graded siltite and overlying rippled “quartzite” lithofacies contain sharp-based, normally 

graded beds of incomplete Bouma (1962) Tbc, Tcde, and Tde

sequences; no complete Ta through Te sequences were observed. 

The graded siltite lithofacies consists of thin beds that grade 

upward from fine-grained metasandstone to siltite. Other features include ripple crosslaminae and soft-sediment deformation 

features—water-escape structures, intrastratal slides, and convoluted laminae.

The rippled quartzite lithofacies consists of thin- to 

medium-bedded metasandstone that contains ubiquitous 
Correlation, Sedimentology, Structural Setting 7

crosslamination and rippled bedding surfaces, and thin interbeds 

of graded siltite or siltite to argillite. Less common features 

include rip-up clasts, scours, sole marks, and horizontal lamination. Soft-sediment deformation structures include “crumpled 

mud cracks,” water-escape structures, wavy and convolute laminae, load structures, and microfaults. In many places, metasandstone beds terminate abruptly against strata deformed during 

soft-sediment deformation (Sobel, 1982).

Sobel (1982) interpreted the strata to be deposits of a submarine fan system, formed of distal turbidites of a basin plain, 

outer fan and interlobe deposits (fan fringe), and mid-fan (suprafan lobes). The intrastratal folding and deformed shrinkage 

cracks were interpreted as deposits formed from gravity sliding 

on a slope. Vertical and horizontal water-escape features of the 

rippled quartzite lithofacies were interpreted to indicate gravity 

sliding and compaction deformation during sedimentation. Sand 

layers commonly were caught between sliding sediment masses, 

resulting in crumpled, contorted, disrupted beds (Sobel, 1982, p. 

144–157).

Many of the sedimentary structures that Sobel (1982) 

described in his study area are widespread in the banded siltite 

unit in the Salmon River Mountains. Some of these—creep and 

slump deposits, “shrinkage cracks,” and dikelets (fig. 6)—are 

genetically related to one another and are examined in this report 

under the heading, “Soft-Sediment Deformation Features.” 

Reconnaissance shows that other rock types, in addition to those 

described by Sobel (1982), occur outside his study area; they are 

described generally in following paragraphs.

Finely Laminated Strata

Finely laminated strata (fig. 5) form sequences 1–10 m or 

more thick and are interlayered with coarser turbidites—coarsegrained siltite to locally fine grained metasandstone—throughout most of the lower strata of the banded siltite unit. The laminae are sharply defined and commonly are 0.3–2 mm thick, 

locally 10 mm thick. Thicker laminae grade from coarse to 

medium silt and to clay-size. Thinner laminae also are graded, 

based on color change of darkening upward. The silt laminae 

are discontinuous, tapering out laterally, and display sharp bases. 

Fluid-escape structures were observed in a few of the thicker 

layers; overlying layers show no deformation. Other, less common features include 1°–2° angular relationships where one 

lamina oversteps underlying laminae; micro-crosslaminae, with 

erosion into underlying laminae; boudinage structures that are 

several centimeters across; micro-load structures, 1–5 mm in 

diameter; and convolute laminae. Laminae are normally graded: 

Current-reworked, soft-sediment deformed turbidites (Yab)

Hummocky crossbedded strata (Yg) 

Shoreface beds (Yg)

Alluvial deposits (Yg)

Turbidites (Yab)

SOUTHWEST

NORTHEAST

OCEAN

 GUNSIGHT

FORMATION

 (Yg)

 APPLE CREEK

 FORMATION

(banded siltite unit)

 (Yab)

Erosion surface

SHORE-NORMAL

SHORE-PARALLEL

D Shoreface beds (Yg)

L 

D - Deep Creek road area

L - Lake Mountain area

Figure 4. Diagrammatic illustration of general facies relationships interpreted for strata within the banded siltite 

unit of the Apple Creek Formation and the overlying Gunsight Formation in the eastern part of the Salmon River 

Mountains. The figure illustrates the relationships for strata southwest of Poison Creek thrust fault as shown in figure 3—that is, on Poison Creek thrust sheet. Shore-normal section shows ocean position during deposition of units. 

No relative thickness of units is implied.
8 Mesoproterozoic Lemhi Group, Central Idaho

no reverse grading was observed. Some single laminae, or several laminae together, thicken and thin laterally over 5± cm; laminae are thin in troughs and thick over what may be ripple crests.

The sharply defined laminae are interpreted as products of 

suspension sedimentation and correspond to the Bouma Td division and the T2 to T4 intervals2 of Stow and Shanmugam (1980). 

The abrupt, intermittent occurrence of the sequences of finely 

laminated strata within coarser grained turbidites suggests a periodically operating process of deposition. The sequences may 

have been deposited from waning, dilute turbidity currents in a 

slope (fan) environment. Alternatively, the finely laminated turbidites may have formed of sediment spilled over a levee that 

flanked a submarine channel down which turbidity currents 

flowed. Spillover levee deposits contain only the finer grained 

sediment of channelized turbidites (Stacey and Bowen, 1988). 

Sediment cored from modern levee deposits of the Amazon 

Channel off Brazil, for example, contains turbidites composed of 

fine silt and clay. From photographs of thin sections (Hiscott and 

others, 1997, fig. 14, p. 72), these turbidites appear to be similar 

to the finely laminated rocks observed near Woodtick Creek.

Soft-Sediment Deformation Features

The lower strata of the banded siltite unit contain several 

sedimentary features that are widespread in the unit and that 

have been examined in some detail by several authors. Sobel 

(1982) described creep and slump structures, and shrinkage 

cracks. Evans (1981) and Connor and Evans (1986) described 

dikelets. Each of these features is described separately, citing 

the appropriate references. From observations throughout the 

banded siltite unit, Tysdal (unpub. data, 1998) determined that 

the three kinds of features are genetically related to one another. 

Dikelets formed during soft-sediment deformation. The “shrinkage cracks” described and pictured by Sobel (1982, his figs. 

33A, 34A) were produced during the same soft-sediment deformation process.

Creep and Slump Structures. Soft-sediment deformation 

structures occur in zones that are sandwiched between undeformed strata, are only a few layers thick, and commonly are disorganized within the deformed beds (Sobel, 1982, p. 43). The 

zones contain wavy and contorted laminae, flame structures, 

convolute bedding, interstratified folds, and crumpled beds that 

appear to have been caught between sliding masses of sediment. 

Crosslamination commonly is deformed. Sobel (1982, p. 46) 

interpreted the deformation to involve rapid sedimentation of 

waterlogged sediment, and his discussion indicates that he 

favored gravity-driven sliding as the cause of the soft-sediment 

deformation.

Shrinkage Cracks. Metasandstone-filled shrinkage cracks 

occur in 1–5 mm thick argillite (siltite) beds. Sobel (1982) 

stated that they display four characteristics: (1) cracks are 

spindle shaped or sinuous; no polygons were observed; (2) 

cracks commonly are associated with ripples, and crosslaminae 

occur in ripple troughs; (3) short cracks are oriented parallel or 

subparallel to one another; and (4) only one generation of cracks 

is present. Stating that the cracks are not like those produced by 

subaerial exposure, he attributed them to syneresis, the subaqueous dewatering of muddy sediment (Sobel, 1982, p. 123). 

Dikelets. The term “dikelets” was introduced by Connor 

and Evans (1986), following recognition of the features by 

Evans (1981), to refer to small, tabular dikes of siltite that generally range from about 5 to10 mm high and from 2 to 5 mm wide, 

and which project downward from the base of beds (fig. 6). 

Evans (1981) called them “***characteristic downward-penetrating, silt-filled ‘dikelets’ of unknown origin.” Their 

depositional environment also was of uncertain origin, but the 

dikelets were considered critical to definition of the banded siltite unit (the “middle subunit of the Yellowjacket Formation” of 

Connor and Evans, 1986). I concluded (Tysdal, unpub. data, 

1998) that the dikelets are not primary depositional structures 

but were produced during soft-sediment deformation. The 

deformation is related to slow mass-movement downslope—

creep—of water-rich sediment.

Upper Strata of the Banded Siltite Unit

Upper strata of the banded siltite unit are well exposed in 

cuts along the Deep Creek road (fig. 3), and along a logging road 

Figure 5. Finely laminated strata in 

banded siltite unit of Apple Creek Formation. Woodtick Creek (fig. 1) in the Salmon 

River Mountains. Pencil segment 5 cm 

long.

2

The classification of Stow and Shanmugam (1980) is for fine-grained 

turbidites. Subdivisions used in the classification range from T0 to T8 and 

correspond approximately to the Tc through Te subdivisions of the Bouma (1962) 

classification. The T2 to T4 intervals correspond approximately to the upper half of 

the Td and lower half of the Te intervals of Bouma (1962). 
Correlation, Sedimentology, Structural Setting 9

metasandstone. Siltite and argillite make up the rest. Other 

features include ripple crosslaminae, local climbing ripples, 

and small channels. He noted that upper surfaces of the crosslaminated siltite and metasandstone beds commonly are eroded 

and may have been reworked by bottom currents. My observations along Deep Creek show that the ripple crosslaminated 

beds contain both complete ripples and ripples with eroded 

crests. Eroded crests have been interpreted as products either 

of bottom currents (Shanmugam and others, 1993; Shanmugam, 1997) or of normal turbidity current processes (Stow 

and others, 1998).

Soft-sediment deformation features are abundant in the 

upper strata of the banded siltite unit in the Deep Creek area 

(figs. 7, 8), a significant difference from their absence in the 

laminated quartzite strata of Sobel’s (1982) small study area 

(fig. 1). Thin, laterally discontinuous beds of light-gray 

metasandstone (or coarse siltite) are interlayered with dark-gray 

fine siltite beds of variable thickness. The thinned and tapered 

beds and the variably thick siltite are here interpreted to indicate flowage of water from water-rich silt. An “extreme” case 

of soft-sediment deformation is shown in figure 9, where a 

metasandstone bed is “broken”—pulled apart. The sand bed 

likely was deposited on top of water-saturated silt and the loading caused the water to flow, rupturing the sand bed.

Folds here attributed to soft-sediment slumping occur 

locally within the upper strata of the banded siltite. Folds as 

much as 30 cm high are underlain and overlain by beds of 

unfolded strata. Dikelets also are present locally within the softsediment deformed rocks and are associated with flowage of 

water-saturated sediment (fig. 10).

The upper strata of the banded siltite unit are here interpreted as turbidites that have been disturbed—partly reworked 

by storm-generated currents and deformed to some extent during adjustment to the high water content of the sediments at the 

time of deposition. Upper beds are interbedded with lower 

rocks of the overlying hummocky strata, interpreted as storm 

deposits, of the Gunsight Formation. They probably were 

deposited on a submarine slope or a fan. The graded beds interlayered in the upper strata of the banded siltite unit display 

Bouma Tbcd features that could be either turbidites or stormgenerated deposits. If turbidites, then some or many of the 

light-gray metasandstone to coarse-grained siltite beds that display reworked upper parts may be turbidites reworked by 

storm-generated currents. Each overlying sharp-based darkgray siltite bed then could be composed of suspension sediments precipitated from a waning storm current. Sobel (1982) 

stated that the vertical bed sequences diagnostic of turbidity 

currents are lacking in his study area, but he did recognize 

current-reworking of beds.

The abundant soft-sediment deformation features of the 

upper strata of the banded siltite unit indicate a high water content for the sediments, particularly the silt-rich strata. Adjustment to the high water content produced such deformation 

features as water-escape structures; metasandstone beds that 

taper normal to the direction of transport (southwest), or are 

abruptly truncated where water-saturated silt flowed across (as 

seen in cross section) “broken” metasandstone beds; local 

slump folds; and local dikelets. The major adjustments took 

west of Deep Creek just below its junction with Pepper Creek 

(this road is not shown in fig. 3). The strata typically form 5–10 

cm thick beds of light-gray fine-grained metasandstone to 

coarse-grained siltite and interlayered dark-gray siltite (metamudstone) or, locally, argillite (fig. 7).

Two prominent characteristics are recorded in the upper 

strata: eroded upper surfaces of metasandstone beds and softsediment deformation features. The lower surfaces of the lightgray beds commonly display sharp contacts. The beds generally 

are planar laminated, but some display planar laminae that grade 

upward into crosslaminae, then into dark-gray siltite—a Bouma 

Tbcd sequence. In other cases, the upper surface of the 

metasandstone beds clearly is reworked, erosional: the upper 

surface of crossbeds is overlain by laminated dark-gray siltite 

deposited across truncated crosslaminae (fig. 8). In still other 

strata, the dark-gray siltite fills, or partly fills, irregularities (such 

as ripple troughs) of the underlying sediment, whereas the upper 

surface of the siltite “fill” is planar, flat laminated. Normal to 

transport direction, the base of metasandstone beds ranges from 

parallel to the underlying siltite to moderate or steep dip of ripple-foreset laminae. In contrast to most of the strata of the 

banded siltite unit, the upper strata generally lack the small (1–5 

mm) load structures, dikelets, and the “syneresis cracks” associated with creep or slump folds. Small-scale slump folds are 

fairly common, however.

As discussed in the section entitled, “Lower Strata of the 

Banded Siltite Unit,” Sobel (1982) recognized three distinct 

lithofacies in his map area (shown in fig. 2). The lower two 

lithofacies—graded siltite and rippled quartzite—were discussed 

in that section. His upper lithofacies—laminated “quartzite”—is 

discussed here because its sedimentary features are similar to 

those described in the previous paragraph. Sobel (1982) 

described his laminated quartzite lithofacies as consisting of 

about 90 percent metasandstone and argillaceous 

Figure 6. Dikelets in banded siltite unit of Apple Creek Formation, 

from Swan Peak (fig. 3). Pencil 13 cm long.

Dikelet Dikelet
10 Mesoproterozoic Lemhi Group, Central Idaho

Some beds along the Deep Creek road display 1–5 mm 

diameter light-gray spots, which appeared as if they may be 

clots of scapolite; samples of the material were collected from 

three outcrops. Two samples are from the uppermost part of 

the banded siltite unit of the Apple Creek Formation, and one 

is from basal strata of the overlying Gunsight Formation. Xray examination of these spotted rocks showed only a trace of 

scapolite; the chief minerals in the two rocks are quartz+phlogopite+albite + potassium feldspar (G.A. Desborough, written commun., 1997; oral commun., 1998). The spots may be 

due to contact metamorphism related to concealed igneous 

rocks. 

The scapolite-rich beds were interpreted, with some question, as metamorphosed volcanogenic material by Connor and 

Evans (1986). This interpretation is consistent with an exhalative process of cobalt mineralization in the Blackbird mining 

district (Hughes, 1983; Hahn and Hughes, 1984; Nash, 1989; 

Nash and Hahn, 1989), and the proposal of Tysdal and Desborough (1997) that the Na- and Cl-rich biotites of the district 

(composition reported by Nash and Connor, 1993) reflect fluid 

movement upward through evaporite rocks of the Yellowjacket 

Formation. The volcanogenic concept does not affect the storm 

deposit interpretation for the non-scapolitic sedimentary rocks 

that make up most of the sequence. 

An alternative explanation for the origin of the scapoliterich beds is that the trace scapolite may be metamorphosed 

NaCl salt crystals. Formation of the crystals requires an evaporite setting wherein NaCl reaches supersaturation within the 

water column or sediment, and is deposited, for example, within 

the upper part of an intertidal to supratidal zone. This 

place during sedimentation or shortly thereafter, as shown by 

deformation structures that are overlapped by undeformed strata.

Scapolite-Rich Beds

Scapolite-rich beds were reported by Connor and Evans 

(1986) in the upper strata of the banded siltite unit of the Apple 

Creek Formation (that is, their “middle subunit of the Yellowjacket Formation”) along the road adjacent to Deep Creek (figs. 

2, 3). They recorded no scapolite from lower in the unit, and I 

also found no scapolite in the lower strata.

The scapolite occurs in the dark-gray siltite that is interbedded with light-gray coarse-grained siltite to fine-grained 

metasandstone of the banded siltite unit. The scapolite porphyroblasts commonly range from 3 to 10 mm in diameter. A few 

beds show a grading in porphyroblast size, from larger diameter 

at the base to smaller at the top. One bed shows smaller diameter porphyroblasts at the base compared to larger at the top. Still 

other scapolitic beds show porphyroblasts of uniform size 

throughout a bed, or of different sizes but with no systematic size 

changes. The scapolitic dark-gray siltite beds do not display 

sedimentary structures that differ from those of the nonscapolitic dark-gray siltite. No scapolite was observed in the 

directly adjacent light-gray clastic beds. I did not find a change 

in sedimentary structures above and (or) below the scapolitic 

beds that would suggest a change in the depositional environment. The scapolitic beds do not appear to display a systematic 

or rhythmic pattern of occurrence within the upper part of the 

banded siltite unit.

Figure 7. Typical outcrop of upper strata of banded siltite unit of Apple Creek Formation, showing interlayered 

beds of light-gray metasandstone and dark-gray siltite. Note laterally discontinuous metasandstone beds, 

some tapering and others abruptly truncated. Irregular thickness of metasandstone and siltite beds is interpreted to indicate soft-sediment deformation, adjustment of strata to escape of water from water-saturated silt during and shortly after deposition of strata. Deep Creek road area of Salmon River Mountains. Hammer head 20 

cm long.
Correlation, Sedimentology, Structural Setting 11

explanation is discounted because the host banded siltite unit is 

believed to represent offshore storm deposits; nevertheless, several arguments are presented to discount the interpretation. The 

dark-gray siltite beds, and the interbedded coarse siltite to finegrained metasandstone beds, do not contain textures of evaporites. The beds lack mudcracks, ripples, small-scale convolute 

lamination, rip-up clasts, and other features suggestive of a tidal 

flat environment. The dark-gray scapolitic and non-scapolitic siltites do not display different sedimentary structures, as if the beds 

had originated from different processes. The dark-gray siltites 

are interpreted as suspension deposits, but the scapolitic and nonscapolitic rocks may have resulted from different original 

sources—the scapolitic rocks from an exhalative source, the nonscapolitic siltites from storm-derived silt. The light-gray clastics 

do not contain scapolite. But if the scapolite formed during 

metamorphism of NaCl precipitated from evaporite fluids, then I 

would expect both the light-gray clastics and the directly overlying dark-gray siltite to contain scapolite. The porous light-gray 

clastics should have contained the evaporite NaCl-bearing precursor of the scapolite just as readily as the directly overlying 

dark-gray siltites.

Correlation. The scapolite-bearing rocks of the Deep Creek 

road area are interpreted as metamorphic products of sediments 

deposited in an environment different from that which existed in 

the area of the principal reference section of the Yellowjacket 

Formation. I examined outcrops along the Deep Creek road to 

determine if these strata contained marble (metalimestone) and 

interbedded sodium-rich scapolitic beds like those of the 

Figure 8. Detailed view of upper strata of banded siltite unit of Apple Creek Formation. Note 

the following: Loc. A—metasandstone deformed into “syncline,” and overlain by undeformed 

siltite and metasandstone. Loc. B—crosslaminae of metasandstone bed show angular discordance with directly overlying metasandstone and siltite. Metasandstone beds 1 and 2 

show minor disturbance, but strata between the two beds show significant disruption, indicating that the soft-sediment deformation ± erosional history of the strata between beds 1 and 2 

differs. Deep Creek road area of Salmon River Mountains. Thickness of strata in photograph 

about 50 cm. 

A

B

1

2
12 Mesoproterozoic Lemhi Group, Central Idaho

Yellowjacket Formation in the vicinity of the principal reference 

section3 near the townsite of Yellowjacket, and along Moyer 

Creek (fig. 1), interpreted as metaevaporites by Tysdal and Desborough (1997). No marble or sodium-rich scapolitic beds were 

found, nor do the Deep Creek strata lie within a depositional 

sequence like that of the Yellowjacket. The Deep Creek scapolitic strata occur within storm-generated rocks of the Apple 

Creek Formation and underlie fluvial rocks of the Gunsight Formation. This contrasts with the metaevaporites of the Yellowjacket, which lie within intertidal to supratidal strata and are 

overlain conformably by quartzites of the Hoodoo Formation, 

interpreted as tidal deposits by Tysdal and Desborough (1997).

Gunsight Formation

Lemhi Range

The Gunsight Formation was named by Ruppel (1975) for a 

sequence of strata in the vicinity of Gunsight Peak in the central 

part of the Lemhi Range (fig. 1). The type locality extends north 

from near Yellow Lake to Gunsight Peak (Ruppel, 1975). 

McBean (1983) examined the rocks of the type area as part of a 

thesis study of the Gunsight Formation. The following summary 

is from Tysdal (2000b) and is based on reconnaissance traverses 

through the type section. The Gunsight strata in the Lemhi 

Range are chiefly of fluvial origin. Trough crossbedded 

metasandstones form units 1–2 m thick that fine upward and give 

way to siltite of overbank deposits. Uppermost strata of the formation lack fine-grained beds of siltite and primarily are 

medium- to coarse-grained quartzite and metasandstone of 

quartz and feldspar. Trough crossbedding is common. These 

uppermost strata could be shoreface deposits transitional upward 

from fluvial strata of the Gunsight into the marine orthoquartzite 

of the overlying Swauger Formation.

Salmon River Mountains

Strata that I here place in the Gunsight Formation previously were mapped as the “upper subunit of the Yellowjacket 

Formation” by Connor and Evans (1986) in their study of the 

Leesburg 15' quadrangle (lat 45° to 45°15' N., long 114° to 

114°15' W.) (in fig. 3, entire area north of lat 45° lies within the 

Leesburg 15' quadrangle). These workers divided their subunit 

into an upper and a lower part, both of which here are reassigned 

to the Gunsight Formation (table 1). The upper part of their 

upper subunit was described as flat-laminated and hummocky 

crosslaminated arkosic quartzite. The lower part of their upper 

subunit was described as interbedded flat-laminated arkosic 

quartzite and micaceous siltite. My observations show fluvial 

deposits on the northwest, in the vicinity of Deep Creek Ridge 

(fig. 3), and a genetically associated sequence of hummocky 

beds, shoreface strata, and lenses of conglomerate to the southeast, in the vicinity of Lake Mountain. Each of these genetically 

associated units is described separately, then integrated into a 

Figure 9. Light-gray metasandstone bed cut by dark-gray siltite during soft-sediment deformation. Structure interpreted to reflect loading of sandstone bed onto water-saturated silt, 

causing flowage of silt and disruption of the sandstone bed. Note soft-sediment deformation 

of strata below disrupted bed and unbroken metasandstone bed above it. Lake Mountain 

area of Salmon River Mountains. Pencil 13 cm long.

3

No type section exists. Naming of the Yellowjacket Formation preceded the 

requirement to designate a type section according to the guidelines of the North 

American Stratigraphic Code (North American Commission on Statigraphic 

Nomenclature, 1983). A section measured by Ross (1934), who named the 

formation for rocks in the vicinity of the townsite of Yellowjacket (fig. 1), was 

designated the principal reference section by Ekren (1988).
Correlation, Sedimentology, Structural Setting 13

(metamudstone) lenses occur between beds. Currents have 

eroded the upper part of some hummocky beds. Ripples are 

fairly common features in the uppermost part of some beds. 

Stacked 1–3 cm thick sets of ripples were observed above one 

eroded hummocky bed. The upper part of each ripple set was 

eroded before deposition of 1–3 mm of dark-gray siltite, and the 

overlying ripple set. The ripples generally show transport to the 

northeast, but a few ripples show transport to the southwest. 

Pebble lags are present along the base of a few hummocky beds 

and along foreset laminae of other beds. These features have 

been reported within hummocky beds by McKie (1994).

The hummocky crossbedded strata are conformable and 

transitional with the upper strata of the underlying banded siltite 

unit of the Apple Creek Formation. The prominent soft-sediment load and slump structures of the upper strata of the banded 

siltite also occur in the lower strata of the hummocky crossbedded unit. This is illustrated in figure 12 where a small hummocky crossbed overlies a succession of slumped beds of finegrained metasandstone. The slumping, or at least most of it, 

took place before deposition of the overlying hummocky crossbedded metasandstone. Figure 13 shows a hummocky crossbedded metasandstone bed interpreted to have been deposited on a 

water-rich silt (now siltite) from which the water flowed and disrupted—pulled apart—the bed of metasandstone. 

Two alternatives for the stratigraphic level of hummocky 

strata in the Gunsight Formation were considered. Because fluvial Gunsight strata grade upward and downward into marine 

strata, hummocky (marine) Gunsight strata might be expected in 

either the upper or lower parts of the Gunsight Formation. In the 

lower part of the hummocky sequence, turbidites and local 

slump structures are interbedded with the hummocky beds. The 

contact of hummocky crossbedded strata with the underlying 

turbidites of the banded siltite unit of the Apple Creek Formation 

general model for deposition. The fluvial strata are described in 

a following section.

The hummocky crossbedded rocks and associated 

metasandstone contrast sharply with the dominantly fine siltite 

and banded black-and-white beds of the underlying banded 

siltite unit of the Apple Creek Formation. I follow Connor and 

Evans (1986) in assigning the hummocky and associated strata 

to the post-Apple Creek strata (that is, post “middle subunit of 

the Yellowjacket Formation” of their nomenclature). But 

because the hummocky strata are gradational with the banded 

siltite unit, I placed them in the lowermost part of the Gunsight 

Formation, in contrast to Connor and Evans’ (1986) placement 

of them in the upper part. The following discussion of stratigraphic and structural relationships shows my reasoning.

Stratigraphic Relationships

Hummocky Crossbedded Strata

Hummocky crossbedded strata (fig. 11) are present in the 

vicinity of Lake Mountain (figs. 2, 3). Typical features of hummocky cross-stratification include the following: (1) crossbed 

sets display lower bounding surfaces that are erosional, commonly sloping at angles of less than 15°; (2) overlying laminae 

are parallel to set boundaries, or nearly so; (3) laminae above the 

erosional base systematically thicken laterally in a set, such that 

their traces on a vertical surface are fan-like; and (4) the directions of dip of erosional set boundaries are scattered (Harms and 

others, 1975; Cheel and Leckie, 1993). The beds, composed of 

an amalgamation of hummocks, typically are separated by 1–5 

mm thick layers of dark-gray siltite. In a few places, siltite 

Figure 10. Dikelets associated with light-gray metasandstone bed cut by dark-gray siltite during 

soft-sediment deformation. Structures interpreted to reflect loading of sandstone bed onto watersaturated silt, causing flowage of silt and thin interbeds of light-gray fine-grained sandstone that 

formed dikelets. Deep Creek area of Salmon River Mountains. Pencil 13 cm long.
14 Mesoproterozoic Lemhi Group, Central Idaho

is gradational and conformable. This indicates that the hummocky strata occur in the lower part of the Gunsight Formation.

I know of no other occurrence of hummocky crossbedded 

strata in the Salmon River Mountains or Lemhi Range (or Beaverhead Mountains or Clearwater Mountains). The upper part 

of the Gunsight in the type section near Gunsight Peak in the 

Lemhi Range (fig. 1), and in the vicinity of Phelan Mountain 10 

km directly north of Lake Mountain (fig. 3), is a medium- to 

coarse-grained quartzite to orthoquartzite sequence that contains trough crossbeds as much as 1 m across. I observed no 

hummocky beds in either area, however, and none were 

reported by McBean (1983) in the type section. In contrast, the 

hummocky and associated strata of the Lake Mountain area are 

not quartzite or orthoquartzite, but fine-grained arkosic 

metasandstone to coarse-grained siltite. In the western part of 

the Lemhi Range, in the Poison Peak area and in the type section near Gunsight Peak, fluvial strata occur downsection from 

the quartzite and orthoquartzite of the upper strata of the Gunsight. These stratigraphic and sedimentologic data argue 

against the occurrence of hummocky strata in the upper part of 

the Gunsight Formation.

Shoreface Strata4

North of Lake Mountain (fig. 3), coarse siltite to finegrained hummocky metasandstone beds give way upsection to 

rocks of similar composition that lack hummocks. The strata 

are mainly composed of fairly clean quartz-feldspar metasandstone of uniformly fine grain size. Beds are 0.5–1 m thick. 

Many beds are characterized by planar lamination. Others display moderate to steeply dipping foreset laminae of megaripples 

(subaqueous dunes); some of the laminae dip to the northeast. 

Beds in some areas pinch and swell (are undulatory) for distances of 1–5 m. Dark-gray fine-grained siltite (metamudstone), in lenses 20–100 cm thick and as much as 1 m long, lie 

between metasandstone beds in a few places. Mud chips occur 

along crossbed laminae locally. Lenses of angular pebbly mudstones are present, but not common, within the metasandstone. 

Rare, local slumped beds of metasandstone show transport to 

the southwest. The shoreface strata contain interstratified amalgamated hummocky beds in a few places on and north of Lake 

Mountain, indicating intertonguing relationship of the two lithofacies. Some of the shoreface deposits may be reworked hummocky strata.

Intraformational Conglomerate

Intraformational conglomerate was reported at three localities in the Lake Mountain area by Connor and Evans (1986), 

and I observed a fourth outcrop about 2.5 km farther northeast 

(fig. 3). J.J. Connor (oral commun., 1987) described the conglomerates as channel-like deposits that contain angular and 

rounded clasts of metasandstone similar to that of the country 

rock, streaked-out mudstone clasts, and local flow-rolls (balland-pillow structures) (fig. 14) as much as 1 m across. The 

matrix of the conglomerates is dark-gray fine-grained siltite, 

4Definition of shoreface used here follows that of Walker (1992), wherein 

shoreface sediment is deposited above fair-weather wave base to high tide line. 

Hummocky crossbedded sediment is deposited below fair-weather wave base. 

(In contrast, some workers include hummocky crossbedded sediment in the lower 

shoreface, even though the sediment is deposited below normal wave base.)

Figure 11. Hummocky crossbeds at Lake Mountain in Salmon River Mountains. Hammer head 20 cm long.
Correlation, Sedimentology, Structural Setting 15

which is well cleaved. The northern occurrence contains fewer 

and smaller clasts, and more matrix, than the other outcrops. 

The deposits have a cross-sectional width of only 10–15 m and 

are 10–15 m thick. The conglomerate about 1.5 km south of 

Lake Mountain occurs within a weathered-in-place rubble pile 

of hummocky crossbedded strata. The three occurrences northeast of Lake Mountain form resistant outcrops isolated within 

covered areas. The nearest host rocks are not hummocky beds, 

but shoreface strata.

J.J. Connor (oral commun., 1987) considered a possible fluvial origin for the conglomerates, but he favored a marine setting 

because of the associated hummocky strata. I concur with his 

interpretation, but note that the three northern occurrences are 

associated with the shoreface deposits. The channel interpretation of Connor and Evans (1986) is appropriate because of the 

limited cross-sectional size and the fine siltite matrix of the 

deposits. The soft-sediment deformation structures attest to the 

contemporaneity of the channels and the host sediment. The 

conglomerates may be channel-bottom deposits, formed of 

poorly sorted clastics transported seaward during storms or during strong tidal action.

Origin

Localization of the hummocky and associated strata to the 

Lake Mountain area of the Salmon River Mountains must be due 

to local depositional parameters or to erosion that could have 

removed such deposits from elsewhere in the region. Hummocky crossbeds generally are interpreted as storm features that 

accumulated below fair-weather wave base (Harms and others, 

1975; Walker and Plint, 1992; Cheel and Leckie, 1993). Preservation of the hummocky strata suggests this as an appropriate 

interpretation for the Lake Mountain area. Examination of the 

hummocky and associated strata was conducted in a reconnaissance manner to determine a plausible interpretation that accommodates the general features observed and a likely reason for 

generation and preservation of the strata. No attempt was made 

to work out details of the depositional environment.

Walker and Plint (1992) noted that most studies of storm 

deposits have been conducted on strata deposited in the Cretaceous Western Interior seaway of North America; a few studies 

are set elsewhere. In these settings, sand bodies that display 

hummocky crossbeds, and their commonly overlying swaley 

crossbedded strata are sheetlike, extend for many hundreds of 

kilometers along depositional strike, may prograde over 300 km 

seaward, and are wave dominated. (Swaley crossbeds are shallow swales—scours—0.5–2 m wide that are filled with concaveupward laminae of sand. The infilling laminae conform to the 

shape of the swale, gradually flattening out upward.) No swaley 

crossbedded strata were observed in the Mesoproterozoic rocks 

of central Idaho. The Cretaceous strandline-generated sheetlike 

sandstone bodies reflect microtidal environments, which are 

wave dominated. Strand plains and long, narrow barrier islands 

are characteristic of such settings. Mesotidal settings also contain barrier islands, although they tend to be short and fairly 

wide, owing to interruption by frequent tidal channels and associated ebb- or flood-tidal deltas. The Lake Mountain area contains the only known occurrence of hummocky and associated 

strata in the Mesoproterozoic strata of central Idaho. No sheetlike sandstone bodies, interpreted to be strand-plain or barrier 

island deposits, are known to be present in the Gunsight Formation of the Lemhi Range or the Salmon River Mountains (or 

Beaverhead Mountains). 

The abundance of the hummocky beds at Lake Mountain 

indicates that the area south of Lake Mountain represents the 

seaward part of the depositional environment, below normal 

wave base, otherwise the hummocky beds would not be 

Figure 12. Slump structure overlain by non-slumped light-gray hummocky crossbedded metasandstone (bed just above point of hammer) and other metasandstone beds. Lower part of hummocky 

crossbedded metasandstone unit, Lake Mountain area of Salmon River Mountains. Hammer handle 

22 cm long.
16 Mesoproterozoic Lemhi Group, Central Idaho

Figure 13. Light-gray hummocky crossbedded metasandstone bed deposited on dark-gray siltite, 

interpreted as water-saturated at time of deposition. Loading of sandstone onto water-saturated silt 

caused flowage of silt and disruption of sandstone. Lake Mountain area of Salmon River Mountains. 

Hammer handle 22 cm long.

Figure 14. Flow-rolls in siltite, interpreted as channel deposit associated with the hummocky crossbedded metasandstone unit. Float block, Lake Mountain area of Salmon River 

Mountains. Hammer handle 22 cm long.
Correlation, Sedimentology, Structural Setting 17

preserved. Thus, the directly overlying strata could represent 

reworked hummocky beds. The small conglomerate-bearing 

channels only a few meters across that lie within the sequence 

may have served as conduits to transport fine sediment to offshore, deeper waters. 

Erosion Surface

The stratigraphic sequence in the Lake Mountain area differs from that in the Deep Creek road area (figs. 3, 4). In the 

Lake Mountain area, the ascending stratigraphic sequence is turbidites—storm reworked deposits—hummocky beds—shoreface 

deposits—erosion surface(?)—fluvial rocks. In the Deep Creek 

road area, the ascending sequence is turbidites—storm-reworked 

deposits—erosion surface—fluvial rocks; no hummocky beds or 

shoreface strata are present.

In a complete regressive sequence, shoreface and beach or 

tidal-flat strata overlie hummocky strata in depositional transition into delta plain fluvial strata. The shoreface beds north of 

Lake Mountain are interpreted as erosionally truncated, a common occurrence within stratigraphic sections (Miall, 1996, p. 

463), wherein upper shoreface and tidal-flat strata are eroded and 

fluvial rocks of the delta plain lie directly on lower shoreface or 

offshore strata.

Along the Deep Creek road (fig. 3), fluvial strata lie on the 

storm-reworked deposits of the banded siltite unit of the Apple 

Creek Formation. The actual contact is concealed within a 25-

m-thick covered interval. No float of hummocky, shoreface, or 

tidal flat strata was observed in the interval. Further, I found no 

evidence for a fault in the covered interval. Although a concealed fault within this interval cannot be excluded, such a fault 

would have to be nearly horizontal or dip to the north at a low 

angle, in contrast to the steeply dipping folds and faults of the 

area. Upstream (southeast) from the southeasternmost hairpin 

turn of the Deep Creek road (fig. 3) to Pepper Creek, and then 

north to the Poison Creek thrust fault, sparse outcrops are of 

shoreface and fluvial strata. These observations suggest that an 

erosion surface may lie between the shoreface and fluvial rocks 

north and northwest of Lake Mountain, and the erosion surface 

cuts downsection westward to the Deep Creek area, where fluvial rocks overlie upper strata of the banded siltite unit of the 

Apple Creek.

Fluvial Strata

Fluvial strata in the eastern part of the Salmon River Mountains were observed in roadcuts along the Deep Creek road, uphill 

from the southeasternmost hairpin turn (fig. 3). Low- angle crossbedding is common. The beds commonly range from 20 to 50 cm 

thick and are coarse-grained siltite to fine-grained metasandstone. 

Thicker beds typically are amalgamations of thinner strata and 

extend farther laterally than the thinner beds. Mudchips, pebbles, 

and gravel are absent. Small channels 1–3 m wide in cross section were observed locally, with nested, upward-decreasing 

trough crossbeds. However, most beds do not define channels; 

they taper out laterally, replaced by other beds, forming complex 

sheets of lenticular beds. Some outcrops clearly show inclined 

surfaces of deposition and erosion, interpreted as lateral accretion 

deposits, whereas other outcrops do not reveal surfaces that cut 

downward. This reflects different orientations of section faces, or 

the faces could reflect different depositional environments within 

the varied depositional settings. The exposed rocks contain a low 

percentage (estimated at 10–15 percent) of fine-grained siltite 

(metamudstone) a few centimeters thick between beds of 

metasandstone. The fine-grained units probably are at least a few 

hundred meters in extent. Other fine-grained units are discontinuous, and sandstones rest directly on other sandstone. The recessive weathering of these fine-grained strata makes the 

metasandstone beds stand out. 

The strata are interpreted as braided or sheet-braided 

deposits, laid down in a fluvial system dominated by sand to 

coarse silt. They correspond to the distal, sheetflood, sand-bed 

river model of Miall (1996, p. 240–241), and display bed characteristics similar to those pictured by Miall (1985, his model 

11), Smith (1970), Cotter (1978), and the medial to distal distributary alluvial fan deposits of Kelly and Olsen (1993). Miall 

(1994, p. 155) interpreted such strata as follows: “The lateral 

amalgamation of many bars of different type, and the absence 

of clearly defined channel margins, suggests the dominance of a 

multiple-channel fluvial style, in which sinuosities were low to 

moderate, and both lateral and downstream accretion were common. Most bars and bar complexes were probably mid-channel 

in origin rather than bank-attached.” The low percentage of 

metamudstone (fine siltite) argues for a lack of overbank deposits, or at least the lack of preservation of such deposits. 

Map Pattern

The map pattern displayed by the lower strata of the Gunsight Formation and that of the underlying banded siltite unit of 

the Apple Creek Formation are closely related. Connor (1990, p. 

9) stated that the banded siltite unit (his “middle subunit of the 

Yellowjacket Formation”) thinned rapidly to the east in the 

Salmon River Mountains, probably to extinction. He attributed 

the thinning to intertonguing at both the top and the bottom of 

the unit, with loss of characteristic strata eastward. These relationships are shown on the maps of Connor and Evans (1986) 

and Connor (1990, fig. 2A). My mapping (fig. 2) also indicates 

that the banded siltite unit thins eastward in the Salmon River 

Mountains. 

 The abrupt end to the banded siltite, hummocky strata, and 

shoreface metasandstone east of Lake Mountain is attributed to 

erosion. From south to north in the Degan Mountain to Deep 

Creek area of the Salmon River Mountains (figs. 2, 3), stratigraphic units gradually appear upsection above the coarse siltite 

unit. In the vicinity of the Twin Peaks mine and Degan Mountain, only the coarse siltite unit is present (fig. 3). The banded 

siltite unit first appears about 3 km north of Degan Mountain. 

The southernmost exposure of basal strata of the Gunsight Formation occurs about 3 km farther north, south of Lake Mountain. 

Both the banded siltite unit and the overlying strata of the 

Gunsight Formation dip moderately northward in the area; thus, 

the outcrops of the two units along the ridge crest from Degan 
18 Mesoproterozoic Lemhi Group, Central Idaho

Mountain to Lake Mountain display the southernmost outcrops 

of each unit.

Structure

Mapping in the east-central part of the Salmon River Mountains was conducted to determine the general structural style of 

the area, in order to resolve stratigraphic and sedimentologic 

problems, to confine possible interpretations of the stratigraphy, 

and to integrate the local stratigraphy into the regional patterns. 

These goals were accomplished by detailed mapping in critical 

areas, with some reconnaissance through the more generally 

mapped terrain shown on the maps of Shockey (1957), Connor 

and Evans (1986), and Evans (1981).

Poison Creek Thrust Fault

The Poison Creek thrust fault, one of the major thrust faults 

of east-central Idaho, trends northwest across the western part of 

the Lemhi Range and into the eastern part of the Salmon River 

Mountains. The fault generally dips about 35°

 SW. In the 

Lemhi Range, the thrust fault placed hangingwall strata of the 

Apple Creek Formation over footwall lower Paleozoic rocks and 

underlying Mesoproterozoic strata of the Swauger and Gunsight 

Formations. Hangingwall rocks are part of the Poison Creek 

thrust sheet, and footwall rocks are part of the McDevitt Creek 

thrust sheet (fig. 2, inset map) (Tysdal, 2002). Recognition and 

mapping of the Poison Creek thrust fault across the part of the 

study area that lies in the Salmon River Mountains (figs. 1–3) 

proved critical to determining that rock units there are juxtaposed 

on different thrust sheets just as they are in the Lemhi Range. 

Some characteristics of the fault are described here because 

structure (1) bears importantly on the thickness of the Gunsight 

Formation in the area and (2) displays continuity of regional 

structural and stratigraphic patterns.

The Poison Creek thrust fault was first delineated across the 

western part of the Lemhi Range by Tysdal (1996a) and Tysdal 

and Moye (1996). K.V. Evans, Falma Moye, and R.F. Hardyman 

(unpub. data, 1996) extended the fault into the eastern part of the 

Salmon River Mountains, vicinity of the Twin Peaks mine (fig. 

2). Evans and associates recognized that an isolated, unnamed 

fault segment originally mapped near the Twin Peaks mine by 

Ekren (1988), and along which Apple Creek strata are juxtaposed 

against Ordovician carbonate rocks, is a short segment of the 

Poison Creek thrust. The fault is concealed by volcanic rocks of 

the Eocene Challis Volcanic Group north of the mine.

The map of Ekren (1988) and the mapping of K.V. Evans, 

Falma Moye, and R.F. Hardyman (unpub. data, 1996) show the 

fault segment near the Twin Peaks mine to trend only a few 

degrees west of north and to dip steeply to the west. North from 

the mine area, I interpret the fault to extend for about 8 km 

beneath volcanic rocks of the Eocene Challis Volcanic Group and 

to connect with the northwest-trending Leesburg fault that 

Shockey (1957, pl. 1) mapped northwest from near the South 

Fork of Williams Creek to west of Phelan Mountain (fig. 3). (I 

consider the Leesburg fault to be a segment of the Poison Creek 

fault; the Poison Creek name is applied to the fault to maintain 

continuity for the regional structure.) The northerly trending segment of the Poison Creek thrust fault coincides with a steepening 

of the fault dip to 60°–65° W. and a downcutting across footwall 

Paleozoic rocks. The northerly trend and the steep dip of the Poison Creek thrust fault are here interpreted as reflecting an oblique 

footwall ramp, which strikes at an angle to the general northeast 

transport direction of the hangingwall of the thrust fault.

The northwest-trending Leesburg segment of the Poison 

Creek thrust fault (northwest from near the South Fork of Williams Creek to west of Phelan Mountain, fig. 3), was interpreted by Shockey (1957, pl. 2) as a reverse fault, up on the 

southwest, with a dip about 70° SW. However, intensely 

developed cleavage of the juxtaposed hangingwall and footwall rocks in the South Fork Williams Creek area indicate a 

southwest dip of 30°–40° for the fault segment. Thin sections of upper Gunsight strata of the footwall reveal protomylonite textures in some places. In contrast, Connor and Evans 

(1986) had reinterpreted the Leesburg fault of Shockey (1957) 

as the southwest limit of a klippe, overthrust from the southwest, and showed its dip inclined to the northeast at a low 

angle. They showed the northeast edge of the klippe to be a 

low-angle, southwest-dipping fault delimiting the northeast 

edge of the Swauger Formation east of Phelan Mountain (fig. 

3). My observations show this Swauger contact to be depositional. The Leesburg name was not used for the fault by Connor and Evans (1986). 

Shockey (1957, pl. 1) showed the northwest-trending part of 

the Leesburg fault segment to be discontinuously exposed, with 

one exposed part southeast of Phelan Mountain (figs. 2, 3) and 

the other near long 114°15' W., near the north edge of the area of 

figure 2. Connor and Evans (1986) mapped the fault segment in 

the same position, but showed the Mesoproterozoic granite pluton in the area near long 114°15' W. (fig. 2) in fault contact, dipping northeast at a low angle, with a klippe. Near Napias Creek, 

I observed intensely cleaved Proterozoic granite (fig. 3) in contact with Gunsight strata and interpret the contact as a fault. The 

cleavage dips southwestward at a moderate angle, and I interpret 

the fault to have the same inclination.

Juxtaposed Strata

Hangingwall strata in the western part of the Lemhi Range 

show the coarse siltite unit of the Apple Creek Formation to be 

the youngest Proterozoic unit preserved in the range. In the 

east-central part of the Salmon River Mountains, in the Twin 

Peaks mine to Degan Mountain area (fig. 2), only rocks of the 

upper part of the coarse siltite unit are present. Younger strata 

gradually appear northward from Degan Mountain. About 3 km 

north of Degan Mountain, the banded siltite unit of the Apple 

Creek crops out. Farther north, about 3 km south of Lake 

Mountain, the hummocky beds appear. Both the banded siltite 

unit and the overlying hummocky beds of the Gunsight Formation dip moderately northward; thus, the outcrops of the two 

units along the ridge crest from Degan Mountain to Lake Mountain display the southernmost outcrops of each unit. North of 
Correlation, Sedimentology, Structural Setting 19

Lake Mountain, the hummocky beds give way to shoreface clastics, then fluvial strata. In the area of the South Fork Williams 

Creek (fig. 3), where the Poison Creek fault changes from a 

northerly to a northwest trend, shoreface strata of the lower 

Gunsight are thrust over quartzite of the uppermost part of the

Gunsight.

Footwall strata of the Poison Creek thrust in the western 

part of the Lemhi Range show the uppermost Gunsight rocks 

and the conformably overlying Swauger Formation to constitute 

the youngest Mesoproterozoic rocks. (These strata lie north of 

the Poison Creek thrust fault, north of lat 44°52'30" N. and near 

long 113°52'30" W., and are not shown in fig. 2.) The same 

footwall sequence is present in the Salmon River Mountains, 

where the Swauger and the directly underlying uppermost Gunsight rocks of the Phelan Mountain–Williams Creek area occur 

(fig. 3). This same structural and stratigraphic setting must exist 

farther northwest, on both sides of the north-south concealed 

fault at long 114° 07'30" W., because the Swauger and upper 

Gunsight strata west of the concealed fault appear to be an 

extension of the strata of the Phelan Mountain–Williams Creek 

area (fig. 3). Hence, west of the concealed fault, the Poison 

Creek thrust fault should lie southwest of the Swauger Formation. The thrust has significant displacement—hence my agreement with Connor and Evans (1986) in interpreting a fault to 

delimit the northeastern boundary of the Proterozoic granite near 

long 114°15' W. (fig. 2).

Tectonic Breccia

Tectonic breccia was observed by me in four widely separated, isolated outcrops near the Poison Creek thrust fault. The 

isolated outcrops are dispersed from near the South Fork Williams Creek to midway along the Deep Creek Ridge (fig. 3). The 

zone of brecciation trends toward the junction area of the Deep 

Creek and Moccasin Creek roads (fig. 3), where it is concealed 

by glacial deposits. The breccias are composed of angular clasts 

of metasandstone tightly cemented within a chloritized matrix of 

the same composition. In some places, breccia fragments are 

composed of mylonitic rock; the mylonite is oriented differently 

in adjacent clasts, indicating reorientation of the mylonite fabric 

during the brecciation process. The breccia is interpreted as a 

product of extensional deformation. The breccias define the general location of a fault, but the limited outcrops do not permit 

determination of a precise location. The extension took place 

subsequent to compressional deformation that produced the Poison Creek thrust fault and likely reflects normal displacement 

within the zone of the Poison Creek thrust. The magnitude of 

the extension is uncertain, however. 

Thickness of Gunsight Formation

The Gunsight Formation in the Salmon River Mountains 

area shown in figure 3 is too thin for a full stratigraphic section 

to be present. That this is likely true can be shown by a 

comparison with the Gunsight thickness in the Lemhi Range. 

Northwest from the South Fork of Williams Creek, extension 

along the trend of the Poison Creek fault subsequent to compressional thrusting accounts for some of the thinned section of the 

Gunsight, yet the same pattern of hangingwall/footwall relationships that exists in the western part of the Lemhi Range is maintained in the Salmon River Mountains. Compressional folding 

and variable dip of the strata also indicate that a full section of 

the Gunsight Formation cannot be present in the area of figure 3, 

as explained in the following paragraphs.

The Gunsight Formation at the type section in the Lemhi 

Range is greater than 1,725+ m thick (measured section of 

McBean, 1983), and as McBean stated, is incomplete owing to a 

normal fault at the top of the section. Although the lower part of 

the McBean (1983) section included about 450 m of the Apple 

Creek Formation, a normal fault may omit more than this thickness of Gunsight strata from the basal part of the Gunsight measured section (Tysdal, unpub. data, 1995). In the Poison Peak 

area of the northwestern part of the Lemhi Range (fig. 1), the 

Gunsight is approximately as thick as the type section, based on 

the mapping work of G.W. Winkler (unpub. data, 1996).

In the vicinity of Lake Mountain, in the Salmon River 

Mountains, thicknesses determined from map patterns show that 

the rocks between the upper strata of the banded siltite unit 

southwest of the mountain and the trace of the Poison Creek 

thrust fault northeast of it are much less than that of the Gunsight in the type area and on the footwall of the Poison Creek 

thrust fault in the Lemhi Range. Northeast of the Lake Mountain thrust fault (figs. 2, 3), which is believed to have only 

minor displacement, dips of the shoreface rocks that overlie the 

hummocky beds generally display low-angle northwesterly 

dips. Steep dips are associated with tight folds in the vicinity of 

the Poison Creek thrust, however. The extensive area of low 

dips between the top of the banded siltite unit and the Poison 

Creek thrust fault testifies to a much too thin sequence of Gunsight strata for the entire formation to be present.

In the Deep Creek Ridge area (fig. 3), thickness determinations also show that the Gunsight is too thin to accommodate the 

entire formation. Assuming a vertical dip, the maximum thickness of the Gunsight in the study area would be about 2,100 m, 

measuring from the top of the banded siltite unit of the Apple 

Creek Formation along the Deep Creek road to the inferred location of the Poison Creek thrust fault in the vicinity of Deep 

Creek Ridge. This thickness is similar to that in the type area 

and in the northwestern part of the Lemhi Range. However, the 

dips of the Gunsight in the Deep Creek Ridge area are not vertical. Dips generally are moderate, the area contains locally tight 

folding, and minor thrust faults cause repetition of strata. Taken 

together, these data indicate that the Gunsight is less than the 

maximum possible thickness, and probably less than half its 

actual thickness in this area.

Other Implications of Existence of Poison Creek Thrust Sheet in 

Salmon River Mountains

Recognition that the Poison Creek thrust fault extends 

northwestward from the Lemhi Range well into the Salmon 

River Mountains has several implications, including the 
20 Mesoproterozoic Lemhi Group, Central Idaho

following: (1) Paleozoic and Mesoproterozoic plutons in the 

northwestern part of the map areas shown in figures 2 and 3 are 

delimited on the northeast by the Poison Creek thrust fault. The 

plutons are part of the Poison Creek thrust sheet and must have 

been transported northeast during thrusting and, therefore, must 

be rootless. (2) The Poison Creek thrust fault displaced older 

rocks over younger rocks, the usual situation. The plutons of the 

Poison Creek thrust sheet, therefore, should represent a deeper 

level of pluton emplacement than do similar plutons present in 

the footwall McDevitt Creek rocks northeast of the thrust fault, 

north of the map area. (3) The stratabound cobalt-bearing mineralized rocks of the Blackbird mining district are part of the 

Poison Creek thrust sheet and are rootless. (4) The banded siltite unit that hosts the mineralized rocks of the Blackbird district 

in the Salmon River Mountains is almost entirely absent from 

the Lemhi Range. It occurs only in a small fault sliver of the 

Poison Creek thrust fault in the westernmost part of the range. 

The banded siltite unit probably existed within the Poison Creek 

thrust sheet in the Lemhi Range but has been eroded from 

exposed rocks. The Blackbird type of mineralization confined to 

the banded siltite unit is, if present in the western part of the 

Lemhi Range, concealed beneath the Gunsight Formation. 

Conclusions

Stratigraphic units of the Mesoproterozoic Apple Creek, 

Gunsight, and Swauger Formations present in the Lemhi Range 

correlate with stratigraphic units in the Salmon River Mountains 

that previously were assigned to the Yellowjacket and Big Creek 

Formations. The “middle subunit of the Yellowjacket Formation” (Connor and Evans, 1986) in the Salmon River Mountains 

is renamed the banded siltite unit and placed in the Apple Creek 

Formation. Strata of the “upper subunit of the Yellowjacket Formation” are correlated with the Gunsight Formation of the 

Lemhi Range and renamed accordingly. Strata previously interpreted as the Mesoproterozoic Big Creek Formation, on a thrustfloored klippe in the eastern part of the Salmon River Mountains, 

are determined to lie depositionally above the Gunsight Formation and are assigned to the Swauger Formation.

The banded siltite unit of the Apple Creek Formation lies 

conformably above the coarse siltite unit of the formation, which 

previously was correlated between the Lemhi Range and the 

Salmon River Mountains. The banded siltite unit is confined 

almost entirely to the Salmon River Mountains. From southwest 

to northeast in the Salmon River Mountains, the stratigraphic 

succession reflects a marine offshore to fluvial onshore 

sequence. The coarse siltite unit and most of the overlying 

banded siltite unit of the Apple Creek Formation are interpreted 

as marine turbidites that were deposited offshore, in a slope or 

fan setting. Upper strata of the banded siltite unit are interpreted 

as turbidites that were reworked by currents, probably related to 

storms. Basal strata of the Gunsight Formation are hummocky 

crossbedded metasandstone interpreted as storm deposits. Preserved hummocky beds must have been deposited below storm 

wave-base. Gradationally overlying beds, with some 

interbedded hummocky crossbedded strata, are termed shoreface strata and are interpreted as shallow marine. Younger 

strata of the lower part of the Gunsight Formation display characteristics of fluvial rocks and likely were deposited in a delta 

plain.

Continuity of the stratigraphic succession from the Lemhi 

Range to the Salmon River Mountains, as well as subdivision of 

the units within the Salmon River Mountains, was accomplished 

in part through geologic mapping in the Salmon River Mountains to determine continuity of structural features there with 

those of the Lemhi Range. The Poison Creek thrust fault, and 

the Poison Creek thrust sheet that forms the hangingwall of the 

thrust fault, extend into the eastern part of the Salmon River 

Mountains. The Apple Creek Formation and overlying basal 

strata of the Gunsight Formation form part of the hangingwall 

Poison Creek thrust sheet in the Salmon River Mountains. Mineral deposits of the Blackbird mining district, as well as Mesoproterozoic granite (fig. 3), also form part of the Poison Creek 

thrust sheet and were displaced to the northeast during orogenic 

activity. Rocks exposed in the footwall of the thrust fault in the 

Salmon River Mountains are upper strata of the Gunsight Formation and the conformably overlying Swauger Formation.

References Cited

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approach to facies interpretation: Amsterdam, Elsevier, 168 p.

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Wright, V.P., ed., Sedimentology Review/1: London, Blackwell, p. 

103–122.

Connor, J.J., 1990, Geochemical stratigraphy of the Yellowjacket Formation (Middle Proterozoic) in the area of the Idaho cobalt belt, Lemhi 

County Idaho, with analytical contributions from A.J. Bartel, E.L. 

Brandt, P.H. Briggs, S. Danahey, D. Fey, D.B. Hatfield, M. Malcolm, 

V. Merritt, G. Riddle, S. Roof, K. Stewart, J. Storey, J.E. Taggert, and 

R.B. Vaughn; Part A—Discussion; Part B—Geochemical data: U.S. 

Geological Survey Open-File Report 90-0234, 30 p.

Connor, J.J., 1991, Some geochemical features of the Blackbird and 

Jackass zones of the Yellowjacket Formation (Middle Proterozoic) 

in east-central Idaho, with analytical contributions from A.J. Bartel, 

P.H. Briggs, R.R. Carlson, J.G. Crock, B.H. Roushey, C.S.E. Papp, D.F. 

Siems, J.E. Taggert, Jr., and E.P. Welsch; Part A—Discussion 

(paper copy): U.S. Geological Survey Open-File Report 91-259–A, 25 

p.

Connor, J.J., and Evans, K.V., 1986, Geologic map of the Leesburg quadrangle, Idaho: U.S. Geological Survey Miscellaneous Field Studies 

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sedimentology: Canadian Society of Petroleum Geologists Memoir 

5, p. 361–383.

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.
Correlation, Sedimentology, Structural Setting 21

Evans, K.V., 1981, Geology and geochronology of the eastern Salmon River Mountains, Idaho, and implication for regional Precambrian 

tectonics: University Park, Pa., The Pennsylvania State University 

Ph. D. dissertation, 222 p.

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 Connor, J.J., 1993, Geologic map of the Blackbird 

Mountain 15-minute quadrangle, Lemhi County, Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF–2234, scale 

1:62,500.

Hahn, G.A., and Hughes, G.J., Jr., 1984, Sedimentation, tectonism, and 

associated magmatism of the Yellowjacket Formation in the Idaho 

cobalt belt, Lemhi County, Idaho, in Hobbs, S.W., ed., The Belt: Montana Bureau of Mines and Geology Special Publication 90, p. 65–67.

Harms, J.C., Southard, J.B., Spearing, D.R., and Walker, R.G., 1975, Depositional environments as interpreted from primary sedimentary 

structures and stratification sequences: Society of Economic Paleontologists and Mineralogists Short Course Notes 2, 161 p.

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flow processes, in Flood, R.D., Piper, D.J.W., and Peterson, L.S., eds., 

Proceedings of the Ocean Drilling Program, Scientific Results: College Station, Texas, Texas A & M University, v. 155, p. 53–78.

Hughes, G.J., Jr., 1983, Basinal setting of the Idaho Cobalt belt, Blackbird 

mining district, Lemhi County, Idaho, in Proceedings of the Denver 

Region Exploration Geologists Society Symposium, the genesis of 

Rocky Mountain ore deposits—Changes with time and tectonics: 

Denver Region Exploration Geologists Society, Denver, Colo., p. 21–

27.

Kelly, S.B., and Olsen, Henrik, 1993, Terminal fans—A review with reference to Devonian examples: Sedimentary Geology, v. 85, p. 339–374.

McBean, A.J., II, 1983, The Proterozoic Gunsight Formation, Idaho-Montana; stratigraphy, sedimentology and paleotectonic setting: University Park, Pa., The Pennsylvania State University M.S. thesis, 235 p.

McKie, Thomas, 1994, Geostrophic versus friction-dominated storm 

flow—Paleocurrent evidence from the Late Permian Brotherton Formation, England: Sedimentary Geology, v. 93, p. 73–84.

Miall, A.D., 1985, Sedimentation on an early Proterozoic continental margin under glacial influence—The Gowganda Formation (Huronian), 

Elliott Lake, Ontario, Canada: Sedimentology, v. 32, p. 763–788.

Miall, A.D., 1994, Reconstructing fluvial macroform architecture from 

two-dimensional outcrops—Examples from the Castlegate Sandstone, Book Cliffs, Utah: Journal of Sedimentary Research, v. B64, p. 

146–158.

Miall, A.D., 1996, The geology of fluvial deposits—Sedimentary facies, 

basin analysis, and petroleum geology: New York, Springer-Verlag, 

582 p.

Nash, J.T., 1989, Geology and geochemistry of synsedimentary cobaltiferous-pyrite deposits, Iron Creek, Lemhi County, Idaho: U.S. Geological Survey Bulletin 1882, 33 p.

Nash, J.T., and Connor, J.J., 1993, Iron and chlorine as guides to stratiform Cu-Co-Au deposits, Idaho cobalt belt, U.S.A.: Mineralium 

Deposita, v. 28, p. 99–106.

Nash, J.T., and Hahn, G.A., 1989, Stratabound Co-Cu deposits and mafic 

volcaniclastic rocks of the Blackbird mining district, Lemhi County, 

Idaho, in Boyle, R.W., Brown, A.C., Jefferson, C.W., Howett, E.C., 

and Kirkham, R.V., eds., Sediment-hosted stratiform copper deposits: Geological Association of Canada Special Paper 36, p. 339–356.

North American Commission on Stratigraphic Nomenclature, 1983, 

North American stratigraphic code: American Association of 

Petroleum Geologists Bulletin, v. 67, p. 841–875.

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., 1975, Precambrian Y sedimentary rocks in east-central 

 Idaho: U.S. Geological Survey Professional Paper 889–A, 23 p.

Shanmugam, G., 1997, Reinterpretation of depositional processes in a 

classic flysch sequence (Pennsylvanian Jackfork Group), Ouachita 

Mountains, Arkansas and Oklahoma—Reply: American Association of Petroleum Geologists Bulletin, v. 81, p. 476–491.

Shanmugam, G., Spalding, T.D., and Rofheart, D.H., 1993, Process sedimentology and reservoir quality of deep-marine bottom-current 

reworked sands (sandy contourites)—An example from the Gulf of 

Mexico: American Association of Petroleum Geologists Bulletin, v. 

77, p. 1241–1259.

Shockey, P.N., 1957, Reconnaissance geology of the Leesburg quadrangle, Lemhi County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 113, 42 p.

Smith, N.D., 1970, The braided stream depositional environment—Comparison of the Platte River with some Silurian clastic rocks, northcentral Appalachians: Geological Society of America Bulletin, v. 81, 

p. 2993–3014.

Sobel, L.S., 1982, Sedimentology of the Blackbird mining district, Lemhi 

County, Idaho: Cincinnati, Ohio, University of Cincinnati M.S. thesis, 

235 p.

Stacey, M.W., and Bowen, A.J., 1988, The vertical structure of density 

and turbidity currents—Theory and observation: Journal of Geophysical Research, v. 93, p. 3528–3542.

Stow, D.A.V., and Shanmugam, G., 1980, Sequence of structures in finegrained turbidites—Comparison of recent deep-sea and ancient 

flysch sediments: Sedimentary Geology, v. 25, p. 23–42.

Stow, D.A.V., Faugeres, Jean-Claude, Adriano, Viana, and Gonthier, 

Elaine, 1998, Fossil contourites—A critical review: Sedimentary 

Geology, v. 115, p. 3–31.

Tysdal, R.G., 1996a, Geologic map of adjacent parts of the Hayden Creek 

and Mogg Mountain quadrangles, Lemhi County, Idaho: U.S. 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 Map 

GQ–1777, scale 1:24,000.

Tysdal, R.G., 1996c, Geologic map of part of May Mountain quadrangle, 

Lemhi County, Idaho: U.S. Geological Survey Open-File Report 96-

537, scale 1:24,000.

Tysdal, R.G., 2000a, Revision of Middle Proterozoic Yellowjacket Formation, central Idaho: U.S. Geological Survey Professional Paper 

1601–A, 14 p.
22 Mesoproterozoic Lemhi Group, Central Idaho

Tysdal, R.G., 2000b, Stratigraphy and depositional environments 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., 2002, Structural geology of western part of Lemhi Range, eastcentral Idaho: U.S. Geological Survey Professional Paper 1659, 33 p.

Tysdal, R.G., and Desborough, G.A., 1997, Scapolitic metaevaporite and 

carbonate rocks of Proterozoic Yellowjacket Formation, Moyer 

Creek, Salmon River Mountains, central Idaho: U.S. Geological Survey Open-File Report 97-268, 26 p.

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.

Walker, R.G., 1992, Turbidites and submarine fans, in Walker, R.G., and 

James, N.P., eds., Facies models—Response to sea level change: 

Geological Association of Canada, p. 239–263.

Walker, R.G., and Plint, A.G., 1992, Wave- and storm-dominated shallow 

marine systems, in Walker, R.G., and James, N.P., eds., Facies models—Response to sea level change: Geological Association of Canada, p. 219–238.

Winston, D.W., Link, K.P., and Hathaway, Nate, 1999, The Yellowjacket is 

not the Prichard and other heresies—Belt Supergroup correlations, 

structure and paleogeography, east-central Idaho, in Hughes, S.S., 

and Thackray, G.D., eds., Guidebook to the geology of eastern Idaho: 

Pocatello, Idaho, Idaho Museum of Natural History, p. 3–20.
Chemical Composition and Provenance of the 

Mesoproterozoic Big Creek, Apple Creek, and 

Gunsight Formations, Lemhi Group, Central Idaho

By David A. Lindsey, Russell G. Tysdal, and Joseph E. Taggart, Jr. 

Chapter B of

Correlation, Sedimentology, Structural Setting, Chemical Composition, 

and Provenance of Selected Formations in Mesoproterozoic Lemhi Group,

Central Idaho 

By Russell G. Tysdal, David A. Lindsey, and Joseph E. Taggart, Jr.

U.S. Geological Survey Professional Paper 1668–B

U.S. Department of the Interior

U.S. Geological Survey
Contents

Abstract ................................................................................................................................................. 25

Introduction .......................................................................................................................................... 25

 Purpose of Investigation.............................................................................................................. 25

 Stratigraphy and Correlation ...................................................................................................... 26

 Acknowledgments........................................................................................................................ 27

Methods and Data................................................................................................................................ 28

 Sample Sets................................................................................................................................... 28

 Analytical Methods ...................................................................................................................... 28

 Statistical Methods....................................................................................................................... 29

Chemical Composition......................................................................................................................... 29

 Major-Oxide Ratios....................................................................................................................... 29

 Geochemical Factor Models and Sediment Provenance ...................................................... 32

 Stratigraphic Variation in Sediment Provenance .................................................................... 32

Conclusions........................................................................................................................................... 38

References Cited.................................................................................................................................. 38

 

Figures

 1. Index map showing localities of samples for geochemical data sets for 

 metasedimentary rocks of the Mesoproterozoic Lemhi Group, central Idaho............. 26

2–5. Scattergrams showing:

 2. Chemical classification of rocks of the Big Creek, Apple Creek, and 

 Gunsight Formations by log (FeTO3/K2O) versus log (SiO2/Al2O3) ............................ 30

 3. Chemical classification of rocks of the Big Creek, Apple Creek, and 

 Gunsight Formations by log (SiO2/Al2O3) versus log (K2O/Na2O) ............................. 31

 4. Major oxides and elements, showing effect of plagioclase content on 

 geochemical covariation in rocks of the Apple Creek and Gunsight Formations, 

 Salmon River Mountains .................................................................................................. 34

 5. Log CaO versus log Na2O, showing effect of plagioclase and calcite 

 content on major-oxide covariation in rocks of the Big Creek and 

 Apple Creek Formations, Lemhi Range.......................................................................... 36

 6. Box plots showing CaO and Na2O content of siltite in the Big Creek and 

 Apple Creek Formations, Lemhi Range................................................................................. 38

Tables

1. Stratigraphic units and relationships in the Mesoproterozoic Lemhi Group, 

 eastern part of the Salmon River Mountains and northern part of the 

 Lemhi Range, central Idaho......................................................................................................... 27

2. Samples of metasedimentary rocks of the Mesoproterozoic Lemhi Group 

 classified by formation (including facies and units) and rock type, 

 Salmon River Mountains and Lemhi Range, central Idaho.................................................... 28

3. R-mode factor analysis of geochemical data, Apple Creek and 

 Gunsight Formations, Salmon River Mountains ..................................................................... 33

4. R-mode factor analysis of geochemical data, Big Creek and Apple Creek 

 Formations, Lemhi Range............................................................................................................. 35

5. Analysis of variance and Fisher’s PLSD test for differences among mean 

 values for factor 2 scores, log CaO, and log Na2O in siltite samples from the 

 Big Creek and Apple Creek Formations, Lemhi Range ........................................................... 37
25

Abstract

Grain size and provenance strongly influence the chemical 

composition of metasedimentary rocks of the Mesoproterozoic 

Big Creek, Apple Creek, and Gunsight Formations of the Lemhi 

Group. In this report, we use major-oxide ratios, statistical analysis of covariation (R-mode factor analysis) among major oxides 

and trace elements, and analysis of variance among stratigraphic 

units to examine the relation between provenance and chemical 

composition. Plagioclase, an important indicator of provenance, 

is widespread and abundant in the the Big Creek, Apple Creek, 

and Gunsight Formations. Plagioclase increases the abundance 

of noncarbonate CaO, Na2O, and Sr, and decreases the ratio of 

K2O to Na2O.

Before metamorphism, rocks of the Big Creek, Apple 

Creek, and Gunsight Formations were for the most part firstcycle clastic sediments derived from a terrane having the majoroxide composition of gneiss like that of the Archean quartzofeldspathic gneiss (also known as the “Dillon Granite Gneiss”) 

of southwestern Montana. Argillite, diamictite, and siltite plot 

mostly in the wacke (graywacke) field of the scattergram log 

(FeTO3/K2O) versus log (SiO2/Al2O3), reflecting the abundance 

of mica and chlorite (originally, clay) versus quartz. Metasandstones tend to plot in the fields of wacke, arkose, and litharenite 

(lithic arenite), suggesting that a wide variety of matrix-rich, 

arkosic, and lithic sandstones were originally present in the 

Lemhi Group. Major-oxide ratios of the Big Creek, Apple 

Creek, and Gunsight Formations are only slightly evolved from 

those of quartzo-feldspathic gneiss.

A notable feature of scattergrams showing log (SiO2/Al2O3) 

versus log (K2O/Na2O) is the strong linear variation in log (K2O/

Na2O). Although this variation may result from mixing of two 

source rocks, one K2O (potassium feldspar)-rich and one Na2O 

(plagioclase)-rich, it more likely reflects varying degrees of 

weathering of a plagioclase-rich source such as gneiss.

Four petrologic processes were interpreted by R-mode factor analysis of geochemical data for the Apple Creek and Gunsight Formations in the Salmon River Mountains. Factors are 

(1) micas and chlorite (grain size of original sediment), (2) rare 

earth minerals (provenance or metamorphism), (3) base-metal 

mineralization, and (4) plagioclase (provenance). The factor 

interpretation for geochemical data from the Big Creek and 

Apple Creek Formations of the Lemhi Range is similar, except 

that mineralization is not well developed and placer concentration of iron oxide minerals is evident in the Big Creek Formation. In both ranges, CaO and Na2O have high loadings on the 

plagioclase factor. In metasandstones of the Salmon River 

Mountains, the linking of CaO and Na2O to plagioclase is 

Chemical Composition and Provenance of the 

Mesoproterozoic Big Creek, Apple Creek, and 

Gunsight Formations, Lemhi Group, Central Idaho

By David A. Lindsey, Russell G. Tysdal, and Joseph E. Taggart, Jr.

confirmed by their correlation with estimates of modal plagioclase in thin section.

To varying degrees, plagioclase was destroyed during 

weathering of the source area and transit to the depositional 

basin. In the Lemhi Range, plagioclase content of the Apple 

Creek and Big Creek Formations is best represented by Na2O. 

Na2O varies among stratigraphic units in the Apple Creek and 

Big Creek Formations, indicating sources of varying composition or varying rates of weathering, erosion, and sediment transport during deposition of this part of the Lemhi Group.

Introduction

Purpose of Investigation

This investigation is aimed at identifying the provenance of 

the Mesoproterozoic Lemhi Group of central Idaho. Provenance 

encompasses the composition of the source, the nature and 

degree of weathering in the source, and the weathering history of 

detritus during transit. Understanding provenance is important 

to the correlation and interpretation of the plate-tectonic setting 

of Mesoproterozoic rocks in Idaho and Montana. Within the 

Lemhi Group, the Mesoproterozoic Big Creek, Apple Creek, and 

Gunsight Formations were selected for provenance studies 

because of their wide distribution and availability of geochemical data.

Previous investigations revealed that many of the metasandstones in Mesoproterozoic formations of central Idaho are arkosic (Lopez, 1981; Ruppel, 1975). In the Lemhi Range, these 

formations include the Mesoproterozoic Big Creek, Apple 

Creek, and Gunsight Formations of the Lemhi Group (Ruppel, 

1975). In the Salmon River Mountains, arkosic metasandstones 

are found in rocks now correlated with the Lemhi Group (Tysdal, 2000a) but formerly assigned to the Mesoproterozoic Yellowjacket Formation (Lopez, 1981; Connor and Evans, 1986; 

Evans and Connor, 1993). In both ranges, as well as in the Beaverhead Mountains, arkosic metasandstones of the Gunsight 

Formation contain abundant plagioclase as well as potassium 

feldspar (McBean, 1983). At Ayers Canyon, about 25 km east of 

Leadore in the Beaverhead Mountains (fig. 1), conglomeratic 

metasandstone assigned to the Gunsight Formation contains 

abundant pebbles of gneiss (McBean, 1983; M’Gonigle, 1993). 

Clasts in the only other conglomerate in the Lemhi Group, the 

diamictite unit of the Apple Creek Formation in the northern 

Lemhi Range, are mostly intrabasinal argillite, siltstone, and 

sandstone (Tietbohl, 1986).
26 Mesoproterozoic Lemhi Group, Central Idaho

In this report, we draw on a detailed statistical analysis of 

two geochemical data sets (Lindsey and others, 2002)1 to infer 

the provenance of the Big Creek, Apple Creek, and Gunsight 

Formations of the Lemhi Group. In these formations, the abundance of noncarbonate CaO, Na2O, and Sr, and the ratio K2O/

Na2O, are dependent upon the abundance of plagioclase, a major 

indicator of provenance in arkosic rocks.

 Although rocks of the Lemhi Group have been metamorphosed to greenschist facies, with metamorphic biotite common 

in the Salmon River Mountains, clastic metasedimentary textures and structures are evident (Lopez, 1981; Tysdal, 2000b). 

Comparison of the major-oxide composition of siltite in the 

more highly metamorphosed rocks (biotite-grade greenschist 

facies) of the Salmon River Mountains with the composition of 

siltite in the less metamorphosed (chlorite-grade greenschist 

facies) rocks of the same unit in the Lemhi Range indicates that 

modification of the original chemical composition is probably 

minimal (Lindsey and others, 2002).

Stratigraphy and Correlation

The stratigraphy of the Mesoproterozoic Lemhi Group in 

the Lemhi Range and rocks prevously assigned to the Yellowjacket Formation in the Salmon River Mountains has been 

recently clarified (Tysdal, 2000a, 2000b; this volume) (table 1). 

The Lemhi Group of the Lemhi Range (Ruppel, 1975) has been 

correlated with rocks previously mapped as Mesoproterozoic 

Yellowjacket Formation in the Salmon River Mountains (Connor 

and Evans, 1986; Connor, 1990; Evans and Connor, 1993). The 

basis for stratigraphic revision and correlation was established 

by geologic mapping of key areas in the northern Lemhi Range 

and the Beaverhead Mountains (Tysdal, 1996a, 1996b; Tysdal 

and Moye, 1996; Tysdal, Evans, and Lund, 2000). During the 

course of mapping in the northern part of the Lemhi Range, the 

Mesoproterozoic Big Creek Formation and the overlying Apple 

Creek Formation were described and subdivided (Tysdal, 2000a, 

2000b). 

The Apple Creek Formation was divided into four units by 

Tysdal (2000a) (table 1). Three of these units crop out over 

extensive areas in the northern Lemhi Range. In ascending 

order, the Apple Creek contains the fine siltite unit, the diamictite unit, and the coarse siltite unit. The diamictite unit thins 

westward in the Lemhi Range and apparently pinches out at the 

east edge of the Salmon River Mountains (Connor, 1990; Tysdal 

and Moye, 1996). Tysdal (2000a, 2000b) correlated the overlying coarse siltite unit with part of the Salmon River Mountains 

sequence of strata that Connor and Evans (1986) and Connor 

(1990) previously had mapped as the “lower subunit of the Yellowjacket Formation,” and applied the Apple Creek name to 

these strata. The conformably overlying “middle subunit of the 

Yellowjacket Formation” of these workers was renamed the 

banded siltite unit and placed in the upper part of the Apple 

Creek by Tysdal (this volume). At Yellow Lake in the Lemhi 

Range, the Apple Creek Formation unconformably overlies the 

Big Creek Formation and is transitional into the overlying Gunsight Formation (Tysdal, 2000b). The Apple Creek Formation 

(undivided) at Yellow Lake is designated the “Yellow Lake unit.”

The Big Creek Formation is light-gray, coarse-grained siltite to medium-grained quartz-feldspar metasandstone. It contains prominent rusty-brown-weathering layers of carbonatecemented sandstone and laminae of dark-gray heavy minerals. 

The formation is about 2,700 m thick in the northern Lemhi 

Range. The fine siltite unit of the Apple Creek Formation is 

planar-laminated siltite, ripple cross-laminated siltite, and 

argillaceous siltite. Many beds are graded, with 1–2 cm thick 

layers of siltite grading upward into argillite. The unit is estimated to be about 1,000 m thick. The diamictite unit of the 

Apple Creek is composed of poorly sorted conglomerate beds, 

1–5 m in thickness, of disorganized, matrix-supported clasts; 

the conglomerate beds are interpreted as turbidites and debris 

flows. Argillite, argillaceous siltite, siltite, and lesser metasandstone form successions as much as 10 m or more thick between 

conglomerate beds. These fine-grained strata are turbidites. 

The thickness range in the northern part of the Lemhi Range is 

about 600–1,500 m. The coarse siltite unit of the Apple Creek 

1

This open-file report contains tables and figures showing details of 

geochemical relationships. An online version, including geochemical data sets 

for the Salmon River Mountains and the Lemhi Range, is available at <http://

geology.cr.usgs.gov/pub/open-file-reports/ofr-02-0310/>.

45°30'

44° 30'

45°00'

114°30' 114°00' 113°30'

0

KILOMETERS

50

SALMON

MOUNTAINS LEMHI

RANGE

Salmon

Challis

MONTANA

IDAHO 93

RIVER

Salmon River

 Mountains

 data set

Lemhi Range

 data set

Yellow Lake

area

LEMHI

VALLEY

BEAVERHEAD

MOUNTAINS

Leadore

28

Area of

figure 1

IDAHO

Figure 1. Index map showing generalized localities of samples for 

geochemical data sets for metasedimentary rocks of the Mesoproterozoic Lemhi Group, Salmon River Mountains and Lemhi Range, central 

Idaho. Sampled areas are shaded.
Chemical Composition and Provenance 27

Diamictite unit

Coarse siltite unit

Fine siltite unit

Banded siltite unit

Big Creek Formation

Apple Creek

 Formation

Gunsight Formation

Swauger Formation

Yellow Lake unit

Eastern part of

 Salmon River

 Mountains

(not exposed)

(not exposed)

Northern part of

 Lemhi Range

Nonmarine

Marine

is composed of medium- to coarse-grained siltite and finegrained metasandstone. Siltite beds are normally graded, and 

soft-sediment deformation structures are common. The unit 

derives its name from distinctive graded beds of sharp-based, 

light-gray, quartz-rich, fine-grained metasandstone interspersed 

within the sequence. The graded bedding of the siltite and the 

sharp-based, light-gray metasandstone beds suggest deposition 

from turbidity currents. The coarse siltite unit is about 2,500 m 

thick (Tysdal, 2000b).

The banded siltite unit is a distinct facies of the Apple 

Creek and is preserved mainly in the Salmon River Mountains 

(fig. 1). Connor (1990) described the rocks as plane-parallel, 

laminated to thin-bedded siltite-argillite couplets that are blackand-white. The white (light-gray) beds are quartz-rich siltite to 

fine-grained metasandstone and the black beds are argillite. The 

planar bedding is strongly disrupted locally by soft-sediment 

deformation. The unit is interpreted as a turbidite sequence by 

Sobel (1982) and Tysdal (this volume), and has an estimated 

thickness of 2,000 m (Connor and Evans, 1986).

The Yellow Lake unit of the Apple Creek Formation (in the 

Lemhi Range) consists mainly of dark-gray medium- and finegrained laminated siltite, and locally argillite in the lower part 

(Tysdal, 2000b). Some siltite beds exhibit scoured bases, graded 

tops, ripple crosslamination, slump folds, and fluid-escape structures. Much of the Yellow Lake unit contains dolomitic beds and 

laminae, which serve to distinguish this unit from other units of 

the Apple Creek Formation. The upper part of the Yellow Lake 

unit also contains interbeds of light-gray feldspathic metasandstone.

The Gunsight Formation was named by Ruppel (1975) for a 

sequence of metasandstone and finer clastic rocks in the Gunsight Peak area of the Lemhi Range. At the type locality, the 

Gunsight is 1,700+ m thick (McBean, 1983). The formation 

consists mainly of fining-upward sequences of crossbedded 

metasandstone and siltite of probable fluvial origin. The lower 

and upper parts of the Gunsight Formation may be marine in 

part. The lowermost Gunsight is transitional into turbidites of 

the Apple Creek Formation. The uppermost Gunsight is transitional into marine orthoquartzite of the Mesoproterozoic 

Swauger Formation (Tysdal, 2000b).

Although various correlations of the Lemhi Group with the 

Mesoproterozoic Belt Supergroup have been proposed (Ruppel 

and Lopez, 1988; Winston and others, 1999), the true stratigraphic relationship between the Lemhi Group and the Belt is 

unknown. Previously proposed correlations were made before 

rocks assigned to the Yellowjacket Formation in the Salmon 

River Mountains were correlated with the Lemhi Group (Tysdal, 

2000a). Differences in the paleogeographic configuration of 

basins and source areas during deposition of the Lemhi Group 

and the Belt Supergroup also pose a challenge to correlation. 

During deposition of sediments that formed the Mesoproterozoic 

Gunsight Formation, the source lay to the northeast, in the 

Archean gneiss terrane of southwestern Montana (R.G. Tysdal, 

D.A. Lindsey, and K.I. Lund, unpub. data, 2002). In the Belt 

Supergroup, the Mesoproterozoic Prichard and Revett Formations were derived from a land mass that lay to the west and 

south, in the area currently occupied by the Lemhi Group 

(Cressman, 1989; Ryan and Buckley, 1999). The depositional 

basin for the Lemhi Group could not have been located in the 

Prichard-Revett source area while the latter was being eroded.

Acknowledgments

We especially acknowledge J.J. Connor, who collected and 

documented samples for the Salmon River Mountains data set 

and part of the Lemhi Range data set, used in this study. Chemical analyses of rocks from the Salmon River Mountains (Connor, 1990) were made by A.J. Bartel, E. Brandt, P.H. Briggs, S. 

Danahey, D. Fey, D.B. Hatfield, M. Malcolm, V. Merritt, G. Riddle, S. Roof, K. Stewart, J. Storey, J.E. Taggart, Jr., and R.B. 

Vaughn, U.S. Geological Survey. Analyses of rocks from the 

Lemhi Range were made by P.H. Briggs, D.E. Detra, D.L. Fey, 

R.T. Hopkins, J.M. Motooka, J.E. Taggart, Jr., and K. Slaughter, 

U.S. Geological Survey. Ten samples from the Lemhi Range 

Table 1. Stratigraphic units and relationships in the Mesoproterozoic Lemhi Group, eastern part 

of the Salmon River Mountains and northern part of the Lemhi Range.

[Includes units sampled for this study (compiled from Tysdal, 1996a, 1996b; Tysdal, 2000a, 2000b; Tysdal and 

Moye, 1996; and Tysdal, Evans, and Lund, 2000)]
28 Mesoproterozoic Lemhi Group, Central Idaho

(Connor, 1991) were analyzed by A.J. Bartel, P.H. Briggs, R.R. 

Carlson, J.G. Crock, C.S.E. Papp, B.H. Roushey, D.F. Siems, 

J.E. Taggart, Jr., and E.P. Welsch, U.S. Geological Survey. K.V. 

Evans and W.R. Miller reviewed the manuscript.

Methods and Data

Sample Sets

Two geochemical sample data sets (online version of Lindsey and others, 2002) were examined for evidence of the petrologic processes, including sediment provenance, that formed the 

Mesoproterozoic rocks in central Idaho. One data set was 

selected from samples collected by J.J. Connor during an earlier 

study of rocks formerly assigned to the Mesoproterozoic Yellowjacket Formation in the Salmon River Mountains (Connor, 

1990) (fig. 1). The Salmon River Mountains data set represents 

rocks of the Apple Creek and Gunsight Formations (table 2). A 

second data set was selected from samples collected by R.G. 

Tysdal, during study of the Lemhi Group in the northern part of 

the Lemhi Range, and augmented by 10 samples from a larger 

set collected by J.J. Connor (Connor, 1991) (fig. 1). The Lemhi 

Range data set represents rocks of the Big Creek and Apple 

Creek Formations (table 2). 

The samples of Connor (1990) from the Salmon River 

Mountains were reassigned to formations of the Lemhi Group by 

R.G. Tysdal (2000a) (table 1). Reassignment became necessary 

after Tysdal (Tysdal, 1996a, 1996b; Tysdal and Moye, 1996) 

mapped the Lemhi Group in the northern part of the Lemhi 

Range and realized that the stratigraphic units there were equivalent to rocks in the Salmon River Mountains, formerly assigned 

to the Yellowjacket Formation. Samples from outlying areas, 

from the type Yellowjacket Formation, and from the Mesoproterozoic Hoodoo Quartzite, all of uncertain stratigraphic relation 

to the Lemhi Group, were excluded from the Salmon River 

Mountains data set. Also excluded were a few samples of 

unusual rock types, including banded iron-formation, tourmalinite breccia, one diamictite, and biotitite (biotite-rich schist). The 

resulting Salmon River Mountains sample set, used in this investigation, contained 98 samples of quartzite (metasandstone, hereafter referenced as “sandstone”), siltite, and argillite (table 2). 

The more or less even distribution of samples, with the exception 

of siltite and argillite in the Gunsight Formation, makes the 

Salmon River data set amenable to statistical comparisons 

among formations and rock types.

Rock types represented in the Lemhi Range data set are 

mostly sandstone, siltite, and diamictite, but more than half the 

samples (78) are siltite (table 2). Most sandstone samples (16) 

are from the Big Creek Formation, whereas diamictite is entirely 

from one unit of the Apple Creek Formation. Most siltite samples are nearly evenly distributed among the Big Creek Formation and two units of the Apple Creek Formation, providing a 

good case for examining stratigraphic variation in those units.

Analytical Methods

The data set from the Salmon River Mountains (Connor, 

1990) contains analyses of major oxides and numerous trace elements, making it ideal for identification of petrogenetic factors 

affecting rock composition. Major oxides were analyzed by Xray fluorescence spectrometry (Taggart and others, 1987). Most 

trace elements were analyzed by inductively coupled plasma–

atomic absorption emission spectroscopy (Lichte and others, 

1987), except thorium and uranium, which were determined by 

the delayed-neutron method (McKown and Millard, 1987). 

Methods of chemical analysis, descriptions of sample sets, and 

estimates of analytical error are given by Connor (1990).

The sample set from the Lemhi Range was originally analyzed for trace elements by the six-step semiquantitative spectrographic method (Golightly and others, 1987), except for 10 

Table 2. Samples of metasedimentary rocks of the Mesoproterozoic Lemhi Group classified by formation (including facies 

and units) and rock type, Salmon River Mountains and Lemhi Range, central Idaho. 

[Ygf, fluvial facies of Gunsight Formation; Ygm, marine facies of Gunsight Formation; Yay, Yellow Lake unit of Apple Creek Formation; 

Yab, banded siltite unit of Apple Creek Formation; Yac, coarse siltite unit of Apple Creek Formation; Yad, diamictite unit of Apple Creek 

Formation; Yaf, fine siltite unit of Apple Creek Formation; Yb, Big Creek Formation. Stratigraphic nomenclature of Tysdal (2000a)]

Formation Total Sandstone Siltite Argillite Diamictite

Salmon River Mountains

Ygf 11 7 0 4 0

Ygm 10 10 0 0 0

Yab 37 10 7 20 0

Yac 40 7 15 18 0

Total 98 34 22 42 0

Lemhi Range

Yay 6 1 3 2 0

Yac 21 0 21 0 0

Yad 49 0 25 0 24

Yaf 15 7 8 0 0

Yb 36 16 20 0 0

Unknown 1 0 1 0 0

Total 128 24 78 2 24
Chemical Composition and Provenance 29

samples reported by Connor (1991), which were analyzed by 

inductively coupled plasma–atomic absorption emission spectroscopy (Lichte and others, 1987). Some samples were analyzed for selected major oxides and trace elements by other 

methods, but these data were not used in this report. The 

formerly incomplete analyses of major oxides were supplanted 

by more recent analyses of all major oxides by X-ray fluorescence spectrometry (Taggart and others, 1987). These majoroxide analyses are available in the online version of Lindsey and 

others (2002). Additional analytical data, except the recently 

completed X-ray fluorescence analyses, are available in CDROM form (Baedecker and others, 1998).

Statistical Methods

We examined geochemical data by comparing scattergrams 

of major-oxide ratios, by statistical analysis of covariation (Rmode factor analysis) among major oxides and trace elements, 

and by analysis of variance of the same data for stratigraphic 

variation. Details of statistical methods are described in Lindsey 

and others (2002).

The major-oxide composition of the Big Creek, Apple 

Creek, and Gunsight Formations was investigated by use of logratio2

 scattergrams (Pettijohn and others, 1972; Lindsey, 1999). 

In sandstones, log (SiO2/Al2O3) is a measure of quartz versus 

feldspar plus clay, and thus is an indicator of chemical and textural maturity. In clay-rich clastic rocks such as siltstone and 

shale, log (SiO2/Al2O3) is essentially a measure of detrital quartz 

versus clay and thus, grain size. Log (K2O/Na2O) reflects the 

nature of the source and the degree of weathering and diagenesis. Plagioclase, especially intermediate to calcic plagioclase, is 

more easily destroyed (converted to clay and soluble ions) by 

chemical weathering than is potassium feldspar, and hence, log 

(K2O/Na2O) increases with weathering of a two-feldspar rock. 

These ratios are combined in scattergrams to classify sandstones 

and other siliciclastic sedimentary rocks (Pettijohn and others, 

1972; Lindsey, 1999).

 A second scattergram, log (FeTO3/K2O) versus log (SiO2/

Al2O3), is useful for distinguishing of some groups of sandstones 

from argillaceous rocks (Herron, 1988). In fine-grained rocks 

and matrix-rich sandstone, log (FeTO3/K2O) is a measure of the 

abundance of iron-bearing chlorite versus potassium-bearing 

clays and micas.

Petrologic processes, including provenance, that affected 

the chemical composition of the Lemhi Group rocks under study 

were investigated using R-mode factor analysis (Lindsey and 

others, 2002). A general discussion of R-mode factor analysis 

can be found in Cooley and Lohnes (1962); a complete discussion of the methods used here is given by Lindsey and others 

(2002). Factor models, expressed as factor loading tables, measure the relationship of major oxides and trace elements to a 

small number of new, uncorrelated variables, called principal 

components.

To produce a factor model for geochemical data, the data 

were first screened for completeness, and frequency distributions 

were examined for skewness. Elements for which a majority of 

the analyses were below the limit of detection were dropped 

from consideration; for other elements, values below the limit of 

detection were replaced by values ranging from 1/2 to 2/3 the 

limit of detection. Values for oxides and elements whose frequency distribution is highly skewed were transformed to logarithms. The resulting data matrix was used to compute a 

correlation matrix. The correlation matrix was solved for its 

latent roots (eigenvalues), and a new matrix specifying uncorrelated vectors (principal component axes) was calculated. A 

small number of principal components were selected for rotation 

and factor interpretation, based on the eigenvalues, on communalities, and on ease of interpretation. Selection of the number 

of principal components to preserve for rotation is somewhat 

subjective. We used the root curve method to compare eigenvalues (SAS Institute Inc., 1998) and also compared communalities 

when various numbers of principal components were selected 

(Lindsey and others, 2002). An evaluation of criteria for selecting the number of principal components for rotation is given by 

Jackson (1993). After rotation, the principal components are 

interpreted as petrologic processes, or factors. Interpretation is 

based on the nature and correlation of elements that load on each 

factor, the likely mineralogical residence of these elements, and 

the known conditions for formation and concentration of these 

minerals. Scattergrams of element pairs and frequency distributions of individual elements are commonly useful in examining 

the causes of factor loadings. 

Stratigraphic variation in provenance was investigated for 

siltite in the Lemhi Range. Analyses of 66 samples of siltite are 

nearly evenly distributed among the Big Creek Formation and 

two units of the Apple Creek Formation (table 2). A simple oneway analysis of variance (Krumbein and Graybill, 1965), 

accompanied by box plots, was used to test and illustrate stratigraphic variation in provenance. Differences among mean values for stratigraphic units also were tested by Fisher’s PLSD 

(Protected Least Significant Difference) method (discussed by 

SAS Institute Inc., 1998).

Chemical Composition

Major-Oxide Ratios

Rocks of varying grain size in the Big Creek, Apple Creek, 

and Gunsight Formations appear to be well classified by the 

scattergram log (FeTO3/K2O) versus log (SiO2/Al2O3) (fig. 2A, 

B). Argillite, diamictite, and siltite plot mostly in the wacke 

(graywacke) field, reflecting abundant mica and chlorite (originally, clay). Very few samples plot in the shale field. In the 

Salmon River Mountains, siltite is distinguished from argillite 

by slightly higher ratios of log (FeTO3/K2O) and log (SiO2/

Al2O3). In both mountain ranges, sandstones tend to plot in the 

fields of wacke (graywacke), arkose, and litharenite (lithic arenite), suggesting that a wide variety of matrix-rich, arkosic, and 

lithic sandstones were originally present in the Lemhi Group. 2All logarithms in this report are to the base 10.
30 Mesoproterozoic Lemhi Group, Central Idaho

LOG (SiO2/Al2O3)

LOG (FeTO3/K2O)

Lemhi Range

B

-1

0

1

2

0 0.5 1 1.5 2

Sublitharenite

Fe-sand

Quartz Wacke

Arkose Subarkose arenite

Fe-shale

Shale

Litharenite

A

Salmon River Mountains

-1

0

1

2

0 0.5 1 1.5 2

LOG (SiO2/Al2O3)

LOG (FeTO3/K2O)

Sublitharenite

Fe-sand

Quartz Wacke

Arkose Subarkose arenite

Fe-shale

Shale

Litharenite

LOG (SiO2/Al2O3)

LOG (FeTO3/K2O)

C

Some possible source rocks

-1

0

1

2

0 0.5 1 1.5 2

Sublitharenite

Fe-sand

Quartz Wacke

Arkose Subarkose arenite

Fe-shale

Shale

Litharenite Igneous

rocks

Basalt

Granite

Siltite

Sandstone

Diamictite

Argillite

EXPLANATION

Archean gneiss,

 southwest 

 Montana

Before metamorphism, rocks of the Big Creek, Apple 

Creek, and Gunsight Formations were for the most part firstcycle arkosic sediments. Low (<1) ratios of log (SiO2/Al2O3) 

indicate that these sediments were chemically and mineralogically immature (silica- and quartz-poor). Comparison with 

Archean quartzo-feldspathic gneiss (“Dillon Granite Gneiss” of 

early reports) of the Tobacco Root and Ruby Ranges (fig. 2C) 

indicates that the composition of the Lemhi Group is only 

slightly evolved from gneiss. Archean quartzo-feldspathic 

gneiss is a major unit in the Precambrian terrane of southwestern 

Montana; for example, the gneiss makes up a major part of the 

southwestern Ruby Range (James, 1990). The major-oxide content of Archean quartzo-feldspathic gneiss in southwestern Montana is also comparable to that of granitic rocks (fig. 2C), which 

may have been the protolith (James, 1990). Alternatively, the 

gneiss may have been formed from both sedimentary and igneous rocks (Garihan and Williams, 1976; Wilson, 1981). On the 

basis of major-oxide content, the quartzo-feldspathic gneiss is a 

possible source, or at least an analog for a source, of detritus in 

the Lemhi Group.

A notable feature of scattergrams showing log (K2O/

Na2O) is the strong linear trend of Lemhi Group rocks, reflecting a wide variation in K2O/Na2O (fig. 3A, B). This variation 

may result from (1) mixing of two source rocks, one K2O 

(potassium feldspar)-rich and one Na2O (plagioclase)-rich; (2) 

varying degrees of weathering of initially Na2O (plagioclase)-

rich rock, either in the source area, during transit to the depositional basin, or both; and (3) postdepositional alteration and 

alkali metasomatism. If log (K2O/Na2O) values represent mixing of two rock types, then the ratios at the ends of the line 

should approximate those of the source rocks. If log (K2O/

Na2O) represents weathering of an originally Na2O (plagioclase)-rich source rock, then the ratio at the Na2O-rich end of 

the trend should approximate the value of the source. If log 

Figure 2. Chemical classification of rocks of the Big Creek, Apple Creek, and Gunsight Formations by log (FeTO3/K2O) versus log (SiO2/Al2O3) (method of Herron, 1988) compared to possible source rocks. A, Salmon River Mountains; B, Lemhi Range; C, some possible source rocks, including 

Archean gneiss, Tobacco Root and Ruby Ranges (Anderson and others, 1990; James, 1990; Wilson, 1981), and common igneous rocks (data from 

Le Maitre, 1976). FeTO3, total iron as Fe2O3.
Chemical Composition and Provenance 31

(K2O/Na2O) represents postdepositional alteration, then the 

ends of the line represent the range between original rock composition and maximum alteration. Commonly, in unmetamorphosed arkosic rocks, weathering of the source area and 

sediment during transit is the probable cause of variation in log 

(K2O/Na2O). (See Van de Kamp and Leake, 1994, for 

example.)

Comparison of log (SiO2/Al2O3) versus log (K2O/Na2O) for 

the Big Creek, Apple Creek, and Gunsight Formations with 

Archean gneiss of the Tobacco Root and Ruby Ranges (fig. 3C) 

0

0.5

1

1.5

2

-1.5 -1 -0.5 0 0.5 1 1.5 2

LOG (K2O/Na2O)

Arkose

Graywacke

0

0.5

1

1.5

2

-1.5 -1 -0.5 0 0.5 1 1.5 2

LOG (K2O/Na2O)

Arkose

Graywacke

0

0.5

1

1.5

2

-1.5 -1 -0.5 0 0.5 1 1.5 2

LOG (K2O/Na2O)

A

C

Siltite

Sandstone

Diamictite

Argillite

EXPLANATION

Archean gneiss,

 southwest 

 Montana

B

Lemhi Range

Some possible

 source rocks

Igneous rocks

Salmon River

 Mountains

LOG (SiO2/Al2O3) LOG (SiO2/Al2O3) LOG (SiO2/Al2O3) Figure 3. Chemical classification of rocks of the Big Creek, Apple Creek, and Gunsight Formations by log 

(SiO2/Al2O3) versus log (K2O/Na2O) (modified method of Pettijohn and others, 1972) compared to possible source 

rocks. A, Salmon River Mountains; B, Lemhi Range; C, some possible source rocks, including Archean gneiss, 

Tobacco Root and Ruby Ranges (Anderson and others, 1990; James, 1990; Wilson, 1981), and common igneous 

rocks (data from Le Maitre, 1976). Also shown in A and B, ranges of representative graywacke and arkose (Pettijohn and others, 1972).
32 Mesoproterozoic Lemhi Group, Central Idaho

is consistent with derivation from gneiss. Values of log (K2O/

Na2O) for Archean gneiss compare closely with the Na2O end 

member of the trend log (SiO2/Al2O3) versus log (K2O/Na2O) 

seen in rocks of the Lemhi Group. Values of log (K2O/Na2O) for 

most of the Lemhi Group could have been produced by preferential weathering of plagioclase (and removal of Na2O) in 

gneiss.

A large contribution of detritus from Archean gneiss is not 

compatible with what is known about the Nd-isotope composition of rocks of the Lemhi Group (previously assigned to the 

Yellowjacket Formation) in the Salmon River Mountains. 

According to Doughty and Chamberlain (1996), most of the 

detritus in rocks of the Lemhi Group of the Salmon River 

Mountains was derived from a Proterozoic terrane, with only 

minor contributions from Archean rocks. Although it may not 

be appropriate to compare the major-oxide composition of rocks 

of the Lemhi Group with that of Archean gneiss in southwestern 

Montana, no other nearby pre-Lemhi Group gneiss terrane is 

available for comparison. Moreover, sedimentary features in the 

Gunsight Formation point to a source in southwestern Montana 

(R.G. Tysdal, D.A. Lindsey, and K.I. Lund, unpub. data, 2002).

Geochemical Factor Models and Sediment Provenance

Four factors were interpreted from R-mode factor analysis 

of geochemical data for the Apple Creek and Gunsight Formations in the Salmon River Mountains (table 3) (Lindsey and others, 2002). Factor interpretations are (1) micas and chlorite 

(grain size of original sediment), (2) rare earth minerals (metamorphism or provenance), (3) base-metal mineralization, and 

(4) plagioclase (provenance). A four-factor solution was 

selected after inspection of eigenvalues on a graph (root curve 

method) and comparison of communalities for various solutions 

involving factors other than four (Lindsey and others, 2002). 

Interpretations are based primarily on the genetic implications 

of oxides and elements that have loadings > 0.50 on one factor—in other words, elements that have at least 0.25 (0.50 

squared) of their total variance explained by one factor.

The provenance implications of factor 4 (plagioclase) will 

be discussed here in preference to those of factor 2 (rare earth 

minerals). The mineralogical residence of factor 2 elements 

remains uncertain, and the genetic significance of factor 2 also 

remains uncertain; therefore, extensive interpretation seems 

unjustified. Factor 2 may reflect sphene, which is commonly 

observed in thin sections. Sphene was formed, or recrystallized, 

during regional metamorphism. If factor 2 elements are present 

mainly in sphene, this mineral, or its precursors, also accounted 

for the premetamorphic concentration of rare earth elements.

The interpretation of factor 4 as plagioclase, a provenance 

indicator in detrital sedimentary rocks, is indicated by the high 

loadings of log CaO and Na2O. Both oxides are highly correlated in argillite and siltite, but they are correlated most strongly 

in sandstones of the Apple Creek and Gunsight Formations of 

the Salmon River Mountains, where currents probably segregated feldspar sand from clay (fig. 4A). The linking of factor 4 

to plagioclase is confirmed by the correlation of volume percent 

plagioclase, estimated from thin sections, with log CaO and 

Na2O in sandstone (fig. 4B). Strontium readily substitutes for 

calcium in plagioclase (Smith and Brown, 1988), and this relationship is evident from the high correlation of log Sr with log 

CaO (fig. 4C). The logarithmic distribution of both elements is 

consistent with substitution in the same mineral. The correlation of Na2O with log Sr is simply a consequence of the correlation of log CaO with both Na2O and log Sr (fig. 4D). The 

moderate loadings of log Y and log Yb on factor 4 (table 3) may 

also reflect substitution of those elements for calcium in plagioclase. The composition of plagioclase was not investigated in 

detail; twin extinction angles observed in thin section suggest a 

composition in the range of andesine and oligoclase.

An R-mode factor analysis of geochemical data for the 

Lemhi Range yields a model similar to that of the Salmon River 

Mountains. A four-factor solution was selected after inspection 

of eigenvalues on a graph (root curve method) and comparison 

of communalities for various solutions involving factors other 

than four (Lindsey and others, 2002). The Lemhi Range data set 

represents the Big Creek and Apple Creek Formations, but not 

the Gunsight. The data set lacks many of the trace elements 

determined in the Salmon River Mountains. Interpretation of 

the Lemhi Range model is essentially the same as that for the 

Salmon River Mountains, but relationships among some trace 

elements are not available. Four factors were identified: (1) 

micas (grain size of original sediment), (2) plagioclase (provenance), (3) chlorite (grain size), and (4) iron-oxide minerals 

(placer concentration or mineralization) (table 4). A factor for 

rare earth minerals (provenance or metamorphism), seen in factor analysis of Salmon River data, was not detected. Cerium 

and neodymium were not analyzed; lanthanum and yttrium, 

which were analyzed, load on factor 1, but their low communalities indicate that they may belong to another, unidentified factor 

or factors. The four-factor solution of Lemhi Range data yields 

two factors for grain size, a somewhat more complicated interpretation than desired.

Factor 2 (plagioclase) of the Lemhi Range data set is the 

same as factor 4 in the Salmon River Mountains. A scattergram 

for log CaO versus log Na2O (both log CaO and log Na2O have 

high loadings on factor 2) shows a weak linear correlation of 

0.446 (fig. 5). Eleven Na2O values that plot at 0.1 percent 

(log –1) have widely varying CaO contents, and many high values for log CaO in siltite and sandstone of the Big Creek Formation (fig. 5B) depart widely from the line of best fit, which 

weakens the correlation. Calcite and dolomite account for high 

CaO values in the Big Creek Formation (Tysdal, 2000b; G.A. 

Desborough, written commun., 2000), and their presence seriously undermines the use of CaO as an indicator of plagioclase 

in the Big Creek. Values for the diamictite unit of the Apple 

Creek Formation, which does not contain carbonate minerals, 

plot close to the line of best fit (fig. 5B).

Stratigraphic Variation in Sediment Provenance

Analysis of variance and tests for differences among means 

were conducted to test for stratigraphic variation in both the 

Salmon River Mountains and Lemhi Range data sets (Lindsey 

and others, 2002). Stratigraphic units for which geochemical 
Chemical Composition and Provenance 33

data are available in the Salmon River Mountains differ from 

those in the Lemhi Range. The Big Creek Formation and the 

diamictite unit of the Apple Creek Formation are not exposed in 

most of the Salmon River Mountains, and the banded siltite unit 

of the Apple Creek Formation is not exposed in the Lemhi 

Range. In the Salmon River Mountains, no differences in plagioclase content (as represented by factor 4—log CaO, Na2O, 

and log Sr) among the various units of the Apple Creek 

Formation and Gunsight Formation were found. In the Lemhi 

Range, significant differences in plagioclase content (as represented by factor 2 and log Na2O) were found among siltites in 

the Big Creek and Apple Creek Formations (table 5; fig. 6). No 

significant stratigraphic differences were found for log CaO at 

the 0.05 level. In the Big Creek Formation, CaO content in part 

reflects calcite and dolomite (Tysdal, 2000b; G.A. Desborough, 

written commun., 2000), so CaO cannot be used to measure 

Table 3. R-mode factor analysis of geochemical data, Apple Creek and Gunsight Formations, Salmon River Mountains. 

[Shown are orthogonal axes, four factors (Varimax rotation), calculated from data of Connor (1990). 

Factor interpretations shown at bottom. Bold type, factor loadings >[0.50] (absolute value); italic type, 

oxides or elements with communality < 0.50. Log oxide or element denotes data that were transformed 

to logarithms before computation of input correlation matrix. (See Lindsey and others, 2002, for further 

explanation.) FeTO3, total iron as Fe2O3; LOI, loss on ignition]

Factor 1—Micas and chlorite (Al2O3, log MgO, K2O, TiO2, LOI, Be, log Cr, log Ga, log Li, Ni, Sc, log 

Th, U, and V) versus SiO2—grain size. P2O5, log Ba, and log Zr have high loadings (> 0.50) but low 

communalities (< 0.50).

Factor 2—Rare earth minerals (log Ce, La, Nd, and log Y)—provenance or metamorphism.

Factor 3—Base metals in MnO (log Co, log Mn, log Pb, and log Zn)—mineralization. Log As, log 

Cu, and log Sb have high loadings (> 0.50) but low communalities (< 0.50).

Factor 4—Plagioclase (log CaO, Na2O, log Sr, log Y, and log Yb)—provenance.

Oxide or element Factor 1 Factor 2 Factor 3 Factor 4

SiO2 -.950 -.086 -.092 -.055

Al2O3 .896 .007 -.028 .117

Log FeTO3 .379 .198 .426 -.438

Log MgO .727 .223 .177 .040

Log CaO .309 .204 .183 .806

Na2O -.268 .053 .107 .785

K2O .703 -.002 -.320 -.415

TiO2 .866 .038 .191 .182

P2O5 .541 .082 .235 .329

LOI .742 .037 .259 -.057

Log As .197 -.137 .594 .110

Log B .394 .236 -.350 .042

Log Ba .549 .009 -.145 -.223

Be .824 .094 -.086 .075

Log Ce .016 .878 .204 .149

Log Co .442 .149 .577 -.073

Cr .710 -.080 .192 .179

Log Cu -.137 .113 .535 .079

Log Ga .912 .055 .007 -.017

La .066 .913 .136 .184

Log Li .732 .358 .224 .074

Log Mn .241 .276 .659 .311

Nd .123 .917 .068 .176

Ni .660 .171 .327 .210

Log Pb -.080 .097 .688 .213

Log Sb .181 .098 .512 .117

Sc .913 -.050 .157 .090

Log Sr .097 .232 .141 .820

Log Th .721 .352 -.022 .035

U .649 .394 .288 .027

V .863 -.137 .199 .018

Log Y .262 .523 .244 .537

Log Yb .443 .250 .253 .571

Log Zn .149 .170 .782 .008

Log Zr .507 .301 .092 -.116
34 Mesoproterozoic Lemhi Group, Central Idaho LOG CaO, IN PERCENT

Na2O, IN PERCENT

 R = .512

All 

R = .753

Sandstone

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

0 1 2 3 4 5 6

A

B

C

DLOG CaO, IN PERCENT

LOG Sr, IN PARTS PER MILLION

R = .696

Na2O, IN PERCENT

LOG Sr, IN PARTS PER MILLION

R = .662

0

1

2

3

4

5

6

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

Siltite

Sandstone

Argillite

EXPLANATION

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

LOG CaO, IN PERCENT

0 5 10 15 20 25 30 35

R = .653, N = 33

0

1

2

3

4

5

Na2O, IN PERCENT

0 5 10 15 20 25 30 35

PLAGIOCLASE, IN PERCENT

R = .597, N = 33

Figure 4. Major oxides and elements, showing effect of plagioclase content on geochemical covariation in rocks of the Apple Creek and Gunsight 

Formations, Salmon River Mountains (calculated from data of Connor, 1990). A, log CaO versus Na2O; B, log CaO and Na2O versus plagioclase in 

sandstone; C, log CaO versus log Sr; D, Na2O versus log Sr. R, correlation coefficient; lines are least-squares fit. All chemical elements in weight 

percent or parts per million; plagioclase in volume percent, estimated by J.J. Connor.
Chemical Composition and Provenance 35

stratigraphic variation in plagioclase content of the Big Creek. 

Probably, Na2O values are the best indicator of plagioclase.

In the Lemhi Range, the distribution of plagioclase differs 

greatly in the diamictite and coarse siltite units of the Apple 

Creek Formation. The diamictite unit exhibits a wide range of 

Na2O values, whereas the coarse siltite unit, deposited by turbidity currents, exhibits a narrow range of Na2O values (fig. 5B). 

The range in CaO values in the diamictite unit is also greater 

than in the coarse siltite unit, but the contrast is less striking. 

When only siltites are compared (fig. 6), the range in Na2O 

values in the diamictite unit is also greater than in the coarse siltite unit. The wide range in plagioclase content of the diamictite 

may reflect incomplete mixing of parent material of varying 

provenance, consistent with redeposition by locally derived 

debris flows. Siltite beds in the diamictite unit could have 

formed by reworking of diamictite, or they could be downstream 

equivalents of diamictite. In contrast to the diamictite unit, the 

uniform plagioclase content as indicated by the Na2O content of 

the coarse siltite unit probably reflects thorough mixing of sediment during redeposition by turbidity flows. 

Oxide or element Factor 1 Factor 2 Factor 3 Factor 4

SiO2 -.566 -.044 -.335 -.644

Al2O3 .584 .161 .378 .417

Log FeTO3 .329 -.157 .155 .698

Log MgO .102 -.120 .839 .057

Log CaO .163 .662 .115 -.443

Log Na2O -.034 .834 .017 .095

K2O .583 -.306 .217 .425

Log TiO2 .703 .302 .340 .324

Log P2O5 .542 .475 .243 .063

Log MnO .088 .405 -.137 -.014

Log LOI .416 -.231 .553 -.061

Log B .776 -.242 .141 -.072

Log Ba .433 -.316 -.128 .592

Log Be .515 -.408 .144 .181

Log Co -.125 .041 .477 .503

Log Cr .670 .294 .293 -.063

Log Cu -.077 -.027 .110 .679

Log Ga .708 .012 .109 .387

Log La .511 .078 -.029 .081

Log Ni .233 .138 .777 .167

Log Pb .170 .194 -.296 .448

Log Sc .784 .119 .059 .354

Log V .785 .159 .120 .234

Log Y .593 .032 -.062 .026

Log Zr .736 .001 .088 -.138

Table 4. R-mode factor analysis of geochemical data, Big Creek and Apple Creek Formations, 

Lemhi Range. 

[Shown are orthogonal axes, four factors (Varimax rotation), calculated from data on samples collected 

by R.G. Tysdal. Factor interpretations shown at bottom. Bold type, factor loadings >[0.50] (absolute 

value); italic type, oxides or elements with communality < 0.50. Log oxide or element denotes data that 

were transformed to logarithms before computation of input correlation matrix. (See Lindsey and others, 

2002, for further explanation.) FeTO3, total iron as Fe2O3; LOI, loss on ignition]

Factor 1—Micas (Al2O3, K2O, log TiO2, log P2O5, log B, log Cr, log Ga, log Sc, log V, and log Zr) versus 

SiO2—grain size. Log Be, log La, and log Y have high loadings (> 0.50) but low communalities (< 0.50).

Factor 2—Plagioclase (log CaO and log Na2O)—provenance.

Factor 3—Chlorite—(Log MgO, log LOI, and log Ni)—grain size.

Factor 4—Iron oxide minerals (log FeTO3, log Ba, log Co, and log Cu) versus SiO2—Placer concentration or mineralization. Log Pb has loading of 0.448 but low communality (< 0.50).
36 Mesoproterozoic Lemhi Group, Central Idaho

The plagioclase content of the Big Creek Formation, as 

expressed by Na2O content, is also variable (fig. 5B). The range 

in Na2O content of siltite in the Big Creek is comparable to that 

of siltite in the diamictite unit of the Apple Creek Formation (fig. 

6). The wide range in plagioclase content of the Big Creek is 

consistent with reworking, and variable degrees of destruction of 

plagioclase, in a nearshore environment. Most of the Big Creek 

represented in the Lemhi Range sample set was deposited in 

intertidal and shallow subtidal environments (Tysdal, 2000b).

The stratigraphic variation in plagioclase in the Lemhi 

Range, as best expressed by variation in log Na2O in siltites 

among stratigraphic units (table 5), is interpreted as variation in 

A

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

LOG CaO, IN PERCENT

-1.5 -1.0 -0.5 0.0 0.5 1.0

LOG Na B 2O, IN PERCENT

Big Creek Formation

Yellow Lake unit

Fine siltite unit

Diamictite unit

Coarse siltite unit

EXPLANATION

Apple Creek Formation

Siltite

Sandstone

Diamictite

Argillite

EXPLANATION

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

-1.5 -1.0 -0.5 0.0 0.5 1.0

R = .446

LOG CaO, IN PERCENT

LOG Na2O, IN PERCENT

Figure 5. Log CaO versus log Na2O, showing effect of plagioclase and calcite content on major-oxide covariation in rocks of the Big Creek and Apple Creek Formations, Lemhi Range. A, Samples classified by rock 

type (grain size); B, samples classified by stratigraphic unit. R, correlation coefficient; line is least-squares 

fit. 
Chemical Composition and Provenance 37

Analysis of variance for factor 2 scores among formations

Degrees of 

freedom

Sum of squares Mean square F-value P-value

Formation 2 19.886 9.943 13.846 <.0001 S

Residual 56 40.213 .718

Mean values of factor 2 scores for formations

Formation Number of samples Mean Standard deviation Standard error

Yac 21 0.766 0.662 0.142

Yad 22 -.505 .675 .144

Yb 16 -.346 1.207 .302

 Fisher's PLSD test for differences in mean values of factor 2 scores 

 among formations

Formation pair Mean difference Critical difference P-value

Yac, Yad 1.273 0.518 <.0001 S

Yac, Yb 1.112 .563 .0002 S 

Yad, Yb -.161 .558 .5949

Analysis of variance for log CaO among formations

Degrees of 

freedom

Sum of squares Mean square F-value P-value

Formation 2 .978 .489 2.673 .0769

Residual 63 11.530 .183

Mean values of log CaO for formations

Formation Number of samples Mean Standard deviation Standard error

Yac 21 -.526 .295 .064

Yad 25 -.748 .421 .084

Yb 20 -.473 .539 .121

 Fisher's PLSD test for differences in mean values of log CaO

 among formations

Formation pair Mean difference Critical difference P-value

Yac, Yad .222 .253 .0849

Yac, Yb -.053 .267 .6956

Yad, Yb -.274 .256 .0355 S

Table 5. Analysis of variance and Fisher’s PLSD test for differences among mean values for factor 2 scores, log CaO, and 

log Na2O in siltite samples from the Big Creek and Apple Creek Formations, Lemhi Range. 

[Yac, coarse siltite unit of Apple Creek Formation; Yad, diamictite unit of Apple Creek Formation; Yb, Big Creek Formation. Number of 

samples for factor 2 scores, 59; for log CaO and log Na2O, 66. F-value, mean square of effect (Formation) divided by residual mean square; 

P-value, probability of no difference among formations; S, significant at <0.05 level; blank, not applicable]

Analysis of variance for log Na2O among formations

Degrees of 

freedom

Sum of squares Mean square F-value P-value

Formation 2 4.155 2.077 10.131 <.0002 S

Residual 63 12.918 .205

Mean values of log Na2O for formations

Formation Number of samples Mean Standard deviation Standard error

Yac 21 .416 .342 .075

Yad 25 -.070 .428 .086

Yb 20 -.172 .570 .127

 Fisher's PLSD test for differences in mean values of log Na2O

 among formations

Formation pair Mean difference Critical difference P-value

Yac, Yad .486 .268 .0006 S

Yac, Yb .588 .283 <.0001 S

Yad, Yb .102 .271 .4562
38 Mesoproterozoic Lemhi Group, Central Idaho

0.01

0.1

1

10

100

CaO AND Na2O, IN PERCENT

CaO Na2O

Yac Yad Yb Yac Yad Yb

Median

25th percentile 

90th percentile 

10th percentile 

Outlying value

75th percentile 

Outlying values

EXPLANATION

Figure 6. Box plots showing CaO and Na2O content of siltite in the Big Creek and 

Apple Creek Formations, Lemhi Range. Major-oxide values are plotted on a logarithmic scale. Yac, coarse siltite unit of Apple Creek Formation; Yad, diamictite unit of 

Apple Creek Formation; Yb, Big Creek Formation.

provenance during deposition of the Big Creek and Apple 

Creek Formations. Relative to quartz and alkali feldspar, plagioclase is less stable during weathering. Most likely, the proportion of plagioclase that was available for deposition varied 

with the degree of weathering during erosion of the source area 

and storage in alluvial and marine environments, prior to final 

deposition. 

Conclusions

The Big Creek, Apple Creek, and Gunsight Formations of 

the Lemhi Group were derived from a terrane having the majoroxide composition of the Archean quartzo-feldspathic gneiss 

(also known as the “Dillon Granite Gneiss”) of southwestern 

Montana. Whether the Archean gneiss of southwestern Montana 

was the source, as suggested by paleocurrents and conglomerate 

in the Gunsight Formation (R.G.Tysdal, D.A. Lindsey, and K.I. 

Lund, unpub. data, 2002), or whether some other gneiss terrane 

also contributed detritus to the Lemhi Group, cannot be 

determined from major-oxide data alone. In any case, derivation 

from a gneiss terrane implies an intracratonic source. 

Sediments of the Big Creek, Apple Creek, and Gunsight 

Formations were rich in plagioclase, which was derived from 

gneiss. Plagioclase content is reflected in values for CaO 

(except in the Big Creek Formation, which contains calcite and 

dolomite) and Na2O, which can be used as proxies for the 

amount of plagioclase. To varying degrees, plagioclase was converted to clays (and CaO and Na2O were depleted) during 

weathering of the source area and transit to the depositional 

basin. In the Lemhi Range, plagioclase content of siltite varies 

in the Big Creek and Apple Creek Formations, indicating varying source areas or variable rates of weathering, erosion, and 

sediment transport over time.

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