Revision of Middle Proterozic Yellowjacket

Formation, Central Idaho, and

Revision of Cretaceous Slim Sam Formation,

Elkhorn Mountains Area, Montana

U.S. Department of the Interior

U.S. Geological Survey

U.S. Geological Survey Professional Paper 1601
Revision of Middle Proterozoic 

Yellowjacket Formation, Central Idaho, and

Revision of Cretaceous Slim Sam Formation,

Elkhorn Mountains Area, Montana

By R.G. Tysdal

A. Revision of Middle Proterozoic Yellowjacket Formation, Central Idaho

By R.G. Tysdal

B. Revision of Cretaceous Slim Sam Formation, Elkhorn Mountains Area, Montana

By R.G. Tysdal

U.S. Geological Survey Professional Paper 1601–A–B

Chapters A and B are issued as a single volume

and are not available separately

U.S. Department of the Interior

U.S. Geological Survey
Revision of Middle Proterozoic Yellowjacket Formation,

Central Idaho

By R.G. Tysdal

Revision of Middle Proterozoic Yellowjacket Formation, Central Idaho, and Revision of

Cretaceous Slim Sam Formation, Elkhorn Mountains Area, Montana

U.S. Geological Survey Professional Paper 1601–A

U.S. Department of the Interior

U.S. Geological Survey
Tysdal, Russell G.

Revision of Middle Proterozoic Yellowjacket Formation, central Idaho, and revision 

of Cretaceous Slim Sam Formation, Elkhorn Mountains area, Montana / by R.G. Tysdal.

p. cm. — (U.S. Geological Survey professional paper ; 1601 A-B)

“Chapters A and B are issued as a single volume and are not available separately.”

Includes bibliographical references.

Supt. of Docs. no.: I 19.16:1601 A-B

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

Formation. 3. Yellowjacket Formation (Idaho) 4. Geology, Stratigraphic—

Cretaceous. 5. Geology—Montana—Slim Sam Formation. 6. Slim Sam Formation (Mont.) I. Title. II. Series.

QE653.5.T968 1999 98–18450

551.7′15′09796—dc21 CIP

U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

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

Box 25286, Federal Center

Denver, CO 80225

This publication is also available online at:

http://greenwood.cr.usgs.gov/pub/ppapers/p1601/

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

First printing December 2000
AI

Contents

Table

1. Correlation diagram for Middle Proterozoic rocks of east-central Idaho

Figures

1. Index map of east-central Idaho

2. Geologic map of contiguous areas in western part of Lemhi Range and

southern part of Salmon River Mountains

.............................................................................. A3

................................................................... 5

Abstract .......................................................................................................................................... A1

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

Stratigraphic Units ........................................................................................................................ 4

Yellowjacket–Quartzite Mountain–Iron Lake Area of Salmon River Mountains....... 4

Yellowjacket Formation (Restricted)........................................................................ 4

Hoodoo Quartzite......................................................................................................... 6

Unnamed Argillaceous Quartzite.............................................................................. 6

Western Part of Lemhi Range ............................................................................................ 6

Big Creek Formation ................................................................................................... 6

Apple Creek Formation............................................................................................... 7

Fine Siltite Unit.................................................................................................... 7

Diamictite Unit .................................................................................................... 7

Coarse Siltite Unit............................................................................................... 7

Gunsight Formation..................................................................................................... 7

Swauger Formation..................................................................................................... 7

Structural and Stratigraphic Patterns ....................................................................................... 8

Regional Patterns................................................................................................................. 8

Iron Lake Fault ...................................................................................................................... 8

Previous Stratigraphic Interpretations...................................................................................... 9

Age of the Yellowjacket Formation (Restricted) ...................................................................... 11

Application to Stratabound Mineralization............................................................................... 11

Conclusions.................................................................................................................................... 11

References Cited........................................................................................................................... 12

Addendum....................................................................................................................... 13

................ A2
A1

Revision of Middle Proterozoic 

Yellowjacket Formation, Central Idaho

By R.G. Tysdal

Abstract

The Proterozoic Yellowjacket Formation is restricted to the 

strata originally assigned to it by Ross (1934). The Yellowjacket 

is conformable beneath the Hoodoo Quartzite, which in turn is 

conformable beneath an unnamed argillaceous quartzite unit. 

The three units constitute a genetically related sequence of strata 

that lies in a structural block delimited on the northeast by the 

Iron Lake fault. This fault and the strata northeast of it are truncated on the west by the younger Quartzite Mountain fault. 

Directly northeast of the Iron Lake fault, strata currently 

assigned by others to the lower subunit of the Yellowjacket are 

correlated with the Apple Creek Formation in the northern part 

of the Lemhi Range. The Yellowjacket name should not be 

applied to the lower subunit or to strata that lie above it.

Mapping in the western part of the Lemhi Range shows 

that the Apple Creek Formation lies depositionally above the 

Big Creek Formation and that no rocks of the Yellowjacket-Hoodoo-unnamed unit stratigraphic sequence are present. In contrast, in the area of the Yellowjacket mapped by Ross (1934) and 

the area directly northeast of the Iron Lake fault, the Big Creek 

Formation is absent, even though it is 2,700 m thick in the 

Lemhi Range. A small sliver of the Big Creek is present in the 

southeastern part of the Salmon River Mountains, however. 

These data indicate that the Iron Lake fault juxtaposed the Yellowjacket-Hoodoo-unnamed unit stratigraphic sequence against 

non-Yellowjacket strata to the northeast.

Introduction

This report builds on recent geologic mapping, stratigraphic, and sedimentologic studies in the northern part of the 

Lemhi Range. Its goal is to correlate Middle Proterozoic strata 

in the Lemhi Range with strata in the Salmon River Mountains 

and reconcile differences of interpretation and nomenclature. 

The initial correlation of the Proterozoic strata in these two 

ranges was made by Ruppel (1975) in conjunction with his mapping in the central part of the Lemhi Range. Ruppel (1975) set 

the stage for subsequent workers on Proterozoic stratigraphy in 

central Idaho by determining the stratigraphic succession during 

the course of his geologic mapping (Ruppel, 1968, 1980; Ruppel 

and Lopez, 1981). Although Ruppel conducted no detailed mapping in the northern part of the Lemhi Range, he (Ruppel, 1975, 

p. 5–6) interpreted strata there to be older than Proterozoic rocks 

to the south and to be the lowest Precambrian rocks exposed in 

central Idaho, which “...are here correlated on the basis of 

lithologic similarity with the Yellowjacket Formation of central 

Idaho as defined by Ross (1934, p. 16).” Subsequent mapping 

by Tysdal (1996a, 1996b) and Tysdal and Moye (1996) showed 

that the Proterozoic strata in the northern part of the Lemhi 

Range are contiguous with strata in the central part, comprised 

of the Lemhi Group (in ascending order, Inyo Creek, West Fork, 

Big Creek, Apple Creek, and Gunsight Formations) and the 

overlying Swauger Formation (table 1). Connor (1990a, 1991) 

previously had correlated some of these same strata in the northern part of the Lemhi Range with rocks in the Salmon River 

Mountains that he considered to be Yellowjacket Formation. 

This report discusses stratigraphic and structural data in the area 

in which the Yellowjacket was originally named by Ross (1934), 

bringing new geologic mapping and sedimentologic data into 

consideration to reinterpret application of the name Yellowjacket Formation and to refine stratigraphic correlations.

The Yellowjacket Formation was named by Ross (1934) for 

a sequence of clastic rocks present in the vicinity of the Yellowjacket mine and the townsite of the same name in the Salmon 

River Mountains of central Idaho (fig. 1). No strata are exposed 

beneath the Yellowjacket Formation, but it is overlain by the 

Hoodoo Quartzite, also named by Ross (1934), and is discussed 

herein because its history is directly related to that of the Yellowjacket.

Ross (1934) mapped about 50 km2

 of Yellowjacket and 

Hoodoo strata south of lat 45°N. in the vicinity of the townsite 

of Yellowjacket (fig. 1). His description of the two units was 

based on a section measured for “about 4.5 mi [7 km] along the 

road up Yellowjacket Creek” (Ross, 1934, p. 15), starting from 

the processing mill for the Yellowjacket mine, which is at the 

edge of the townsite of Yellowjacket (fig. 2). The route included 

about 2.5 km of traverse within his map area and extended about 

5 km beyond, to the northeast. No type section or locality was 

formalized for either formation,1

 but Ekren (1988) designated 

the measured section of Ross (1934) as the principal reference 

section for both formations.

The upper contact of the Yellowjacket is transitional into 

the Hoodoo in the general area, but was reported to be a thrust 

fault in some places (Ross, 1934). Ekren (1988) concluded this 

contact is depositional except where the two formations are juxtaposed along high-angle faults, one of which is at the base of 

steeply dipping beds of Hoodoo in the measured section of Ross 

(1934). The high-angle faults display a component of lateral slip 

(Ekren, 1988).

1

 Naming of the Yellowjacket Formation preceded the necessity of designation of 

a type section according to the guidelines of the North American Stratigraphic 

Code (North American Commission on Stratigraphic Nomenclature, 1983).
A2 Revision of Middle Proterozoic Yellowjacket Formation and Cretaceous Slim Sam Formation Table 1. Correlation diagram for Middle Proterozoic rocks of east-central Idaho. [Diagram is separated into upper and lower parts. Some workers interpreted the Yellowjacket Formation to underlie the Lemhi Group and superjac

typeYellowjacket (of Ross, 1934) (leftmost column) to be separated by a fault from other rocks. I interpret the type Yellowjacket, Hoodoo, and unnablock juxtaposed against the Lemhi Group and superjacent rocks. Upper part of diagram shows lateral correlation of rock units. Lower part of diagraunits of Ross (1934); the other units of the lower part are a mix of units of Ross (1934) and units of the Lemhi Group and superjacent strata, as explYellowjacket Formation as defined and mapped by Ross (1934) and rock units of subsequent workers that I interpret to be the same formation. Lighas defined and mapped by Anderson (1961) and rock units of subsequent workers that I interpret to be the same formation. Interpretation is necessary binclude rocks that I believe are of formations other than those to which they were assigned due to unrecognized faults, contacts, misinterpretation onames introduced subsequent to an author's mapping. No thickness of formations is implied. “Height” of boxes is greater for some units to accommdivisions of formations] Ross (1947), upper column. Ross (1934), lower column. Ruppel (1975) Lopez (1981) Ruppel and Lopez (1988) Bennett (1977) Connor and Evans (1986); Connor (1990A); Evans and Connor (1993); Evans (1999) Ekren (1988) Winston and Link 1993) Tysdal 

(1996)(1996aand THIS 

Lawson Creek Fm Lawson Creek Fm Lawso

Swauger Fm Swauger Fm Swauger Fm Swauger Fm Swauger Fm Swaug

Gunsight Fm Gunsight Fm Yellow- Mem E Gunsig

Apple Apple Mem B, C, D Apple 

Lemhi Creek Creek jacket Creek 

Fm Fm Fm Fm fBig Big Big 

Creek Fm Creek Fm Hoodoo Quartzite Creek Fm Big CrWest Fork Fm West Fork Fm West Fork Fm West FInyo Creek Fm Inyo Creek Fm Inyo Creek Fm Inyo C

F

Unnamed unit Unnamed unit UnnamHoodoo Quartzite THRUST FAULT FAULT THRUST FAULT Hoodoo Quartzite HoodoYellowjacket Fm Yellow- Yellow- Mem E Yellow- Mem E Yellowjacket Yellow- U Yellowjacket Fm Yellow

Mem D Mem D Fm Phylite L M

jacket jacket Mem C jacket Mem C jacket L

Mem B Mem B L

Fm Fm Mem A Fm Mem A Fm L

Quartzite

Yellow- Yellow

HoodooQuartzite
Revision of Middle Proterozoic Yellowjacket Formation, Central Idaho A3

Yellowjacket

 Challis 

Salmon 

Leadore 

Area mapped by 

Ross (1934) (part) 

Quartzite

Mountain

114°30' 

0 10 20 MILES 

0 10 20 30 KILOMETERS 

IDAHO 

BLACKBIRD

MINE

YELLOWJACKET

MINE

Iron

Lake

CLEARWATER MOUNTAINS 

MONTANA 

IDAHO 

BEAVERHEAD 

MOUNTAINS 

LEMHI

LOST 

RANGE 

RIVER 

RANGE 

SALMON 

MOUNTAINS 

RIVER

Lemhi 

Hayden 

River 

Creek 

Pahsimeroi 

River 

Salmon 

River

Moyer Creek

Iron

Creek

N. Fk Iron Cr

Hat Creek 

Porphyry Cr

Musgrove Cr

Yellowjacket Creek

Shovel Cr

Lake Cr

Hoodoo Cr

Panther 

Creek

Middle

Fork Salmon

River 

Salmon 

River 93

93

114° 113°30'

45°30'

45°

44°30'

Area of figure

28

N.Fk Hat Cr

Figure 1. Index map of east-central Idaho, showing major geographic features. Shading indicates mountain ranges.

The upper contact of the Hoodoo along the measured section was found to be gradational into a unit of banded, slightly 

calcareous quartzite. The quartzite unit is not present in the 50 

km2 area mapped by Ross, although present north of his map 

area, and was not assigned a formal name; Ekren (1988) did not 

apply a formal name to the unit either. The conformable succession of the Yellowjacket of Ross (1934), Hoodoo Quartzite, and 

the overlying unnamed unit constitute a genetically related 

sequence of strata mainly deposited in shallow subtidal, intertidal, and perhaps supratidal environments.
A4 Revision of Middle Proterozoic Yellowjacket Formation and Cretaceous Slim Sam Formation

North and east of the area of Ross’ (1934) work, several 

authors (Vhay, 1948; Bennett, 1977; Hughes, 1983; Hahn and 

Hughes, 1984; Lopez, 1981; Connor and Evans, 1986; Ekren, 

1988; Fisher and others, 1992; Evans and Connor, 1993; and 

Evans, 1999) applied the Yellowjacket name to strata in an 

extensive area of the Salmon River Mountains (table 1). Application of the Yellowjacket name in this extensive area is called 

into question for the following reasons: (1) The stratigraphic 

sequence of the Yellowjacket, Hoodoo, and overlying unnamed 

unit lies in a structural block (sheet?) delimited on the northeast 

by the Iron Lake fault and on the west by the Quartzite Mountain 

fault (fig. 2). (2) Rocks of the stratigraphic sequence were deposited in shallow subtidal to upper intertidal environments in contrast with deeper water turbidite strata directly north and east of 

the faults and in contrast with the turbidites that constitute some 

of the rocks described by Lopez (1981), Sobel (1982), Hughes 

(1983), and Hahn and Hughes (1984) and assigned to the Yellowjacket in the general area that includes the Blackbird mine 

and the North Fork of Iron Creek (figs. 1, 2). (3) North of the 

Iron Lake fault and east of the Quartzite Mountain fault, the 

lower subunit of the Yellowjacket, as used by Connor and Evans 

(1986), Connor (1990a), and Evans and Connor (1993), is correlatable with the Apple Creek Formation (named by Anderson, 

1961) in the northern part of the Lemhi Range (Connor, 1990a, 

1991). Mapping in the Lemhi Range (Tysdal, 1996a, 1996b, in 

press; Tysdal and Moye, 1996) revealed that the Apple Creek lies 

depositionally above the Big Creek Formation (named by Ruppel, 1975) and that no “type” Yellowjacket, Hoodoo, or unnamed 

post-Hoodoo strata are present. The Big Creek, about 2,700 m 

thick in the Lemhi Range, is absent from the area of the Yellowjacket as mapped by Ross (1934) and from the area directly 

north of the Iron Lake fault near its junction with the Quartzite 

Mountain fault. Two slivers of the Big Creek are present in the 

southeastern part of the Salmon River Mountains, one of which 

is in fault contact with the Swauger Formation (fig. 2, long 

114°W., near Lem Peak fault).

The purpose of this paper is to restrict the Yellowjacket 

name to the strata beneath the Hoodoo Quartzite, as the name 

was applied originally by Ross (1934). Strata directly above the 

Hoodoo remain unnamed. The most recent maps of the extensive 

area of the Salmon River Mountains directly north of the Iron 

Lake fault are those of Connor and Evans (1986), Connor 

(1990a, 1991), and Evans and Connor (1993). These maps were 

completed prior to mapping and other studies in the northern 

part of the Lemhi Range (Tysdal, 1996a, 1996b, 1996c, in press) 

that permit correlation and reconciliation of strata in the two 

mountain ranges. The nomenclatural changes advocated here 

reflect this more recent work. Strata mapped and assigned to the 

lower subunit of the Yellowjacket by Connor and Evans on their 

maps here are reassigned to the Apple Creek Formation, with 

which they are contiguous (table 1), as Connor (1990a, 1991) 

recognized. No correlation of the post-Apple Creek strata previously assigned to the Yellowjacket Formation is undertaken here, 

and the middle and upper units of the Yellowjacket, as the name 

is used by Connor and Evans in their papers, are not renamed. 

The Yellowjacket name should not be applied to them, however, 

because the mapping and other work of Connor and Evans determined that these strata constitute a conformable sequence above 

their lower subunit of the Yellowjacket.

The change in status of the name Yellowjacket Formation 

advocated in this report restricts the name to the strata to which 

it was applied originally by Ross (1934). An alternative is to 

raise the name Yellowjacket to Supergroup status, thus including 

all of the rocks to which the Yellowjacket name is now applied, 

and including the Lemhi Group as a subdivision. This latter 

approach has merit because the Yellowjacket name has been 

applied to virtually all of the strata within the Salmon River 

Mountains, including correlatives of units of the Lemhi Group 

that lie within the Salmon River Mountains and some units 

within the Lemhi Range and Beaverhead Mountains as well. 

However, this approach still requires that a new name be applied 

to each mappable unit that presently has Yellowjacket in its 

name (for example, lower subunit of Yellowjacket Formation). 

Furthermore, a new name would have to be applied to strata of 

the Yellowjacket Formation as originally mapped by Ross 

(1934). Restricting the Yellowjacket name to the stratigraphic 

unit to which it was applied originally requires less revision of 

nomenclature than elevating the name to Supergroup status.

A brief description of the formations discussed in this 

report is presented first, followed by structural and stratigraphic 

arguments that led to the interpretations presented here; this is 

followed by a section that reconciles the interpretations presented with previous interpretations of other workers. Measurements in this manuscript are reported in metric units. English 

units are given, in parentheses following metric units, only 

where an original measurement was in English units.

Stratigraphic Units

The brief descriptions of stratigraphic units presented here 

are separated into those (1) in the vicinity of the townsite of Yellowjacket to Quartzite Mountain to the Iron Lake area of the 

Salmon River Mountains, and (2) in the western part of the 

Lemhi Range (fig. 2). Formations described under the Salmon 

River Mountains heading occur only in those mountains; the 

other formations described are present in both the Lemhi Range 

and the Salmon River Mountains.

Yellowjacket–Quartzite Mountain–

Iron Lake Area of Salmon River Mountains

Yellowjacket Formation (Restricted)

The Yellowjacket Formation (restricted) is the formation as 

delimited originally by Ross (1934) who described it as consisting of about 2,743 m (9,000 ft) of strata, of which the lower 518 

m (1,700 ft) are calcareous beds, the remainder quartzitic beds. 

The calcareous beds were described as containing differing 

amounts of carbonates and lime silicates, intertonguing with 

quartzitic beds, and dying out along strike. The strata have been 

metamorphosed to the upper greenschist facies. Some beds are 

banded, others are mottled with clusters of dark metamorphic 

minerals, including scapolite. Carter (1981) determined that the 

calcareous rocks form discrete lenses. The quartzite beds above 
Revision of Middle Proterozoic Yellowjacket Formation, Central Idaho A5

Yaq 

Yh 

Yh 

Pz 

Yac 

Tc 

Qu & Tc 

Tc 

Yellowjacket 

Yy 

Yy 

Tc 

Yy 

Yh & Yaq 

 Iron 

Lake 

Tc 

Yy 

Yh 

Tg Yy 

Y? 

Tc 

Tc 

Tc 

Tc 

FAULT 

LAKE 

IRON 

Yh 

Yac 

Yu 

Tc 

Ym 

Yac 

BLACKBIRD 

 MINE 

Yu 

Os 

Yu 

Yu 

Yu 

Yu 

Qu 

Yac 

Yac 

Qu Tc 

Tc 

Yac 

Tc 

Tc 

Tc 

Yu 

Tc 

Qu 

Ys 

Tc 

Ys 

Tc 

Tc 

Ys 

Ym 

Ym 

Qu 

Qu 

Yh 

Tc 

Tc 

Yg Yaf 

Tc 

Yg 

Yg 

Yaf 

Yb 

Y? 

Ys 

Yb 

Ys 

? 

Tc 

Yad 

Yaf 

Yac 

Yaf 

Tc 

Tc 

Tc 

Yb 

Tc 

Ya

Y

Tc 

Salmon 

River 

ON RIVER 

LEM 

PEAK 

FAULT 

Hat 

Cr 

Tc 

Tc 

FAULT 

Qu 

Ym 

QUARTZITE 

MOUNTAIN 

FAULT Yh 

Yellowjacket 

Creek 

N. Fk Hat Cr 

PANTHER CREEK GRABEN 

 0 5 10 15 KILOMETERS 

0 5 10 MILES 

EXP

Qu Quaternary sediments, TcTgPzOs

 Middle Proterozoic units 

Ylc Lawson Creek FormaYs Swauger Formation Yg Gunsight Formation Yu

 Formation as used 

Ym

 Formation as used 

 Apple Creek Formation 

Yac Coarse siltite unit Yad Diamictite unit Yaf Fine siltite unit Yb Big Creek Formati

Yag Unnamed unit (argilla

Yh Hoodoo Quartzite 

Yy Yellowjacket Formatio

 (unit as defined by 

Y? Unit of uncertain affin

Contact Normal fault; ball-and-bar on downthrown side Thrust fault; teeth on upper plate Thrust fault with later normal fault displacement Syncline; showing trace of axial plane Quartzite Mountain 

Yad 

Yaf 

114°30' 114°15' 114°

45°

44°52'30"

44°45'

 Eocene Challis Volca

Tertiary granitic rock

Paleozoic rocks, undi

Ordovician granitic roc

Unnamed unit (upper Unnamed unit (middle 

Figure 2 . Geologic map of contiguous areas in western part of Lemhi Range and southern part of Salmon River Mountains. Compiled from Bennett (1977), Co

Ekren (1988), Evans and Connor (1993), Fisher and others (1992), Tysdal (1996a, 1996b, unpub. mapping), Tysdal and Moye (1996).
A6 Revision of Middle Proterozoic Yellowjacket Formation and Cretaceous Slim Sam Formation

the calcareous sequence are dark gray, dark bluish gray, or 

locally white and, in general, contain about 70 percent quartz, 15 

percent biotite (or chlorite altered from biotite), with the remainder mainly feldspar (Ekren, 1988). Reconnaissance examination 

of some of the lower strata of the Yellowjacket in the principal 

reference section led to the recognition of metaevaporite strata 

(Tysdal and Desborough, 1997).

The upper few hundred meters of the Yellowjacket Formation (restricted), that is, the Yellowjacket Formation of Ross 

(1934), was examined in several places by the author. These 

strata consist of fine- to medium-grained siltite, and severalmeter-thick sequences of interbedded siltite and 2- to 10-mmthick layers of argillite (metaclaystone). The most characteristic 

feature of the upper strata is ripple crosslamination. Other common features include mudchips, fluid-escape structures, local 

herringbone crosslamination, climbing ripples, mudcracks, and 

syneresis cracks. Load casts only a few millimeters thick also 

are present. Some argillite layers show “pull-apart” structures, 

as if clay layers shrank and pulled apart while drying. Locally, 

mudchips are imbricated in opposing directions, suggesting 

reversing currents.

From examination of Yellowjacket (restricted) strata in the 

vicinity of Yellowjacket Creek, Ekren (1988) concluded that the 

rocks were deposited in a shallow-marine environment, thus 

agreeing with Ross (1934), Anderson (1953), and Carter (1981). 

The sedimentary structures and sharply bounded beds of (1) 

finely laminated argillite and mudcracked argillite alternating 

with (2) coarse-grained ripple crosslaminated siltite is indicative 

of deposition from currents of alternating flow intensities, suggesting a tidal environment.

The name Yellowjacket Formation as used henceforth in 

this report refers to the unit as restricted to the sequence named 

by Ross (1934). For other strata that have been assigned to the 

Yellowjacket, the name will be qualified: for example, the Yellowjacket of Connor and Evans (1986).

Hoodoo Quartzite

The Hoodoo Quartzite was named by Ross (1934) for 

strata exposed near Hoodoo Creek, northwest of the townsite 

of Yellowjacket (fig. 1). Ross (1934) measured 1,085 m 

(3,560 ft) of the Hoodoo Quartzite in his section along Yellowjacket Creek. Ekren (1988) stated that the Hoodoo averages about 85 percent well-rounded quartz grains, 5–15 

percent feldspar, and 0–10 percent biotite and sericite, indicative of upper greenschist facies metamorphism. The unit is 

generally light gray to white but locally is brownish gray to 

somber gray. The quartzite is massive, beds indistinct, but 

crosslamination is distinct. Oscillatory and current ripples are 

present throughout the unit. Ekren (1988) described local thin 

units of marble interbedded with calcareous quartzite in the 

basal part of the formation. He stated that the units consist of 

quartzite a few centimeters thick, with little or no calcite, that 

alternate with strata containing more calcite and calc-silicate 

minerals than quartz.

Near Lake Creek (fig. 1, about 7 km west of the townsite of 

Yellowjacket), Ekren (1988) determined the basal part of the 

Hoodoo is transitional throughout a 200-m interval into the 

underlying Yellowjacket. The Hoodoo transition zone consists 

of thin beds of dark-gray argillaceous quartzite and siltite and 

fine-grained gray quartzite typical of the Yellowjacket that alternate with thin beds of light-gray, brownish-gray, and white clean 

quartzite typical of the Hoodoo. The Hoodoo also is demonstrably transitional into the Yellowjacket near McEleny Mountain 

(about 5 km northwest of the Yellowjacket mine, fig. 1) (K.V. 

Evans, oral commun., 1997). The Hoodoo is interpreted as a 

shallow subtidal to intertidal deposit. The absence of silt and 

clay, and the ubiquitous high-angle crosslamination, suggest 

high-energy conditions.

Unnamed Argillaceous Quartzite

Lower strata of the post-Hoodoo rocks form the uppermost 

strata in the measured section of Ross (1934, p. 16). He recognized these rocks as being different from the formations below 

and did not apply a formal name to them. Ross described these 

rocks, which lie outside his map area (fig. 1), as distinctly 

banded, gray, slightly calcareous quartzites that are conformable 

above the Hoodoo. The rocks have been metamorphosed to the 

upper greenschist facies. Ekren (1988, p. 8), who also left the 

unit unnamed, described the change from the Hoodoo as transitional through “several hundred meters.” He stated that the unit 

is mainly bluish-gray to gray, argillaceous, fine-grained quartzite and siltite. My observations show abundant cross-beds, ripple crosslaminations, and local rip-up clasts. The unit contains 

beds of quartzite and quartz-rich metasandstone that are interlayered with beds of fine- to medium-grained argillaceous siltite. These alternating lithologies reflect contrasting energy 

conditions and, combined with the sedimentary structures, suggest intertidal deposition.

Western Part of Lemhi Range

Big Creek Formation

The Big Creek Formation is generally light-gray, coarsegrained siltite to medium-grained and locally coarse-grained 

metasandstone composed of quartz, feldspar, and auxiliary 

tourmaline with lesser other heavy minerals. Distinguishing 

characteristics throughout much of the formation include carbonate laminae that are oxidized to pale yellowish brown, and 

dark-gray heavy mineral laminae (tourmaline and zircon), typically only 1–2 grains thick. Cross-beds are widespread and 

record both northeast and southwest transport. Dark-gray 

argillaceous siltite zones a few meters thick are present in the 

upper part and display flaser and lenticular bedding. Beds 

commonly are about 1 m thick. The upper contact is unconformable, marked by the abrupt appearance of gray-green fine 

siltite of the Apple Creek Formation. The formation is about 

2,700 m thick, deposited in shallow subtidal and intertidal 

environments (Tysdal, 2000).
Revision of Middle Proterozoic Yellowjacket Formation, Central Idaho A7

Apple Creek Formation

The Apple Creek Formation was named by Anderson 

(1961) for strata near Hayden Creek on the northeast flank of the 

central part of the Lemhi Range (fig. 1). The original name was 

the Apple Creek Phyllite. Tietbohl (1981, 1986) separated a unit 

of diamictite from the Apple Creek, studied its depositional 

environment, and mapped part of its extent. Tysdal (1996a) 

remapped the diamictite and other rocks of the Apple Creek of 

Anderson (1961) and contiguous rocks to the west (Tysdal, 

1996b; Tysdal and Moye, 1996).

Within the Apple Creek Formation, the geologic map of 

Anderson (1961) included three genetically related gravity-flow 

units as well as strata of the Big Creek Formation that are in 

fault contact with the Apple Creek. Tysdal (1996a, 1996b; Tysdal and Moye, 1996) mapped the three units separately and 

referred to them as informal units, which in ascending order are 

as follows: fine siltite unit, diamictite unit, and coarse siltite unit. 

The fine siltite unit is unconformable above the Big Creek Formation (Tysdal, 1996a, 1996b; Tysdal and Moye, 1996). In the 

central part of the Lemhi Range, the map of Ruppel (1980) generally included the fine siltite unit in the upper part of the Big 

Creek Formation, but Ruppel did not describe its character as 

being distinct from that of most of the Big Creek (Ruppel 1975; 

Ruppel and Lopez, 1988). The diamictite unit, conformable 

above the fine siltite unit (Tysdal, 1996b), was assigned to the 

lower subunit of the Yellowjacket strata by Evans (1999). The 

coarse siltite unit, conformable above the diamictite unit, correlates with the lower subunit of the Yellowjacket Formation of 

Connor and Evans (1986), Connor (1990a), Evans and Connor 

(1993), and Evans (1999) in the Salmon River Mountains west 

of the Lemhi Range, as Connor (1990a, 1991) recognized previously.

The fine siltite unit is greenish-gray to olive-gray, planarlaminated and ripple crosslaminated, fine-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 in planar-laminated strata. Many beds are graded, with 1- to 2-cm-thick strata 

of light-gray, medium- to fine-grained siltite grading upward into 

dark-gray fine-grained siltite. The upper part of unit contains 

sparse, local, matrix-supported, gravel-size argillite clasts in 

horizons 1–2 clast diameters thick. The fine silt and clay content 

is greater in the upper part of the unit than in the lower part. The 

upper contact is conformable, marked by the abrupt appearance 

of beds of diamictite. The thickness is estimated at 1,000 m.

The diamictite unit is generally intensely cleaved, obscuring sedimentary features, and is composed of gray-green argillite, argillaceous siltite, and fine- to medium-grained siltite. 

Graded beds are obvious where deformation is not intense. Pebbly strata form sequences a few meters thick that alternate with 

nonpebbly argillite and siltite. Pebbles are dispersed, matrixsupported, and typically 1–5 cm in diameter. The upper contact 

is gradational into the coarse siltite unit. The thickness is uncertain due to deformation but is estimated to have a range of about 

600 m on the west (fig. 1, near the Salmon River) to 1,000–

1,500 m in the east (Hayden Creek area) (Tysdal, 2000).

The coarse siltite unit is composed of gray-green mediumto coarse-grained siltite and fine-grained sandstone, metamorphosed to lower greenschist facies. It contains distinctive, erosionally based, graded beds, as thick as 50 cm, of light-gray 

quartz-rich coarse-grained siltite to fine-grained metasandstone 

that grades upward into gray-green, medium-grained siltite. 

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

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

siltite that locally contain soft-sediment deformation structures. 

The preserved thickness of the unit is about 2,000–2,500 m 

(Tysdal, 2000).

Gunsight Formation

The description of the Gunsight Formation is summarized 

from McBean’s (1983) study of the type section in the central 

part of the Lemhi Range. The Gunsight, 1,800+ m thick, is 

mainly a sandstone unit comprised of feldspar and quartz that is 

metamorphosed to the lower greenschist facies. The feldspar 

content ranges from 25 to 55 percent, although in the uppermost 

100+ m, transitional into the Swauger Formation, the formation 

is 80–90 percent quartz. The matrix content ranges from 0 to 8 

percent, except in the lower 450 m where it is 2–40 percent and 

the formation is comprised of interbedded siltite, argillite, and 

very fine grained metasandstone. The remaining 1,375 m of the 

Gunsight is pale brown to gray, very fine grained to mediumgrained metasandstone that coarsens upward through the section. Sedimentary structures include trough and planar crosslaminated strata; parallel ripple and climbing ripple laminations; 

dewatering structures; straight-crested, asymmetrical ripples and 

oscillation ripples. Heavy minerals also are present. McBean 

(1983) concluded that the sequence was deposited in a nearshore 

environment; however, reconnaissance examination of all but 

the lower and uppermost part of the type section of this formation suggests a fluvial depositional environment (D.A. Lindsey,

oral commun., 1996). The lowest part, with its higher content of 

fines, may represent subtidal and intertidal environments.

Swauger Formation

The Swauger Formation is light-gray, pale-green, to palered-purple, medium- to coarse-grained orthoquartzite (at least 

95 percent quartz) or quartzite (90 percent or more quartz), 

metamorphosed to lower greenschist facies. Quartz grains are 

well rounded, well sorted, tightly cemented, and glassy. Beds 

are about 1 m thick and appear massive or display foresets of 

crosslamination. Current ripples are present on some bedding 

Fine Siltite Unit

Diamictite Unit

Coarse Siltite Unit
A8 Revision of Middle Proterozoic Yellowjacket Formation and Cretaceous Slim Sam Formation

planes, and partings of dark-gray siltite or argillite separate beds 

in some areas (Tysdal, 2000). The lower contact is gradational 

with the underlying Gunsight Formation. The thickness in the 

Lemhi Range was estimated at 3,100 m (10,000 ft) by Ruppel 

(1975, 1980).

Structural and Stratigraphic Patterns

Regional Patterns

The stratigraphic sequence of the Yellowjacket Formation 

(restricted), Hoodoo Quartzite, and overlying unnamed unit in 

the southern part of the Salmon River Mountains has never been 

found in depositional contact with other Proterozoic rocks, and 

no bottom or top is known (Ekren, 1988, p. 10). Rocks of this 

sequence were deposited mainly in shallow-water and tidal-flat 

environments, in strong contrast to the turbidites that constitute 

some of the strata assigned to the Yellowjacket Formation by 

other workers in the general area that includes the North Fork of 

Iron Creek and the Blackbird mine (figs. 1, 2). A change in rock 

types takes place along the northeast side of Quartzite Mountain 

(fig. 2), across a northwest-trending normal fault (Ekren, 1988, 

pl. 1) at the northeast limit of the stratigraphic sequence. Bennett 

(1977, pl. 1 and p. 30) previously mapped a 2.5-km-long segment of this fault northwest of the Panther Creek graben (fig. 2), 

considering the fault to be a thrust that dips southwest. Evans 

and Connor (1993) showed this fault as a nearly vertical structure of limited extent. The Panther Creek graben is 11 km wide 

and contains a thick sequence of volcanic rocks of the Eocene 

Challis Volcanic Group (Fisher and others, 1992; Fisher and 

Johnson, 1995; Janecke and others, 1997).

The Iron Lake fault, mapped in the Iron Lake area east of 

the graben by Ekren (1988), is on the same trend as the fault 

northeast of Quartzite Mountain and displays the same structural 

and stratigraphic relationships. These two faults are here considered segments of a single structure, and the Iron Lake name is 

applied to both. This interpretation is in concert with the findings 

of Ekren (1988) that the Hoodoo Quartzite, deformed into a 

faulted syncline southwest of Quartzite Mountain, is an extension of the southeast-plunging syncline in the Hoodoo Quartzite 

of the Iron Lake area southeast of the graben. (My mapping in 

the Quartzite Mountain area shows the syncline contains an axial 

plane fault, not recognized as such by Ekren (1988); the synclinal axis is not shown on the geologic map of fig. 2.)

Correlation of the coarse clastic unit of the Apple Creek 

Formation in the Lemhi Range with part of the lower subunit of 

the Yellowjacket Formation of Evans and Connor (1993) indicates that, adjacent to Quartzite Mountain, strata are missing 

from between the Apple Creek Formation northeast of the Iron 

Lake fault and the Hoodoo-overlying unnamed unit southwest of 

the fault (fig. 2). These missing strata are the Big Creek Formation and the fine siltite and diamictite units of the Apple Creek 

Formation, which lie beneath the coarse clastic unit in the northern part of the Lemhi Range (Tysdal, 1996a, 1996b, in press; 

Tysdal and Moye, 1996) (table 1). (The Inyo Creek and West 

Fork Formations, which underlie the Big Creek Formation in the 

Lemhi Range, are not discussed here because they occur southeast of the area of my mapping, shown in fig. 2.) The diamictite 

unit thins westward in the Lemhi Range and could be absent due 

to nondeposition in the southeastern part of the Salmon River 

Mountains (except for a small outcrop area adjacent to the 

Salmon River, fig. 2) and westward in the Quartzite Mountain 

area. It is unlikely that the fine siltite unit of the Apple Creek 

Formation and the underlying Big Creek Formation are absent 

due to nondeposition in the Quartzite Mountain area, however, 

because the two units combined total about 3,700 m of strata in 

the western part of the Lemhi Range. Further, about 500 m of 

the fine siltite unit is present 2–5 km east of the Iron Lake fault 

in the Iron Lake area of the Salmon River Mountains (fig. 2), as 

discussed below.

The stratigraphic-structural succession mapped in the 

western part of the Lemhi Range also occurs in the southeastern 

part of the Salmon River Mountains. (The succession in the 

Salmon River Mountains is offset southward from that in the 

Lemhi Range along the north-south Salmon River normal fault 

at the western edge of the Lemhi Range (fig. 2) (Tysdal and 

Moye, 1996).) From south to north in the western part of the 

Lemhi Range (fig. 2, east of long 114°W.), the stratigraphicstructural succession is Swauger Formation–Lawson Creek Formation–Lem Peak normal fault (south side down)–Big Creek 

Formation–Apple Creek Formation (fine clastic unit, diamictite 

unit, coarse clastic unit) (Tysdal, 1996a, 1996b; Tysdal and 

Moye, 1996). The same stratigraphic-structural succession is 

present in the southeastern part of the Salmon River Mountains 

(fig. 2, long 114°W.), where its westernmost extent is about 2–5 

km southeast of Iron Lake, directly east of the YellowjacketHoodoo-unnamed unit stratigraphic sequence delimited by the 

Iron Lake fault (fig. 2).

In the southeastern part of the Salmon River Mountains, 

much of the Swauger, and all of the Lawson Creek, have been 

cut out along the Lem Peak normal fault: only the lower part of 

the Swauger Formation and upper strata of the underlying Gunsight Formation are present in the area about 8 km southeast of 

Iron Lake (fig. 2) (headwaters of North Fork of Hat Creek, fig. 

1). The Lem Peak fault itself is concealed beneath volcanic 

rocks in most of this area, but the stratigraphic succession 

requires that it be present.

In one of the isolated exposures in the area east of the Iron 

Lake fault (fig. 2), about 50 m of the upper part of the Big Creek 

Formation and about 500 m of the depositionally overlying fine 

siltite unit of the Apple Creek Formation are present. These isolated exposures of Proterozoic rocks reflect not only erosion 

through extensive volcanic cover but also the intersection of the 

pre-volcanic, northwest-trending Lem Peak normal fault and the 

syn- and post-volcanic, north-south normal faults of the area. 

The isolated exposures in the vicinity of the North Fork of Hat 

Creek were first mapped by Ekren (1988, pl. 2; Fisher and others, 1992). The formations recognized within the isolated outcrop areas differ in part from the formation names assigned to 

them by Ekren (1988), however, who made his interpretations 

before the stratigraphy and sedimentology of the strata in the 

western part of the Lemhi Range had been worked out.

Iron Lake Fault

The Iron Lake fault is discussed at some length in this section because the structure is a key feature in reconciling the 
Revision of Middle Proterozoic Yellowjacket Formation, Central Idaho A9

stratigraphy of the Yellowjacket and associated strata of the type 

area with the stratigraphy of the non-Yellowjacket rocks that 

have been assigned to the formation in the Salmon River Mountains and the Lemhi Range. The Iron Lake name was introduced 

by Ekren (1988) for the fault east of the Panther Creek graben, 

taking precedence over Moyer Creek thrust fault that Connor 

(1990a) applied to a segment of it. The name Porphyry Creek 

thrust fault was given by Bennett (1977) to a 2.5-km-long segment of the fault northwest of the graben. Although this name is 

the first used, Ekren (1988) recognized the continuity of the fault 

and the strata southwest of it; thus, the Iron Lake name is preferred for the entire structure. In the vicinity of Iron Lake (fig. 2), 

the Iron Lake fault appears to be a thrust fault that underwent 

later normal (back-slip) displacement. Such a reversal of movement is a pattern observed along some faults in the Lemhi Range 

(Tysdal, 1996a, 1996b; Tysdal and Moye, 1996). Farther west 

along the Iron Lake fault, from the Panther Creek graben to near 

Quartzite Mountain, the Iron Lake fault is nearly vertical and the 

youngest movement is down on the southwest side.

The Iron Lake fault delimits the northeastern extent of the 

Hoodoo Quartzite in the vicinity of Iron Lake, where it displaced Hoodoo Quartzite northeastward over strata here 

assigned to the Apple Creek Formation (fig. 2) (Ekren, 1988; 

Fisher and others, 1992). Ekren (1988, p. 13, pl. 2) labeled it a 

reverse fault and determined that fractures on the west side of 

Iron Lake suggested a component of right-slip displacement that 

occurred after reverse movement. South from Iron Lake, the 

north-south segment of the Iron Lake fault dips 60° southwest. 

The fault previously was interpreted as a southwest-dipping 

thrust on the map of Rember and Bennett (1979). The footwall 

of the Iron Lake fault is characterized by closely spaced macroscopic folds that strike at a small angle to the fault (Ekren, 1988, 

pl. 2). This is a pattern typical for the footwall plate of thrust 

faults in the Lemhi Range (Tysdal, 1996a, 1996b) and probably 

in the Salmon River Mountains as well.

An area of interstratified scapolite-rich rock and metalimestone (marble) is present along Moyer Creek, about 8 km 

northwest of Iron Lake (figs. 1, 2). Connor (1990a, fig. 2a) 

placed these strata at the leading edge of a thrust plate, later 

downdropped locally by northeast-trending faults of the Panther 

Creek graben. Mapping of Tysdal and Desborough (1997) determined that the unit of interstratified scapolite-rich rock and metalimestone is bounded on the northeast by a nearly vertical 

normal fault. A cross section (Connor, 1990a, fig. 3) showed the 

scapolitic rock and metalimestone to lie directly above the Hoodoo Quartzite. Connor (1990a, fig. 3) labeled these rocks as part 

of his lower subunit of the Yellowjacket Formation. However, 

the interbedded metalimestone and scapolite-rich rocks are 

unlike any of the strata within the unnamed unit that directly and 

conformably overlie the Hoodoo Quartzite. They are similar to 

strata present locally within the lower strata of the Yellowjacket 

Formation (restricted), as reported by Ross (1934), Carter 

(1981), and Ekren (1988), and observed by the author. The scapolite-rich rocks and metalimestone, therefore, are interpreted as 

strata of the Yellowjacket Formation (restricted), which lies conformably beneath the Hoodoo (Tysdal and Desborough, 1997).

On the northeast side of Quartzite Mountain, northwest of 

the Panther Creek graben, a 2.5-km-long segment of the Iron 

Lake fault was first mapped by Bennett (1977), who interpreted 

it as a southwest-dipping thrust. The fault was extended farther 

northwest by Ekren (1988), who interpreted it as a nearly vertical west-side-down normal fault. The fault was shown as a westdipping thrust by Connor (1990a, fig. 2), later interpreted as a 

vertical fault by Evans and Connor (1993), who indicated a 

south-side-down relationship on the cross section of their reconnaissance map.

The Quartzite Mountain fault truncates the Iron Lake fault 

near the north end of Quartzite Mountain (fig. 2) (R.G. Tysdal, 

unpub. mapping, 1995). My mapping in the vicinity of Quartzite 

Mountain extended the Quartzite Mountain fault southwest from 

Musgrove Creek to connect with an unnamed fault that Ekren 

(1988, pl. 1) mapped from the area of Yellowjacket Creek southward to directly west of the townsite of Yellowjacket (fig. 2). The 

map of Ekren (1988, pl. 1), following Ross (1934), showed the 

unnamed segment of the Quartzite Mountain fault has a left-slip 

of about 1.5 km near the townsite of Yellowjacket, and two 

related faults (splays?) in the same area have an additional cumulative left-slip of about 1.5 km. Northeast from Yellowjacket 

Creek (fig. 2) to near Quartzite Mountain, I remapped the location of the fault and determined that Hoodoo Quartzite on the 

southeast side is structurally downthrown relative to Hoodoo and 

underlying Yellowjacket Formation (restricted) on the northwest.

The White Ledge shear zone (not shown on figs. 1 or 2) is 

the name applied by Shenon and others (1955) to a fault zone as 

wide as 250 m (800 ft) that was originally mapped by Vhay 

(1948). The zone trends north from the vicinity of the Blackbird 

mine (fig. 2) and southwestward from the mine toward Musgrove Creek. Bennett (1977) speculated that the southwestern 

end of the White Ledge fault connected with a 2.5-km-long segment of the Iron Lake fault (his Porphyry Creek thrust fault) that 

he mapped southeast from Quartzite Mountain (figs. 1, 2). Such 

an extension of the White Ledge fault zone would cross through 

strata of the Apple Creek Formation: no such fault connection is 

known to exist, as shown by my mapping (R.G. Tysdal, unpub. 

mapping, 1995) and mapping of Evans and Connor (1993). Further, Evans and Connor (1993) do not show the White Ledge 

shear zone to extend south of the Blackbird mine area, and 

unpublished mapping of R.G. Tysdal, K.V. Evans, and K.I. Lund 

(1997) indicates that the White Ledge shear zone does not connect with the Quartzite Mountain fault.

Previous Stratigraphic Interpretations

The previous sections of this report have focused on resolving the conflicting interpretations of stratigraphic units assigned 

to the Yellowjacket Formation in the Salmon River Mountains 

and the western part of the Lemhi Range. The foregoing stratigraphic and structural data set the stage for a closer examination 

of some specific previous usages, correlations, and miscorrelations of Yellowjacket strata. They are reviewed to aid workers in 

reconciling some previous concepts and ideas on Middle Proterozoic stratigraphy of central Idaho with those presented here.

Lopez (1981) made one of the first attempts to subdivide 

the Yellowjacket Formation, designating five informal members, 

A–E, in ascending order. Lopez’s (1981) reference sections for 

members A and B were measured where Ross (1934) had 
A10 Revision of Middle Proterozoic Yellowjacket Formation and Cretaceous Slim Sam Formation

measured his sections when naming the Yellowjacket, Hoodoo, 

and overlying unnamed unit. Lopez (1981) interpreted the Hoodoo to be the Swauger Formation and to be bounded above and 

below by faults. He considered Ross’ (1934) unnamed unit as 

the oldest part of the Yellowjacket Formation and designated it 

his informal member A. The section of Yellowjacket measured 

by Ross (1934) along the road from the mill at the townsite of 

Yellowjacket northward to the fault at the base of Hoodoo 

Quartzite, was considered by Lopez (1981) to be a younger part 

of the Yellowjacket, and he designated it member B. Because the 

Yellowjacket-Hoodoo-unnamed unit stratigraphic sequence is a 

conformable sequence (Ross, 1934; Ekren, 1988), Yellowjacket 

member B of Lopez (1981) is older than member A, in contrast 

to the conclusion of Lopez.

During the late 1980’s, a compilation of findings concerning Proterozoic rocks in the United States were summarized as 

part of the Decade of North American Geology (DNAG), sponsored by the Geological Society of America. Data on the Proterozoic rocks of central Idaho were compiled by Winston and 

Link (1993), within a necessarily limited timeframe to meet 

DNAG publication goals. Their summary was based on limited 

field study prior to mapping and sedimentological studies that 

led to the present report. Interpretations of Yellowjacket stratigraphy and correlation of rock units made by Winston and Link 

(1993, p. 502) during their compilation, necessarily built on limited (and therefore speculative) data, introduced some confusion 

into the stratigraphic picture. They correctly believed it probable 

that two quite separate units had been mapped as Yellowjacket 

Formation. They considered the type Yellowjacket in the vicinity 

of the townsite of Yellowjacket to constitute one unit, which they 

stated lay west of the White Ledge shear zone; the other unit lay 

east of the shear zone. Their regional map (Winston and Link, 

1993, their fig. 10) shows a north-south orientation for the White 

Ledge shear zone, which they extended directly south from 

where Bennett (1977) mapped its southern end at Musgrove 

Creek. As stated previously, mapping (R.G. Tysdal, unpub. mapping, 1995) subsequent to the work of Winston and Link (1993) 

showed that the White Ledge shear zone does not extend south 

of the vicinity of the Blackbird mine (fig. 2) and that the northeast-trending Quartzite Mountain fault delimits the mutual contact of the “type” Yellowjacket Formation (on the west) and the 

Apple Creek Formation (on the east) northward from Quartzite 

Mountain. The Quartzite Mountain fault connects southwestward with a fault of Ekren (1988, pl. 1) that lies west of the area 

of the townsite of Yellowjacket. The principal reference section 

of the Yellowjacket Formation lies east of the Quartzite Mountain fault. The reference section would lie west of a southward 

projection of the White Ledge shear zone. To place the reference 

section west of the White Ledge shear zone, Winston and Link 

(1993) may have followed the suggestion of Bennett (1977, p. 

30, pl. 1), and perhaps the map of Connor (1990a, fig. 2a), that 

show that the White Ledge shear zone and the southeast-trending 

segment of the Iron Lake fault near Quartzite Mountain are the 

same structure. In any case, Winston and Link (1993) knowingly 

excluded the type area of the Yellowjacket Formation as mapped 

by Ross (1934) from their “eastern Yellowjacket” rocks, an 

exclusion that is in agreement with the findings of this report.

Winston and Link (1993, p. 502) also stated that Lopez 

(1981) and Ruppel and Lopez (1988) subdivided the eastern 

Yellowjacket (emphasis added here) into five informal members. 

As noted above, Lopez (1981) did divide the Yellowjacket into 

five members, but he included strata both east and west of the 

theoretically connected White Ledge shear zone–Iron Lake 

fault. His members A and B lie west of the connected structures. 

Winston and Link (1993) were correct in excluding member A 

from the Yellowjacket, but erred in assigning it to the Hoodoo 

Quartzite because Lopez’s (1981) key reference section for 

member A is in the same sequence that Ross (1934) measured 

for his unnamed unit, which is conformable above the Hoodoo. 

Winston and Link (1993, table 1) omitted member A from their 

“eastern Yellowjacket,” but included member B at the base. This 

inclusion is spurious because (1) member B lies west of a theoretically connected White Ledge shear zone–Iron Lake fault, or 

on both sides of the Quartzite Mountain fault as I have mapped 

it, and (2) Lopez’s (1981) key measured section for Yellowjacket member B is the same section that Ross (1934) measured 

when originally naming the Yellowjacket Formation. Winston 

and Link (1993, table 1) thus correlated the principal reference 

section of the Yellowjacket Formation with lowermost strata of 

the Apple Creek Formation, even though their stated intention 

was to restrict the type Yellowjacket to west of the White Ledge 

shear zone (west of a theoretically connected White Ledge shear 

zone–Iron Lake fault, as their intentions are interpreted here).

Winston and Link (1993) (table 1) equated the Yellowjacket Formation, exclusive of member A of Lopez (1981), with 

the Lemhi Group, correlating it with the Apple Creek and Gunsight Formations. I believe that exclusion of member B as well 

as member A of Lopez (1981) from their table, thus equating 

members C–E with the Apple Creek and Gunsight, is the correct 

correlation. Similarly, Lopez (1981, fig. 50, p. 147) measured a 

section along Hayden Creek on the northeast flank of the Lemhi 

Range (fig. 1) and assigned the strata to his member B of the 

Yellowjacket. These rocks, mapped by Tysdal (1996a), are in 

the upper part of the Big Creek Formation and the lower part of 

the overlying fine siltite unit of the Apple Creek Formation.

Connor (1990a, 1990b) correlated the Jackass mineralized 

zone (banded iron-formation and cobaltian pyrite) along the 

North Fork of Iron Creek (Salmon River Mountains, fig. 1) at 

the top of his lower subunit of the Yellowjacket Formation with 

strata near Hayden Creek in the Lemhi Range. Connor (1990a, 

p. 22) stated that, in the Lemhi Range, argillitic siltite remarkably similar to that of the Jackass mineralized zone intertongues 

with diamictite mapped by Tietbohl (1986) in the Lemhi Range. 

Connor (1990a) further noted that Tietbohl (1986) assigned the 

diamictite to the Lemhi Group, and stated “I prefer, however, to 

equate these Yellowjacket-like strata, at least in part, with the ... 

lower subunit of the Yellowjacket ....” Connor (1990a, 1990b, 

1991) did not equate any part of his lower subunit of the Yellowjacket with the Big Creek Formation of the Lemhi Group as was 

stated by Winston and Link (1993, p. 503).

Comparing the text and maps of Lopez (1981, pl. 5) with the 

maps of Connor and Evans (1986), Connor (1990a), and Evans 

and Connor (1993), I find that, in general, Lopez’s (1981) members C and D correspond with the post-Hoodoo lower, middle, and 

upper subunits of the Yellowjacket Formation of these authors. I 

am not sure about the correlation, or validity, of member E.

Ruppel (1975) and Ruppel and Lopez (1988) described 

biotite as a characteristic mineral of the Yellowjacket Formation 
Revision of Middle Proterozoic Yellowjacket Formation, Central Idaho A11

and did not include it in their descriptions of other Proterozoic 

formations. Biotite cannot be used to distinguish the Yellowjacket from other formations. It is a metamorphic mineral that 

formed in other Proterozoic formations of appropriate composition and metamorphic conditions in the Lemhi Range. In any 

case, Ruppel (1980) did not apply the criterion rigorously 

because biotite is lacking from virtually all of the units shown as 

Yellowjacket in the Patterson 15-minute quadrangle in the 

Lemhi Range.

As a result of geologic mapping in the Lemhi Range, Ruppel (1975) designated several new Proterozoic formations and 

introduced the name Lemhi Group. He (Ruppel, 1975, p. 6) 

believed the Yellowjacket Formation to lie beneath the Lemhi 

Group and extended the Yellowjacket name to the Lemhi Range. 

Ruppel and Lopez (1988, p. 11) implied, and Winston and Link 

(1993, p. 502) stated, that Anderson (1956) extended the Yellowjacket name from its type area to Proterozoic strata in the 

Lemhi Range and Beaverhead Mountains. But Anderson (1956, 

1959, 1961) applied only the name “Belt Series” to strata of the 

Lemhi Range. Ross (1962, p. 20) stated that “one is tempted to 

correlate ... with the Yellowjacket ... but this is not advisable 

until more mapping is done.”

The Hoodoo Quartzite was shown to occur within the Yellowjacket on the map of Fisher and others (1992), but, in a discussion of the rocks of the map, Hobbs and Cookro (1995, p. 14) 

appear to consider the unnamed argillaceous quartzite unit to be 

the upper strata of the Yellowjacket. The Hoodoo was interpreted as a lens within the Yellowjacket Formation by Evans and 

Ekren (1985). Ekren (1988) apparently did not continue this 

interpretation—his stratigraphic succession showed the Yellowjacket to lie entirely beneath the Hoodoo. Connor (1990a) and 

Evans and Connor (1993) continued to interpret the Hoodoo as a 

lens within the Yellowjacket. The reconnaissance map of Evans 

and Connor (1993) miscorrelated the Yellowjacket Formation 

(restricted) beneath the Hoodoo (Yellowjacket Creek area, fig. 

2), showing it to be contiguous with Yellowjacket strata (Apple 

Creek Formation of this report) along the northeast side of 

Quartzite Mountain, northeast of the Iron Lake fault. Then, in 

the area from southeast of Quartzite Mountain westward to 

Shovel Creek, these strata are contiguous with the post-Hoodoo 

unnamed unit of Ross (1934). This interpretation cannot be valid 

because the pre-Hoodoo Yellowjacket cannot be contiguous with 

post-Hoodoo strata, unless the nearly 1,100-m-thick Hoodoo 

becomes 0 m thick at both ends of the outcrop belt between 

Shovel Creek and the Yellowjacket Creek area to the northwest 

(fig. 2). It also is untenable on a sedimentological basis. The preHoodoo strata of the Yellowjacket Formation were deposited in 

shallow water and tidal-flat environments, as were the strata of 

the post-Hoodoo unnamed unit. The map of Evans and Connor 

(1993), however, shows these two units are contiguous with turbidite strata, here assigned to the Apple Creek Formation, northeast of the Iron Lake fault, east of Quartzite Mountain.

The Hoodoo Quartzite was interpreted as Swauger Formation by Lopez (1981), probably because both units are composed of orthoquartzite. He believed the orthoquartzite of the 

Yellowjacket area to be fault bounded. The presence of two 

orthoquartzite units within the Middle Proterozoic strata of central Idaho raises an interesting question: could the Hoodoo and 

the Swauger be the same unit? If the Hoodoo and the Swauger 

are the same age, a possibility for which I have no evidence, 

then, theoretically at least, it is possible for the Hoodoo and 

Swauger to be lithic and time equivalents of one another. If the 

sequence of the Yellowjacket (restricted)-Hoodoo-unnamed unit 

was thrust into its present position from some distance to the 

west, then the units underlying and overlying the Hoodoo and 

Swauger need not be (and are not) lithic equivalents, but they 

could be time equivalents. Hence, stratigraphic names applied 

to the overlying and underlying units in the two areas cannot be 

the same. Strata of the Yellowjacket (restricted) beneath the 

Hoodoo and the unnamed unit that overlies the Hoodoo are 

unlike strata of the Gunsight Formation that lies beneath the 

Swauger or strata of the Lawson Creek Formation that overlies 

the Swauger.

Age of the Yellowjacket Formation 

(Restricted)

Because the Yellowjacket-Hoodoo-unnamed unit stratigraphic sequence mapped by Ross (1934) is on a structural 

block or thrust plate separate from that of other formations in the 

Salmon River Mountains, the original spatial and depositional 

relationships of the sequence relative to the Apple Creek and 

other Lemhi Group rocks to the northeast is uncertain. If the 

Iron Lake thrust fault follows a typical thrust pattern of olderover-younger displacement, then the stratigraphic sequence of 

the thrust plate is older than rocks of the Lemhi Group. However, if normal (back-slip) displacement took place later along 

the Iron Lake thrust fault, as it has along some thrusts in the 

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

and Moye, 1996), then the relative age is uncertain because the 

amount of thrust and (or) normal displacement on the Iron Lake 

fault is unknown. The Yellowjacket-Hoodoo-unnamed unit 

sequence may be older or younger than the Lemhi Group.

Application to Stratabound Mineralization

Restricting the Yellowjacket name as advocated here places 

the Co-Cu-Au deposit of the Blackbird mine area (figs. 1, 2) 

within strata that conformably overlie the coarse siltite unit of 

the Apple Creek Formation. The Co-bearing mineralized rocks 

in the vicinity of the North Fork of Iron Creek (fig. 1) lie within 

the coarse siltite unit of the Apple Creek or lower strata of the 

conformably overlying rocks, instead of the upper part of the 

lower subunit or lower part of the middle subunit of the Yellowjacket Formation, as is the case with the present nomenclature.

Conclusions

The Yellowjacket Formation, Hoodoo Quartzite, and overlying unnamed unit of Ross (1934) are a conformable sequence 

that is of shallow subtidal, intertidal, and perhaps supratidal 

strata. These strata are separate and distinct from the turbidites 

that have been assigned to the Yellowjacket by other workers. 
A12 Revision of Middle Proterozoic Yellowjacket Formation and Cretaceous Slim Sam Formation

With these observations in mind, the major conclusions of this 

report are as follows:

1. The name Yellowjacket Formation is restricted to the

stratigraphic unit to which it was applied originally by

Ross (1934).

2. The Hoodoo Quartzite is reconfirmed as a formation in

its own right, changed from a unit within the Yellowjacket Formation.

3. The unnamed unit of argillaceous quartzite of Ross

(1934), which conformably overlies the Hoodoo, is

removed from the Yellowjacket Formation.

4. The turbidite-bearing strata of the lower subunit of the

Yellowjacket Formation, as the name was used and

mapped by Connor and Evans (1986) and Evans and

Connor (1993) in the Salmon River Mountains, is correlated with the coarse siltite unit of the Apple Creek Formation of the Lemhi Group and is renamed to so

indicate.

5. In the Salmon River Mountains, stratigraphic units that

lie conformably above the strata here newly assigned to

the Apple Creek Formation, and to which the Yellowjacket name has been applied previously, are removed

from the Yellowjacket Formation. New names should be

given to these units; they likely correlate with formations

of the Lemhi Group and, perhaps, overlying units.

References Cited

Anderson, A.L., 1953, Gold-copper-lead deposits of the Yellowjacket district, Lemhi County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 94, 41 p.

———1956, Geology and mineral resources of the Salmon quadrangle, 

Lemhi County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 

106, 102 p.

———1959, Geology and mineral resources of the North Fork quadrangle, Lemhi County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 118, 92 p.

———1961, Geology and mineral resources of the Lemhi quadrangle, 

Lemhi County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 

124, 111 p.

Bennett, E.H., 1977, Reconnaissance geology and geochemistry of the 

Blackbird Mountain–Panther Creek region, Lemhi County, Idaho: 

Idaho Bureau of Mines and Geology Pamphlet 167, 108 p.

Carter, C.H., 1981, Geology of part of the Yellowjacket mining district, 

Lemhi County, Idaho: Moscow, University of Idaho, M.S. thesis, 

131 p.

Connor, J.J., 1990a, Geochemical stratigraphy of the Yellowjacket Formation (Middle Proterozoic) in the area of the Idaho cobalt belt, Lemhi County Idaho, with analytical contributions from Bartel, A.J., 

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

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

J.E., and Vaughn, R.B., Part A—Discussion, Part B—Geochemical 

data: U.S. Geological Survey Open-File Report 90-0234, 30 p.

———1990b, Large-scale mineral zonation in the Yellowjacket Formation (Middle Proterozoic), Lemhi County, Idaho [abs.]: Geological 

Society of America Abstracts with Programs, Rocky Mountain 

Section, v. 22, no. 6, p. 6.

———1991, Some geochemical features of the Blackbird and Jackass 

zones of the Yellowjacket Formation (Middle Proterozoic) in eastcentral Idaho, with analytical contributions from Bartel, A.J., Briggs, 

P.H., Carlson, R.R., Crock, J.G., Roushey, B.H., Papp, C.S.E., Siems, 

D.F., Taggert, J.E., Jr., and Welsch, E.P., Part A—Discussion [paper 

copy]: U.S. Geological Survey Open-File Report 91-259A, 25 p.

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

Map MF-1880, scale 1:62,500.

Ekren, E.B., 1988, Stratigraphic and structural relations of the Hoodoo 

Quartzite and Yellowjacket Formation of Middle Proterozoic age 

from Hoodoo Creek eastward to Mount Taylor, central Idaho: U.S. 

Geological Survey Bulletin 1570, 17 p.

Evans, K.V., 1999, The Yellowjacket Formation of east-central Idaho, in

Berg, R.B., ed., Belt Symposium III: Montana Bureau of Mines and 

Geology Special Publication 112, p. 17–30.

Evans, K.V., and Connor, J.J., 1993, Geologic map of the Blackbird Mountain 15-minute quadrangle, Lemhi County, Idaho: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-2234, scale 1:62,500.

Evans, K.V., and Ekren, E.B., 1985, Hoodoo Quartzite revisited—Stratigraphic relationships near Yellowjacket, Idaho [abs.]: Geological 

Society of America Abstracts with Programs 1985, v. 17, no. 4, p. 217.

Fisher, F.S., and Johnson, K.M., eds., 1995, Geology and mineral 

resource assessment of the Challis 1°×2° quadrangle, Idaho: U.S. 

Geological Survey Professional Paper 1525, 204 p., 23 maps.

Fisher, F.S., McIntyre, D.H., and Johnson, K.M., 1992, Geologic map of 

the Challis 1°×2° quadrangle, Idaho: U.S. Geological Survey Miscellaneous Investigations Series Map I-1819, 39 p., 1 sheet, scale 

1:250,000.

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

associated magmatism of the Yellowjacket Formation in the Idaho 

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

Hobbs, S.W., and Cookro, T.M., 1995, Proterozoic terrane, in Fisher, F.S., 

and Johnson, K.M., eds., Geology and Mineral Resource Assessment of the Challis 1°×2° Quadrangle, Idaho: U.S. Geological Survey 

Professional Paper 1525, p. 12–17.

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

mining district, Lemhi County, Idaho, in The Genesis of Rocky Mountain Ore Deposits—Changes with Time and Tectonics: Denver 

Region Exploration Geologists Society Symposium, p. 21–27.

Janecke, S.U., Hammond, B.F., Snee, L.W., and Geissman, J.W., 1997, 

Rapid extension in an Eocene volcanic arc—Structure and paleogeography of an intra-arc half graben in central Idaho: Geological 

Society of America Bulletin, v. 109, p. 253–267.

Lopez, D.A., 1981, Stratigraphy of the Yellowjacket Formation of eastcentral Idaho: U.S. Geological Survey Open-File Report 81-1088, 218 

p.

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

North American Commission on Stratigraphic Nomenclature, 1983, 

North American Stratigraphic Code: American Association of 

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

Rember, W.C., and Bennett, E.H., compilers, 1979, Geologic map of the 

Challis quadrangle, Idaho: Idaho Bureau of Mines and Geology, 

Geologic Map Series, Challis 2° quadrangle, scale 1:250,000.

Ross, C.P, 1934, Geology and ore deposits of the Casto quadrangle, 

Idaho: U.S. Geological Survey Bulletin 854, 135 p.
Revision of Middle Proterozoic Yellowjacket Formation, Central Idaho A13

———1947, Geology of the Borah Peak quadrangle, Idaho: Geological 

Society of America Bulletin, v. 58, p. 1085–1160.

———1962, Stratified rocks in south-central Idaho: Idaho Bureau of 

Mines and Geology Pamphlet 125, 125 p.

Ruppel, E.T., 1968, Geologic map of the Leadore quadrangle, Lemhi County, Idaho: U.S. Geological Survey Geologic Quadrangle Map GQ-733, 

scale 1:62,500.

———1975, Precambrian Y sedimentary rocks in east-central Idaho: 

U.S. Geological Survey Professional Paper 889-A, 23 p.

———1980, Geologic map of the Patterson quadrangle, Lemhi County, 

Idaho: U.S. Geological Survey Geologic Quadrangle Map GQ-1529, 

scale 1:62,500.

Ruppel, E.T., and Lopez, D.A., 1981, Geologic map of the Gilmore quadrangle, Lemhi County, Idaho: U.S. Geological Survey Geologic Quadrangle Map GQ-1543, scale 1:62,500. 

———1988, Regional geology and mineral deposits in and near the central part of the Lemhi Range, Lemhi County, Idaho: U.S. Geological 

Survey Professional Paper 1480, 122 p.

Shenon, P.J., Full, R.P. Snyder, D.M., and Bauer, H.L., 1955, Geologic 

study of the Blackbird–Big Deer Creek area, Blackbird mining district, Lemhi County, Idaho: unpublished consultant report for Northfield Mines, Inc.

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

County, Idaho: Cincinnati, University of Cincinnati, M.S. thesis, 235 p.

Tietbohl, D.R., 1981, Structure and stratigraphy of the Hayden Creek area, 

Lemhi Range, east-central Idaho: University Park, Pennsylvania 

State University, M.S. thesis, 121 p.

———1986, Middle Proterozoic diamictite beds in the Lemhi Range, 

east-central Idaho, in Roberts, S.M., ed., Belt Supergroup: Montana 

Bureau of Mines and Geology Special Publication 94, p. 197–207.

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

and Mogg Mountain quadrangles, Lemhi County, Idaho: U.S. Geological Survey Miscellaneous Investigations Series Map I-2563, 

scale 1:24,000.

———1996b, Geologic map of the Lem Peak quadrangle, Lemhi County, 

Idaho: U.S. Geological Survey Geologic Quadrangle Map GQ-1777, 

scale 1:24,000.

———1996c, Geologic map of part of May Mountain quadrangle, Lemhi 

County, Idaho: U.S. Geological Survey Open-File Report 96-537, 

scale 1:24,000.

———2000, Stratigraphy and sedimentology of Middle Proterozoic 

rocks in northern part of Lemhi Range, Lemhi County, Idaho: U.S. 

Geological Survey Professional Paper, 40 p.

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

carbonate rocks of Proterozoic Yellowjacket Formation, Moyer 

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

Tysdal, R.G., and Moye, Falma, 1996, Geologic map of the Allison Creek 

quadrangle, Lemhi County, Idaho: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1778, scale 1:24,000.

Vhay, J.S., 1948, Cobalt-copper deposits in the Blackbird district, Lemhi 

County, Idaho: U.S. Geological Survey Strategic Minerals Investigations Preliminary Report 3-219, 26 p.

Winston, D., and Link, P.W., 1993, Middle Proterozoic rocks of Montana, 

Idaho, and eastern Washington—The Belt Supergroup, in Reed, 

J.C., Jr., Bickford M.E., Houston, R.S., Link, P.K., Rankin, D.W., Sims, 

P.K., and van Schmus, W.R., eds., Decade of North American Geology, Precambrian—Conterminous U.S.: Geological Society of America, The Geology of North America, v. C-2, p. 487–517.

Addendum

As this manuscript was in the process of final editing for 

publication, Link and others (1997) published an abstract in 

which they correlated the Yellowjacket Formation in the vicinity 

of the townsite of Yellowjacket (the “type” area) in the Salmon 

River Mountains with the West Fork Formation of the Lemhi 

Group, southwest flank of the Lemhi Range. The West Fork and 

conformably underlying Inyo Creek Formation occur below the 

Big Creek Formation on the southwest flank and are not discussed in the present report because they occur south of the area 

of mapping shown in fig. 2. Correlation of the Yellowjacket with 

the West Fork Formation does not appear to conflict with the 

general conclusions of this report.

In addition, these authors suggested that the Hoodoo 

Quartzite (plus unnamed argillaceous quartzite?) may correlate 

with the Big Creek Formation. (Bennett, 1977, also suggested 

correlation of the Hoodoo and Big Creek Formations.) If the 

Hoodoo and associated strata of the Salmon River Mountains 

were thrust into their present position from some distance to the 

west, then the Yellowjacket and Hoodoo plus unnamed unit need 

not be lithic equivalents of the West Fork and Big Creek Formations of the Lemhi Range, although they may be time equivalents. Hence, stratigraphic names applied to the units in the two 

areas should not be the same. I think that the orthoquartzite of 

the Hoodoo is sufficiently different from the Big Creek that the 

two formations should retain their current names, even if it is 

determined that they are time equivalents.

Structural setting needs to be considered with respect to 

strata of the Lemhi Range. Within the entire area of the range, 

the Inyo Creek and West Fork Formations are known only along 

the southwest flank, and only in the area northeast of the Pahsimeroi River (fig. 1). The flank is characterized by compressional faults and folds, followed by extensional normal faulting. 

This raises the possibility that these two formations constitute 

strata of the leading edge of a thrust plate that later was severed 

by normal faults, downdropped, and now lies largely concealed 

beneath sediments of the Pahsimeroi Valley. One needs to 

determine if the Inyo Creek plus West Fork sequence is in structural contact or in depositional contact with the Big Creek Formation before correlating the Hoodoo with the Big Creek 

because, in the thrusting scenario, the two formations would be 

on different plates.

Link, P.K., Winston, D., and Hathaway, N., 1997, The Yellowjacket is not 

the Prichard—Revised correlation of Lemhi Group, Yellowjacket 

Formation and Belt Supergroup, Mesoproterozoic, Montana and 

Idaho [abs.]: Geological Society of America Abstracts with Programs, v. 29, no. 6, p. A-408.
Revision of Cretaceous Slim Sam Formation,

Elkhorn Mountains Area, Montana

By R.G. Tysdal

Revision of Middle Proterozoic Yellowjacket Formation, Central Idaho, and Revision of

Cretaceous Slim Sam Formation, Elkhorn Mountains Area, Montana

U.S. Geological Survey Professional Paper 1601–B

U.S. Department of the Interior

U.S. Geological Survey
BII

Contents

Abstract .......................................................................................................................................... B1

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

Slim Sam Formation (Restricted) ....................................................................................... 3

Eagle Sandstone................................................................................................................... 3

Telegraph Creek Formation ................................................................................................ 4

Cody Shale............................................................................................................................. 4

Age................................................................................................................................................... 4

Discussion and Conclusions ....................................................................................................... 5

References Cited............................................................................................................. 7

Figures

1. Index map showing location of Elkhorn Mountains, neighboring mountain

ranges, and towns ................................................................................................................ B1

2. Geologic map showing outcrop area of Slim Sam Formation as originally

defined by Klepper and others (1957) ............................................................................... 2

3. Correlation chart showing relationships in the Elkhorn Mountains .......................... 6
B1

Helena

MONTANA

0 10 20 30 MILES

0 20 40 KILOMETERS

Livingston

Bozeman

Townsend

Helena

112° 111° 47°

46°30'

46°

45°30'

Elkhorn

Mountains

Bridger

Range

Big Belt Mountains

z

15

z

90

287

Revision of Cretaceous Slim Sam Formation,

Elkhorn Mountains Area, Montana

By R.G. Tysdal

Abstract

The Upper Cretaceous Slim Sam Formation of the Elkhorn 

Mountains area is revised. Strata of the lower part correlate with 

the regionally recognized marine Telegraph Creek Formation 

and the overlying marine to marginal marine Eagle Sandstone. 

The Eagle in the region generally consists of upper and lower 

sandstone units separated by a shaly to coaly unit. Only the 

lower sandstone of the Eagle is present in the Elkhorn Mountains study area, and it is preserved discontinuously. The nonmarine volcanic and volcaniclastic rocks of the upper part of the 

Slim Sam as originally defined retain the name Slim Sam Formation. These rocks, mainly of sedimentary origin, are genetically related to the Elkhorn Mountains Volcanics. The lower 

contact of the Slim Sam (restricted) is unconformable above the 

Eagle Sandstone, or more commonly, above the Telegraph 

Creek Formation. The upper contact is conformable with the 

Elkhorn Mountains Volcanics.

Introduction

This paper is an outgrowth of mineral-resource-assessment studies conducted in the Helena National Forest of westcentral Montana. The Elkhorn Mountains (fig. 1) of the southwestern part of the forest contain the Upper Cretaceous Slim 

Sam Formation, a stratigraphic name not used beyond the 

Elkhorn Mountains and adjacent area (fig. 2). This unit contains magnetite-bearing fossil beach-placer deposits in and 

adjacent to the forest. Examination of the stratigraphic 

sequence associated with the deposits revealed that the Slim 

Sam as originally defined by Klepper and others (1957) is comprised of the Telegraph Creek Formation, the younger Eagle 

Sandstone, and an overlying nonmarine volcanic and volcaniclastic unit.

The Slim Sam Formation was named by Klepper and others (1957, p. 28) for a sequence of fine- to coarse-grained clastic rocks present along the eastern flank of the Elkhorn 

Mountains. The type locality is in the Slim Sam Basin at the 

southeast corner of the mountain range. Klepper and others 

(1957) recognized a lower and an upper part to the Slim Sam. 

They described the lower part as consisting mainly of quartzchert sandstone, containing crystal-lithic tuff, sedimentary tuff, 

and black shale that commonly includes thin lenses of coarse 

feldspar crystals, probably of sedimentary origin. Klepper and 

others (1971, p. 11) reported discontinuous occurrences of one 

or more beds of detrital titaniferous magnetite in the uppermost 

strata of the lower part. The upper part of the formation was 

described as crystal-lithic tuff, lithic tuff, volcaniclastic sandstone, and at least two intervals of hard mudstone (Klepper and 

others, 1957, 1971).

The geologic studies of Klepper and coworkers (Klepper 

and others, 1957, 1971; Freeman and others, 1958; Smedes, 

1966) in the Elkhorn Mountains dates from the 1950’s and 

remain the standard references for the area. The revision of the 

Slim Sam in this report builds on their careful mapping and 

related detailed studies and reflects subsequent areal, regional, 

and correlation work carried out by many people.

The Slim Sam Formation as defined by Klepper and others 

(1957) lies conformably beneath the Upper Cretaceous Elkhorn 

Mountains Volcanics and above a black shale unit of the Colorado Group (rank raised). The black shale is herein described 

and correlated with the Cody Shale that is present in the Livingston area and elsewhere in southwestern Montana. Reasons for 

assignment to the Cody rather than the Marias River Shale are 

presented in the section on the Cody Shale.

Figure 1. Index map showing location of Elkhorn Mountains, 

neighboring mountain ranges, and towns.
B2 Revision of Middle Proterozoic Yellowjacket Formation and Cretaceous Slim Sam Formation

Elkhorn

Radersburg

Winston

East

Helena

Helena

 Qu 

 Kg 

 Ke Cretaceous Elkhorn Mountains Volcanics and

 related intrusions

 Ks Cretaceous Slim Sam Formation (restricted),

 Telegraph Formation, and Eagle Sandstone 

 MzPz Mesozoic and Paleozoic rocks, undivided

 Yu Middle Proterozoic rocks, undivided

EXPLANATION

Yu

Ke

Ke

Qu

Qu

Ke

Ke

Ke

Ks

MzPz

MzPz

Ks

MzPz

MzPz

Kg

Kg

Kg

Qu

Qu

Kg

Kg

MzPz

MzPz

Qu

Kg

Kg

Ks

Corral Creek

Sheep

Creek

Indian

Cr

Weston Cr

Crow

Creek

0 1 2 3 4 5 6 MILES

0 2 4 6 8 KILOMETERS

112° 111°45'

46°30'

46°15' Slim Sam

Basin

Iron

Cross

mine

Measured

section

Big

Mountain

287

15z

ELKHORN MOUNTAINS

 

 Cretaceous granite and related intrusions

Quaternary sediments, undivided

Figure 2. Geologic map showing 

outcrop area of Slim Sam Formation as originally defined by Klepper and others (1957) and as 

herein redefined to include the 

Telegraph Creek Formation, the 

discontinuously present Eagle 

Sandstone, and the Slim Sam Formation (restricted). The Indian 

Creek reference section of the 

Slim Sam (restricted) is indicated. 

Compiled from maps of Klepper 

and others (1957, 1971), Freeman 

and others (1958), Smedes (1966), 

and Knopf (1963).
Revision of Cretaceous Slim Sam Formation, Elkhorn Mountains Area, Montana B3

The purpose of this paper is to redefine the Slim Sam, 

restricting the name to the volcanic and volcaniclastic rocks 

and applying the two regionally established formation names 

to the other units. The Slim Sam Formation (restricted), the 

Eagle Sandstone, and Telegraph Creek Formation are separated from the Slim Sam as originally defined—these formations and the Cody Shale beneath the sequence are described, 

correlated, and discussed in descending order in the following sections. All identifications of megafossils were made by 

W.A. Cobban.

Slim Sam Formation (Restricted)

The Slim Sam Formation (restricted) consists of mediumgray to gray-green nonmarine volcanic and volcaniclastic 

rocks that constitute the upper part of the Slim Sam as originally defined by Klepper and others (1957). The Slim Sam 

(restricted) contains a few beds of pyroclastic tuff but mainly 

consists of volcanic debris reworked by sedimentary processes. Some beds appear massive and are fine- to mediumgrained epiclastic tuff composed almost wholly of angular 

grains of plagioclase, small fragments of fine-grained volcanic rocks, and less abundant grains of quartz, mica, hornblende, and opaque minerals, set in a microcrystalline 

groundmass altered to chlorite and sericite (Klepper and others, 1957, 1971). Sandstone beds commonly are amalgamated 

into units that are clearly stratified, exhibit both planar and 

trough crosslamination, local lenses of mudstone or rip-up 

clasts of mudstone, and zones of well-rounded volcanic pebbles. Some volcaniclastic sandstone units are laterally lenticular over several tens of meters. Scouring and channeling were 

noted by Smedes (1966) in the northwestern part of the outcrop area. The above described strata tend to be resistant and 

form outcrops. Poorly exposed sandstone, siltstone, mudstone, and locally porcellanite, form recessive strata. Most 

beds of the formation are noncalcareous.

Measured thicknesses include 155 m (508 ft) near Sheep 

Creek in the northwestern extent of the formation (Smedes, 

1966) and 201 m (659 ft) near Indian Creek (this study) (fig. 

2). The upper unit of the Slim Sam of Smedes (1966) represents the restricted Slim Sam of this study. Thicknesses measured by Klepper and others (1957) do not distinguish the 

upper part (Slim Sam as herein restricted) from the lower part 

(Eagle Sandstone and Telegraph Creek Formation). A readily 

accessible reference section for the Slim Sam (restricted) is 

the Indian Creek section of Klepper and others (1957, p. 30–

31) (fig. 2; N1

/2 SW1/4 and S1/2 NW1/4 sec. 6, T. 6 N., R. 1 E.), 

starting about 122 m (400 ft) above the base.

Eagle Sandstone

Rocks here assigned to the Eagle Sandstone constitute the 

uppermost strata of the lower part of the Slim Sam Formation 

as originally defined by Klepper and others (1957). The formation is described and discussed because (1) its local and 

regional character aid demonstration of the unconformable 

nature of the upper contact, thus providing a basis for 

interpreting the stratigraphic history of the sequence of rocks 

and for restricting rocks assigned to the Slim Sam, (2) it is 

used as the key stratigraphic unit for regional correlation, and 

(3) it hosts strandline placer deposits.

The Eagle was named by Weed (1899) from exposures in 

north-central Montana, where the formation consists of an 

upper thin-bedded buff-colored sandstone unit, a middle unit of 

coaly and carbonaceous rocks, and a lower ledge-forming unit 

of hard white sandstone. The lower sandstone unit later was 

named the Virgelle Sandstone Member (Bowen, in Stebinger, 

1914). In the Elkhorn Mountains, only strata of the Virgelle 

and lowermost part of the middle unit are preserved.

The Eagle of the Elkhorn Mountains is here correlated 

with Eagle strata in the vicinity of Livingston, Mont. (fig. 1), 

where the Eagle has been carefully examined and mapped over 

a broad area (Roberts, 1972, and maps cited therein). In the 

Livingston area, the sandstones and intervening coal-bearing 

strata of the Eagle have been mapped separately from overlying somber-colored volcaniclastic strata, which were assigned 

to the conformably overlying Cokedale Formation of the Livingston Group (Roberts, 1972; Weed, 1893). The Eagle consists mainly of delta-plain and beach deposits. The basal 

sandstone, the Virgelle Sandstone Member, is as much as 34 m 

(110 ft) thick and is conformable with the underlying Telegraph Creek Formation (Roberts, 1972).

The Eagle Sandstone thins northwestward from the Livingston area, ranging from about 183 m (600 ft) in the southern part of the Bridger Range to about 30 m (100 ft) in the 

northern part (McMannis, 1955) (fig. 1). McMannis (1955, p. 

1407) stated that the formation is prominent in the southern 

and middle parts of the range but becomes inconspicuous and 

difficult to locate in the northern part. The basal contact is not 

well exposed.

Northward, within and adjacent to the southernmost part 

of the Big Belt Mountains, Skipp and Peterson (1965) reported 

that the Eagle ranges from 30–91 m (100–300 ft) thick. The 

Eagle, and locally the underlying Telegraph Creek Formation, 

are unconformably overlain by volcanic and volcaniclastic 

rocks of the Maudlow Formation, which is considered to be an 

isolated remnant of the same volcanic field of which the 

Elkhorn Mountains Volcanics are a part (Skipp and Peterson, 

1965; Skipp and McGrew, 1977).

It is clear from the above data that the basal sandstone of 

the Eagle decreases in thickness and prominence northwestward from the Livingston area toward the Elkhorn Mountains 

and that the formation is directly overlain by volcanic and volcaniclastic rocks. As noted by Roberts (1972, p. 31), rocks 

above the Eagle in central and western Montana range from 

marine to nonmarine, and, where they are nonmarine, the top 

of the Eagle generally is placed at the lithologic change where 

volcanic detritus becomes abundant. The restriction of the Slim 

Sam as defined herein follows this general relationship and parallels the stratigraphic and sedimentologic usage of the Livingston area in and adjacent to the southernmost part of the Big 

Belt Mountains.

In the Elkhorn Mountains, the basal Virgelle Sandstone 

Member of the Eagle is present only locally. The best 
B4 Revision of Middle Proterozoic Yellowjacket Formation and Cretaceous Slim Sam Formation

exposure, and thickest known preserved sequence, is in a bulldozer trench west of Big Mountain (fig. 2). In the trench, and 

in the roadcut of the access road leading to it, the Eagle consists of feldspar-chert-quartz sandstone overlain by silty sandstone, and then medium-grained magnetite-bearing sandstone 

that also contains hornblende, chert, quartz, and feldspar. The 

latter intertongues with, and is overlain by, planar-bedded finegrained sandstone that is devoid of trace fossils. Upward, this 

sandstone contains lenses of mudstone that display traces of 

plant rootlets. These uppermost exposed strata are interpreted 

as basal strata of the middle unit of the Eagle.

The magnetite-bearing sandstone and the sandstone with 

which it intertongues are interpreted as back-beach deposits. 

This interpretation is in agreement with that of Klepper and 

others (1957, 1971) and Houston and Murphy (1977) for magnetite-bearing deposits at the Iron Cross mine west of Radersburg (fig. 2), from which ore has been mined (Reed, 1951). 

The planar-bedded sandstone grades upward into the mudstone and rootlet-bearing strata interpreted as delta-plain 

deposits. Sandstone in the roadcut leading to the trench possibly also was deposited in a beach, but its poor and discontinuous exposure prevents making a reliable determination. 

Volcanic and volcaniclastic rocks of the Slim Sam overlie this 

sequence of rocks.

Foreshore (beach) deposits were observed about 1 km (0.6 

mi) southeast of the top of Big Mountain and along the road 

directly south of Weston Creek (fig. 2). At both locations, the 

basal Eagle is light-gray, fine- to medium-grained, quartz and 

feldspar (“salt-and-pepper”) sandstone that has very little 

matrix and displays planar and trough crosslaminated beds. At 

these two localities, the lower Eagle is overlain by volcanic 

and (or) volcaniclastic rocks of the Slim Sam Formation 

(restricted). Upper foreshore deposits, such as magnetite-bearing sandstone and delta-plain strata, are absent.

Telegraph Creek Formation

The Telegraph Creek Formation is transitional between 

the Cody Shale and the overlying Eagle Sandstone. In the 

Elkhorn Mountains, the transitional position is reflected in the 

upward-changing makeup of the formation. The lower part 

consists of interbedded dark-gray mudstone and shale, similar 

to that of the underlying Cody Shale, and interbedded clayey 

and silty fine-grained sandstone. The sandstone content 

increases upsection; the grain size generally increases from 

fine to medium grained; and the clay and silt content of the 

sandstone decreases. Sandstone is calcareous throughout the 

formation. Trace fossils are common throughout the formation, changing in type upward and recording shallowing depositional conditions. Horizontal traces, indicative of lower 

shoreface and offshore environments (Pemberton and others, 

1992), are common on sandstone beds in lower strata of the 

formation. In the upper strata, steeply inclined to vertical burrows of Ophiomorpha and Diplocraterion, indicative of middle and upper shoreface environments, and thoroughly 

burrowed (churned) sandstone units are interstratified with planar, unburrowed sandstone beds.

Cody Shale

Basal strata of the Telegraph Creek Formation are conformable and gradational with the underlying shale that was 

assigned to the upper black shale unit of the Colorado Group 

by Klepper and others (1957). This upper black shale unit is 

here correlated with the Cody Shale of the Livingston area 

(fig. 1), where it directly underlies the Eagle Sandstone. Three 

sections of the Cody measured by Klepper and others (1957, 

p. 28; their upper black shale unit of the Colorado Group) in 

the eastern part of the Elkhorn Mountains range from 81 to 

119 m (265 to 390 ft) thick.

The black shale is assigned to the Cody rather than to the 

Marias River Shale that constitutes the upper shale of the Colorado Group in north-central Montana for the following reasons. The Marias River consists of four members, which, in 

descending order, are the Kevin, Ferdig, Cone, and Floweree 

Shale Members. The Floweree (mainly marine fissile shale) 

and Cone Members (chiefly calcareous shale) (Cobban and 

others, 1976) are absent in the Elkhorn Mountains. The Ferdig (mainly dark bluish-gray shale with a medial sandy 

shale—Cobban and others, 1976) also is absent in the Elkhorn 

Mountains. The time zone of the Ferdig is represented by 

sandstone, which is more similar to sandstone of the Frontier 

Formation than shale of the Ferdig. The Kevin lithology 

(mainly shale, bentonitic in the lower part, and containing 

limestone concretions of differing character in different parts 

of the member—Cobban and others, 1976) is present in the 

Elkhorn Mountains, where it contains more siltstone and fewer 

concretions than is reported to the north by Cobban and others (1976) and Schmidt (1978). This lithology also is typical 

of the Cody. The Cody name is used for the shale unit in the 

Elkhorn Mountains because (1) the Frontier is succeeded by 

the Cody Shale in southwestern Montana and the type sections of both formations in Wyoming, and (2) the Marias River 

Shale (or the Kevin Member) nomenclature is not used for 

rocks overlying the Frontier.

Age

Based on the limited data available at the time of their 

field studies, Klepper and others (1957, p. 37) stated that the 

basal part of the Elkhorn Mountains Volcanics could be as old 

as the Telegraph Creek Formation because the Slim Sam Formation (as originally defined), which contains “Niobrara” 

(age) fossils (i.e., Telegraph Creek age) in its lower part, 

“appears to have been deposited rapidly” and to grade upward 

into the volcanic rocks at some localities. Klepper and others 

(1971) later stated that the Slim Sam could be as young as the 

Eagle Sandstone, but no younger, because the overlying 

Elkhorn Mountains Volcanics are only slightly younger than 

the Eagle. We now know that the Elkhorn Mountains Volcanics are younger than the Eagle Sandstone and the Slim Sam 

(restricted). The Elkhorn Mountains Volcanics are above the 

Slim Sam (restricted) and lie unconformably on the Telegraph 

Creek and the Eagle; thus, they are younger than all of the 

clastic-bearing formations.
Revision of Cretaceous Slim Sam Formation, Elkhorn Mountains Area, Montana B5

The maximum age of strata previously assigned to the Slim 

Sam Formation by Klepper and others (1957) is limited by the 

age of the Cody Shale, which underlies the Telegraph Creek Formation. The bivalve Cremnoceramus deformis (Meek) (previously Inoceramus deformis), from the basal middle Coniacian 

zone of Forresteria alluaudi, was collected from basal strata of 

Fauna recovered from somewhat higher in the Cody include 

Scaphites ventricosus Meek and Hayden and associated middle 

Coniacian fossils Baculites asper Morton, Volviceramus 

umbonatus (Meek and Hayden), and Pseudoxybeloceras dispar

Kennedy and Cobban (Klepper and others, 1957; Kennedy and 

Cobban, 1991).

Fossils recovered from the Telegraph Creek range from 

middle Coniacian through upper Santonian (fig. 3). Scaphites cf. 

S. ventricosus Meek and Hayden was recovered from the basal 

15 m (50 ft) of the Telegraph Creek (Slim Sam of Klepper and 

others, 1957) (Klepper and others, 1971). Scaphites sp. and Magadiceramus subquadratus crenelatus Seitz of the slightly 

younger Scaphites depressus zone were recovered during this 

study from shale near the base of the Telegraph Creek Formation 

southeast of Big Mountain (fig. 2). The pelecypod Cymbophora 

arenaria (Meek) (previously Mactra arenaria) was recovered 

from as high as 116 m (380 ft) above the base of the Telegraph 

Creek Formation (Klepper and others, 1957, p. 29), as well as 

about 60 m (200 ft) above the base at another locality (Klepper 

and others, 1971, p. 11–12). From near Big Mountain, Cymbophora arenaria (Meek) was recovered during this study from the 

uppermost exposed part of the Telegraph Creek Formation, about 

20 m (66 ft) below the base of exposed strata of the Eagle Sandstone. (Strata within the 20 m (66 ft) is covered; hence, the base 

of the Eagle could be, and probably is, less than 20 m (66 ft) 

above the horizon that yielded Cymbophora arenaria (Meek).) 

Clioscaphites cf. C. novimexicanus (Reeside), an ammonite 

probably from one of the upper Santonian Desmoscaphites zones 

(W.A. Cobban, written commun., 1994), was obtained from 

about 22 m (72 ft) below the base of exposed Eagle strata at the 

same locality.

The above fauna from the Telegraph Creek Formation 

indicates that the Eagle Sandstone is no older than late Santonian. The preserved Eagle strata likely represent only a slightly 

younger age, but the time span of the eroded Eagle strata is not 

well confined. A minimum age for preserved Eagle Sandstone, 

the Slim Sam Formation (restricted), and the time span represented by the unconformity between the two is limited by the 

age of the overlying Elkhorn Mountains Volcanics. Eruption of 

these rocks took place from about 81 to 76 Ma (Rutland and 

others, 1989). The most recent radiometric dates on the volcanics are two potassium-argon determinations of Tilling and others (1968) from rocks within the lower member of the 

sequence. The dates are 79.6 and 80.2 Ma, and average 79.9 

Ma (recalculated from 77.6 and 78.2, and 78.0+1.7 Ma, respectively, using decay constants of Steiger and Jager, 1977, and 

conversion tables of Dalrymple, 1979). With these radiometric 

dates, the time span between the base of the Eagle and rocks 

within the lower member of the Elkhorn Mountains Volcanics 

is about 4 m.y.

Tilling (1974, p. 1927) suggested that the Elkhorn Mountains Volcanics may have been reheated during intrusion of the 

Boulder batholith; thus, the dates may be reduced. However, 

unreheated volcanic rocks of the apparently genetically related 

rocks of the Maudlow Formation at the southern end of the Big 

Belt Mountains yielded a radiometric date of 80.9 Ma from the 

lower part of the formation (Skipp and McGrew, 1977) (recalculated from 78.9+1 Ma, using decay constants of Steiger and 

Jager, 1977, and conversion tables of Dalrymple, 1979). This 

date suggests that the dates on the Elkhorn Mountains Volcanics 

are reasonable.

Discussion and Conclusions

The nonmarine volcanic- and volcaniclastic-rich Slim Sam 

Formation (restricted) records early history of the Elkhorn 

Mountains Volcanics. The sandstones were formed of volcanic 

particles eroded from the initial extrusives and were redeposited 

by sedimentary processes. Even though the rocks are mainly of 

sedimentary origin, they are nonmarine strata genetically related 

to the Elkhorn Mountains Volcanics and not to marine strata of 

the Telegraph Creek Formation and marine to marginal marine 

delta-plain deposits of the Eagle Sandstone. This interpretation 

helps to explain the geographically variable thickness of the nonmarine Slim Sam (restricted); the variability of the sedimentary 

versus volcanic components of the unit; and internal discontinuities within the Slim Sam (restricted) in some areas, but apparent 

conformity in other areas. Further, the interpretation is consistent 

with the regional pattern of marine Eagle Sandstone being separate and distinct from directly overlying volcanic and volcaniclastic rocks.

The lower part of the sequence of strata that constituted the 

Slim Sam Formation as originally defined by Klepper and others 

(1957) is everywhere represented by marine strata of the Telegraph Creek Formation, deposited in the zone of transition 

between the offshore deposits of the Cody Shale and the strandline to lowermost delta-plain deposits of the basal part of the 

Eagle Sandstone. Regional data show the Eagle to consist of a 

basal and upper sandstone and an intermediate coaly to carbonaceous unit. Only the basal sandstone and lowermost strata of the 

middle part were observed in the Elkhorn Mountains, and they 

are preserved only locally. The absence of part or all of the 

Eagle, and deposition of the nonmarine rocks of the Slim Sam 

(restricted) directly on lower shoreface to offshore marine strata 

of the Telegraph Creek Formation throughout much of the area 

reveals that an unconformity lies at the base of the Slim Sam 

(restricted), even though no angular discordance is evident in 

some areas.

Klepper and others (1957, 1971) stated that the contact of 

the Slim Sam Formation (as originally defined) with the overlying Elkhorn Mountains Volcanics ranged from conformable to 

unconformable. They believed this contact was conformable 

along the eastern flank of the Elkhorn Mountains, northward 

from near Radersburg, Mont. (fig. 2). Southward from near Radersburg, and elsewhere in the Elkhorn Mountains, Klepper and 

others (1971, p. 21) stated that the Slim Sam and older rocks 

were folded and eroded before extrusion of the volcanics. Separation of the Slim Sam (as they defined the formation) into the 
B6 Revision of Middle Proterozoic Yellowjacket Formation and Cretaceous Slim Sam Formation

Baculites scotti

Baculites gregoryensis

Baculites perplexus (late form)

Baculites gilberti

Baculites perplexus (early form)

Baculites sp. (smooth)

Baculites asperiformis

Baculites maclearni

Baculites obtusus

Baculites sp. (weak flank ribs)

Baculites sp. (smooth)

Scaphites hippocrepis III

Scaphites hippocrepis II 

Scaphites hippocrepis I

Desmoscaphites bassleri

Desmoscaphites erdmanii

Clioscaphites choteauensis

Clioscaphites vermiformis

Inoceramus undulatoplicatus

 Clioscaphites saxitonianus

Scaphites depressus, Protexanites

 bourgeoisianus

Scaphites ventricosus

Forresteria alluaudi, Scaphites

 preventicosus

Forresteria peruana 

WESTERN INTERIOR ZONE FOSSILS

 (No specific Ar-Ar age has been 

 determined for fossils in braces)

MILLION YEARS (m.y.)

 STAGE

(based on zone fossils)

Ar-Ar AGE (m.y.)

80.54

78

80

82

84

86

88

81.71

83.91

84.88

86.92

88.34

ELKHORN MOUNTAINS

 This 

report

Klepper and

others (1957)

?

?

?

?

 Elkhorn

Mountains

 Volcanics

 (part)

 Elkhorn

Mountains

 Volcanics

 (part)

Eagle Sandstone

Telegraph

 Creek

Formation

 Slim

 Sam

Formation

Cody

Shale

Upper black

shale unit of

 Colorado

 Formation

Coniacian Santonian Campanian (part)

Slim Sam Fm

(restricted)

Figure 3. Correlation chart showing relationships in the Elkhorn Mountains. Boundaries of the Upper Cretaceous 

stages, Western Interior fossil zones, and 40Ar/39Ar radiometric ages are adapted from Obradovich (1993). Ages in 

the Ar-Ar column correspond to specific, indicated zone fossils in the column headed “Western Interior Zone Fossils.” 

Fossils within braces in the latter column are not tied to specific ages, and no age (or age range) is implied other than 

biostratigraphic position within the sequence and the age constraints afforded by zone fossils that have been dated. 

Individual zone fossils, or sequences of zone fossils, within braces could be slightly younger or older than indicated by 

position in the figure. In the columns headed “Elkhorn Mountains,” a query symbol indicates uncertainty in correspondence of the formation contact with a fossil zone or radiometric age. The column subheaded “Klepper and others 

(1957)” shows lithofacies correlation, but not necessarily age correlation, with the column subheaded “this report.”
Revision of Cretaceous Slim Sam Formation, Elkhorn Mountains Area, Montana B7

Telegraph Creek, Eagle, and Slim Sam (restricted) indicates that 

erosion, and probably folding, occurred before deposition of the 

Slim Sam (restricted). The reasoning is as follows.

The upper contact of the Slim Sam (restricted) is conformable with the overlying Elkhorn Mountains Volcanics along the 

east flank of the Elkhorn Mountains, as determined by Klepper 

and others (1957, 1971) and by my observations as well. The 

upper contact also is conformable in the northernmost part of the 

Elkhorn Mountains, in the Sheep Creek to Corral Creek area 

(fig. 2) (Smedes, 1966, p. 21). The upper contact is an angular 

unconformity along the southern flank of the Elkhorn Mountains, several kilometers west of Radersburg, where the Elkhorn 

Mountains Volcanics lie on the Slim Sam Formation (Klepper 

and others, 1957, pl. 1). Similarly, the upper contact is unconformable southward from near Radersburg, where the Elkhorn 

Mountains Volcanics lie directly on Slim Sam and older strata 

(Freeman and others, 1958, pl. 1). But the Slim Sam of the maps 

of Klepper and others (1957) and Freeman and others (1958) in 

both of these areas may be Telegraph Creek Formation, and the 

Slim Sam (restricted) could be absent. If volcanic rocks rest 

directly on Telegraph Creek strata in these areas, then deformation before or after deposition of the Slim Sam (restricted) cannot be determined from these areas.

The lower contact of the Slim Sam (restricted) is unconformable above older strata. In the Indian Creek area (fig. 2), it 

rests on lower shoreface deposits of the Telegraph Creek Formation. In the Crow Creek–Weston Creek area, and in the Big 

Mountain area, it rests on the Telegraph Creek and, locally, on 

lower strata of the Eagle Sandstone. These data show that the 

Slim Sam (restricted) does not represent continuous deposition from Telegraph Creek or Eagle strata and indicates that 

erosion and perhaps deformation took place prior to deposition of the Slim Sam (restricted). Further work in the Elkhorn 

Mountains may show that the strata remaining within the Slim 

Sam Formation may more properly be assigned to the Elkhorn 

Mountains Volcanics.

References Cited

Cobban, W.A., Erdmann, C.E., Lemke, R.W., and Maughn, E.K., 1976, 

Type sections and stratigraphy of the members of the Blackleaf 

and Marias River Formations (Cretaceous) of the Sweetgrass 

Arch, Montana: U.S. Geological Survey Professional Paper 974, 

66 p.

Dalrymple, G.B., 1979, Critical tables for conversion of K-Ar ages from 

old to new constants: Geology, v. 7, p. 558–560.

Freeman, V.L., Ruppel, E.T., and Klepper, M.R., 1958, Geology of part of 

the Townsend Valley, Broadwater and Jefferson Counties, 

Montana: U.S. Geological Survey Bulletin 1042-N, p. 481–556.

Houston, R.S., and Murphy, J.F., 1977, Depositional environment of 

Upper Cretaceous black sandstones of the Western Interior: U.S. 

Geological Survey Professional Paper 994-A, 29 p.

Kennedy, W.J., and Cobban, W.A., 1991, Coniacian ammonite faunas 

from the United States Western Interior: The Paleontological 

Association, Special Papers in Paleontology, no. 45, 96 p.

Klepper, M.R., Ruppel, E.T., Freeman, V.L., and Weeks, R.A., 1971, 

Geology and mineral deposits, east flank of the Elkhorn Mountains, Broadwater County, Montana: U.S. Geological Survey Professional Paper 665, 66 p.

Klepper, M.R., Weeks, R.A., and Ruppel, E.T., 1957, Geology of the 

southern Elkhorn Mountains, Jefferson and Broadwater Counties, Montana: U.S. Geological Survey Professional Paper 292, 82 

p.

Knopf, Adolph, 1963, Geology of the northern part of the Boulder bathylith and adjacent area, Montana: U.S. Geological Survey Miscellaneous Investigations Series Map I-381, scale 1:48,000.

McMannis, W.J., 1955, Geology of the Bridger Range, Montana: Geological Society of America Bulletin, v. 66, p. 1385–1430.

Obradovich, J.D., 1993, A Cretaceous time scale, in Caldwell, W.G.E., 

and Kauffman, E.G., eds., Evolution of the Western Interior Basin: 

Geological Association of Canada, Special Paper 39, p. 379–396.

Pemberton, S.G., MacEachern, J.A., and Frey, R.W., 1992, Trace fossil 

facies models; environmental and allostratigraphic significance, 

in Walker, R.G., and James, N.P, eds., Facies Models—Response 

to Sea Level Change: Geological Association of Canada, p. 47–72.

Reed, G.C., 1951, Mines and mineral deposits (except fuels), Broadwater County, Montana: U.S. Bureau of Mines Information Circular 

7592, 58 p.

Roberts, A.E., 1972, Cretaceous and Early Tertiary depositional and 

tectonic history of the Livingston area, southwestern Montana: 

U.S. Geological Survey Professional Paper 526-C, 120 p.

Rutland, Carolyn, Smedes, H.W., Tilling, R.I., and Greenwood, W.R., 

1989, Volcanism and plutonism at shallow crustal levels; the 

Elkhorn Mountains Volcanics and the Boulder batholith, southwestern Montana, in Hyndman, D.W., ed., Cordilleran Volcanism, 

Plutonism, and Magma Generation at Various Crustal Levels, 

Montana and Idaho: 28th International Geological Congress, Field 

Trip Guidebook T337, p. 16–31.

Schmidt, R.G., 1978, Rocks and mineral resources of the Wolf Creek 

area, Lewis and Clark and Cascade Counties, Montana: U.S. Geological Survey Bulletin 1441, 91 p.

Skipp, Betty, and McGrew, L.W., 1977, The Maudlow and Sedan Formations of the Upper Cretaceous Livingston Group on the west 

edge of the Crazy Mountains Basin, Montana: U.S. Geological 

Survey Bulletin 1422-B, 68 p.

Skipp, Betty, and Peterson, A.D., 1965, Geologic map of the Maudlow 

quadrangle, southwestern Montana: U.S. Geological Survey Miscellaneous Investigations Series Map I-452, scale 1:24,000.

Smedes, H.W., 1966, Geology and igneous petrology of the northern 

Elkhorn Mountains, Jefferson and Broadwater Counties, Montana: U.S. Geological Survey Professional Paper 510, 116 p.

Stebinger, Eugene, 1914, Titaniferous magnetite on the Blackfeet Indian Reservation, Montana: U.S. Geological Survey Bulletin 540-H, 

p. 329–337.

Steiger, R.H., and Jager, E., 1977, Subcommission on geochronology—Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359–

362.

Tilling, R.I., 1974, Composition and time relations of plutonic and associated volcanic rocks, Boulder batholith region, Montana: Geological Society of America Bulletin, v. 85, p. 1925–1930.

Tilling, R.I., Klepper, M.R., and Obradovich, J.D., 1968, K-Ar ages and 

time span of emplacement of the Boulder batholith, Montana: 

American Journal of Science, v. 266, p. 671–689.
B8 Revision of Middle Proterozoic Yellowjacket Formation and Cretaceous Slim Sam Formation

Weed, W.H., 1893, The Laramie and the overlying Livingston Formation 

in Montana: U.S. Geological Survey Bulletin 105, 68 p.

———1899, Description of the Fort Benton quadrangle, Montana: U.S. 

Geological Survey Geological Atlas, Folio 55, 9 p.

Published in the Central Region, Denver, Colorado

Manuscript approved for publication March 12, 1998

Edited by Richard W. Scott, Jr.

Graphics by R.G. Tysdal

Photocomposition by Gayle M. Dumonceaux