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Stratigraphy and Depositional Environments 

of Middle Proterozoic Rocks, Northern Part 

of the Lemhi Range, Lemhi County, Idaho

U.S. Geological Survey Professional Paper 1600

U.S. Department of the Interior 

U.S. Geological Survey
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Stratigraphy and Depositional Environments of 

Middle Proterozoic Rocks, Northern Part of the 

Lemhi Range, Lemhi County, Idaho

By R.G. Tysdal

U.S. Geological Survey Professional Paper 1600

U.S. Department of the Interior 

U.S. Geological Survey
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director

First Printing 2000

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

Box 25286, Federal Center

Denver, CO 80225

This report is also available online at 

http://greenwood.cr.usgs.gov/pub/ppapers/pl600

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.

Stratigraphy and depositional environments of Middle Proterozoic rocks, northern 

part of the Lemhi Range. Lemhi County, Idaho / by R.G. Tysdal

p. cm.—(U.S. Geological Survey professional paper ; 1600)

Includes bibliographical references (p. - ).

1. Geology, Stratigraphic—Proterozoic. 2. Geology—Idaho—Lemhi 

County. I. Title. II. Series. 

QE653.5.T97 1999 

551.7'15'097678—dc21 98-18116

CIP
CONTENTS

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

Introduction .................................................................................................................... 2

Geologic Setting...................................................................................................... 3

Terminology............................................................................................................ 3

Big Creek Formation....................................................................................................... 5

North of Lem Peak Fault......................................................................................... 5

Light-Gray Heavy-Mineral-Bearing Metasandstone ...................................... 6

Dark-Gray Siltite Strata .................................................................................. 8

Strata Directly Above Measured Section........................................................ 9

Channel Deposit.............................................................................................. 11

Other Strata of Mapped Area.......................................................................... 11

Volcanic Flow.................................................................................................. 11

South of Lem Peak Fault......................................................................................... 13

Upper Contact......................................................................................................... 15

Origin...................................................................................................................... 16

Apple Creek Formation................................................................................................... 17

North of Lem Peak Fault......................................................................................... 18

Fine Siltite Unit............................................................................................... 18

Current Ripples....................................................................................... 18

Floating Argillite (Mudstone) Clasts ...................................................... 18

Graded-Stratified Siltite.......................................................................... 19

Diamictite Unit........................................................................................ 20

Siltite....................................................................................................... 23

Matrix-Supported Conglomerate............................................................ 23

Grain-Supported Conglomerate.............................................................. 24

Magnetite Bands ..................................................................................... 24

Coarse Siltite Unit........................................................................................... 24

Light-Gray Metasandstone...................................................................... 25

Gray-Green Siltite................................................................................... 27

Origin of Apple Creek Formation North of Lem Peak Fault.......................... 28

South of Lem Peak Fault......................................................................................... 28

Yellow Lake Unit............................................................................................ 28

Carbonate Beds....................................................................................... 29

Metasandstone......................................................................................... 31

Contacts........................................................................................................... 31

Origin of Apple Creek Formation South of Lem Peak Fault.......................... 31

Correlation ...................................................................................................... 31

Gunsight Formation........................................................................................................ 32

Origin...................................................................................................................... 32

SwaugerFormation......................................................................................................... 33

South of Lem Peak Fault......................................................................................... 33

Dunes and Ripples .......................................................................................... 33

Transition Strata.............................................................................................. 35

Origin...................................................................................................................... 35

Lawson Creek Formation................................................................................................ 36

Metasandstone and Orthoquartzite.......................................................................... 36

Siltite....................................................................................................................... 37

Origin...................................................................................................................... 37

Conclusions..................................................................................................................... 37

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

	in
IV CONTENTS

FIGURES

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

2. Map showing quadrangle boundaries, major faults, and features referred to in text............................................... 4

3. Graphic log of measured section of Big Creek Formation...................................................................................... 7

4-7. Photographs showing:

4. Reactivation surfaces in Big Creek Formation................................................................................................. 9

5. Heavy minerals along scour surface between two dunes of Big Creek Formation.......................................... 9

6. Asymmetrical ripples on dune of Big Creek Formation ................................................................................ 10

7. Small two-dimensional dune in upper part of Big Creek Formation ............................................................. 10

8. Sketch of three-dimensional dune in upper part of Big Creek Formation............................................................. 11

9-34. Photographs showing:

9. Tidal channel deposit in upper part of Big Creek Formation......................................................................... 12

10. Upper strata of tidal channel deposit of Big Creek Formation....................................................................... 12

11. Heavy-mineral lens in lower part of Big Creek Formation............................................................................ 14

12. Small two-dimensional dunes in Big Creek Formation of type area.............................................................. 14

13. Reactivation surfaces in dunes of Big Creek Formation in type area............................................................. 15

14. Double heavy-mineral laminae in Big Creek Formation................................................................................ 15

15. Current ripples in fine siltite unit of Apple Creek Formation......................................................................... 19

16. Ripple crosslaminae in fine siltite unit of Apple Creek Formation................................................................ 20

17. Ripples in fine siltite unit of Apple Creek Formation..................................................................................... 20

18. Planar clast fabric in conglomerate of fine siltite unit of Apple Creek Formation......................................... 21

19. Stratified siltite in fine siltite unit of Apple Creek Formation......................................................................... 21

20. Sequence of structures in graded stratified siltite of fine siltite unit of Apple Creek Formation.................... 22

21. Ripple crosslaminated siltite in fine siltite unit of Apple Creek Formation.................................................... 22

22. Fluid-escape structure in fine siltite unit of Apple Creek Formation.............................................................. 23

23. Light-gray metasandstone bed in coarse siltite unit of Apple Creek Formation ............................................ 25

24. Disrupted siltite (mudstone) layer in coarse siltite unit of Apple Creek Formation....................................... 25

25. Convolute lamination in coarse siltite unit of Apple Creek Formation........................................................... 26

26. Climbing ripples in coarse siltite unit of Apple Creek Formation.................................................................. 26

27. B all- and-pillow and pillar structures in coarse siltite unit of Apple Creek Formation .................................. 27

28. Flute and groove casts and syneresis cracks in coarse siltite unit of Apple Creek Formation........................ 28

29. Slump folds in Yellow Lake unit of Apple Creek Formation.......................................................................... 30

30. Lenticular bedding in Yellow Lake unit of Apple Creek Formation............................................................... 30

31. Pseudonodules in Yellow Lake unit of Apple Creek Formation..................................................................... 31

32. Dune in orthoquartzite of Swauger Formation............................................................................................... 34

33. Antidune in orthoquartzite of Swauger Formation......................................................................................... 34

34. Ripples on surface of orthoquartzite bed of Swauger Formation................................................................... 35

TABLES

1. Stratigraphic sequence of Middle Proterozoic units exposed in vicinity of mapped area............................................ 6

2. Oxides of major elements from amygdaloidal andesite flow within Big Creek Formation....................................... 13
Stratigraphy and Depositional Environments of Middle 

Proterozoic Rocks, Northern Part of the Lemhi Range,

Lemhi County, Idaho

By R.G. Tysdal

ABSTRACT

Middle Proterozoic rocks in a region of detailed geologic mapping in the northern part of the Lemhi Range are 

separated into two packages by the Tertiary Lem Peak normal fault. Strata north of the fault, the main focus of this 

report, contain thicknesses and lithofacies of the Big Creek 

and Apple Creek Formations of the Lemhi Group that contrast with those south of the fault. Strata south of the fault, 

examined mainly in reconnaissance, may have been thrust 

eastward prior to being downdropped.

The Big Creek north of the Lem Peak fault is light-gray 

coarse-grained siltite to medium-grained metasandstone 

with prominent silty laminae of rusty-brown-weathering 

carbonate and dark-gray heavy-mineral laminae. Many beds 

comprise two-dimensional dunes (megaripples) that display 

planar (tabular) crosslamination; dune wavelengths are as 

much as 3 meters. Beds composed of three-dimensional 

dunes that display trough crosslamination occur in the upper 

part of the sequence. Dark-gray argillaceous siltite forms 

sequences less than 5 meters thick in the upper part of the 

formation. Reactivation surfaces and double clay drapes 

were observed locally and are indicative of tidal current processes. Bipolar current activity is evident in the sedimentary 

structures. Other sedimentary structures accord with the 

tidal interpretation: flaser and lenticular bedding, herringbone crosslamination, interbedded dunes and siltite (mudstone) lenses and beds, and tidal channel deposits. An 

amygdaloidal andesite flow 2-4 meters thick occurs within 

the upper part of the Big Creek in one area.

The Big Creek south of the fault, in the type area of the 

formation, was examined in reconnaissance traverses. In the 

lower part of the formation, gently inclined planar strata of 

dark-gray heavy minerals form prominent laminae in gently 

inclined trough crosslaminae, and in 2-3 centimeter thick 

lenses that extend laterally as much as 100 meters or more. 

These strata are interpreted as beach deposits. The uppermost part of the Big Creek in the type area is composed of 

two-dimensional dunes that are stacked one atop another. 

The dunes display wavelengths of 2-3 meters. The

dominant current transport in these upper strata was to northeast; subordinate current transport was to southwest. Reactivation surfaces and double clay drapes are present within the 

dunes, indicating deposition from tidally controlled currents.

The Apple Creek Formation north of the Lem Peak fault 

consists of three gravity-flow units that, in ascending order, 

are the fine siltite unit, the diamictite unit, and the coarse siltite unit. The thickness of the three units totals 4,000-5,000 

meters. South of the fault, the Apple Creek is only 700+ 

meters thick and is a single lithofacies. The Apple Creek is 

unconformable above the Big Creek Formation in both areas.

North of the fault, the fine siltite unit of the Apple Creek 

is planar-laminated siltite, ripple crosslaminated siltite, and 

argillaceous siltite. Many beds are graded, with 1-2 centimeter thick layers of siltite grading upward into argillite. The 

unit locally contains matrix-supported granule- to pebblesize argillite clasts. Strata attributed to reworking by bottom 

currents form ripples within the fine siltite unit. Gradedstratified siltite is common, forming bands of coarse-grained 

siltite to fine-grained 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 sequence of graded turbidite beds.

Abundant beds of poorly sorted conglomerate—diamictite—characterize a succession of otherwise fine-grained 

strata within the Apple Creek north of the Lem Peak fault. 

Most of the conglomerate 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 meters or 

more thick between conglomerate 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 

strata are attributed to a turbidite origin.

The coarse siltite unit is composed of gray-green 

medium- to coarse-grained siltite and fine-grained metasandstone. Siltite beds observed are normally graded. Soft-sediment deformation structures are widespread: ball-and-pillow
MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

structures are fairly common; flute casts and tool marks were 

observed only locally. 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 sharpbased light-gray metasandstone beds suggest deposition 

from turbidity currents.

Apple Creek strata south of the Lem Peak fault consist 

of dark-gray medium- and fine-grained finely laminated siltite, and locally argillite in the lower part. The siltite is very 

finely planar laminated and many of the fine-grained siltite 

beds are graded; ripple crosslaminae are common. Other beds 

exhibit scoured bases and graded tops. Slump folds are common and indicate gravity-driven downslope movement. Silty 

dolomite forms thin distinctive interlayered beds in the bottom half of the unit. Some of the dolomite forms the base of 

beds that fine upward to noncalcareous siltite. Other distinctive characteristics include the overall fine to medium grain 

size of the siltite, and in the upper part of the section, sporadic 

interbeds of sharp-based, graded, light-gray metasandstone. 

The unit is generally coarser grained in its upper third, 

reflected both in the coarser grain size of the siltite beds and 

in the presence of the metasandstone beds. The assemblage 

of sedimentary structures in the lower part of the Apple Creek 

south of the fault is indicative of turbidites. Strata generally 

are finer grained than the lithofacies units north of the fault 

and likely were deposited from lower energy turbidity currents. Metasandstone of the upper part of the unit suggests 

shallow water deposition, perhaps influenced by waves. This 

would be in accord with shallowing of the depositional environment and a depositional transition into the overlying Gunsight Formation.

The Gunsight Formation, present only in a small part of 

the mapped area north of the Lem Peak fault, consists of crosslaminated orthoquartzite lenses as thick as 10 meters within 

siltite and silty feldspathic metasandstone. The lenses are 

interpreted as channel deposits of meandering streams within 

a flood plain environment, in which siltite strata constitute a 

large part of the rocks and the channels make up a small part.

The Swauger Formation, exposed only south of the Lem 

Peak fault within the mapped area, is mainly crosslaminated 

orthoquartzite of medium to coarse well-rounded grains. 

Two-dimensional dunes occur locally and display wavelengths of 4-6 meters, heights of 0.75-1 meter. The dominant current transport direction was to the northeast. Upper 

surfaces of some dunes contain asymmetrical ripples that 

show transport to the southwest, capping dune foreset laminae inclined to the northeast. Planar-laminated to ripple 

crosslaminated siltite, some beds containing flaser and lenticular bedding, is interbedded with orthoquartzite in the 

upper part of the Swauger. Herringbone crosslamination, 

formed by bipolar currents, occurs in close stratigraphic 

association with dunes that contain reactivation surfaces, 

indicating the herringbone structures to be of tidal origin.

The interlayering of 0.5-1 meter thick beds of orthoquartzite 

and fine-grained siltite units indicates deposition from currents of contrasting flow speeds, and suggests that the transition strata were formed by tidal processes.

The Lawson Creek Formation, present only south of the 

Lem Peak fault in the mapped area, is conformable above the 

Swauger. It is generally quartz-rich metasandstone but 

ranges from about equal parts feldspar and quartz to locally 

orthoquartzite; a fine-grained matrix is typical. Ripple crosslaminae are common; rip-up clasts of argillite chips "float" 

within quartzite beds locally. Siltite and argillaceous siltite, 

interlayered with thin beds of metasandstone, form 

sequences as thick as 10 meters between beds chiefly composed of metasandstone and orthoquartzite. These strata 

range from planar laminated to ripple crosslaminated and 

display abundant flaser and lenticular bedding. The Lawson 

Creek is an intertidal sequence. Many of the sedimentary 

structures are typical of those found in tidal flats.

INTRODUCTION

The Lemhi Range trends southeastward for about 150 

km from the vicinity of Salmon in the eastern part of central 

Idaho (fig. 1). The backbone of the range is generally 

1,200-1,500 m higher than the valley floors to the northeast

114'

44° —

Figure 1. Index map of east-central Idaho.
INTRODUCTION

and southwest. Paleozoic strata are the chief rocks exposed 

in the southeastern part of the range and gradually give way 

to Proterozoic strata in the northwestern part.

The stratigraphic sequence for Proterozoic strata in the 

central part of the Lemhi Range was established by Ruppel 

(1975) during the course of geologic mapping of three 15- 

minute quadrangles—Leadore (Ruppel, 1968), Patterson 

(Ruppel, 1980), and Gilmore (Ruppel and Lopez, 1981) (fig. 

2). The basis for this report is from mapping of two 7.5- 

minute quadrangles; a quadrangle-size area composed of the 

northern part of the Mogg Mountain quadrangle and the 

adjacent southern part of the Hay den Creek quadrangle (Tysdal, 1996a); and the northern part of the May Mountain 

quadrangle (Tysdal, 1996c). The Mogg Mountain quadrangle coincides with the northwest corner of the Patterson 15- 

minute quadrangle, and the Hayden Creek quadrangle lies 

directly north of it. The Lem Peak (Tysdal, 1996b) and Allison Creek (Tysdal and Moye, 1996) quadrangles, and the 

May Mountain quadrangle (Tysdal, 1996c), lie northwest of 

the area of Ruppel's work.

The 7.5-minute quadrangles, and the area equivalent 

to a 7.5-minute quadrangle, were mapped as part of a 

project to study the regional setting of strata associated 

with cobalt-bearing rocks. Stratabound cobaltiferous rocks 

in the Salmon River Mountains west of the Lemhi Range 

were assigned to the Middle Proterozoic Yellowjacket Formation (Hughes, 1983; Hahn and Hughes, 1984) and delineated in quadrangle maps (Bennett, 1977; Lopez, 1981; 

Connor and Evans, 1986; Evans and Connor, 1993). Quadrangle mapping in the northern part of the Lemhi Range 

was undertaken at the behest of Connor to establish the 

stratigraphic relationship of the Yellowjacket Formation of 

the Salmon River Mountains to the Middle Proterozoic 

sequence of the central part of the Lemhi Range. Mapping 

was conducted in several quadrangles by Tysdal (1996a, b, 

c), Tysdal and Moye (1996), K.V. Evans (in progress), and 

G.R. Winkler (in progress). This report presents a general 

description of the Proterozoic sedimentary units mapped by 

Tysdal and a general interpretation of the depositional 

environment for each of them.

GEOLOGIC SETTING

Rocks in the vicinity of the 7.5-minute quadrangles 

mapped by Tysdal (1996a, b, c) and Tysdal and Moye (1996) 

are largely composed of the Middle Proterozoic Lemhi 

Group and overlying Swauger and Lawson Creek Formations (table 1). Small areas of lower Paleozoic strata occur 

locally, and fairly large areas of Tertiary volcanic rocks flank 

these strata on the north. The Proterozoic and Paleozoic 

strata underwent compressional deformation, probably in the 

Cretaceous, creating a series of thrust faults and related 

folds. The trace of the Poison Creek fault, a major thrust that 

dips to the southwest, extends east-west across the

northernmost part of the Lem Peak quadrangle (fig. 2). Subsequent Tertiary and (?)Cretaceous extension in the region 

was accompanied by formation of normal faults, some with 

major displacement. This Cretaceous to Tertiary deformation is in addition to (1) Middle Proterozoic compressional 

deformation reported for local areas of the general region 

(for example, Evans, 1986; Evans and Zartman, 1990), and 

(2) Late Proterozoic extension (Ruppel and Lopez, 1988), 

both of which may have affected rocks in the Lemhi Range.

The Lem Peak normal fault trends eastward across the 

southern part of the Allison Creek, Lem Peak, and Mogg 

Mountain quadrangles and dips steeply to the south (fig. 2). 

It underwent a large amount of displacement, south side 

down, as indicated by juxtaposition of Swauger Formation 

on the south against Big Creek Formation on the north. Several thousand meters of displacement are implied by stratigraphic separation (Tysdal, 1996a, b; Tysdal and Moye, 

1996). Swauger strata adjacent to the fault locally contain 

tightly folded rocks formed during compression, but directly 

adjacent Big Creek strata north of the Lem Peak fault lack 

corresponding folded rocks. This contrast suggests that the 

structural features of the Swauger directly south of the fault 

were formed prior to juxtaposition against the Big Creek 

Formation along the Lem Peak normal fault. The Swauger 

and other strata south of the Lem Peak fault are believed to 

have been thrust eastward, then downdropped, accounting 

for the contrast in structures, the contrast in thickness of the 

Big Creek and Apple Creek Formations on opposite sides of 

the fault (table 1), and the contrast in lithofacies of each of 

these formations on opposite sides of the fault.

The Inyo Creek and West Fork Formations, named and 

described by Ruppel (1975), are known only from the southwest flank of the central part of the Lemhi Range, in the 

vicinity of creeks by these names (fig. 2). The formations are 

not present in the mapped quadrangles or the directly adjacent region and are not described or considered further here. 

The reader is referred to Ruppel (1975, 1980) and Ruppel 

and Lopez (1988) for data on these formations. The Yellowjacket Formation, as defined and mapped by Ross (1934) in 

the Salmon River Mountains, is not present in or adjacent to 

the mapped area and is not considered further here. Yellowjacket stratigraphy as interpreted by some authors since the 

work of Ross (1934) was reinterpreted and revised by Tysdal 

(2000) and is summarized briefly in this report in the section 

on the Apple Creek Formation.

TERMINOLOGY

Proterozoic rocks throughout most of the northern part 

of the Lemhi Range discussed here have been metamorphosed to the lower greenschist facies. Biotite-grade rocks 

exist in the central to northern part of the Allison Creek 

quadrangle (fig. 2), where biotite has replaced some to all of
MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

114° 113°45' 113°30'
BIG CREEK FORMATION

Figure 2 (facing page). Map showing major faults and 

geographic features referenced in text. Bar and ball on downthrown 

side of normal fault; sawteeth on upthrown side of thrust fault; 

dotted where concealed. 7.5-minute quadrangles: AC, Allison 

Creek; HC, Hayden Creek; LP, Lem Peak; M, May Mountain; MM, 

Mogg Mountain; YP, Yellow Peak. 15-minute quadrangles: 1, 

Lemhi; 2, Patterson; 3, Leadore; 4, Gilmore. X-X', line of measured 

section in Big Creek Formation.

the muscovite. In this report, the following terminology is

used for the metamorphosed clastic rocks.

Argillite. Metamorphosed claystone.

Siltite. Metamorphosed siltstone or mudstone.

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.

Orthoquartzite. Metamorphosed sandstone composed of at 

least 95 percent quartz grains. 

This terminology differs from the "standard" usage for

metamorphosed Proterozoic clastic rocks, wherein "quartzite" denotes a metamorphosed sandstone regardless of

composition.

BIG CREEK FORMATION

The Big Creek Formation was named by Ruppel (1975) 

for strata exposed in the central part of the Lemhi Range, on 

the southwest side. The type locality is directly west of the 

North Fork of Big Creek (Ruppel, 1975) (fig. 2). Rocks of 

the type area lie south of the Lem Peak fault, on a structural 

block that contains Big Creek strata somewhat different in 

character from Big Creek north of the fault. The following 

descriptive sections illustrate the variety of sedimentary features present within the Big Creek north of the Lem Peak 

fault. For comparison, a brief description and a depositional 

interpretation for Big Creek strata of the type area are presented at the end of this section.

Because some users of this report will not be familiar 

with some of the sedimentologic terms that are used, definitions of several terms used in the interpretation of depositional environments of the Big Creek Formation are 

presented.

Dune. Flow-transverse bedforms that develop on a sediment bed under a unidirectional current. They display a spacing of greater than 0.6 m, thus are larger 

than ripples. (In some reports, the term megaripple 

was previously used for these features.) Dunes that 

have straight crests are termed two-dimensional; 

those that are lunate or have sinuous crests are 

termed three-dimensional (Ashley, 1990). 

Reactivation surface. A gently inclined erosional surface 

that separates foresets [in ripples or dunes] of similar

orientations. The erosion surface is produced when 

one ripple or dune overtakes [migrates over] another, 

or is produced during current reversal in a tidal environment. When the tide again reverses to the original 

direction, sediment deposition is reactivated to form 

more foresets (McCabe and Jones, 1977; de Mowbray 

andVisser, 1984).

Clay drape. In a diurnal tidal environment, a strong (dominant) current and a subsequent weak (subordinate) current exist during the course of a tidal cycle. A slackwater stage follows both the dominant current stage 

and the weak current stage as the tide reverses. Clay 

particles deposited during the slack-water stage following the dominant current may be deposited on top of 

existing sedimentary structures, forming a "clay 

drape." A thin layer of sediment deposited during the 

subsequent weak current stage may overlap the clay 

drape. If clay also is deposited during the slack-water 

stage that follows the subordinate (weak) tidal current, 

then a "double clay drape" is formed. Double-clay 

drapes are indicative of a tidal environment (Visser, 

1980; de Mowbray and Visser, 1984; Nio and Yang, 

1991).

NORTH OF LEM PEAK FAULT

The Big Creek north of the Lem Peak fault is mainly 

light gray coarse-grained siltite to medium-grained 

metasandstone composed of quartz, feldspar, auxiliary muscovite and tourmaline, and matrix. Distinguishing characteristics throughout much of the formation include silty laminae 

of limestone and dolomite that weather pale yellowish brown 

(rusty brown), and dark-gray heavy-mineral laminae, typically 1-2 grains thick. Large-scale tabular and trough crosslamination is widespread and was formed from straightcrested and sinuous-crested dunes. Beds commonly are 

about 1 m thick. Dark-gray argillaceous siltite zones, generally less than 5 m thick, are present locally and contain current ripples, and flaser and lenticular bedding. An 

amygdaloidal andesite flow 2-4 m thick lies within the upper 

part of the formation west of Wright Creek (fig. 2) (Tysdal, 

1996a).

The sequence has been metamorphosed to the lower 

greenschist facies throughout most of the mapped area, but 

biotite-grade rocks exist in the central to northern part of the 

Allison Creek quadrangle (fig. 2), where biotite has replaced 

some to all of the muscovite. The rusty-weathering carbonate laminae, a prominent criterion for recognition of Big 

Creek strata, are not so obvious in the biotite-grade rocks, 

possibly due to the increased metamorphic grade. The upper 

contact of the formation is unconformable (discussed in a 

later section of this report), marked by the abrupt appearance 

of gray-green siltite of the Apple Creek Formation. The Big 

Creek Formation is about 2,700 m thick.
MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO 

Table 1. Stratigraphic sequence of Middle Proterozoic units exposed in vicinity of mapped area.

[Yellow Lake unit of Apple Creek Formation south of Lem Peak fault is not necessarily the Stratigraphic equivalent of all three 

units of the Apple Creek north of fault—see text for discussion. Leaders (--), unit not present or not exposed; +, minimum thickness, due to erosion or fault]

Group

Lemhi 

Group

North of Lem Peak fault

Formation

Swauger Formation

Gunsight Formation

Coarse siltite unit 

Apple

Creek Diamictite unit

Formation 

Fine siltite unit

Big Creek Formation

Thickness 

(meters)

....

3,100(?)

(?)

'2,500+ 

1 600-1,500 

1 1,000

'2,700

....

South of Lem Peak fault

Thickness 

(meters)

2300+

3 3,100

4 1,725+

5700+

33,100

6600

6380+

Formation

Lawson Creek Formation

Swauger Formation

Gunsight Formation

Yellow Lake unit

Big Creek Formation

Apple 

Creek 

Formation

West Fork Formation

Inyo Creek Formation

Tysdal (1996a, b), Tysdal and Moye (1996).

2Tysdal(1996c).

3Ruppel (1975).

4McBean (1983).

5Thickness measured by R.G. Tysdal, G.R. Winkler, K.V. Evans, and J.J. Connor.

6Thickness measured by R.G. Tysdal, G.R. Winkler, K.V. Evans.

On the ridge between the headwaters of Wright and 

Kadletz Creeks (fig. 2), a section measured (fig. 3) in the 

upper part of the Big Creek contains many features 

representative of those found in much of the formation. The 

measured section, based at the Lem Peak fault, displays thick 

successions of metasandstone, dunes, and thin interlayered 

zones of dark-gray fine-grained siltite. The section characterizes the sedimentological interplay of the dominantly 

metasandstone succession as it intertongues with siltite, and 

provides the basis for much of the following discussion.

LIGHT-GRAY HEAVY-MINERAL-BEARING 

METASANDSTONE

Metasandstone of the measured section is chiefly composed of 50-60 percent quartz, 10-20 percent feldspar, 5-15 

percent matrix, and as much as 5 percent heavy minerals of 

tourmaline, zircon, and ilmenite that are dispersed within 1-2 

mm thick laminae. Carbonate laminae are common, interpreted as originally composed of detrital grains. Recrystallized during metamorphism, the carbonate grains form

laminae that range from mainly carbonate with minor siliciclastic grains, to carbonate that cements mainly siliciclastic 

grains. Planar-layered beds 1-2 m thick are interstratified 

with planar and trough crosslaminated dunes that display 

wavelengths of 1-3 m and heights of 0.3-0.5 m. Beds of ripple crosslaminated metasandstone also are present.

About 200-300 m laterally southeast of the line of the 

measured section, contiguous strata display planar-laminated 

beds 1.5-3 m thick. The beds are composed of two-dimensional dunes that display planar (tabular) crosslamination in 

sets 0.5-1 m high; dune wavelengths are as much as 3 m and 

heights are 0.3-0.5 m. Reactivation surfaces were observed 

in some beds of the two-dimensional dunes, locally marked 

by a 0.1-1 cm thick siltite (mudstone) layer (fig. 4). Dominant current transport was to the southwest. Prominent bedthick foresets indicating transport to the northeast are common within these strata, although the beds are thinner and 

less abundant than beds with southwest-directed foresets.

Figure 3 (facing page). Graphic log of partial thickness of Big 

Creek Formation, between Wright and Kadletz Creeks, May Mountain and Lem Peak quadrangles (fig. 2).
BIG CREEK FORMATION

UNIT DESCRIPTION

275 -

225 '

200

175 J

150 -

125 -

100 -

75 -

25 -

1 Siltite, medium- and dark-gray; flaser bedding, ripple crosslamination; transport direction to 

both the northeast and southwest

Metasandstone, light- to medium-gray, fine-grained; 0.5-1.0 m thick crossbeds; some 

dark-gray heavy-mineral laminae; sparse rusty-brown-weathering carbonate laminae

Interbedded siltite and metasandstone in units 1 -2 m thick. Siltite, dark-gray; flaser

bedding; metasandstone, light-gray, fine-grained; ripple crosslaminae, carbonate laminae

Metasandstone, medium-gray, fine-grained; two-dimensional dunes with wavelength 1.5-2.0 m,

height about 0.5 m; rusty-brown-weathering carbonate laminae; sparse dark-gray heavy

minerals in laminae 1 grain thick; ripples show transport to northeast

Siltite, dark-gray, medium- to coarse-grained; rusty-brown-weathering carbonate laminae

Andesite flow; chalcedony-filled amygdules, most abundant and largest in upper part of

flow; flow is well cleaved, in continuity with the sedimentary sequence. Chemical analyses

showing major oxides are in table 2

Siltite, dark-gray, medium- to coarse-grained

Metasandstone, medium-gray, fine-grained; local rusty-brown-weathering carbonate laminae;

1-2 m thick zones of siltite with flaser bedding

Siltite, dark-gray, medium- to coarse-grained; rusty-brown-weathering carbonate laminae;

ripple crosslaminated; asymmetrical ripples on bedding surfaces show transport to northeast;

sparse interbeds of siltite in zones about 1 m thick

Metasandstone, medium-gray, fine-grained; two-dimensional dunes with wavelength of 1-2 m, 

height about 0.5 m; rusty-brown-weathering carbonate laminae; ripple crosslaminated; blocky 

weathering

Orthoquartzite, light-gray to medium-light-gray, medium-grained; rounded grains 

Siltite, grayish-yellow-green, fine-grained

Metasandstone, medium-gray, fine-grained; two-dimensional dunes with wavelength 1-2 m, height 

about 0.5 m; rusty-brown-weathering carbonate laminae; ripple crosslaminated; blocky weathering

10

13

14

15

16

Siltite, green and gray-green, abundant muscovite

Siltite, olive-gray, fine- to medium-grained, finely laminated; rusty-brown-weathering carbonate 

laminae; well cleaved

Metasandstone, fine-grained, to coarse-grained siltite, grayish yellow green, in beds 0.5-2.0 m 

thick; rusty-brown-weathering carbonate laminae in lower 10m; crossbeds in sets 0.5-1.0 m high; 

some dark-gray heavy-mineral laminae; one 4 cm thick lens of heavy minerals—zircon, tourmaline, 

and ilmenite

17 Siltite, gray-green, fine- to medium-grained; planar laminated to very small ripple crosslaminae

18 Metasandstone, fine-grained, to coarse-grained siltite, grayish yellow green, in beds 0.5-2.0 m 

thick; rusty-brown-weathering carbonate laminae; crossbeds in sets 0.5-1.0 m high; some darkgray heavy-mineral laminae

19 Siltite, medium-gray, medium- to coarse-grained, finely laminated to ripple crosslaminated; some 

rusty-brown-weathering carbonate laminae;contains metasandstone planar-laminated interbeds 

like those in upper part of unit 18; muscovite prominent on some bedding surfaces

20 Metasandstone, fine-grained, to coarse-grained siltite, medium gray; platy weathering; poor outcrop; 

topmost surface of unit displays two-dimensional dunes, wavelength about 2 m, height about 0.3 m

21 Siltite, medium-gray, planar and ripple crosslaminated; flaser bedding

22 Metasandstone, olive-green, fine-grained, platy-weathering, planar- laminated to ripple crosslaminated; dark-gray heavy-mineral laminae; rusty-brown-weathering carbonate laminae

23 Covered

Fault at base
MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

Beds comprising three-dimensional dunes are present 

locally within the sequence, are spaced 1-2 m apart, and have 

heights of 0.3-0.5 m. Heavy-mineral laminae are common 

in basal strata of these trough crosslaminated structures. In 

one outcrop, heavy minerals form a 5-10 mm thick layer 

directly above a scour surface that separates two dunes. The 

scour surface is inclined to the northeast, opposite to the 

transport direction indicated by the foreset laminae of the 

dunes that lie above and below the surface, suggesting 

reversing tidal currents (fig. 5).

Siltite lenses as long as 2 m and as thick as 15 cm occur 

locally between the beds of metasandstone (fig. 4). In one 

outcrop a siltite lens is underlain by asymmetrical ripples that 

show transport to the southwest, which is opposite to that 

indicated by the large-scale foresets of the 1 m thick underlying part of the bed (fig. 6). The ripples have a wavelength 

of about 8 cm and height of about 1 cm.

The uppermost few millimeters of planar-laminated 

beds locally contain soft-sediment load structures. The 

loading is of sediment that contains abundant heavy minerals. The upper part of the load structures has been eroded. 

The sequence of events apparently is as follows: deposition 

of the lower bed, by a dominant current; erosion of the 

upper surface; deposition of the heavy-mineral layer, by a 

weak current; erosion of the upper part of the deformed 

heavy-mineral layer, (?)by a weak current; and, subsequently, deposition of the overlying bed, by a dominant current. In another case, a heavy-mineral layer about 2 mm 

thick occurs between two 0.5 m thick beds in which the 

bed-thick foreset laminae are inclined to the southwest. 

The heavy-mineral layer is inclined to the northeast in concert with the underlying erosion (scour) surface. The layer 

is a subordinate current cap, a depositional structure produced on the crest of a dune by a subordinate current (de 

Mowbray and Visser, 1984, p. 817).

Northwest of Wright Creek, beds on strike with those of 

the measured section show that, over a distance of several 

tens to hundreds of meters parallel to bedding, 1-3 m thick 

beds of metasandstone taper laterally and intertongue with 

other metasandstone beds, producing slight angular relationships. Some beds are eroded deep into underlying beds. In 

other places metasandstone fills 1-1.5 m deep by 2-5 m wide 

channels cut into underlying metasandstone.

Aim thick quartzite bed (fig. 3, unit 11), and underlying 0.5 m thick dark-gray fine-grained siltite, occur near the 

middle of a 75 m thick succession of metasandstone. The 

quartz-rich bed is medium grained, about 85 percent subangular to rounded quartz grains, 10 percent feldspar, and 5 

percent matrix. Basal strata of one metasandstone, directly 

above siltite of unit 19 (fig. 3), contain dark-gray heavy minerals in 1 -lamina-thick layers in some beds, although local 

4-5 cm thick planar laminated layers are present. These 

heavy-mineral laminae comprise mostly zircon, tourmaline, 

and ilmenite(?) grains. These strata show sorting and

selective concentration of the kinds of grains that generally 

are dispersed or within laminae of the metasandstone of the 

formation.

Two-dimensional dunes are well exposed on a ridge 

east of Buck Creek (figs. 2, 7). The dunes are in fine- to 

medium-grained metasandstone, display wavelengths of 2-3 

m and heights of 0.5-1 m, and indicate sediment transport to 

northeast. Zones of dark-gray siltite are not common in this 

area and are only 1-2 m thick where they do occur. The size 

of the dunes and the scarcity of fine-grained sediment suggest high-energy conditions. The structures here are larger 

than those of the section measured near Wright Creek (figs. 

2, 3) and stratigraphically are higher in the formation. In the 

upper 200-300 m of the formation in the Buck Creek area, 

the metasandstone is fine grained, crosslamination is small 

scale, heavy minerals are nearly absent, and thin zones of 

dark-gray siltite are fairly common. Argillaceous rip-up 

clasts of pebble to cobble size are present in the basal part of 

a few thick metasandstone beds.

DARK-GRAY SILTITE STRATA

The upper part of the measured section (fig. 3, units 1 

and 3) displays dark-gray argillite and fine-grained siltite 

interbedded with light-gray coarse-grained siltite to finegrained metasandstone. Rocks of these contrasting colors 

and grain sizes alternate in layers generally ranging from 2-3 

mm thick to 3-4 cm thick, although beds as thick as 10-15 

cm occur locally. The finer grained rocks tend to be planar 

laminated and the coarser grained rocks are ripple crosslaminated. Both flaser bedding and lenticular bedding are common, as are rippled bedding surfaces. Ripples are mainly 

longitudinal current forms and typically have wavelengths of 

2-5 cm and heights of 0.5-1 cm. Ripple transport directions 

are generally to the northeast, but a few beds indicate transport to the southwest; herringbone crosslamination displays 

the opposing transport directions indicated locally by the ripples of bedding surfaces. Some of the coarser grained beds 

display small soft-sediment deformation features, including 

ball-and-pillow structures 1-3 cm across, flame structures, 

and water-escape pillar structures.

The thick dark-gray siltite of units 1 and 3 (fig. 3) of 

the upper part of the measured section contains a wider 

variety of structures than the other, thinner siltite zones. 

But the structures of all zones within the measured section, 

and throughout the formation, show similar size features 

and ripples that generally indicate transport to the northeast. 

The zones of dark-gray siltite in the formation are lensshaped and laterally discontinuous. Some zones extend for 

only a few tens of meters; thicker zones extend for a few 

hundred meters. None was physically traceable from one 

ridge to the next.
BIG CREEK FORMATION

Figure 4. Reactivation surfaces 

(indicated by arrows) of dune in 

Big Creek Formation in Kadletz 

Creek area (fig. 2), vicinity of 

measured section of figure 3. 

Siltite (mudstone) along each 

reactivation surface is up to 1 

cm thick. Lens of siltite at 

upper right occurs between 

dunes and has maximum thickness of about 10 cm, length of 

about 3 m; layering in this lens 

is cleavage. (See fig. 6 for 

closeup.) Hammer for scale, 

handle 24 cm long.

Figure 5. Dunes with foreset 

laminae (short-dashed lines) indicating transport to the southwest (to left), separated by a 

scour surface that is inclined to 

the northeast. Heavy minerals 

form a 5-10 mm thick layer 

along the scour surface (intermittent heavy dashed line). 

Dunes are in Big Creek Formation in Kadletz Creek area (fig. 

2), vicinity of measured section of figure 3. Hammer for 

scale, handle 24 cm long.

STRATA DIRECTLY ABOVE MEASURED SECTION

Stratigraphically directly above the measured section, 

extending north about 1,000 m to the Hayden Creek fault 

(not shown in fig. 2), zones of dark-gray fine-grained siltite 

are absent. The sequence consists of fine- to mediumgrained metasandstone that contains two-dimensional dunes 

with spacing of ?>-4 m and heights of about 1 m; threedimensional dunes that display spacing of 1—1.5 m and 

heights of about 0.5 m; and interbeds 1-2 m thick, planar

laminated, and generally lacking in heavy minerals. Softsediment slump structures are common in the plane-bedded 

rocks, and in one outcrop include double clay drapes.

Three-dimensional dunes are well developed in finegrained metasandstone in the upper part of the Big Creek 

Formation. At the head of Payne Creek (fig. 2), for example, 

three-dimensional dunes occur in 1-2 m thick beds and display trough crosslamination. The basal half of each bed is 

finely planar laminated and grades abruptly upward into 

trough crosslamination of dunes that have spacing of 1-3 m
10 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

Figure 6. Closeup of upper 

surface of dune and siltite lens 

(directly above hammer) in 

upper right of photograph of 

figure 4. Upper surface of dune 

displays asymmetrical ripples 

that indicate transport to the 

southwest (to upper left), 

which is opposite to that indicated by reactivation surfaces 

within the dune. Hammer for 

scale, handle 24 cm long.

Figure 7. Small two-dimensional dune in upper middle 

part of Big Creek Formation in 

vicinity of Buck Lakes (west of 

Buck Creek, fig. 2). Wavelength is about 1.5 m and 

height about 0.3 m. Transport 

was to the northeast, to right in 

photograph. Hammer for scale, 

handle 24 cm long.

and heights about 0.5 m (fig. 8). Some beds display ripple 

crosslamination directly above the basal strata. Rustyweathering carbonate laminae are common in the crosslaminated strata but generally are lacking from the planar-laminated lower strata. Dark-gray fine- to medium-grained siltite 

locally forms isolated lenses as much as 10 cm thick and 1 m 

long, within troughs. Chips of argillite and argillaceous siltite are present locally within basal strata of the metasandstone beds, and dark-gray heavy-mineral laminae occur

within trough crosslaminated strata. Sparse load structures 

formed where trough crosslaminated sediment settled into 

planar-laminated sediment.

The lower, planar strata of each bed are interpreted as 

deposits of the upper flow regime, overlain by sediments 

deposited from decelerating currents of the lower flow 

regime. The local lenses of dark-gray siltite were deposited 

during slack water or ebb stages of sedimentation. The 

lenses may be remnants from extensive siltite beds that were
BIG CREEK FORMATION 11

Figure 8. Sketch of features in small threedimensional dune in upper strata of Big Creek 

Formation. Near Payne Creek (fig. 2).

Crosslaminated fine-grained 

metasandstone; contains 

rusty-weathering carbonate 

laminae, dark-gray heavymineral laminae; local siltite 

(mudstone) lens as much as 

10 cm thick at top

Ripple crosslaminated metasandstone; contains rustyweathering carbonate laminae

Planar-laminated fine-grained 

metasandstone; no carbonate 

laminae

largely eroded during a subsequent flood tide. Load structures in planar-laminated rocks indicate water-saturated 

sediments.

earthquake activity. In the Lemhi Range, the localization of 

the affected strata and the interlayering of distorted and 

nondistorted beds argue against such a catastrophic origin.

CHANNEL DEPOSIT

West of Buck Lakes (not shown in fig. 2, but directly 

west of central part of segment of Buck Creek shown), large 

soft-sediment slump structures occur within a 50+ m thick 

sequence that has an exposed width of 50+ m—its full width 

is not exposed. The strata are composed of fine-grained 

metasandstone that contains carbonate laminae, emphasized 

by differential weathering. Deformed strata form zones 

0.5-4 m thick between undeformed beds of planar-laminated 

metasandstone (fig. 9). Individual slump folds are as thick 

as the thickest bed and are 2-3 m across. The largest slumps 

show progressively less deformation (less dewatering) from 

base to top. In the upper part of the sequence, zones 0.3-0.5 

m thick contain massive to planar-laminated metasandstone 

that grades upward into interbedded metasandstone and 1 cm 

thick dark-gray argillite (fig. 10). Strata of these zones overlie deformed metasandstone, although the zones themselves 

typically are not deformed.

The above sequence is confined to the one locality and 

is interpreted as sediment deposited within a tidal channel in 

an intertidal setting. The largest soft-sediment deformation 

structures are in the lower part of the sequence, the interstratified metasandstone and argillite layers in the upper part. 

This sequence suggests a channel that contains thick beds of 

rapidly deposited, water-saturated sediment in its lower part, 

giving way upward to thinner beds of water-saturated sediment, and in the upper part to sediment that included silt and 

clay deposited during lower energy sedimentation or slack 

water conditions. Interbeds of undeformed strata reflect 

deposition of nonsaturated sediments.

Features similar to those described, and shown in figures 9 and 10, have been described by Glendinning (1988) 

in Upper Proterozoic rocks of Scotland. There, however, 

the dewatering structures are not confined to a local zone 

but form sheets. Glendinning attributed their origin to

OTHER STRATA OF MAPPED AREA

The stratigraphically lowest exposed Big Creek strata 

north of the Lem Peak fault are in the southwesternmost corner of the Lem Peak quadrangle (fig. 2). The sequence is 

chiefly fine grained to medium-grained metasandstone. 

Some beds are quartz rich and contain well-rounded quartz 

grains, associated with potassium feldspar, plagioclase, 1-2 

percent matrix, and heavy-mineral laminae. Trough crossbeds have a wavelength of 1-3 m and height of 0.3-0.5 m. 

Dark-gray lenticular and flaser crosslaminated fine-grained 

siltite in zones 1-3 m thick is fairly common in this part of 

the formation. Some zones are directly overlain by 

metasandstone that displays 0.5 m high sets of trough crosslamination. Argillite chips occur in the basal part of some 

metasandstone beds. These strata probably were deposited 

in an environment similar to that of the Payne Creek area, 

described previously.

VOLCANIC FLOW

A volcanic flow composed of amygdaloidal andesite 

lies within the upper part of the Big Creek Formation adjacent to the upper reaches of Wright Creek. The flow has a 

strike length of about 2 km, assuming that it is continuous 

beneath glacial deposits in the valley of Wright Creek. The 

flow is 2 m thick southeast of Wright Creek, within the measured section (fig. 3), and attains a maximum observed thickness of about 4 m on the ridge northwest of the creek. 

Amygdules are most abundant and largest in the upper part 

of the flow. Northwest of Wright Creek, amygdules also 

form zones lower in the flow and are about parallel to layering. All amygdules are filled with chalcedony. The flow 

rocks are well cleaved, in continuity with the enclosing sedimentary sequence. Major-oxide chemical analyses of the
12 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

Figure 9. Outcrop interpreted 

as strata within a tidal channel 

deposit, upper part of the Big 

Creek Formation in vicinity of 

Buck Lakes (fig. 2). Bedding 

surfaces accentuated by dashed 

lines. Upper part of sequence 

(right side of photo) shows softsediment deformation— slumping due to dewatering. Note that 

lower part of the main "slump" 

(vicinity of hammer) shows 

greater deformation than upper 

part of structure, and that deformation of beds within slump decreases upward. This suggests 

that dewatering of strata was a 

process ongoing at time of deposition, with least dewatering 

taking place in upper sediment 

of slump. This also is indicated 

to right of hammer where upper 

strata, in area of "anticline," 

overlie erosionally truncated 

lower strata. Lower part of sequence is nearly planar layered 

strata. Photo is looking approximately along axis of transport. 

Hammer for scale, handle 24 cm 

long.

Figure 10. Outcrop interpreted as upper strata within a tidal 

channel deposit, upper part of 

the Big Creek Formation in vicinity of Buck Lakes (fig. 2). 

Lower part of photo shows softsediment deformation—slumping due to dewatering—prior to 

deposition of overlying strata. 

Photo is looking approximately 

along axis of transport. Hammer 

for scale, handle 24 cm long.

flow are shown in table 2. Care must be taken in use of the 

analyses because the loss-on-ignition value is high, indicating significant mobility of the alkali elements (Ca, Na, K).

Southwest of Wright Creek the flow directly overlies 1 

m of fine-grained siltite, into which its base locally forms

small load casts. The flow is overlain by 0.5 m of the siltite. 

Northwest of Wright Creek the flow directly overlies finegrained crosslaminated metasandstone. No fragments from 

the flow were found in strata directly above the flow. The 

Big Creek of the region was rotated to near-vertical dips
BIG CREEK FORMATION 13

Table 2. Oxides of major elements from amygdaloidal andesite flow within Big Creek Formation. 

[<, less than; analysts: D.F. Siems, I.E. Taggart]

Field

No.

89ITzl07

90ITz34B

Lat

N.

44-45-01

44-45-22

Long 

W.

113-48-27

113-49-31

Major oxides (percent)

SiO2

52.4

52.7

A1203

10.8

15.8

FeTO3

11.0

11.4

MgO

6.15

8.77

CaO

5.39

0.63

Na2O

<0.15

3.64

K2O

3.16

0.53

TiO2

0.91

1.38

P2°5

0.17

0.21

MnO

0.19

0.02

LOI

9.90

5.15

during Cretaceous deformation, and the lower and upper 

contacts of the flow commonly served as slip planes during 

rotation; hence, contact relationships are clear only locally. 

The upward increase in the number and size of the 

amygdules is interpreted to indicate top of the igneous 

body at the time of emplacement, reflecting decreasing 

pressure while the flow was fluid. This is consistent with 

top direction determined from sedimentary structures of the 

host clastic strata. These observations could indicate either 

a subaerial flow or an intrusion at a very shallow depth in 

unconsolidated sediments. No other extrusives are known 

in the Big Creek Formation; thus the intrusion interpretation is favored.

SOUTH OF LEM PEAK FAULT

Big Creek strata of the type area were examined in 

reconnaissance traverses. The lowermost strata, exposed in 

an unnamed cirque about 1.5 km northeast of the head of 

Inyo Creek (fig. 2; Yellow Peak 7.5-minute quadrangle), are 

light-gray fine-grained quartz-rich metasandstone that contains dark-gray heavy minerals. Strata on the floor of the 

southwestern part of the cirque display planar and trough 

crossbeds and minor heavy-mineral laminae; calcareous 

metasandstone beds are common. The heavy minerals typically form laminae 1-2 mm thick. Repetition of the following cyclical sequence occurs in intervals 0.3-1 m thick:

Top

Scour surface

Metasandstone, quartz-rich

Fine-grained metasandstone, heavy-mineral laminae 

Metasandstone, calcareous 

Scour surface

Base

In a few places, beds 0.5-1 m thick of argillite to finegrained siltite lie upsection of quartz-rich metasandstone 

beds; pebbles of argillite locally are associated with these 

strata.

On the floor of the northeastern part of the cirque, 

gently inclined to planar strata of dark-gray heavy-mineral 

laminae are exposed. Trough crossbedding and calcareous

metasandstone are less common. The heavy minerals form 

prominent laminae 1-2 mm thick in very gently inclined 

trough crosslaminae, and in lenses 2-3 cm thick in flatplanar-laminated strata (fig. 11). The flat-planar-laminated 

layers that host the units are at a slight angle to one another. 

Heavy minerals of the lenses extend laterally for as much as 

100 m or more. Thin sections show the heavy minerals to be 

zircon, tourmaline, and hematite. Polished sections examined by G.A. Desborough (oral commun., 1990) revealed 

ilmenite surrounded by hematite that has replaced titaniferous magnetite.

Reconnaissance examination of strata higher in the Big 

Creek of the type area revealed repetition of trough crosslaminated metasandstone beds that commonly are about 1 m 

thick. Crosslamination of these beds is more steeply inclined 

than that described in the preceding paragraph, approaching 

the angle of repose, with widths of several meters. Darkgray heavy minerals in bands 1 mm thick form laminae of the 

troughs; no lenses of heavy minerals were observed. These 

crosslaminated beds commonly have erosional bases. Calcareous beds are common. In other areas the erosional base 

of an overlying bed contains a zone 2-3 cm thick of interlaminated dark-gray heavy minerals and quartz grains; the zone 

gives way upward to the broadly trough shaped strata of the 

remainder of the bed. In still other areas, beds either do not 

contain trough crosslamination or are composed of very gently inclined laminae. The beds truncate one another at very 

low angles.

About 2 km southwest of Yellow Lake (fig. 2), the 

uppermost 50 m of the Big Creek, and probably a much 

greater thickness, is composed of two-dimensional dunes 

that are stacked atop one another. The dunes display heights 

of 0.5-1.5 m and wavelengths of 2-3 m (fig. 12). Individual 

beds pinch and swell, such that the thickest parts of some 

beds are overlain by thinner parts of succeeding beds. The 

beds are 1-1.5 m thick and are composed of quartz-rich 

metasandstone that is medium to coarse grained; they lack 

the rusty-brown-weathering calcareous metasandstone that 

characterizes other strata of the Big Creek Formation in the 

cirque northeast of the head of Inyo Creek. No siltite beds 

were observed in the sequence.

The dominant current transport was to northeast, as 

shown by northeast inclination of foresets of the dunes.
14 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

Figure 11. Heavy-mineral lens, 

about 2 cm thick, in lower part 

of Big Creek Formation in type 

area. Lens is at very low angle to 

lamination of host fine-grained 

metasandstone. Hammer head at 

top of lens; handle for scale, 

about 24 cm long.

Figure 12. Small two-dimensional dunes in upper strata of 

Big Creek Formation in type 

area. Dashes emphasize bedding. Wavelength is about 1 m, 

height about 0.2 m. Hammer for 

scale, handle 24 cm long.

Subordinate current transport was to southwest, shown by 

smaller scale crosslaminated beds that are thinner and dip to 

the southwest. The rippled upper surface of some dunes contains two-dimensional asymmetrical ripples that show transport to the southwest. Subordinate currents deposited 

sediment directly to the southwest of the crestal area of some 

dunes; the strata show oppositely directed (that is, southwestdipping) laminae. The upper parts of beds are erosionally 

truncated; thus, topsets of crosslaminae have been removed.

Strata and reactivation surfaces are inclined at slightly lower 

angles than foreset lamination of dunes (fig. 13).

Some dunes display closely spaced double laminae of 

heavy minerals, separated by a lamina of quartz and feldspar 

grains (fig. 14). The laminae may represent deposition during slack water and weak current tidal stages, analogous to 

double clay drapes. Toes of some of the foreset laminae contain small mudchips and pebbles. Bottomset laminae are 

tangential with the underlying strata; some show slumping.
BIG CREEK FORMATION 15

Figure 13. Reactivation surfaces (dashed lines) in dunes of 

Big Creek Formation in type 

area. Overlying dune (dotted 

lines) has wavelength of about 

2.5 m and height about 0.25 m. 

Hammer for scale, handle 24 

cm long.

Figure 14. Double heavymineral laminae that may have 

been deposited by slack water 

and weak tidal current stages. 

Upper strata of Big Creek Formation in type area. Pencil for 

scale, 13 cm long.

UPPER CONTACT

The contact surface of the Big Creek Formation with 

the overlying fine siltite unit of the Apple Creek Formation 

is a discontinuity and is here considered to be an unconformity. The contact is interpreted as a flooding surface—"a 

surface separating younger from older strata across which 

there is evidence of an abrupt increase in water depth" (Van 

Wagoner and others, 1990, p. 8). Shallow-water strata of the 

Big Creek are abruptly overlain by "deep" water turbidites of

the Apple Creek. As defined by Van Wagoner and others 

(1988; 1990, p. 22) in their discussions of the concepts of 

sequence stratigraphy, an unconformity is "a surface separating younger from older strata across which there is evidence 

of subaerial erosional truncation and, in some areas, correlative submarine erosion, or subaerial exposure, with a significant hiatus indicated." These authors stated that deepening 

may be accompanied by minor submarine erosion or nondeposition. I found no direct evidence for subaerial exposure, such as a transgressive lag deposit, ravinement surface,
16 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

transgressive sandstone capping the Big Creek, or indications of a paleosol at the contact. But I do not believe the surface has been examined over a sufficiently wide area to be 

certain if it underwent subaerial erosion, nor to determine if 

angularity exists. Further study in the region may cause this 

interpretation to be reevaluated.

ORIGIN

The Big Creek contains many features in common with 

modern sediments deposited in intertidal and shallow subtidal environments. Several studies cited in paragraphs following indicate general conditions under which the Big 

Creek strata may have been deposited. None of the examples 

of modern-day depositional environments is considered to be 

a direct analogy; rather, each contains a variety of sedimentary features that are present in Big Creek strata. The features suggest that Big Creek strata were tidally controlled, 

possibly deposited in a macrotidal setting (daily tidal range 

greater than 4 m). A brief interpretation of depositional environments for the Big Creek of the type area, south of the Lem 

Peak fault, closes this section.

Sedimentary features unique to tidal processes include

(1) reactivation surfaces, (2) double clay drapes, and (3) sediment cyclicity patterns of thickness variation and sequence 

repetition that correspond to diurnal and neap/spring tidal 

cycles (Nio and Yang, 1991). Sedimentary features (1) and

(2) are present in the Big Creek strata of the Lemhi Range; 

no investigation of item (3), cyclicity patterns, was undertaken. These features (1-3) are preserved mainly in a subtidal environment (Nio and Yang, 1991).

Other sedimentary structures present in the Big Creek, 

but which by themselves are not unique to tidal environments, include (4) flaser and lenticular bedding, (5) herringbone crosslamination, (6) interbedded dunes and siltite 

(mudstone) lenses and beds, and (7) tidal channel deposits. 

These features generally are preserved in shallow subtidal to 

upper intertidal environments. They occur within wavedominated environments as well as tide-dominated environments, but their direct and intimate association with Big 

Creek structures diagnostic of tidal origin indicates that they 

also are of tidal origin.

Most tidal deposits display sedimentary structures that 

indicate deposition from bipolar currents. The structures are 

mainly formed during a dominant direction of sediment 

transport, reflecting either a flood current or an ebb current. 

Strong tidal asymmetry is indicated for Big Creek strata, with 

dominant transport to the southwest, and the opposite direction for sedimentary structures produced by subordinate tidal 

currents.

Dunes of both the two-dimensional and three-dimensional types make up a significant part of the rocks of the Big 

Creek Formation. Measured two- and three-dimensional

dunes are mainly small (wavelength 0.6-5 m), although a 

few two-dimensional dunes are of medium size (wavelength 

5-10 m). Tabulations of dune wavelength and height in 

many modern environments clearly show that dune size generally increases as water depth increases, and that small to 

medium dunes generally occur on intertidal flats (Dalrymple 

and Rhodes, 1995, p. 367-369). (These authors stated that 

large to very large dunes (wavelength >10 m) generally 

occur in deep subtidal channels and inlets.)

Three-dimensional dunes are present mainly in the 

upper part of the Big Creek Formation north of the Lem 

Peak fault and are of the small size. Flume studies have 

shown that three-dimensional dunes are formed at greater 

current speeds, over a greater range of speeds, and in sediment of a wider range of grain sizes, than two-dimensional 

dunes—the latter have a fairly narrowly restricted range of 

formation (Southard and Boguchwal, 1990). This suggests 

that upper strata of the Big Creek, the stratigraphic horizon 

of the upper part of the measured section (fig. 3) and above, 

were deposited under higher energy conditions than lower 

strata. Such conditions reflect an intertidal setting rather 

than a subtidal one.

Dalrymple and Rhodes (1995, p. 365) stated that threedimensional dunes develop in the most seaward extent of 

intertidal sediments exposed during low tides. They are particularly abundant in tide-dominated estuaries, where they 

are widespread on elongate sand bars or shoals in areas of 

tidal inlets, tidal channels, and in open-mouthed estuaries. 

The three-dimensional dunes of the Big Creek resemble, for 

example, dunes pictured by Dalrymple and others (1978, 

1990: their type 2 dunes) in the seaward intertidal deposits 

of the Bay of Fundy, Nova Scotia, although the Big Creek 

dunes are smaller; dunes pictured by Elliott and Gardiner 

(1981) in the macrotidal beaches of the Loughor Estuary, 

Wales; and those pictured by Hawley (1982) in the inner 

rough facies of the unbarred macrotidal beaches of the north 

coast of the Bristol Channel, Wales.

Discontinous layers and lenses of siltite (mudstone) 

interbedded with dunes composed of fairly clean metasandstone of the Big Creek north of the Lem Peak fault are 

indicative of highly contrasting flow velocities. The discontinuous layers and lenses of siltite may be analogous to sediments in The Wash, a large (>600 km2), square-shaped 

embayment of the North Sea into the southeast side of 

England. The Wash has a tidal range of 5-6 m. During low 

tide about half of the sediment in The Wash is exposed, 

including a complex of offshore banks and shoals toward the 

central part of the embayment (Wingfield and others, 1978). 

Sand is the dominant surface sediment of the subtidal areas 

and the sand flats and offshore banks that are exposed at low 

tide (Ke and others, 1996).

Cores from the sand banks of The Wash are characterized by sand-with-mud-laminae. The mud laminae are flat 

lying, usually less than 4 mm thick, and have sharp tops and
APPLE CREEK FORMATION 17

diffuse bases. They occur in groups, as isolated laminae, 

and frequently are cut out by erosion. Much of the area of 

sand-with-mud-laminae is exposed at extreme low tides 

(Wingfield and others, 1978). Mud also is present in subtidal areas, as thin discontinuous layers (flasers) within the 

sands (Ke and others, 1996). The sand of the banks is 

medium grained (Wingfield and others, 1978) but sandwith-mud-laminae occur in both fine and medium sands 

according to maps of Wingfield and others (1978), and Ke 

and others (1996). Further, McCave and Geiser (1978) 

reported the sand banks to be composed of well-sorted fine 

sand with mud lenses 5 cm thick and 100 m in lateral extent 

in the outer banks of The Wash.

By analogy with the preceding data on modern depositional environments, the mud lenses and layers of the Big 

Creek may indicate an upper subtidal to lower intertidal depositional environment. The nonpillowed andesite flow of the 

Big Creek measured section area (fig. 3) suggests extrusion 

onto subaerially exposed strata, supporting an intertidal depositional environment for the directly adjacent flaser and lenticular bedded siltite. In addition, the sparse occurrence of 

double clay drapes in the Big Creek north of the Lem Peak 

fault is suggestive of an intertidal setting. The presence of 

mainly a single clay drape suggests that only the drape of 

slack water at high tide was deposited. The tidal channel 

deposit of the Buck Lakes area is in the upper strata of the 

Big Creek and is interpreted as an intertidal feature.

Big Creek strata of the type area, south of the Lem Peak 

fault, differ from strata north of the fault in being coarser 

grained, cleaner, richer in quartz, containing lenses of heavy 

minerals, and lacking layers and lenses of dark-gray siltite 

(mudstone). The abundance and concentration of heavy 

minerals in the cirque directly north of the head of Inyo 

Creek (fig. 2) are indicative of the upper part of a beach. The 

very broad, low-angle crosslamination of some of the strata 

is suggestive of beach cusps. Sequences of strata that contain troughs, steeply inclined crosslamination, and beds that 

cut deeply into one another are suggestive of beds seaward 

of an upper beach environment. In the cirque, the depositional picture is of a sea shallowing from southwest to northeast, from shoreface to beach.

About 2 km southwest of Yellow Lake (fig. 2), uppermost beds of the Big Creek are composed of small twodimensional dunes (wavelength 0.6-5 m) that are stacked 

atop one another. Strong tidal asymmetry is indicated, with 

a dominant northeast transport and subordinate transport in 

the opposite direction. Fine sediment is represented only by 

the thin clay drapes and sparse pebbles, indicating that fines 

were generally flushed from the environment. Double clay 

drapes are more common here than in dunes of the Big Creek 

north of the Lem Peak fault, suggesting that the dunes of the 

type area were formed in a subtidal environment.

APPLE CREEK FORMATION

The Apple Creek Formation was named by Anderson 

(1961) for strata in the southern part of the Lemhi 15-minute 

quadrangle, on the northeast flank of the central part of the 

Lemhi Range, north of the Lem Peak normal fault. Anderson (1961) designated the type area in the vicinity of Apple 

Creek, a short tributary of Hayden Creek (fig. 2). Originally 

called the Apple Creek Phyllite, the name was changed to 

Apple Creek Formation by Ruppel (1975) because the phyllitic appearance is due to intensely developed cleavage in the 

type area; the intense cleavage, thus phyllitic appearance, is 

not characteristic everywhere. Tietbohl (1981, 1986) separated a unit of conglomerate (diamictite) from the Apple 

Creek, studied its depositional environment, and mapped 

part of its extent. I remapped the conglomerate (Tysdal, 

1996a) and other rocks of the Apple Creek in the southern 

part of the Lemhi quadrangle map area of Anderson (1961) 

and contiguous rocks to the west (Tysdal, 1996b; Tysdal and 

Moye, 1996). I recognized three genetically related gravityflow units within the Apple Creek. Two of the units, as well 

as Big Creek strata that are in fault contact with the Apple 

Creek, were mapped as Apple Creek by Anderson (1961). I 

mapped the three gravity-flow units separately and gave 

them informal names, which in ascending order are the fine 

siltite unit, the diamictite unit, and the coarse siltite unit.

The fine siltite unit is unconformable above the Big 

Creek Formation and conformable beneath the diamictite 

unit (Tysdal, 1996a, b; Tysdal and Moye, 1996). In the 

central part of the Lemhi Range, north of the Lem Peak 

fault, Ruppel (1980) generally mapped the fine siltite in the 

upper part of the Big Creek Formation, but he did not recognize its character as being distinct from that of most of 

the Big Creek (Ruppel, 1975; Ruppel and Lopez, 1988). 

The diamictite unit, conformable beneath the coarse siltite 

unit (Tysdal, 1996b), was assigned to the lower subunit of 

the Yellowjacket strata by Evans (in press). The diamictite 

was reassigned to the Apple Creek by Tysdal (1996a, b, 

2000). The coarse siltite unit also correlates with the lower 

subunit of the Yellowjacket Formation of Connor and 

Evans (1986), Connor (1990; 1991), Evans and Connor 

(1993), and Evans (in press) in the Salmon River Mountains west of the Lemhi Range, as Connor (1990, 1991) recognized previously. The name Apple Creek Formation, 

coarse siltite unit, was applied to these rocks in the Salmon 

River Mountains by Tysdal (2000).

The Apple Creek name was applied to rocks in the 

west-central part of the Lemhi Range, in the vicinity of Yellow Lake (fig. 2), by Ruppel (1975, 1980). These strata lie 

directly above the Big Creek Formation, as do Apple Creek 

strata north of the Lem Peak fault. The Apple Creek is thinner south of the fault than north of it (table 1) and constitutes 

a single lithofacies that generally differs from each of the 

three lithofacies north of the fault. Apple Creek strata south 

of the Lem Peak fault are described beginning on page 28.
18 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

NORTH OF LEM PEAK FAULT

FINE SILTITE UNIT

The fine siltite unit was so named to convey that its general grain size is less than that of either of the other two units 

of the Apple Creek Formation north of the Lem Peak fault. 

The major part of the unit is composed of turbidites. Deposits that may be turbidites or debris flows are interspersed 

within the sequence and contain stratified clasts of coarse silt 

to pebble size.

The fine siltite unit is planar-laminated and ripple crosslaminated fine- to medium-grained siltite and argillaceous 

siltite. In some beds planar laminations grade upward to 

small-scale (1-3 cm) sets of ripple crosslaminated siltite. 

Water-escape structures are present locally. Sole marks were 

not observed. Many beds are graded, with 1-2 cm thick 

strata of light-gray, medium- to fine-grained siltite grading 

upward into dark-gray fine-grained siltite and argillite. The 

unit locally contains matrix-supported granule- to pebblesize argillite clasts in horizons 1-2 clast diameters thick; 

thicker units occur locally in the upper part. Fine silt and clay 

content is greater in the upper part of the unit than in the 

lower part. The unit is approximately 1,000 m thick.

The fine siltite unit generally is greenish gray, although 

the lower part in particular contains strata that are a distinctive medium bluish gray, grayish blue green, and locally light 

bluish gray. The greenish-gray color is typical of the entire 

Apple Creek Formation, but the blue-gray colors are unique 

to the fine siltite unit. The greenish-gray and blue-gray colors contrast sharply with the light- to medium-gray colors of 

the underlying Big Creek Formation. The contrast is especially noticeable from the air and where the two formations 

are extensively exposed in the rugged, nearly treeless terrain 

of the western part of the Lem Peak quadrangle and adjacent 

eastern part of the Allison Creek quadrangle (fig. 2). The 

blue tints are lost in the central and western part of the Allison Creek quadrangle where the metamorphic grade 

increases from lower greenschist facies to biotite-grade 

greenschist facies.

The lower contact is marked by the change in color and 

by change from the coarser grained, clay-free underlying 

rocks. The upper contact is conformable, marked by the 

abrupt appearance of beds of diamictite.

CURRENT RIPPLES

Two kinds of current ripples are present within the fine 

siltite unit. Both kinds occur within zones a few meters thick 

and are made up almost entirely of rippled, well-sorted 

medium- to coarse-grained siltite. Grains are chiefly quartz 

and lesser feldspar, 1-2 percent nonmagnetic opaque

minerals in some rocks, and trace tourmaline. Clay matrix is 

absent.

1. Ripples of the McKim Creek outcrop area (fig. 15), 

in the westernmost part of the Lemhi Range (fig. 2), are similar to those pictured by Shanmugam and others (1994, fig. 

12) from the North Sea, Shanmugam and others (1993, figs. 

9,10; 1995) from the Gulf of Mexico, and Ito (1996, fig. 5B) 

from the Sea of Japan. They include migrating ripples, some 

of which are draped by thin clay layers. A few ripples have 

a sigmoidal shape and display one- to several-laminae-thick 

clay "offshoots" (discontinuous dark-colored clay "drapes" 

on ripples or in ripple crosslaminae). The ripples exhibit 

multidirectional crosslamination and both preserved and 

eroded tops.

2. In the Bear Valley Creek area (fig. 2), ripples 

observed only in one place contain heavy-mineral laminae, 

one lamina thick, at the base of crosslaminae. These current 

ripples generally are about 0.3-1 cm high and have a wavelength of about 1-3 cm (figs. 16 and 17). The ripple crosslamination is multidirectional. No clay in ripple troughs or 

clay "offshoots" were observed in the Bear Valley Creek 

area.

The two kinds of rippled strata are attributed to reworking by bottom currents for the following reasons. The siltites 

are well sorted and generally lack a matrix. This is in marked 

contrast to the associated turbidites of the sequence, which 

contain poorly sorted silt grains and several percent clay in 

the matrix. The differing dip directions of crosslaminae suggest multiple current directions, in contrast to the generally 

unidirectional crosslaminae of ripples of turbidity current 

origin. The ripples form zones within the turbidite sequence 

and are not at the top of a depositional unit such as a Bouma 

turbidite Ta_e succession. The clay "offshoots" of some ripples suggest fluctuation of high and low energy conditions. 

The association of both preserved and eroded ripple crests 

(fig. 15) also suggests variable current speeds. In their discussions of bottom-current reworked sands (contourites), 

Shanmugam and others (1993, 1995) and Ito (1996, 1997) 

argued that all these features, as well as others not observed 

in the fine siltite unit of the Lemhi Range, are indicative of 

traction-formed structures that are different from those of 

turbidites.

FLOATING ARGILLITE (MUDSTONE) CLASTS

The fine siltite unit contains some beds characterized by 

isolated clasts of argillite. The beds are much like those 

termed "slurried beds" by Wood and Smith (1959, p. 173, 

and pi. VIII, fig. 4). In the classification scheme of Picketing 

and others (1988), these beds include (but may not be limited 

to) graded-stratified pebbly siltite and fine-grained 

metasandstone and stratified gravel.

The common feature of the beds in the Lemhi Range is 

argillite clasts that are isolated ("floating") in a matrix of 

coarse-grained siltite to medium-grained metasandstone.
APPLE CREEK FORMATION 19

Figure 15. Current ripples, 

some of which are draped by 

thin clay layers, in fine siltite 

unit of Apple Creek Formation. 

The ripples exhibit multidirectional crosslamination. Near 

McKim Creek, western part of 

Lemhi Range (fig. 2). Pen for 

scale, 13 cm long.

The matrix is stratified but generally is not graded, although 

grading of argillite clasts was observed at one locality in the 

eastern part of the mapped area. The clasts are elongate and 

range in size from granules to pebbles as large as about 10 

cm in longest dimension. The packing—or closeness—ranges from clasts that touch to those that are widely 

dispersed within the matrix. Floating clasts occur at specific 

horizons within a sequence and are isolated from one another 

at that horizon (fig. 18). The clasts are not graded within a 

bed—the largest clasts may occur at any horizon within a 

bed. The beds exhibit planar clast fabric—clasts are oriented 

such that the flat side is about parallel to bedding.

The beds range from about 15 to 100 cm thick. Lateral 

dimensions range from about 1 m for thin beds to several 

tens of meters for thicker beds. The beds occur in zones and 

are more abundant in some parts of the section than others, 

but they do not appear to have a regular pattern. Their base 

can be erosional or nonerosional. Beds erosional into underlying turbidites were observed only where argillite clasts are 

closely spaced; conversely, not all beds of closely packed 

clasts display erosional bases. Some beds are sharply based, 

and soles of beds in one area north of Mill Mountain (fig. 2) 

show lineation.

The argillite clasts are foreign to the depositional setting in which they occur—no argillite beds were observed 

within the fine siltite unit of the Apple Creek Formation. 

Further, the matrix of these beds generally contains wellrounded grains of quartz sand, including some grains that 

exhibit overgrowths.

Interpretations of conglomeratic gravity-flow deposits 

that display the previously described features are in dispute. 

According to Shanmugam (1996), planar clast fabric, which

characterizes the beds, is indicative of laminar flow conditions, the dominant flow process in debris flows. Preservation of rafted clasts requires deposition en masse by freezing, 

a property characteristic of plastic debris flows rather than 

fluidal turbidity currents (Shanmugam and others, 1995, p. 

483; Shanmugam, 1996; Shanmugam and Moiola, 1997). In 

contrast, Lowe (1997) pointed out that the importance of 

"floating" mudstone clasts in turbidites and debris flow 

deposits has been discussed by many workers over the last 

few decades and remains poorly understood. He stated that 

mudstone clasts cannot be used as an unambiguous indicator 

of flow rheology. No data for choosing between the two 

interpretations is presented here.

GRADED-STRATIFIED SILTITE

Graded-stratified siltite (classification of Picketing and 

others, 1988, p. 58) is prominent throughout the areal extent 

of the fine siltite unit and is the most abundant rock type. 

The siltite consists mainly of bands, which are very prominent planar, parallel, and horizontal layers. The bands are 

0.5-2 cm thick, and locally as thick as 10-20 cm, within the 

lower 0.5 m to the entire thickness of a bed; beds range up to 

about 1 m thick (fig. 19). The bands consist of coarsegrained siltite to fine-grained metasandstone that are of finest grain size in the upper part; uppermost parts of some 

bands grade to fine siltite, but argillite is absent. The bands 

are rhythmically stratified repetition of Bouma Tb and Tc 

intervals (Tbcbc, for example). Grading is normal; light-colored, coarser grained laminae locally have eroded into 

underlying dark-colored, finer grained laminae. Some beds
20 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

Figure 16. Ripple crosslaminae in fine siltite unit of Apple Creek 

Formation. Bear Valley Lakes area (fig. 2). Pen for scale, 13 cm 

long.

give way upward to large ripples (fig. 20), succeeded upward 

by small ripple crosslaminated siltite (figs. 20 and 21), then 

planar-laminated siltite. In some areas, the beds formed of 

the graded siltite are gently undulatory, with wave lengths of 

several meters and heights of only a few centimeters. These 

strata fit the Bouma sequence: the bed of bands as Tbc layers, 

grading upward to only Tc and locally T^ layers. Bases of 

some beds contain a Ta layer of granule- to pebble-size clasts 

of argillite (mudstone).

Soft-sediment deformation structures are fairly common. Fluid-escape structures are the most abundant and typically extend through several bands. The largest structures 

observed were about 30 cm high and their cross section is 

teepee shaped (fig. 22). Dish structures were observed only 

locally. Convolute laminations are uncommon but include 

local slump folds that are recumbent.

The graded-stratified siltite beds are interpreted as turbidite deposits. The repeated Tbc strata that make up most (to 

all of some) beds are similar to the vacillatory turbidites 

described and pictured by Larue and Speed (1983) and Larue 

and Pro vine (1988). Vacillation refers to the repetition of the 

Bouma Tbc intervals within what is regarded as a single bed 

or a single flow event (Larue and Provine, 1988).

DIAMICTITE UNIT

Abundant beds of poorly sorted conglomerate characterize a succession of otherwise fine-grained strata within the 

Apple Creek Formation north of the Lem Peak fault. These 

pebbly strata were first reported by Anderson (1961) in his 

Apple Creek map unit in the Apple Creek-Hayden Creek

Figure 17. Ripples of type that 

produced crosslaminae illustrated in figure 16. Pen for 

scale, 13 cm long.
APPLE CREEK FORMATION 21

Figure 18. Conglomerate bed 

(across middle of photograph) 

in upper part of fine siltite unit 

of Apple Creek Formation, 

about 4 km northwest of Mill 

Mountain (fig. 2). Bed exhibits 

planar clast fabric—clasts 

(light gray) are oriented such 

that the flat side is about parallel to bedding, but they are not 

graded according to size within 

bed. Note that most clasts are 

not touching, but rather are 

floating within the argillaceous 

matrix. Hammer for scale, 

handle 24 cm long.

Figure 19. Rhythmically graded beds of stratified siltite within 

fine siltite unit of Apple Creek Formation. Light-gray layers are 

coarser grained siltite, dark-gray finer grained. Youngest strata are 

at top of photograph. From eastern part of Salmon River Mountains 

(fig. 2). Pen for scale, 13 cm long.

area of the Lemhi 15-minute quadrangle (fig. 2). The succession was separated as a distinct unit by Tietbohl (1981, 

1986), who used the term diamictite for the poorly sorted, 

matrix-rich conglomerate beds and applied the name generally to the entire unit. Tietbohl confirmed the sedimentary 

origin of this pebbly mudstone-bearing unit, mapped its distribution in the vicinity of Hayden Creek-Apple Creek, and 

attributed its origin to submarine slumping in combination 

with normal sedimentation. The diamictite unit generally is 

intensely cleaved where it crops out on the eastern flank of 

the Lemhi Range, commonly obscuring sedimentary features. Tietbohl (1986) interpreted the cleavage as a tectonic 

overprint. The upper contact of the diamictite unit is gradational through tens of meters into the coarse siltite unit of the 

Apple Creek Formation.

The thickness of the diamictite unit is uncertain due to 

deformation, but is estimated to be 1,000-1,500 m in the 

vicinity of Hayden Creek on the east and about 600 m near 

the Salmon River on the west, where it is depositionally thin
22 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

Figure 20. Planar-laminated 

stratified siltite of Bouma Tb 

layer that gives way upward to 

large ripple (form shown by 

dashed line) of Tc layer, wavelength about 0.6 m; succeeded 

upward by siltite containing 

abundant small crosslaminated 

rippes (above and to right of 

ripple trough), then by planarlaminated siltite of Td layer. 

Hammer spans basal part of 

overlying bed. Hammer for 

scale, handle 24 cm long. Apple Creek Formation.

Figure 21. Closeup of ripple 

crosslaminated siltite like that 

above trough axis of large ripple shown in figuie 20. Exposed part of hammer handle is 

about 22 cm long. Apple Creek 

Formation.

(Tysdal, 1996b; Tysdal and Moye, 1996). The regional, 

along-strike, outcrop pattern of the diamictite unit thus is a 

fan shape. The conglomerate is tightly folded in the footwall 

of the Bear Valley thrust fault (not shown in fig. 2) in the 

Hayden Creek area.

Rocks here assigned to the diamictite unit of the Apple 

Creek Formation, and mapped as such by Tysdal (1996a), 

were shown as Yellowjacket Formation by Ruppel (1980) in

the northwest quarter of the Patterson 15-minute quadrangle. 

Ruppel and Lopez (1988, p. 15) recognized the diamictite 

unit of the Patterson map as being structurally deformed and 

stated that in places the strata resemble those of the Apple 

Creek Formation. They apparently did not recognize the 

conglomeratic nature of the unit, however, stating (Ruppel 

and Lopez, 1988, p. 76) that on Mill Mountain " ***the Big 

Creek [Formation] is thrust over a coarse turbidite facies of
APPLE CREEK FORMATION 23

Figure 22. Fluid-escape structure in stratified siltite of fine 

siltite unit of Apple Creek 

Formation, eastern part of 

Salmon River Mountains (fig. 

2). Hammer for scale, handle 

24 cm long.

the Yellowjacket Formation, and is strongly sheared and 

brecciated." The area shown as sheared and brecciated by 

Ruppel (1980) is intensely cleaved conglomerate of the 

diamictite unit, and locally of the underlying fine siltite unit, 

of the Apple Creek Formation.

SILTITE

Nonconglomerate beds of the diamictite unit are mainly 

composed of gray-green argillite, argillaceous siltite, fine- to 

medium-grained siltite, and lesser metasandstone. The beds 

form successions as thick as 10 m or more between 

conglomerate beds. Graded beds are obvious where 

deformation is not intense, and Bouma sequences are most 

prominent in the upper part of the unit. Beds are generally 

only a few centimeters thick and are finely laminated. Tietbohl (1986) observed sole marks along the base of some 

metasandstone beds that directly overlie conglomerate beds. 

He also reported load structures and convolute beds, the latter including some that were truncated by overlying 

diamictite beds. Water-escape structures occur locally. I 

observed sparse laminae and bands of magnetite as thick as 

2 cm in the Wright Creek area. Paleocurrent directions were 

determined from current ripples. The few determinations 

revealed transport from both the northeast and the southwest. 

The significance of the few observations is uncertain. These 

fine-grained strata are attributed to a turbidite origin, based 

on the Bouma sequences and the associated sedimentary 

structures.

MATRIX-SUPPORTED CONGLOMERATE

Most of the conglomerate beds are composed of disorganized matrix-supported clasts of argillite, siltite, 

metasandstone, and rock fragments. Clasts are commonly 

1-2 cm in diameter but locally are as large as 20-25 cm; one 

50 cm long clast was observed. Clasts range from subangular to well rounded. The concentration of clasts is variable, 

and the larger clasts are dispersed within deposits that also 

contain abundant pebbles; hence, deposits are poorly sorted. 

Of more than 500 clasts examined in thin section by Tietbohl 

(1986), about 90 percent were detrital rock fragments, which 

ranged from argillite to fine-grained metasandstone. Foliated low-grade metamorphic rock fragments also were 

observed by Tietbohl (1986), who believed that the foliation 

formed before the clasts were incorporated into the conglomerate. I observed a 5 cm diameter clast of white bull quartz 

in a conglomerate bed near the junction of Hayden and Bear 

Valley Creeks. The matrix of the conglomerate beds is composed of sericite, muscovite, chlorite, and silt of quartz and 

plagioclase, as well as local medium-grained, rounded, 

quartz sand grains (Tietbohl, 1986).

Fine layering within the matrix-supported conglomeratic beds, or imbrication of clasts in the beds, was not 

observed everywhere in the unit. The sequence of Proterozoic rocks in the Lemhi Range was deformed during thrusting, and the silt in the matrix of the conglomerate units 

"flowed." Fine layering is commonly obscured or destroyed, 

and clasts are rotated such that their long axes are oriented 

about parallel to cleavage. Conglomeratic beds have a tabular geometry in the few places where exposures are sufficient
24 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

to view the beds over a broad area and are tens to several tens 

of meters wide.

These conglomerate beds contain many of the features 

described under "Floating Argillite (Mudstone) Clasts," pertaining to beds of the fine siltite unit. The beds are either turbidites or debris flows; distinguishing between these two 

kinds of gravity flows is a matter in dispute, as stated in the 

aforementioned section of this report.

GRAIN-SUPPORTED CONGLOMERATE

Conglomerate that contains a small amount of clay was 

observed on the ridge west of Wright Creek (fig. 2). The 

beds are composed of poorly sorted granules and pebbles of 

claystone and mudstone that are more or less in contact with 

one another—grain supported—thus are similar in packing 

and clast composition to some conglomerate beds in the 

underlying fine siltite unit of the Apple Creek Formation. 

The clasts are tabular shaped and are aligned such that their 

broadest sides are about parallel to bedding. They range 

from a few millimeters to as much as 3 cm across and are disorganized (no grading, no stratification, no strongly preferred fabric). In a few outcrops, crosslaminated 

metasandstone forms the basal part of a bed of mudstonegranule conglomerate, which is ungraded and of a uniform 

granule size. No grading is evident within the conglomerate.

Silt constitutes about 75 percent of the matrix of grainsupported conglomerate beds and is chiefly fine grained to 

coarse-grained quartz and feldspar. Sand grains in the matrix 

are entirely quartz of fine and medium grain size; many of the 

quartz grains are well rounded and some display overgrowths. Sericite makes up 1-3 percent of the matrix. Beds 

of the granule conglomerate commonly are 10-15 cm thick, 

lack internal lamination, and show little evidence of erosion 

into the underlying strata.

These strata probably are either turbidites or debris flow 

deposits, as discussed in the section entitled "Floating Argillite (Mudstone) Clasts."

MAGNETITE BANDS

Magnetite bands are present locally within the finely 

laminated, fine- to medium-grained siltite that occurs in 

sequences between beds of conglomerate on the ridge north 

of Wright Creek. The bands generally are less than 5 mm 

thick, although a 2 cm thick band was found at one locality. 

Some of the thicker bands locally sag into the underlying siltite, forming small load casts. Magnetite appears to form the 

basal 1-5 mm of 1-2 cm thick graded beds; dark-gray finegrained magnetite and magnetic siltite grade upward into 

nonmagnetic siltite.

Magnetite bands present in the conglomerate unit are 

considered to be primary features. Millimeter-thick 

microbands of magnetite are undisturbed, implying accumulation below wave-base of storms and not reworked by bottom currents. The magnetite laminae may have been 

deposited directly on the sea floor as a chemical sediment. In 

addition, or alternatively, magnetite laminae may have precipitated directly onto the ocean floor and subsequently were 

resedimented as turbidites, as proposed by Barrett and Fralick (1985; 1989) for Late Archean rocks of Ontario, Canada.

Structural relationships indicate a predeformation timing for formation of the magnetite bands in the Lemhi 

Range. At one place, a 1 cm thick band of magnetite within 

the core of a fold 4 m across is deformed into small folds of 

1-2 cm wavelength. Axial plane cleavage of the large fold 

cuts the magnetite bands. These structural features and 

relationships indicate that the magnetite layers predate 

deformation. Magnetite grains that are considerably coarser 

than host siltite grains also are scattered throughout some 

beds. These late-formed magnetite grains are undeformed 

and postdate cleavage.

Magnetite-bearing beds were reported previously in 

strata of central Idaho. Connor (1990, 1991), in his interpretation of mineralization in the Salmon River Mountains, 

tentatively considered the diamictite unit of the Lemhi 

Range to be related to his lower subunit of the Yellowjacket 

Formation, which I (Tysdal, 2000) consider to be the coarse 

siltite unit of the Apple Creek Formation. Within this unit 

near Iron Creek in the Salmon River Mountains, Modreski 

(1985, p. 211) had reported magnetite in "thin laminae 

along bedding planes, in irregular streaks and veinlets, and 

in layers composed entirely of massive magnetite." Connor 

(1991) stated that the rocks of the Iron Creek area are variably magnetic. Connor (1990, p. 20-21) discussed the iron 

content of the unit, stating that euhedral grains of magnetite 

are disseminated in the siltite and that the euhedral form of 

the magnetite suggested diagenetic or metamorphic crystallization. He followed Hughes (1983), Hahn and Hughes 

(1984), Nash and Hahn (1989), and Nash (1989) in attributing mineralization in the Salmon River Mountains to submarine thermal venting.

COARSE SILTITE UNIT

The coarse siltite unit, present north of the Lem Peak 

fault, is composed of gray-green medium- to coarse-grained 

siltite and fine-grained sandstone, metamorphosed to the 

lower greenschist facies. Distinctive graded beds of lightgray quartz-rich fine-grained metasandstone to coarsegrained siltite, as thick as 1 m, are interspersed within the 

sequence. These beds are most abundant in the lower part of 

the unit. The coarse siltite unit of the Apple Creek Formation 

derives its informal name from these light-gray beds. Upper
APPLE CREEK FORMATION 25

Figure 23. Sharp-based finegrained light-gray metasandstone bed (Tb) within siltite of 

coarse siltite unit of Apple 

Creek Formation. Metasandstone grades upward into rippled siltite (Tc). Siltite clast is 

present within upper part of 

bed. From Basin Lake area (fig. 

2). Pen for scale, 13 cm long.

Figure 24. Light-gray finegrained metasandstone bed (Tb) 

that contains disrupted mudstone layer in lower part (Ta). 

Siltite layer was broken into 

pieces when still soft mudstone, 

as indicated by curved and 

sharply folded shapes of some 

pieces. From coarse siltite unit 

of Apple Creek Formation, Basin Lake area (fig. 2). Hammer 

for scale, handle 24 cm long.

strata of the unit generally are fine- to medium-grained siltite 

that locally contain soft-sediment deformation structures. 

The most complete sequence of the coarse siltite unit extends 

from near the easternmost Bear Valley Lake to Basin Lake, 

upsection from south to north, in the westernmost part of the 

Lem Peak quadrangle (fig. 2). The preserved thickness of 

the unit in the northern part of the Lemhi Range is 

2,000-2,500 m (Tysdal, 1996b; Tysdal and Moye, 1996).

LIGHT-GRAY METASANDSTONE

Light-gray metasandstone beds are the unifying characteristic of the coarse siltite unit. The beds first appear 

abruptly at the base of the unit, and except for one known 

occurrence of a few thin beds within the upper part of the 

underlying diamictite unit, are absent from other strata of the 

Apple Creek Formation north of the Lem Peak fault. No
26 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

Figure 25. Convolute lamination of Bouma Tc layer, including 

pillar structures between Ushaped areas. Coarse siltite unit 

of Apple Creek Formation, Basin 

Lake area (fig. 2). Hammer for 

scale, exposed part of hammer 

head 10 cm long.

Figure 26. Climbing ripples 

in Bouma Tc subdivision of 

graded bed that give way upward to convolute lamination, 

emphasized by dashed lines, 

then to planar lamination of Td 

layer. Coarse siltite unit of Apple Creek Formation, Basin 

Lake area (fig. 2). Exposed part 

of pen (arrow) is 11 cm long.

diamictite beds are present within the coarse siltite unit. The 

light-gray metasandstone beds are commonly 10-30 cm 

thick, although some as thick as 1 m occur locally. They are 

sharp based and generally nonerosive (fig. 23), except for 

some thick beds that fill scours cut into underlying strata. 

Load casts occur at the base of some beds. Ripple crosslamination is a common feature of the metasandstone and is 

most pronounced and thickest in the thicker beds.

The light-gray metasandstone beds are graded. The 

grain size ranges from sand to silt, but sand-size grains

constitute well over half of each bed, hence the general name 

of metasandstone. Grain size grades upward from very fine 

grained or fine-grained sand into coarse silt, and, in about the 

top 10 percent of each bed, into gray-green silt that is 

medium to fine grained. The metasandstone is medium gray 

on fresh fracture, but weathers light gray and contrasts 

sharply with the gray-green color of associated strata. The 

light-gray metasandstone and coarse siltite are 80-90 percent 

quartz, the remainder mainly plagioclase and 1-2 percent 

muscovite. The beds display Bouma T^ and
APPLE CREEK FORMATION 27

Figure 27. Soft-sediment deformation features in upper part 

of coarse siltite unit of Apple 

Creek Formation, including 

ball-and-pillow structures and 

pillar structures. Basin Lake 

area (fig. 2). Exposed part of 

pen (upper right corner) is 10 

cm long.

sequences. The beds are planar laminated or structureless in 

the lower part, grading upward to ripple crosslaminated, then 

to planar-laminated gray-green siltite. Pebbles (Bouma Ta) 

were observed at the base of a few beds. Water-escape structures were observed in a few beds.

Chips of dark-gray fine-grained siltite occur in a few 

beds, where they are dispersed within a matrix of light-gray 

fine-grained metasandstone or coarse-grained siltite. The 

chips generally are 0.1-0.5 cm long, although some tabular 

argillite clasts as long as 5 cm were observed; their flat sides 

are parallel to bedding. Some "clasts" are disrupted, broken 

beds that apparently were soft during downslope gravitydriven transport; they are folded (fig. 24).

The best exposures of the light-gray metasandstones are 

on the ridge north of the cirque that contains the Bear Valley 

Lakes (fig. 2). The metasandstone beds are most abundant, 

thickest, and display the greatest range of sedimentary features on the southern slope of this ridge. In one zone, 

metasandstone beds are stacked one atop another. Strata of 

this area are in the upper part of the lower third of the coarse 

siltite unit. Up- and downsection from this area the lightgray metasandstones are less common and thinner, but beds 

are present at least every several tens of meters.

GRAY-GREEN SILTITE

Gray-green siltite constitutes most strata of the coarse 

siltite unit, which mainly contains siltite, local beds of 

argillite, and fine-grained metasandstone. The siltite beds 

observed are normally graded. Siltites are composed predominantly of quartz, plagioclase, and less than 10 percent

matrix that has been recrystallized to fine mica. The coarser 

grained siltites generally occur in the thicker beds. The beds 

are as thick as 1 m, but most commonly are about 10-25 cm 

thick. In some areas beds are 1 cm or less thick, their grading 

indicated by a color change from medium gray to dark gray 

upward in each bed. Magnetite bands, about 0.5 cm thick, 

were observed in the coarse siltite unit in two places: directly 

above the diamictite unit along the Hayden Creek Road 

(downstream from junction with Bear Valley Creek) and in 

the northeast part of the Sl/2 of Hayden Creek quadrangle 

(fig- 2).

Soft-sediment deformation structures are widespread in 

this unit. They are particularly common in the Basin Lake 

area (fig. 2), which contains the stratigraphically highest 

(youngest) rocks of the unit in the mapped area. Some of the 

fine-grained siltites and argillites were very water rich when 

deposited and host spectacular convolute lamination structures (fig. 25), the largest of which is about 50 cm high. Dish 

and flame structures are typical of some beds. Some thick 

beds show an upward change from massive metasandstone at 

the base through dish structures, thin pillars, to convolute 

bedding in the upper part (generally 1/4-1/3 of the entire 

bed). Ripple crosslamination characterizes many of the turbidites in the coarse clastic unit. Climbing-ripple crosslaminations, in the Bouma Tc subdivision of a few graded beds, 

occur directly above nonclimbing ripples and give way 

upward to metasandstone displaying convolute lamination 

structures 1-5 cm across (fig. 26).

Ball-and-pillow structures, which are generally 5-10 

cm across but as large as 25 cm across, are common (fig. 27). 

Pillar structures (Lowe, 1975) are present between some of
28 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

Figure 28. Flute and groove 

casts and syneresis cracks in 

coarse siltite unit of Apple 

Creek Formation. Bear Valley 

Lakes area (fig. 2). Hammer for 

scale, handle 24 cm long.

the ball-and-pillow structures, shown in the figure. Differential compaction and formation of ball-and-pillow structures 

took place before deposition of directly overlying beds, 

which show no soft-sediment deformation.

Syneresis cracks (fig. 28) are fairly common in the siltite beds of this unit, in contrast with their absence from the 

other two units of the Apple Creek Formation north of the 

Lem Peak fault. Flute casts and tool marks were observed 

locally (fig. 28).

ORIGIN OF APPLE CREEK FORMATION NORTH OF 

LEM PEAK FAULT

The graded bedding and sharp bases of the light-grayweathering metasandstone beds indicate deposition from turbidity currents or storm currents. The close spatial association of the gray-green siltites and interbedded light-gray 

metasandstones with the resedimented material in debris 

flows/turbidites of the diamictite unit favors a turbidite interpretation. The gray-green siltites display characteristics of 

turbidites. Soft-sediment deformation recorded by the convolute folds and associated pillar structures (figs. 25, 27) is 

similar to that commonly found at the head of pro-delta slope 

deposits. The general lack of wave reworking of sediment of 

the unit indicates deposition below normal wave base.

The few pebble beds in the sequence could be either turbidites or debris flows. See section on the "Fine Siltite Unit" 

(of the Apple Creek Formation) for discussion concerning 

controversy of interpretation of origin of the types of 

deposits.

A turbidite origin was proposed by Sobel (1982), 

Hughes (1983), and Hahn and Hughes (1984) for strata in 

the vicinity of the Blackbird mine (not shown in fig. 1) and 

directly to the east, in the Salmon River Mountains, about 

30 km west of Salmon (fig. 2). These strata were assigned 

to the Yellowjacket Formation by the above cited workers, 

and to a lithofacies mapped as the "middle subunit of the 

Yellowjacket" by Connor and Evans (1986). Strata of this 

lithofacies conformably overlie rocks that Tysdal (2000) 

correlated with the coarse siltite unit of the Apple Creek 

Formation, and therefore are younger than the Apple Creek 

lithofacies preserved in the Lemhi Range.

A main goal of this report was to determine the general 

depositional environment for the strata of the Apple Creek 

Formation. No attempt was made to determine the regional 

structural setting at the time of deposition because the 

mapped area is only a small part of the region within which 

the formation is distributed. However, my working hypothesis is that the Apple Creek strata were deposited in an extensional structural setting, not a compressional setting in which 

thrusting was ongoing at the time of deposition. This interpretation is based in part on the work of Hahn and Hughes 

(1984) and coworkers in the Blackbird area of the Salmon 

River Mountains.

SOUTH OF LEM PEAK FAULT

YELLOW LAKE UNIT

The description of this unit is based mainly on a section 

measured on the ridge directly southwest of Yellow Lake
APPLE CREEK FORMATION 29

(fig. 2). (This lake was called Golden Trout Lake by Ruppel 

(1975), is unnamed on the Yellow Peak 7.5-minute topographic quadrangle published in 1989, but is known as Yellow Lake to U.S. Forest Service personnel.) The section is 

in the Patterson 15-minute quadrangle that was mapped by 

Ruppel (1980), who (Ruppel, 1975) first applied the Apple 

Creek name to these strata. It lies about 1 km west of the 

principal reference section designated by Ruppel (1975, fig. 

2) and was selected because it affords nearly complete exposure of moderately to steeply north dipping strata of an anticlinal limb. The measured section does not include the entire 

formation due to structural complications in the lower strata. 

However, reconnaissance traverses, combined with the mapping of Ruppel (1980), suggest that this is the most complete 

sequence. Ruppel (1975) stated that the entire Apple Creek 

(that is, mainly in the Patterson 15-min. quadrangle that he 

mapped) "***is present, unfaulted, high on the ridge 

between Patterson Creek and its East Fork." Examination of 

Ruppel's (1980) map shows the formation to be much thinner there than in the Yellow Lake area, and my reconnaissance indicates that a normal fault accounts for the absence 

of lower strata of the unit in the Patterson Creek area.

The Apple Creek strata in the Yellow Lake area 

mainly consist of dark-gray medium- and fine-grained 

finely laminated siltite, and locally argillite in the lower 

part. Carbonate strata are the unifying characteristic of the 

sequence and serve to distinguish this unit from the other 

units of the Apple Creek Formation. Other distinctive characteristics include the overall fine to medium grain size of 

the siltite, and in the upper part of the section, sporadic 

interbeds of light-gray metasandstone. Dark-gray and reddish-purple colors reported by Ruppel (1975, 1980) and 

Ruppel and Lopez (1988) in the lower part of the Yellow 

Lake lithofacies characterize many beds of fine siltite and 

argillite. The lithofacies is generally coarser in its upper 

third, reflected both in the coarse grain size of the siltite 

beds and in the presence of the metasandstone beds.

The fine-grained siltite is very finely planar laminated 

in layers that are 1-2 mm thick, typically forming beds 5-20 

cm thick. Beds 1-5 cm thick are repetitively interlayered 

with the siltite in the lower part of the lithofacies. Carbonate 

laminae 1-5 mm thick typically occur at irregular intervals 

throughout much of the upper two-thirds of the section. 

Many of the fine-grained siltite beds are graded; slabbing of 

rock probably would show that most of the beds are finely 

laminated and graded. Ripple crosslamination is common in 

the unit. Mudcracks reported by Ruppel (1975, 1980) and 

Ruppel and Lopez (1988) are here termed syneresis cracks: 

their origin cannot be demonstrated to indicate subaerial 

exposure.

Some siltite beds are composed of interlaminated layers 

of fine- to medium-grained siltite and argillite. Laminae are 

commonly 1-2 mm thick and normally graded. Other beds 

exhibit scoured bases and graded tops. Ripple crosslamination is fairly common. These siltite strata commonly occur

in beds as thick as 0.5 m, appearing abruptly above other 

strata.

Slump folds are common within the sequence and indicate gravity-driven downslope movement (fig. 29). Movement apparently was toward the northeast, but further 

examination is needed to determine the general direction 

with some certainty.

Lenticular bedding is present locally, composed of siltite ripples that are isolated within argillite beds (starved ripples of some authors) (fig. 30). The same photograph also 

shows graded fine siltite and argillite layers 0.5-1 cm thick, 

which are directly associated with some of these strata. 

Where the isolated ripples have settled into underlying 

water-saturated clay (now argillite), they formed load structures called pseudonodules (Stow and Shanmugam, 1980) in 

which the isolated ripples are separated farther from one 

another (fig. 31). In other cases, the pseudonodules formed 

from beds a few millimeters thick of horizontally laminated 

silt that settled into underlying water-saturated clay, breaking into segments separated by upward-protruding flame 

structures.

Larger structures related to water-saturated sediment 

are fairly common. Local fluid-escape structures, generally 

5-10 cm high but some 25-30 cm high, show dewatering 

across several beds. Convolute lamination takes the shape of 

ball-and-pillow structures in a few beds and U-shaped softsediment deformation structures flanked by pillars in finegrained siltite beds as thick as 10 cm.

Thickness of the Apple Creek in the area mapped by 

Ruppel (1980) was calculated at 760-900 m (Ruppel, 1975; 

Ruppel and Lopez, 1988). The thickness of 725 m that colleagues and I measured (table 1) in the section near Yellow 

Lake does not include the entire unit.

CARBONATE BEDS

Carbonate beds in the Yellow Lake lithofacies of the 

Apple Creek include dolomite and locally calcareous dolomite. Ruppel (1975, 1980) reported that the carbonates are 

ferrodolomite, but X-ray examination of five samples from 

the unit revealed only dolomite: no significant amounts of 

ferrodolomite, ankerite, siderite, or other iron-carbonate 

minerals were detected (G.A. Desborough, written commun., 1996). The outcrop surface of many of the dolomite 

beds weathers a rusty-brown color, indicating the presence 

of iron. The iron likely precipitated from rainwater during 

chemical reaction with the limestone; iron in the rainwater 

probably was derived from siltite beds upsection (G.A. Desborough, oral commun., 1996).

Silty dolomite forms distinctive interlayered beds 1-5 

cm thick in the bottom half of the unit. Carbonate attains a 

maximum presence about 300 m above the base of the measured section where a 4 m thick unit of slightly silty
30 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

Figure 29. Slump folds in 

Yellow Lake unit of Apple 

Creek Formation. Slump folds 

are accentuated by cleavage, 

but note that beds directly beneath folds are not folded, thus 

folds are not of tectonic origin. 

Stud-finder magnet at right for 

scale, 12 cm long.

Figure 30. Lenticular bedding of siltite ripples isolated 

within argillite. Some ripples 

are fading ripples (Stow and 

Shanmugam, 1980), in which 

silt laminae of ripple crests pass 

gradually into ripple trough of 

argillite. Paleocurrent is from 

left to right. Photo also shows 

graded layers, 0.5-1 cm thick, 

of fine siltite and argillite. 

Yellow Lake unit of Apple 

Creek Formation, near Yellow 

Lake (fig. 2). Pen for scale, 13 

cm long.

calcareous dolomite crops out. The dolomite contains 

sparse laminae of siltite that are planar bedded to ripple 

crosslaminated. Dewatering structures, in zones as thick as 

25-30 cm, occur locally in siltite beds associated with these 

carbonate beds. Some of the thicker carbonate beds contain 

"floating" medium sand grains of rounded quartz and silt of 

fine- to medium-grained quartz and feldspar. Upsection,

carbonate decreases in abundance and in bed thickness, 

forming laminae only a few millimeters thick. Some of the 

carbonate is at the base of 3-5 cm thick graded beds, which 

fine upward to noncalcareous siltite. Carbonate laminae 

and silty carbonate beds weather more readily than siltite, 

causing the carbonate layers to be recessive, prominently so 

in some beds.
APPLE CREEK FORMATION 31

Figure 31. Pseudonodules, 

load structures formed of isolated ripples of siltite that have 

settled into underlying watersaturated clay (argillite). Horizontal displacement (paleocurrent from left to right in 

photograph) of siltite layer 

took place during deposition, 

as shown by the general "tailing off of layer to left, forming threads (tails) that connect 

many psudonodules; other 

pseudonodules are isolated, 

their threads broken. Yellow 

Lake unit of Apple Creek Formation, near Yellow Lake (fig. 

2).

METASANDSTONE

Light-gray metasandstones and interbedded siltite in 

the upper part of the unit are sharp based, graded, and commonly 20-50 cm thick. They typically are fine grained and 

either plane bedded or massive in the lower part, grade 

upward to ripple crosslaminated siltite in the middle, and are 

capped by planar-laminated gray-green siltite. The 

metasandstone is 40-60 percent quartz, 25-30 percent potassium feldspar, 5-15 percent plagioclase, and lesser rock 

fragments; matrix is 0-3 percent. Reconnaissance examination of strata in the uppermost 100 m of the section showed 

chiefly fine grained metasandstone. Some beds display linguoid ripples and others straight-crested ripples. A few 

metasandstone beds as thick as 3 m are graded, capped by 

siltite that locally displays ripples.

CONTACTS

The Apple Creek is conformable and transitional into 

the overlying Gunsight Formation (Ruppel, 1975, 1980; 

Ruppel and Lopez, 1988; McBean, 1983), although the contact is sharp and not gradational at the one place I observed 

it northeast of Yellow Lake. Ruppel (1975) placed the contact of the two formations at the highest bed of siltite that 

contains lenses of dolomite-cemented metasandstone, even 

though dark-gray siltite interbeds occur within the overlying 

fine-grained metasandstone. Ruppel (1975) reported that the 

Yellow Lake unit of the Apple Creek Formation is conformable with the underlying Big Creek Formation in the central 

part of the Lemhi Range. The contact is a flooding surface 

and is here considered to be an unconformity, as in the northern part of the Lemhi Range, as discussed in the section on

the Big Creek Formation. The lower contact is sharp, not 

gradational, and is exposed on the nose of a ridge, at about 

lat 44°32'36" N., long 113°33'30" W. (this location is in the 

Yellow Peak quadrangle (fig. 2), which is unsurveyed and 

thus is not divided into townships and ranges), southwest 

from the base of the measured section.

ORIGIN OF APPLE CREEK FORMATION SOUTH OF 

LEM PEAK FAULT

The assemblage of sedimentary structures present in the 

lower part of the Yellow Lake section is indicative of turbidites. The fine-grained siltite and argillite are organized silts 

and muddy silts in the classification scheme of Picketing and 

others (1988), who generally attributed such strata to transport by low-concentration turbidity currents. In general, 

lower strata of the section were deposited from lower energy 

turbidity currents than those in the three lithofacies of the 

Apple Creek north of the Lem Peak fault. Many of the sedimentary structures are of the types illustrated by Stow and 

Shanmugam (1980) in fine-grained turbidites of base-ofslope to basin plain environments. Reconnaissance examination suggests that the upper, metasandstone strata of the section may have been deposited in wave-influenced shallow 

water. This would be in accord with a conformable contact 

with the overlying Gunsight Formation observed by Ruppel 

(1975).

CORRELATION

Thin sections show that rounded quartz grains, and 

locally plagioclase grains, of medium-sand size float in the 

carbonate beds of the Yellow Lake strata. These grains
32 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

indicate that the carbonate is not a chemical precipitate but 

likely was resedimented from shallow water. The rounded 

quartz grains may establish a genetic tie between the Yellow 

Lake lithofacies and the diamictite and coarse clastic units of 

the Apple Creek, the largely silt matrix of which contains 

scattered well-rounded quartz grains.

A geochemical study was made by Connor (1991) of the 

rocks assigned to the (1) Yellowjacket Formation in the 

Salmon River Mountains, (2) strata of the same lithofacies in 

the western part of the Lemhi Range, here considered to be 

Apple Creek Formation, and (3) Apple Creek Formation in 

the Yellow Lake area. Connor reported that the Yellow Lake 

unit differed (1) in lacking chlorite and biotite, and (2) in 

containing twice as much B and Ca, and marginally more Cr, 

Li, and Zr. He (Connor, 1991, p. 7) stated, however, that the 

chemical differences alone were not sufficient to consider the 

Yellow Lake strata to be a different stratigraphic unit, and 

that only geologic mapping could determine correlation of 

the Yellow Lake strata with those of the Salmon River 

Mountains and the western part of the Lemhi Range. The 

contrasting lithofacies of the Apple Creek strata north and 

south of the Lem Peak fault, representing different depositional settings, may account for the differing chemical 

makeup found by Connor (1991).

Because much of the Apple Creek is not readily accessible, Ruppel (1975) referred interested parties to a large outcrop near the hamlet of Ellis (now only a Post Office) on U.S. 

Highway 93, near the junction of the Pahsimeroi and Salmon 

Rivers (figs. 1, 2). This outcrop lacks carbonate strata and is 

composed of beds that I interpret as being of shallow-water 

origin, probably intertidal. This contrasts with the turbidite 

origin that I attribute to the Apple Creek in the Yellow Lake 

area. The Ellis outcrop may be strata of the West Fork 

Formation.

GUNSIGHT FORMATION

The Gunsight Formation was named by Ruppel (1975) 

for a sequence of strata in the central part of the Lemhi 

Range. The type locality extends north from near Yellow 

Lake to Gunsight Peak (Ruppel, 1975), which is south of the 

Lem Peak fault (fig. 2). Strata assigned to the Gunsight in the 

7.5-minute quadrangles that I mapped crop out only in the 

northeastern part of the Lem Peak quadrangle, in the footwall 

of the Poison Creek thrust fault, where only about a 100 m 

thick, poorly exposed sequence is present (Tysdal, 1996b). 

Gunsight of the type area was examined only in reconnaissance, but is discussed briefly to place the thin Gunsight of 

the Lem Peak quadrangle in perspective. A summary 

description of the formation is presented first, based on 

McBean's (1983) study of the type section.

McBean's (1983) measured section shows that the Gunsight is 1,700+ m thick and is mainly a metasandstone unit 

composed of feldspar and quartz. The feldspar content 

ranges from 25 to 50 percent, although in the uppermost

100+ m, transitional into the Swauger Formation, the Gunsight is 80-90 percent quartz. The matrix content ranges 

from 0 to 8 percent, except in the lower 450 m where it is 

2^0 percent and the formation comprises interbedded siltite, 

argillite, and very fine grained metasandstone. The remaining 1,275 m of the measured Gunsight is pale-brown to gray, 

very fine grained to medium-grained metasandstone that 

coarsens upward through the section. Sedimentary structures 

include trough and planar crosslaminated strata; parallel lamination; ripple and climbing ripple laminations; dewatering 

structures; and straight-crested, asymmetrical, and oscillation ripples. Heavy minerals also are present. McBean 

(1983) concluded that the sequence was deposited in a nearshore environment. The lowest part, with its higher content 

of fines, was considered subtidal and the remainder intertidal 

to possibly subaerial. McBean (1983) indicated that the 

Gunsight strata were derived from the northeast.

North of the Lem Peak fault, the Gunsight in the northeastern part of the Lem Peak quadrangle is not well exposed. 

It consists of lenses of orthoquartzite that are interstratified 

with siltite and silty feldspathic metasandstone, metamorphosed to the lower greenschist facies. The orthoquartzite is 

light gray to pale red purple, fine to medium grained, and 

composed of well-rounded quartz grains. Crosslamination 

includes both trough and tabular types and forms sets 20-50 

cm thick, which are erosionally truncated at the top. Lenses 

are as thick as 10 m and form resistant ledges, in contrast to 

the adjacent generally poorly exposed slope-forming siltite 

and fine-grained metasandstone. Siltite is olive gray to yellowish gray, argillaceous, medium to coarse grained, and ripple crosslaminated. The feldspathic metasandstone is fine to 

medium grained, planar laminated, and locally contains 

dark-gray heavy-mineral laminae. Local ripple crosslaminae 

are in sets 2-10 cm high. The siltite and metasandstone are 

interbedded in some outcrops.

ORIGIN

Reconnaissance through much of the type section suggests that the Gunsight deposits are chiefly of fluvial origin; 

the trough crossbedded metasandstones occur in units 1-2 m 

thick that fine upward and give way to siltite of overbank 

deposits (D.A. Lindsey, oral commun., 1996). Uppermost 

strata of the formation lack fine-grained beds of siltite and 

are primarily medium grained to coarse-grained 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. Lowermost strata of the Gunsight may be marine, as 

suggested by McBean (1983), transitional into turbiditic 

strata of the Yellow Lake unit of the Apple Creek Formation. 

The lowermost part of the Gunsight and uppermost part of 

the Apple Creek were tightly folded and faulted during
SWAUGER FORMATION 33

compressional deformation, as McBean (1983) recognized. 

My reconnaissance suggests that some lower strata of the 

Gunsight (but above the lower contact) may be missing due 

to normal faults(s) formed during subsequent extensional 

deformation. Thus, some shallow-marine strata of the lower 

Gunsight, transitional from the below-wave-base turbidites 

of the Apple Creek to the nonmarine fluvial Gunsight, may 

be omitted from the type section.

The crossbedded orthoquartzite lenses of the northern 

part of the Lem Peak quadrangle are interpreted as fluvial 

deposits. They may be channel deposits of meandering 

streams within a flood plain environment, in which siltite 

strata constitute a large part of the rocks and the channels 

make up a small part. Gunsight strata of the Lem Peak quadrangle, therefore, correspond to the fluvial deposits that 

make up the maj or part of the formation of the type area. The 

two areas are, however, on different thrust plates and the 

strata of each plate likely were deposited in somewhat different fluvial settings. The orthoquartzite composition of the 

lenses of the Lem Peak quadrangle suggests a sedimentologic tie to the orthoquartzite of the conformably overlying 

Swauger Formation, which G.R. Winkler (oral commun., 

1991) has mapped in the Poison Peak quadrangle north of the 

Lem Peak quadrangle.

SWAUGER FORMATION

SOUTH OF LEM PEAK FAULT

The Swauger Formation was named by Ross (1947) for 

strata in the vicinity of the Swauger ranch near the hamlet of 

Goldburg on the southwest flank of the central part of the 

Lemhi Range (fig. 2). The formation, metamorphosed to the 

lower greenschist facies, is resistant to erosion and forms 

steep-sided ridges and cliffs that make up the backbone of 

the northern part of the range. Within the mapped area, the 

Swauger is exposed only south of the Lem Peak fault.

The Swauger displays a wide variety of colors, including light gray, pale to dark blue green, pale red purple, pink, 

and pale orange. The strata are mainly medium grained to 

coarse-grained orthoquartzite or quartzite with 5-10 percent 

feldspar, and locally metasandstone (less than 90 percent of 

rock is quartz). Quartz grains are well rounded, well sorted, 

tightly cemented, and glassy. Some orthoquartzite contains 

1-2 percent matrix; grains in such rocks are not as well 

sorted; plagioclase makes up 1-2 percent of the rock. Beds 

are typically 0.5-1 m thick but range up to 2 m. Some beds 

appear massive, but others display crosslamination. All 

these features were reported previously by Ross (1947) and 

Ruppel (1975); Farooqui (1996) described similar compositional and textural features.

A speckled appearance of many beds is caused by 0.5-3 

mm diameter spots of hematite, locally limonite, and

uncommonly chlorite. The density of the spots is variable. 

Some spots are concentrated along crosslaminae, others 

along the coarser grains within rocks of variable grain size. 

As pointed out by Ross (1947), thin sections show the ironoxide to form a coating on and between grains. It coats chlorite of metamorphosed matrix of some rocks, is concentrated 

along fractures, and occurs along folia of some sheared 

rocks. These data indicate that the spots formed well after 

lithification and deformation of their host rocks.

Throughout most of the formation exposed in the 

mapped area, sets of crosslaminae commonly are as thick as 

the host bed (0.5-2 m) and are erosionally truncated. Each 

bed in turn is overlain by another bed of like composition and 

structure. Siltite or argillite laminae 1- 2 mm thick occur 

between some beds, but fines are lacking between most beds.

Dark-gray laminae of heavy minerals of tourmaline, 

zircon, and sphene were observed locally in the upper part of 

the Swauger, most commonly in fine-grained orthoquartzite 

or metasandstone in beds as thick as 1.5 m. Some of the 

heavy minerals formed laminae in broad, gently inclined 

troughs 1-3 m across, although it was difficult to determine 

if all such crosslaminae were trough shaped. In one area, 

heavy minerals and quartz grains form 1-2 cm thick layers. 

Some beds in the upper strata of the formation are capped by 

planar-laminated fine-grained metasandstone that becomes 

rippled in the uppermost part and also contains heavy-mineral laminae.

The upper contact is conformable with the Lawson 

Creek Formation, and the lower contact is gradational with 

the underlying Gunsight Formation (Ruppel, 1975). The 

thickness in the central part of the Lemhi Range was estimated at 3,100 m (10,000 ft) by Ruppel (1980).

DUNES AND RIPPLES

Dunes (defined in section on Big Creek Formation, p. 

5) are spectacularly displayed directly west of Mill Lake (fig. 

2) where they are stacked atop one another into beds that are 

composed of medium- to coarse-grained orthoquartzite. The 

beds pinch and swell, commonly range between 1.0 and 1.5 

m thick normal to dune strike, and gradually pinch out in the 

downcurrent direction. The thickest dimension of some beds 

is complemented by the thinnest dimension of the directly 

overlying bed.

The dunes are two-dimensional and display wavelengths of 4-6 m and heights of 0.75-1 m (fig. 32), although 

one dune had a measured wavelength of 8 m and a height of 

0.7 m. The dunes are small (wavelength 0.6-5 m) to medium 

(wavelength 5-10 m) in size according to the classification 

of Ashley (1990). The dominant current transport direction 

was to the northeast in the Mill Lake area, shown by northeast-inclined foresets and by asymmetry of dune crests. Bottomset laminae are tangential with the underlying strata. 

Upper parts of beds are erosionally truncated; thus, topsets of
34 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

Figure 32. Dune (megaripple) 

in orthoquartzite of Swauger 

Formation. Transport direction 

is to northeast, toward the viewer. Wavelength is about 5 m, 

height about 1 m. Dashed line 

shows upper surface of dune. 

Near Mill Lake (fig. 2).

Figure 33. Antidune in orthoquartzite of Swauger Formation. 

Foreset laminae of dune are inclined to northeast (right), except at top of left-center of bed 

where antidune (anticlinal 

shape) shows laminae inclined 

to southwest (left) as well. Bed 

is about 1 m thick. Mill Lake 

area (fig. 2).

crosslaminae have been eroded. Reactivation surfaces 

(defined in section on Big Creek Formation, p. 5) occur in 

many dunes and generally are inclined at a slightly lower 

angle than the lamination of dune foresets. Herringbone 

cros shedding characterizes some strata.

One antidune was observed in an orthoquartzite bed 

about 0.75 m thick. Foresets of the bed are inclined to the 

northeast, except in one part where laminae show a buildup 

of layers into an "anticlinal" shape that verges to the southwest, forming an antidune (fig. 33).

Double clay drapes (defined in section on Big Creek 

Formation, p. 5) were not observed in the orthoquartzites of 

the Swauger Formation. In fact, no mud chips, pebbles, or 

siltite beds were observed within most strata of the Swauger. 

However, tidal flat strata are present within the upper part of 

the Swauger in the upper Cow Creek area and north of the 

upper extent of Allison Creek (fig. 2), in the transition strata 

described in the following section.

Some strata display northeast-inclined foresets of dunes 

that are capped by a thin bed with southwest-inclined
SWAUGER FORMATION 35

Figure 34. Ripples on eroded 

surface of orthoquartzite bed of 

Swauger Formation. Foreset 

laminae of bed are inclined to 

northeast, indicating transport 

to northeast; slightly asymmetrical ripples indicate transport 

to southwest (toward upper left 

of photograph). Ripple wavelengths are 5-7 cm, heights 

about 1 cm. Cow Creek area 

(fig. 2).

foresets. The thin bed is a subordinate current cap, a depositional structure produced on the crest of a dune by a subordinate (weak) tidal current (de Mowbray and Visser, 1984, p. 

817). Beds as thick as 0.3 m display foresets that are inclined 

to the southwest.

Ripples on the upper surface of erosionally planed 

orthoquartzite beds throughout the formation are twodimensional and show a range of sizes: wavelength 12-15 

cm and height 2-3 cm to wavelength of 2 cm and height of 

0.3-0.5 cm (fig. 34). Transport directions are to the southwest for most beds; however, some are to the northeast. A 

few ripples are symmetrical. Linguoid ripples were 

observed locally in siltite beds in the upper part of the 

formation.

Rippled upper surfaces of some dunes in the Mill Lake 

area (fig. 2) contain two-dimensional asymmetrical ripples 

that show transport to the southwest, capping dune foreset 

laminae that are inclined to the northeast. Small longitudinal 

ripples occur within the troughs of a few dunes of the Mill 

Lake area. Ripple crests of the larger of these ripples (wavelength 10-20 cm, height 1-1.5 cm), are oriented about parallel to those of the dune crests. Crests of smaller ripples show 

transport about parallel to the trough axis of the dune. Virtually no siltite beds or lenses were observed in this area, 

only local fine-grained siltite partings on some beds.

TRANSITION STRATA

Siltite is interbedded with orthoquartzite in the upper 

part of the Swauger where a bed, or a few-meter-thick 

sequence of orthoquartzite beds, abruptly alternates with 

sequences of siltite. The siltite is planar laminated to ripple

crosslaminated, some beds containing flaser and lenticular 

bedding.

On the ridge directly north of Allison Creek, and about 

400 m below the upper contact, the Swauger contains an 

approximately 35 m thick sequence of interbedded flaser and 

lenticular siltite, metasandstone, and lesser orthoquartzite. 

The sequence could be mistaken for strata of the overlying 

Lawson Creek Formation if it were isolated within a fault 

sliver. North of Allison Creek, however, the sequence is 

overlain by about 250 m of well-exposed orthoquartzite that 

forms a rugged part of the ridge, showing that the 35 m thick 

sequence is within the Swauger.

Some upper beds of the formation display a repetitive 

sequence, from base to top, of beds 0.7-1 m thick: basal 

30-70 cm of medium-grained massive orthoquartzite; 5-15 

cm of fine-grained orthoquartzite; 1-5 cm of siltite, or interlaminated siltite and fine-grained orthoquartzite. Beds of 

fine-grained orthoquartzite commonly display planar lamination and locally are amalgamated, as shown by 1-2 mm 

thick interlaminae. Other transition beds, 0.5-1.5 m thick, 

are feldspathic metasandstone, grading upward into siltite in 

the uppermost part. The siltite contains current, oscillatory, 

and locally interference ripples.

ORIGIN

The Swauger strata are believed to have formed by tidecontrolled depositional processes. The following interpretations are based on concepts and reasoning more fully 

explained in the section on the Big Creek Formation, to 

which the reader is referred.
36 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

The nearly 100 percent quartz content of most beds of 

the Swauger probably indicates extensive recycling and 

concentration of the quartz grains; other minerals were 

removed and exited the system. A high energy depositional environment for at least some of the strata is attested 

by the small to medium size two-dimensional dunes that 

have wavelengths as much as 8 m and amplitudes reaching 1 m. Flume experiments (Southard and Boguchwal, 

1990) demonstrate that antidunes, observed in the Mill 

Lake area (fig. 2), form under the highest flow speeds, 

higher than those of ripples, dunes, and plane beds, indicating that these features formed by tidal rather than wavedominated processes. Reactivation surfaces of the dunes 

are diagnostic evidence of tidal processes. Double clay 

drapes, also diagnostic of tidal deposits (Visser, 1980), 

were not observed. The general absence of fine-grained 

metasediment in the Swauger could indicate that highstrength tidal currents eroded silt and clay that may have 

formed drapes, as suggested by Mellere (1996, p. 260) for 

other tidal deposits.

Herringbone crosslamination, formed by bipolar currents, occurs in close stratigraphic association with dunes 

that contain reactivation surfaces, indicating that the herringbone structures are of tidal origin. Rippled surfaces present 

at the top of some thick beds of orthoquartzite reflect transport direction and current speed that contrast with those of 

the underlying bed, thus providing another argument for tidal 

origin. Flaser and lenticular bedding, which can be produced 

in a wave-dominated or tide-dominated environment, occurs 

in the transition strata of the upper part of the formation. The 

interlayering of 0.5-1 m thick beds of orthoquartzite and 

fine-grained siltite units indicates deposition from currents of 

highly contrasting flow speeds, and suggests that the transition strata were formed by tidal processes. Heavy-mineralbearing trough crosslaminated metasandstone observed 

locally in the upper part of the Swauger in the Allison Creek 

area (fig. 2) is stratigraphically not far below the transition 

strata of the formation. The metasandstone probably was 

deposited in an upper intertidal (upper shoreface) 

environment.

Most of the orthoquartzite of the Swauger is believed to 

be of tidal origin, based on the arguments presented. The I 

m thick beds of orthoquartzite that make up most of the formation may be largely dune remnants, with the dune shapes 

not readily recognized because their topset and part of their 

foreset laminae have been eroded by strong tidal currents. 

Only steep foreset lamination generally is preserved. Conversely, the dunes of the Mill Lake area may represent tidal 

channel deposits, and other beds of the Swauger could have 

formed by tidal processes in other depositional settings, such 

as low-tidal sand flat (Tankard and Hobday, 1977, p. 151; 

Klein, 1977, p. 81), or an intertidal sand bar (Klein, 1970). A 

tidal-dominated estuarine setting (Bay of Fundy, for example) where currents are concentrated and of high speed is 

favored for the orthoquartzite strata. Reactivation surfaces

indicate reversing tidal scour over dunes subjected to a 

combined constructional-destructional history. This repeated 

tidal action also may have acted to remove fines and nonresistant grains, but overgrowths on quartz grains indicate 

recycling of many of the grains of the Swauger.

LAWSON CREEK FORMATION

The Lawson Creek Formation was named by Hobbs 

(1980) for outcrops in the northernmost part of the Lost River 

Range. The name was first applied to strata in the Lemhi 

Range by Tysdal (1996b) and Tysdal and Moye (1996). In 

the mapped area, the Lem Peak fault has cut out most of the 

Lawson Creek: it occurs only south of the fault. The maximum thickness of the Lawson Creek observed is about 300 

m, on a ridge between the headwaters of Wright Creek on the 

north and Kadletz Creek on the south, in the May Mountain 

quadrangle (fig. 2). The lower part of the formation is fairly 

well exposed on the Wright-Kadletz ridge, directly south of 

the measured section within the Big Creek Formation (fig. 2). 

In general, strata of this area revealed interbeds of a variety 

of rocks—fine-grained siltite to coarse-grained orthoquartzite, well sorted to poorly sorted, metamorphosed to the lower 

greenschist facies. The rocks of this area serve as the basis 

for the following description.

The contact of the Lawson Creek is conformable and 

gradational with the underlying Swauger Formation. The 

contact was chosen where the thick beds of Swauger orthoquartzite give way to a Lawson Creek sequence that contains 

abundant beds of metasandstone and siltite, and orthoquartzite constitutes only a small percent of the rocks. The change 

takes place fairly abruptly in most places within the mapped 

area.

METASANDSTONE AND ORTHOQUARTZITE

Metasandstone is generally quartz rich but ranges from 

about equal parts feldspar and quartz to locally orthoquartzite; it commonly contains a fine-grained matrix that is chloritized. Beds range from 1 cm to 1.5 m thick. Orthoquartzite 

is gray, gray green, and blue green, is fine to coarse grained, 

and contains a high percent of well-rounded quartz grains, 

some with overgrowths. It is massive to planar laminated, in 

beds as thick as 1 m.

A few beds within the Lawson Creek are composed of 

light-gray fine-grained quartz-rich metasandstone that is 

finely laminated, contains dark-gray heavy minerals in laminae 1-2 grains thick, and locally is crosslaminated. Other 

metasandstone is poorly sorted, composed of well-rounded 

medium- to coarse-grained quartz, minor plagioclase, and 

medium- to coarse-grained silt, and 2-3 percent matrix. 

Some well-sorted fine-grained quartz-rich beds contain a
CONCLUSIONS 37

recrystallized clay matrix that constitutes 10-15 percent of 

the rock.

Rusty-brown-weathering metasandstone beds or laminae occur intermittently throughout the exposures. Ripple 

crosslaminae are fairly common in the metasandstone beds. 

A few orthoquartzite and quartzose metasandstone beds display crosslaminae about 1 m thick. Rip-up clasts of argillite 

chips locally "float" within quartzite beds.

SILTITE

Medium- to dark-gray siltite and argillaceous siltite, 

interlayered with thin beds of metasandstone, form 

sequences as thick as 10 m between sequences of beds 

chiefly composed of metasandstone and orthoquartzite. The 

siltite sequences display alternate interlayering of the siltite 

and metasandstone beds, which range from 0.5 to 5 cm thick. 

The siltite is fine to coarse grained, and the interbedded 

metasandstone and orthoquartzite generally are fine grained. 

These strata range from planar laminated to ripple crosslaminated and display abundant flaser and lenticular bedding. 

Siltite beds generally contain a higher percent of feldspar 

than the metasandstone. Some strata of the sequence display 

fine- to medium-grained siltite that fills ripple troughs of the 

underlying metasandstone. The upper surface of the siltite 

layer is planar. Cut-and-fill structures are also present within 

the interbedded siltite and metasandstone sequences.

ORIGIN

The Lawson Creek Formation is interpreted as an intertidal sequence, displaying sedimentary structures typical of 

those on tidal flats. The flaser and lenticular bedding, and 

the sequences of interbedded fine- and coarse-grained rock, 

indicate deposition under alternately high and low energy 

conditions.

At the type section of the Lawson Creek Formation, 

near Lawson Creek in the Lost River Range (fig. 2), the contact with the underlying Swauger is covered (Hobbs, 1980, 

fig. 3). Above the covered interval is a 250 m thick sequence 

that Hobbs called the transition zone, which consists of 

metasandstone that he considered similar to that of the 

Swauger. My examination shows the transition strata to be 

stream deposits, perhaps part of a braid delta, in contrast to 

the marine Swauger orthoquartzite. Northwest from the type 

section, in a reference section near Shep Creek (fig. 2), 

Hobbs (1980) noted the absence of the transition unit, as 

well, and that rocks like those of the middle unit of the type 

section were deposited directly on the Swauger, with which 

they are conformable. The middle unit of the type section, 

and the section at Shep Creek, are interpreted by me as an 

intertidal sequence, as is the upper unit of the formation.

In the Lemhi Range, a nonmarine transition unit of 

stream deposits was not observed beneath the intertidal 

strata—its absence is not surprising, considering the local 

occurrence of the unit in the Lost River Range. The Lawson 

Creek strata of the Lemhi Range differ in lacking the upper 

unit described by Hobbs (1980). This unit mainly consists of 

thin-bedded siltite that contains abundant mudcracks, rip-up 

clasts, and other features suggestive of the upper intertidal 

zone of a tidal flat. Such a unit may be absent from the 

incomplete formation in the Lemhi Range due to faulting. 

The general variability of intertidal depositional environments in coastal areas may account for differences in the two 

ranges as well.

Intertidal strata of the Lost River Range are chiefly reddish brown to pale red purple, whereas those in the Lemhi 

Range are mainly pale green and light to dark gray. The 

color differences indicate oxidation states of the rocks and 

probably reflect conditions after the time of deposition of the 

sediments. Color is not necessarily a reliable criterion for 

correlation.

Intertidal strata assigned to the Lawson Creek Formation in the Lemhi Range are similar to strata described in the 

section on the Swauger Formation, from the upper part of the 

Swauger in the Allison Creek area. The two sequences could 

be confused with one another without extensive exposures. 

The sequence within the Swauger is thinner and is overlain 

by a thick sequence of orthoquartzite. As the depositional 

environment began to change from that of the Swauger to 

that of the Lawson Creek, intertonguing of strata took place. 

Some intertonguing of strata likely occurred only locally, 

thus is not regionally extensive. Hence, choosing the 

Swauger-Lawson Creek contact may be difficult in some 

areas where exposures are poor.

CONCLUSIONS

1. The Lemhi Group and overlying Middle Proterozoic 

formations of the mapped area lie on two structural blocks 

that are believed to be two thrust plates now juxtaposed by 

the Tertiary Lem Peak normal fault.

2. The Big Creek, Swauger, and Lawson Creek Formations are marine units that were deposited in tidally controlled environments.

3. The small area of Gunsight Formation in the northern part of the Lem Peak quadrangle is interpreted as a fluvial sequence in which siltite strata constitute a large part of 

the rocks and the channels make up a small part.

4. The Apple Creek Formation is composed of marine 

turbidites and possibly debris flow deposits. The sequence 

north of the Lem Peak fault is several times thicker than 

equivalent strata south of the fault. Three lithofacies occur 

north of the fault, one south of it.
38 MIDDLE PROTEROZOIC ROCKS, NORTHERN LEMHI RANGE, IDAHO

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Manuscript approved for publication March 10, 1998 

Published in the Central Region, Denver, Colorado 

Graphics by Norma J. Macs and L.M. Carter 

Photocomposition by Norma J. Macs 

Edited by L.M. Carter

* U.S. GOVERNMENT PRINTING OFFICE: 2000 — 573-047 / 20189 Region No. 8
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