Mississippian and Pennsylvanian 

Stratigraphy in Southwest Montana 

and Adjacent Idaho 

U.S. GEOLOGICAL SURVEY BULLETIN 1656 

Mississippian and Pennsylvanian 

Stratigraphy in Southwest Montana 

and Adjacent Idaho 

Edited by Wi II iam J. Sando 

A. Revision of Mississippian Stratigraphy, Northern Tendoy Mountains, 

Southwest Montana, by William j. Sando, Charles A. Sandberg, and 

William j. Perry, Jr. 

B. New Mississippian-Pennsylvanian Stratigraphic Units in Southwest 

Montana and Adjacent Idaho, by Bruce R. Wardlaw and 

William C. Pecora 

U.S. GEOLOGICAL SURVEY BULLETIN 1656 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1985 

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304 

Library of Congress Cataloging in Publication Data 

Main entry under title: 

Mississippian and Pennsylvanian stratigraphy in 

southwest Montana and adjacent Idaho 

(Geological Survey bulletin ; 1656AB) 

Contents: Revision of Mississippian stratigraphy, 

northern Tendoy Mountains, southwest Montana I by William J. Sando, Charles A. Sandberg, and William j. Perry, Jr. 

- New Mississippian-Pennsylvanian stratigraphic units in 

southwest Montana and adjacent Idaho I by Bruce R. Wardlaw and William C. Pecora. 

Bibliography: p. A9, B8. 

Supt. of Docs. no.: 119.3:1656A,B 

1. Geology, Stratigraphic-Mississippian. 2. Geology, 

Stratigraphic-Pennsylvanian. 3. Geology-Montana-Tendoy Mountains Region. 4. Geology-Idaho. I. Sando, William jasper. Revision of Mississippian stratigraphy, northern Tendoy Mountains, 

southwest Montana. 1986.11. Wardlaw, Bruce R. New 

Mississippian-Pennsylvanian stratigraphic units in southwest Montana and adjacent Idaho. 1986. Ill. Series. 

QE75.B9 no. 1656AB 557.3 s 85-600090 

[QE672] [551.7'51 '097688] 
Chapter ARevision of Mississippian Stratigraphy, 

Northern Tendoy Mountains, 

Southwest Montana 

By William j. Sando, Charles A. Sandbe·rg, 

and William j. Perry 

U.S. GEOLOGICAL SURVEY BUllETIN 1656 

MISSISSIPPIAN AND PENNSYLVANIAf'J STRATIGRAPHY 

IN SOUTHWEST MONTANA AND ADJACENT IDAHO 

CONTENTS 

Abstract Al 

Introduction Al 

Subjacent rocks Al 

Three Forks Formation Al 

Tendoy Group Al 

Paine Limestone A2 

Middle Canyon Formation A5 

Mission Canyon Limestone A5 

McKenzie Canyon Limestone A 7 

Snowcrest Range Group A 7 

Kibbey Sandstone A 7 

Lombard Limestone A 7 

Conover Ranch Formation A9 

Quadrant Sandstone A9 

References cited A9 

FIGURES 

1. Location of northern Tendoy Mountains and Bell Canyon and Bell-McKenzie Divide measured sections of Mississippian rocks A2 

2. Chart comparing Mississippian stratigraphic units recognized in this report 

with stratigraphic units previously recognized in the northern Tendoy 

Mountains A2 

3. Composite section of Mississippian rocks in northern Tendoy Mountains 

(type section of Tendoy Group) based on measurements at Bell Canyon and 

Bell-McKenzie Divide A3 

4. Geologic map of the Bell Canyon area in sec. 17, T. 11 S., R. 10 W., 

Beaverhead County, Mont., showing section traverses A4 

5. Geologic map of the Bell-McKenzie Divide area in sec. 21, T. 11 S., R. 10 

W., Beaverhead County, Mont., showing section traverses A6 

6. Type section of McKenzie Canyon Limestone measured on north side of 

Bell Canyon in sec. 17, T. 11 S., R. 10 W., Beaverhead County, Mont. AS 

Contents iii 

Revision of Mississippian Stratigraphy, 

Northern Tendoy Mountains, Southwest Montana 

By William J. Sando, Charles A. Sandberg, and William j. Perry, Jr. 

Abstract 

A complexly faulted and folded sequence of Mississippian carbonate rocks more than 1,000 m thick is exposed in the northern Tendoy Mountains in southwest 

Montana. These rocks, previously assigned to the Madison Croup, Big Snowy Croup, and Amsden Formation, 

are herein referred to the Tendoy Croup (new name) and 

the Snowcrest Range Croup (new name, Wardlaw and 

Pecora, Chapter B, this bulletin). The Tendoy Croup 

comprises, in ascending order, the Paine limestone 

(raised in stratigraphic rank), Middle Canyon Formation, 

Mission Canyon limestone (previously assigned to Madison Croup in this area), and McKenzie Canyon limestone 

(new name). The Tendoy Croup represents a hitherto unrecognized facies belt near the craton margin. The overlying Mississippian Snowcrest Range Croup consists of, in 

ascending order, the Kibbey Sandstone (reassigned from 

the Big Snowy Croup) and two newly named units, the 

Lombard limestone and Conover Ranch Formation 

(Wardlaw and Pecora, Chapter B, this bulletin). 

INTRODUCTION 

A complexly faulted and folded sequence of Devonian to Permian rocks is exposed along the front of 

the northern Tendoy Mountains in Beaverhead 

County, Mont. (fig. 1). Rocks of Mississippian age in 

this area were previously assigned to the Madison 

Group, Big Snowy Group, and Amsden Formation 

by Scholten, Keenmon, and Kupsch (1955) and 

Scholten (1957) (fig. 2). Recent biostratigraphic 

studies and geologic mapping of the Mississippian 

sequences by the writers and by B. R. Wardlaw and 

W. C. Pecora revealed a need for new nomenclature 

for these rocks. This paper proposes new nomenclature for the sequence previously regarded as Madison 

Group and incorporates new nomenclature proposed 

by Wardlaw and Pecora (Chapter B, this bulletin) for 

the rocks previously assigned to the Big Snowy 

Group and Amsden Formation. 

Complex thrust-faulting and folding in the area 

make measurement of the Mississippian sequence 

difficult. However, overlapping stratigraphic sections 

were measured at Bell Canyon in sec. 17, T. 11 S., R. 

10 W., and on the divide between Bell Canyon and 

McKenzie Canyon in sec. 21, T. 11 S., R. 10 W. (fig. 

1 ). A composite of these two stratigraphic sections 

characterizes the Mississippian sequence in the 

northern Tendoy Mountains (fig. 3). 

We are indebted to B. L. Mamet for determination 

of foraminifer zones. 

SUBJACENT ROCKS 

Three Forks Formation 

The Mississippian sequence rests disconformably 

on shallow-water carbonate rocks and terrigenous 

rocks assigned to the Three Forks Formation, which 

is exposed at the base of the Bell-McKenzie Divide 

section (fig. 3). Although the base of the Three Forks 

is not exposed in this section because the formation 

rests in fault contact on the Mission Canyon Limestone, all three members (ascending order: Logan 

Gulch, Trident, and Sappington) present in the Three 

Forks type section can be identified. The Trident 

Member contains Cyrtospirifer monticola and 

Strophopleura raymondi. 

TENDOY GROUP 

The Tendoy Group, named for exposures in the 

Tendoy Mountains, is proposed for the sequence of 

Mississippian rocks between the Three Forks Formation of Late Devonian age and the Snowcrest Range 

Group, which is of Late Mississippian age in this area 

(see Wardlaw and Pecora, Chapter B, this bulletin). 

Revision of Mississippian Stratigraphy, Tendoy Mountains, Montana A1 
Idaho Falls Jackson 0 

Figure 1. location of northern Tendoy Mountains and 

Bell Canyon (1) and Bell-McKenzie Divide (2) measured 

sections of Mississippian rocks. 

The Tendoy Group includes, in ascending order, the 

Paine Limestone, Middle Canyon Formation, Mission Canyon Limestone, and McKenzie Canyon 

Limestone. This unique combination of stratigraphic units represents a hitherto unrecognized Mississippian facies belt near the craton margin. The type 

section of the group is the composite section (fig. 3) 

measured at Bell Canyon (fig. 4) and Bell-McKenzie 

Divide (fig. 5), where the group is 810 m thick. The 

known areal extent of the Tendoy Group is the northem Tendoy Mountains between McKnight Canyon 

and the Clark Canyon Reservoir (fig. 1). 

Paine Limestone 

The Paine Limestone was originally described by 

Weed (1899a,b) as a subdivision ("Paine shale") of 

the Madison Limestone in the Little Belt Mountains, 

Mont. Subsequently, the Paine became a member of 

the Lodgepole Limestone in the Madison Group 

throughout parts of Montana, Wyoming, Idaho, and 

Utah (see Sando and Dutro, 1974, p. 2, for historical 

discussion). 

In the northern Tendoy Mountains, the name 

Paine Limestone is applied to a unit of formational 

rank at the base of the Tendoy Group (fig. 3). The 

Paine Limestone has the essential lithologic characteristics and contacts of the Paine Member of the 

Lodgepole Limestone elsewhere in Montana. It consists of 228 m of thin-bedded, dark-gray, silty micrite 

and rare crinoidal micrite containing Zoophycos and 

rare hom corals. Shaly partings are a conspicuous 

SCHOLTEN. KEENMON, SANDO, SANDBERG, 

AND KUPSCH, 1955; AND PERRY, 

SCHOLTEN, 1957 THIS REPORT 

Amsden Formation Q) Conover Ranch Formation Ol 

c 

a:: "' a. Lombard Limestone .... ::J 

Big Snowy Group ~ e 

t(!) 

~ 0 Kibbey Sandstone c 

I- (/) 

-

McKenzie Canyon 

Limestone 

Mission Canyon 

Formation 

Mission Canyon 

Limestone 

a. a. ::J ::J e e 

(.9 (!) c > 0 0 Middle Canyon en "'C :0 c Formation 

"' Q) 

~ 1-

Lodgepole 

Formation 

Paine Limestone 

Figure 2. Mississippian stratigraphic units recognized in 

this report compared with stratigraphic units previously 

recognized in the northern Tendoy Mountains. 

A2 Mississippian and Pennsylvanian Stratigraphy, Montana and Idaho 
PENNSYLVAN IAN 

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DEVONIAN 

Quadrant Sandstone 

(part) 

Conover Ranch 

Q. 1---.:....Fo:::;r:...:.m:...:.a:.:t:.:.::io::..:n..:...._ __ l-- 1 OOO m 

.... . . . .. . . .. 

... . . . . 

6 I <6 lr 

Brown, crossbedded sandstone and quartzite. Late Chesterian 

conodonts in lower 7 m. 

*32m. Red siltstone and mudstone with minor silty micrite and sandstone. 

Late Chesterian conodonts. 

5 above base ,_-=-r-~ .... -~--t ~ ~~'~/ *118 m. Gray silty micrite and biomicrite with shaly partings, and 

minor encrinite and black shale. Chesterian brachiopods, corals, 

and conodonts in upper half. Late Meramecian corals in lower 

half. Oncolites common. 

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c: 

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Lombard 

Limestone 

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Kibbey 

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McKenzie Canyon 

Limestone 

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Mission 

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Limestone 

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Middle 

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Paine 

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A ......_ "'' /'-II I 

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*76 m. Yellow siltstone, sandstone, and mudstone with rare silty 

micrite in lower half. Meramecian conodonts in lower 10m. 

*140 m. Gray, partly cherty micrite, dismicrite, and pelmicrite, 

with minor crinoidal wackestone, encrinite, pelsparite, 

evaporite-solution breccia, and silty dolomitic limestone. Biooopelsparite with early Meramecian corals and foraminifers at 

top. Early Meramecian corals and foraminifers rare in lower 

part. Cloghergnathus conodonts throughout. 

*74 m. Brown, dolomitic, crinoidal wackestone and minor 

encrinite. Abundant early Meramecian corals, rare foraminifers, 

and C/oghergnathus conodonts. 

*102 m. Light gray, partly cherty encrinite and crinoidal 

wackestone. Osagean conodonts, brachiopods, and corals rare 

to common . 

128 m. Dark gray, silty, mostly recrystallized micrite and ribbony 

chert beds and nodules with minor debris flows of encrinite and 

crinoidal wackestone. Rare Osagean corals and brachiopods, 

and common Osagean conodonts. 

17m. Tabular dark gray chert and minor micnte . 

1 

... ~..._~ .... ~~ ..... ~~--;' 121 m. Dark gray, silty micrite and ribbony chert beds and 

nodules with minor debris flows of encrinite and crinoidal 

wackestone. Rare Osagean corals and common Osagean 

.... ..... ,., - conodonts. 

228 m. Dark gray silty micrite and rare crinoidal micrite with very 

rare chert nodules. Deformed and brecciated zone 61 m above 

base, probably penecontemporaneous with sedimentation. 

Ichnofossils common, corals very rare. Osagean conodonts in 

upper part and Kinderhookian conodonts and agglutinated 

~~~ foraminifers at base. 

0--~=-----_j f-:.-:---:---: Orange and red siltstone and mudstone, crinoidal wackestone, evaporitesolution breccia, and silty dolomite. Late Devonian brachiopods. 

Three Forks Formation 

(part) 

Figure 3. Composite section of Mississippian rocks in northern Tendoy Mountains (type section of Tendoy Group) based 

on measurements at Bell Canyon and Bell-McKenzie Divide. (See fig. I for locations of section traverses.) Thicknesses 

and descriptions of formations of Snowcrest Range Group from B. R. Wardlaw and W. C. Pecora (written commun., 1984). 

Asterisks indicate thicknesses measured at Bell Canyon; other thicknesses were measured at Bell-McKenzie Divide. 

Revision of Mississippian Stratigraphy, Tendoy Mountains, Montana A3 
0 

EXPLANATION 

~ Alluvium (Quaternary) 

EJ Terrace gravels and alluvial fans (Quaternary) 

Snowcrest Range Group (Upper Mississippian): 

~ Lombard Limestone 

l!!i[[i[!ii:M~~i:J:['[!j Kibbey Sandstone 

Tendoy Group (Upper and Lower Mississippian): 

~~~j McKenzie Canyon Limestone 

F~~-~~ ] Mission Canyon Limestone 

t~~J Middle Canyon Formation 

A4 Mississippian and Pennsylvanian Stratigraphy, Montana and Idaho 

0.5 MILE 

---······ Contact-Dotted where concealed 

~ Strike and dip of bedding 

__.__- Thrust fault-Sawteeth on upper plate 

---- High-angle transverse fault 

! Anticline 

l Syncline 

< < < < Stratigraphic section traverse 
feature. The Paine Limestone in this area contains 

less crinoidal debris, fewer megafossils, and less chert 

than in other areas of Montana and lacks the crinoidal wackestone that is commonly found at the base. 

In the Bell-McKenzie Divide section, the Paine contains breccia and folded beds 61 m above the base 

interpreted as resulting from penecontemporaneous 

soft sediment deformation (slumping). 

The Paine Limestone is regarded as Early Mississippian (Kinderhookian and Osagean) in the northern Tendoy Mountains. Kinderhookian conodonts 

are present at the base of the Paine, and most of the 

formation is considered to be Kinderhookian on the 

basis of correlation with the Paine Member of the 

Lodgepole Limestone, which has been dated as 

Kinderhookian only by conodonts and foraminifers 

elsewhere in the Cordilleran region (see Gutschick, 

Sandberg, and Sando, 1980), but Osagean conodonts 

were recovered from the upper 20 m of the Paine at 

Bell-McKenzie Divide. 

Middle Canyon Formation 

The Middle Canyon Formation was originally described by Huh (1967) from exposures in the southern Lemhi Range, Idaho, where it overlies beds now 

assigned to the McGowan Creek Formation 

(Sandberg, 1975) and underlies the Scott Peak Formation. The Middle Canyon is recognized as a deepwater foreslope facies in much of south-central Idaho north of the Snake River Plain, where it ranges in 

age from Early Mississippian (Osagean) to Late Mississippian (Meramecian) (Skipp, Sando, and Hall, 

1979), mainly on the basis of conodonts. 

The Middle Canyon Formation overlies the Paine 

Limestone in the northern Tendoy Mountains and 

consists of 266 m of predominantly dark-gray, thinbedded, silty, very cherty, mostly recrystallized 

micrite and minor debris flows of encrinite and crinoidal wackestone (fig. 3) similar to beds assigned to 

the formation in south-central Idaho. Three informal 

members are recognized in the northern Tendoy 

Mountains: a lower member 121 m thick distinguished by ribbony chert beds and nodules, a middle 

member 17 m thick distinguished by thin, tabular 

chert beds that make up more than half the unit, and 

an upper member 128 m thick characterized by ribbony chert beds and nodules like those of the lower 

member. Top and bottom contacts of the formation 

are conformable and gradational into the units above 

and below. 

<IIIII Figure 4. Geologic map of the Bell Canyon area in sec. 

17, T. 11 S., R. 10 W., Beaverhead County, Mont., showing section traverses. Base from Red Rock and Kidd 

7 1

h- minute quadrangles. 

The Middle Canyon Formation is entirely Osagean 

in age in the northern Tendoy Mountains. The lower 

member contains conodonts of the Lower Gnathodus 

typicus Zone and rare deep-water horn corals ( Cyathaxonia, Rotiphyllum) and tabulate corals (Michelinia). The upper part of the middle member and all 

of the upper member contain conodonts of the Upper 

G. typicus Zone and rare horn corals (Vesiculophyllum) and brachiopods. 

Mission Canyon Limestone 

The Mission Canyon Limestone was originally described by Collier and Cathcart ( 1922) as a formation 

in the Madison Group in the Little Rocky Mountains, Mont. Subsequently, the formation was recognized over an extensive area in Montana, western 

Wyoming, and southeast Idaho. The Mission Canyon in that area consists of a complex of shallowwater shelf limestone facies (lithofacies E of Sando, 

1980) in the lower part and evaporite facies (lithofacies G of Sando, 1980) in the upper part. Outside of 

the Tendoy Mountains, the formation is bounded 

above by a regional unconformity and is overlain by 

the Snowcrest Range Group as far east as the Horseshoe Hills (Wardlaw and Pecora, Chapter B, this bulletin, fig. 1) or Big Snowy Group in central Montana. 

In the area studied, the Mission Canyon conformably overlies the Middle Canyon Formation and consists of 17 6 m of medium- to thick-bedded, partly 

cherty encrinite and crinoidal wackestone (fig. 3). It 

is divided into a lower member consisting of thickbedded, light-gray, partly cherty encrinite and crinoidal wackestone and an upper member of mediumbedded brown dolomitic, crinoidal wackestone and 

minor encrinite. The formation is overlain conformably by the McKenzie Canyon Limestone. 

The lower member of the Mission Canyon probably is entirely Osagean in age. It contains conodonts 

of the eotaphrid biofacies of the Upper Gnathodus 

typicus and Scaliognathus anchoralis-Doliognathus 

latus Zones, brachiopods, and corals ( Vesiculophyllum, Sychnoelasma, Syringopora). The coral Ankhelasma occurs together with the conodont Eotaphrus 

burlingtonensis in the upper third of the lower member. The upper member contains the corals 

Diphyphyllum, Syringopora, and Vesiculophyllum 

and a Cloghergnathus conodont fauna that are compatible with an early Meramecian age. Mamet Zone 

12 foraminifers were recovered 0.2 m below the top 

of the upper member, the first record of such fossils 

in an uninterrupted carbonate sequence in the western interior region. 

Revision of Mississippian Stratigraphy, Tendoy Mountains, Montana AS 
0 

EXPLANATION 

~ Alluvium and terrace gravels (Quaternary) 

Tendoy Group (Upper and Lower Mississippian): 

~ McKenzie Canyon Limestone 

[~:w·~J Mission Canyon Limestone 

~ Middle Canyon Formation 

1:::::::::::!~:.:::::::1 

-' ' 

Paine Limestone 

Three Forks Formation (Devonian) 

A6 Mississippian and Pennsylvanian Stratigraphy, Montana and Idaho 

·.·.·.: 

0.5 MILE 

---····· Contact-Dotted where concealed 

Strike and dip of bedding 

~ lnchned 

-+- Vertical 

~ Overturned 

~- Thrust fault-Sawteeth on upper plate 

<<<< 

High-angle transverse fault 

Syncline 

Stratigraphic section traverse 
McKenzie Canyon Limestone 

The new name McKenzie Canyon Limestone is 

proposed for carbonate rocks at the top of the 

Tendoy Group between the underlying Mission Canyon Limestone and the overlying Kibbey Sandstone 

of the Snowcrest Range Group. The name is derived 

from McKenzie Canyon (fig. 1), where the beds are 

exposed. The type section (fig. 6) is in Bell Canyon 

(fig. 4). 

The McKenzie Canyon is composed of 140 m of 

mostly gray thin- to medium-bedded sparsely cherty 

micrite, dismicrite, and pelmicrite with minor crinoidal wackestone, encrinite, pelsparite, evaporite-solution breccia, and silty dolomitic limestone. This 

combination of lithic features is indicative of intertidal and supratidal deposition (Shinn, 1983). Both 

the top and the bottom of the formation are conformable with overlying and underlying units. 

Shelly benthos are generally rare in the McKenzie 

Canyon Limestone, but small hom corals (Hapsiphyllum) found 34 m above the base and a diversified 

and abundant coral fauna ( Clisiophyllum, 

Ekvasophyllum, Canadiphyllum, H apsiphyllum?, 

Multithecopora?, Stelechophyllum banffensis?) recovered from the upper 12 m indicate a Late Mississippian (early Meramecian) age for the formation. 

Mamet Zone 12 foraminifers occur 34 m above the 

base, and a Mamet Zone 12/13 boundary fauna was 

collected from the upper 10 m of the formation. A 

Cloghernathus conodont fauna occurs throughout the 

McKenzie Canyon. 

The McKenzie Canyon Limestone represents a 

hitherto unrecognized facies belt deposited near the 

shoreline of the karst plain formed on the craton 

during post-Mission Canyon, pre-Snowcrest Range 

(Big Snowy) time (see Sando, Gordon, and Dutro, 

1975, fig. 15; Sando, 1976, fig. 6, cycle III, phase 1; 

Gutschick, Sandberg, and Sando, 1980, fig. 1 0). It is 

the only known record of continuous carbonate deposition from cycle I to cycle III of Sando (1976), a 

veritable "missing link" in the history of Mississippian sedimentation in the northern Cordilleran region. 

Fossils recovered from the McKenzie Canyon 

(Mamet Zone 12) are younger than any of the highest 

fossils found in the Charles Formation (Mamet 

Zones 10 and 11) in the Williston Basin (Sando, 

1978; Sando and Mamet, 1981). 

~ Figure 5. Geologic map of the Bell-McKenzie Divide 

area in sec. 21, T. 11 S., R. 10 W., Beaverhead County, 

Mont., showing section traverses. Base from Kidd 7 112-

minute quadrangle. 

SNOWCREST RANGE GROUP 

Wardlaw and Pecora (Chapter B, this bulletin) are 

proposing the new name Snowcrest Range Group for 

beds previously assigned to the Big Snowy and Amsden Formations in southwest Montana. The Snowcrest Range Group consists of 226 m of shallowwater carbonate and terrigenous rocks between the 

Tendoy Group and the Quadrant Sandstone in the 

area studied. The group includes, in ascending order, 

the Kibbey Sandstone, Lombard Limestone, and 

Conover Ranch Formation. 

Kibbey Sandstone 

The Kibbey Sandstone was originally proposed by 

Weed (1899a,b) for exposures in the Little Belt 

Mountains, Mont., and was later included in the Big 

Snowy Group by Scott (1935). It is now included in 

the Snowcrest Range Group by Wardlaw and Pecora 

(Chapter B, this bulletin) in the area studied. 

The Kibbey consists of 76 m of yellow siltstone, 

sandstone, and mudstone conformably overlying the 

McKenzie Canyon Limestone and conformably underlying the Lombard Limestone at Bell Canyon (fig. 

3). It represents a shallow-water, probably lagoonal, 

embayment marking the initiation of cycle III eastward transgression of the Cordilleran sea onto the 

cratonic karst plain during post-Mission Canyon 

time (see Sando, Gordon, and Dutro, 1975, and 

Sando, 1976). 

Meramecian conodonts recovered by Wardlaw and 

Pecora from the Kibbey in the Bell Canyon section 

are the only marine fossils known from the formation, although plant fossils also occur in the lower 

part at Bell Canyon. The ages of the formations 

above and below indicate that the Kibbey is probably 

entirely of middle Meramecian age. This age for the 

Kibbey is slightly older than the late Meramecian age 

proposed for the formation in this area by Sando, 

Gordon, and Dutro (1975, p. A58) on the basis of 

regional stratigraphic synthesis. 

Lombard Limestone 

The name Lombard was originally proposed by 

Blake ( 1959) as a facies of the Big Snowy Group in 

southwest Montana. Wardlaw and Pecora (Chapter 

B, this bulletin) have applied the name to a formation between the Kibbey Sandstone and the overlying 

Conover Ranch Formation in the Snowcrest Range 

Group. The Lombard Limestone consists of 118 m of 

gray, silty, oncolitic micrite and biomicrite with shaly 

Revision of Mississippian Stratigraphy, Tendoy Mountains, Montana A7 
y=·~ 7-·---: a: 

w g m (/) LITHOLOGIC DESCRIPTION 

... ::-:-:::- ~ (/) 

Kibbey Sandstone .. :::-::-:-:- :::> w z z 

(part) -·-;- ~ -·- ...._ ... :-:-.·.-: u Siltstone, sandstone, and calcareous mudstone, dark yellowish orange, =.·.= z z k poorly exposed. --- :::> ...._ 

·-·-

- 140 m above base 0 0 1s 1 Evaporite-solution breccia, dolomitic, and peloidal and pisolitic limestone, yellowish brown. 

SAMPLES 1 & II 25 3 Micrite and dismicrite, recrystallized, and intraformational breccia, brownish gray, with large, I II .t. 

USGS 28603-PC rET-4,5~ ''""'I-• 1'. brownish black cannonball chert nodules. 

28607-PC BET-2 J .,.0

, _, ~ '• 24 7.5 Bio-oopelsparite, olive gray. Clisiophyllum, Ekvasophyllum, Canadiphyllum, _ 130 28973-PC BET-1 J , ,

1 Hapsiphyllum ?, Multithecopora?, Stelechophyllum banffensis?, advanced 

29312-PC BET-3 ' ""~ o ~ Cloghergnathus conodont fauna, Mamet Zone 12/13 foraminifers. :t ~ 

~ 

f-120 ~ 

~ 23 35 Covered. Float indicates micrite, dismicrite, and evaporite-solution breccia. 

f--110 

~ 

'75" 

f--100 c;-

j t 

~ L'22 1 Micrite, recrystallized, pale yellowish brown, with calcite veins. 

f--90 l 21 2 Covered. 

JJ ll 20 10 Micrite, dismicrite, pelmicrite, and pelsparite, medium yellowish brown and 

CD II II light gray. Ostracodes near base. 1: ) II ~ 0 f--80 ... II II • CD 

E II Micrite, dismicrite, pelmicrite, and pelsparite, olive gray and dark yellowish :.:; II Q II 19 12.5 

1: II brown, bed of black chert at base, possible solution-breccia near middle. 

~f--70 .a.ll4 ... J ... 1: 

CIJ Micrite, dismicrite, pelsparite, and porous evaporitic limestone, brownish u 18 5.5 CD u II gray and yellowish brown. Ostracodes near base. 

·;;; 

~I --.>-.:1 ... 3 Pelsparite, coarse, and intraformational breccia, brownish gray; cut by channel-fill of i ,___ 60 17 

~ USGS 29311-PC, BET-17 ;Y f'\. conglomeratic crinoidal wackestone. Horn corals. 

u 

:E 16 10.5 Micrite, dismicrite, and pelmicrite, yellowish gray, brownish gray, and very 

II light gray. 

-50 

15 1 Micrite and pel micrite, recrystallized, olive gray. 

I A II • 14 

14 5 Micrite, recrystallized, pale yellowish brown, chert bed at top. I ll 

\ I Micrite and pelmicrite, partly recrystallized, olive gray and medium -40 II 13 8.5 brownish gray, large chert nodules. 4 l II Vcrinoidal wackestone, yellowish brown. Hapsiphyllum, Cloghergnathus conodont fauna, Mamet 

USGS 28606-PC, BET-16,16A II ..... ... .... 12 1 Zone 12 foraminifers . 

2931Q-PC II 11 2.5 Micrite, recrystallized, yellowish brown. 

-30 ~~u '"P 10 3 Pel micrite, recrystallized, medium yellowish brown, with laths of anhydrite or gypsum. 

II .. 114 9 3 Micrite, yellowish brown, with chert beds at top. 

8 4 Micrite, recrystallized, yellowish brown, with large cannonball chert nodules. 

-20 BET-15,15R , I /' 11-- 7 3 Crinoidal wackestone and encrinite, dark yellowish brown. Cloghergnathus conodont fauna. 

BET-15A 4 II,~ 6 3 Micrite, olive gray, with patches of wackestone and cannonball chert. 

II 5 3 Micrite, recrystallized, laminated, weathering light gray. 

~10 11 4 ;1 4 4 2.5 Dolomitic limestone, dark brownish gray to brownish black, with irregular interbeds and nodules of 

f- ' light brown wood-grained chert. 

~ 3 4.5 Dolomitic limestone and calcitic dolomite, dark yellowish brown to brownish gray. Horn corals. 

~ BET-14C {\.rl ...... , I~ 2 2 ,crinoidal wackestone, olive gray. Amplexizaphrentis, C/oghergnathus conodont fauna. 

~ ./ . ....:. 1 3 r--'- 0 ,Dolomitic siltstone and silty dolomite, yellowish gray, platy . USGS .. ""! /1 '"'11 ->.J.J._/ 

Mission Canyon 29309-PC 1~11 8 Calcitic dolomite, brownish gray. Diphyphyllum, Mamet Zone 12 foraminifers. 

Limestone BET-148 II »!J....f ~ /1 

(upper member, BET-14A 11 11 v, Silty dolomite or dolomitic siltstone, yellowish brown, poorly exposed. 

part) USGS , ........ I .u. 4 Crinoidal wackestone and encrinite, brownish gray. Diphyphyllum and Syringopora. 29308-PC~ I .>.~.~o-1 

A8 Mississippian and Pennsylvanian Stratigraphy, Montana and Idaho 
partings and minor encrinite and black shale in the 

area studied (fig. 3). It represents shallow-water carbonate deposition in slightly deeper and clearer water 

than do the sediments of the Kibbey Sandstone. 

The lower half of the Lombard contains brachiopods and corals (Faberophyllum, Clisiophyllum, 

Canadiphy/lum?, "Pseudodorlodotia'") of late Meramecian age. The upper half of the formation contains brachiopods and corals (Siphonophyllia, Amplexizaphrentis) of Chesterian age. Chesterian 

conodonts were also recovered from the upper part of 

the formation by Wardlaw and Pecora. 

Conover Ranch Formation 

The new name Conover Ranch Formation is being 

proposed by Wardlaw and Pecora (Chapter B, this 

bulletin) for rocks previously assigned to the Amsden 

Formation in southwest Montana. The Conover 

Ranch conformably overlies the Lombard Limestone 

and conformably underlies the Quadrant Sandstone. 

The Conover Ranch Formation consists of 32 m of 

red siltstone and mudstone with minor silty micrite 

and sandstone in the area studied (fig. 3). It represents increased terrigenous deposition at the craton 

margin presaging the flood of quartz sand represented by the overlying Quadrant Sandstone. 

Wardlaw and Pecora (Chapter B, this bulletin) recovered late Chesterian (Late Mississippian) conodonts from the formation in the Tendoy Mountains. The Conover Ranch is as young as early 

Morrowan (Early Pennsylvanian) along the Jefferson 

River, Mont., outside of the Tendoy Mountains. 

QUADRANT SANDSTONE 

The Quadrant Sandstone was originally described 

by Peale (1893) from exposures in the Gallatin Range 

in the northwest comer of Yellowstone Park, Wyo. 

Subsequent usage has extended the name over most 

of Montana for sandstone of Pennsylvanian age between the Amsden Formation and the Phosphoria 

Formation. 

The Quadrant is a resistant unit of brown quartz 

sandstone and quartzite and minor carbonate beds 

whose thickness was not measured in the area studied 

(fig. 3). Wardlaw and Pecora (Chapter B, this bulle-

~ Figure 6. Type section of McKenzie Canyon Limestone 

measured on north side of Bell Canyon in sec. 17, T. 11 

S., R. 10 W., Beaverhead County, Mont. (see figs. 1 and 

3 for location of section traverse). 

tin) recovered conodonts of latest Chesterian age 

from the lower 7 m of the formation. 

REFERENCES CITED 

Blake, 0. D., 1959, Big Snowy stratigraphy in the area 

adjacent to the Rocky Mountain front [Montana], in 

Billings Geological Society Guidebook, 1Oth Annual 

Field Conference, 1959, p. 64-68. 

Collier, A. J., and Cathcart, S. H., 1922, Possibility of 

finding oil in laccolithic domes south of Little Rocky 

Mountains, Montana: U.S. Geological Survey Bulletin 

736-F, p. 171-178. 

Gutschick, R. C., Sandberg, C. A., and Sando, W. J., 1980, 

Mississippian shelf margin and carbonate platform 

from Montana to Nevada, in Fouch, T. D., and 

Magathan, E. R., eds., Paleozoic paleogeography of 

west-central United States: Rocky Mountain Section, 

Society of Economic Paleontologists and Mineralogists, West-central United States Paleogeography Symposium 1, Denver, Colorado, June, 1980, p. 111-128. 

Huh, 0. K., 1967, The Mississippian System across the 

Wasatch Line, east-central Idaho, extreme southwestem Montana: Montana Geological Society, 18th Annual Field Conference Guidebook, p. 31-62, 1 pl. 

Peale, A. C., 1893, The Paleozoic section in the vicinity of 

Three Forks, Mont.: U.S. Geological Survey Bulletin 

110, 56 p. 

Sandberg, C. A., 1975, McGowan Creek Formation, new 

name for Lower Mississippian flysch sequence in eastcentral Idaho: U.S. Geological Survey Bulletin 

1405-E, 11 p. 

Sando, W. J., 1976, Mississippian history of the northern 

Rocky Mountains region: U.S. Geological Survey 

Journal ofResearch, v. 4, no. 3, p. 317-338. 

__ 1978, Coral zones and problems of Mississippian 

stratigraphy in the Williston Basin: Montana Geological Society, Williston Basin Symposium, Billings, 

1978, p. 231-237, 1 pl. 

__ 1980 [1981], The paleoecology of Mississippian corals in the western conterminous United States: Acta 

Palaeontologia Polonica, v. 25, no. 3-4, p. 619-613. 

Sando, W. J., and Dutro, J. T. Jr., 1974, Type sections of 

the Madison Group (Mississippian) and its subdivisions in Montana: U.S. Geological Survey Professional Paper 842, 22 p., 1 pl. 

Sando, W. J., Gordon, McKenzie, Jr., and Dutro, J. T. Jr., 

197 5, Stratigraphy and geologic history of the Amsden 

Formation (Mississippian and Pennsylvanian) of Wyoming: U. S. Geological Survey Professional Paper 848-

A, 83 p., 11 pls. 

Sando, W. J., and Mamet, B. L., 1981, Distribution and 

stratigraphic significance of Foraminifera and algae in 

well cores from Madison Group, Williston basin, Montana: U.S. Geological Survey Bulletin 1529-F, 12 p. 

Scholten, Robert, 1957, Paleozoic evolution of the geosynclinal margin north of the Snake River Plain, IdahoMontana: Geological Society of America Bulletin, v. 

68, no. 2, p. 151-170, 1 pl. 

Revision of Mississippian Stratigraphy, Tendoy Mountains, Montana A9 

CONTENTS 

Abstract Bl 

Introduction Bl 

Newunits B2 

Snowcrest Range Group B2 

Lombard Limestone B2 

Conover Ranch Formation B4 

Railroad Canyon Formation B4 

Measured sections B4 

Type section of Lombard Limestone B4 

Type section of Conover Ranch Formation B6 

Type section of Railroad Canyon Formation B7 

Conclusions B8 

References cited B8 

FIGURES 

1. Nomenclature applied to Mississippian and Pennsylvanian rocks in central 

Montana, southwest Montana, and east-central Idaho B2 

2. Location map showing areas in southwest Montana and adjacent Idaho 

cited in report B3 

3. Columnar sections of the Snowcrest Range Group and Railroad Canyon 

Formation showing relationships to the Scott Peak Formation in Idaho and 

the Amsden Formation and Group in Montana BS 

Contents iii 

New Mississippian-Pennsylvanian Stratigraphic Units 

in Southwest Montana and Adjacent Idaho 

By Bruce R. Wardlaw andWilliam C. Pecora1 

Abstract 

The Kibbey Sandstone, lombard limestone (new 

unit), and Conover Ranch Formation (new unit) constitute 

the Snowcrest Range Group (new unit) in southwest Montana. This nomenclature better describes the rocks that 

are between the Mississippian Madison Group and related rocks and the Mississippian and Pennsylvanian Quadrant Sandstone. The Railroad Canyon Formation (new 

unit), found in southwesternmost Montana and adjacent 

Idaho, is a fine-grained carbonate and terrigenous unit 

largely equivalent to the Snowcrest Range Group. 

INTRODUCTION 

Presently available nomenclature applied to rocks 

equivalent to the Amsden Formation of Wyoming 

and to the dominantly clastic rocks of the Big Snowy 

Group and lower part of the Amsden Group in central Montana (Sando and others, 197 5) does not adequately apply to rocks dominated by carbonates in 

southwest Montana and adjacent Idaho. For this reason, the new name Snowcrest Range Group is proposed for these southwest Montana rocks. This short 

paper, in which new terminology is defined, is accompanied by a paper by Sando and others (Chapter 

A, this bulletin) defining new units that underlie the 

Snowcrest Range Group in the Tendoy Mountains. 

These two papers together attempt to clarify the Mississippian stratigraphy of southwest Montana and adjacent Idaho. Detailed studies of the Snowcrest 

Range Group by Wardlaw and Pecora are in progress; these focus on the Mississippian-Pennsylvanian 

boundary, the lack of unconformities in rocks that 

include the system boundary in the northern Rocky 

Mountains, and the stratigraphic and structural relations of the Snowcrest Range Group and Railroad 

Canyon Formation. 

I William C. Pecora is from Minerals Management Service, Reston, 

VA 22092. 

Mississippian limestones were first described in 

southwest Montana by Hayden (1872). Though the 

rocks described here do not adequately fit into any 

previous nomenclature, the units of Upper Mississippian-Pennsylvanian rocks described by Scott (1935) 

and modified by Maughan and Roberts (1967) have 

generally been applied to them (fig. 1). Scott (1935) 

divided the clastic rocks previously referred to the 

Quadrant Formation in central Montana into the Big 

Snowy Group (lower part) and Amsden Formation 

(middle part) and restricted the Quadrant to the upper part. The Big Snowy Group consisted of the Kibbey Sandstone and Otter Formation (previously 

members of the Quadrant of Weed, 1892, 1899), and 

the Heath Formation. Maughan and Roberts (1967) 

raised the Amsden Formation to group status in central Montana and divided the Amsden Group into, in 

ascending order: Tyler Formation (of Freeman, 

1922), Alaska Bench Limestone (of Mundt, 1956), 

and Devils Pocket Formation (of Gardner, 1959). 

Maughan and Roberts ( 1967) also attempted to apply 

this terminology to southwest Montana by recognizing the Big Snowy Formation and Amsden Formation throughout the area, including Railroad Canyon 

in adjacent Idaho. Though the rocks included in 

these two units may vary from author to author, 

these constitute the previously applied nomenclature. 

Skipp, Hogan, and others ( 1979) suppressed the application of Amsden Formation to the Railroad Canyon area of Idaho in preference to the newly named 

Bluebird Mountain Formation. The Big Snowy Formation was retained for a lack of a better name for 

thin-bedded argillaceous and limey rocks between the 

Scott Peak Formation and the Bluebird Mountain 

Formation. 

The Kibbey Sandstone is identified as the lower 

unit of both the clastic-dominated Big Snowy Group 

of central Montana and the carbonate-dominated 

Snowcrest Range Group of southwest Montana. It is 

New Mississippian-Pennsylvanian Stratigraphic Units, Montana and Idaho 81 
similar in both areas and represents initial transgressive sedimentation of subtidal to supratidal silt, sand, 

and evaporites of both groups. 

The age relations in this paper are based on conodont, foraminifer, coral, and brachiopod faunas. 

The conodont data are abundant but, as yet, unpublished. Davis ( 1983) presented some of the new conodont data and confirmed the age assignments of 

this study and the more widespread unpublished collections of Wardlaw. Two important points are derived from this information: ( 1) The Tyler Formation 

is Late Mississippian throughout most of its regional 

extent, and (2) the lowest beds of the Quadrant 

Sandstone range in age from latest Mississippian to 

Early Pennsylvanian and are generally younger 

northeastward in southwest Montana. Conodonts of 

latest Chesterian age were identified from the lower 7 

m of the formation. 

NEW UNITS 

Snowcrest Range Group 

The Snowcrest Range Group, named for the Snowcrest Range in Beaverhead and Madison Counties, 

Montana, is composed of the Kibbey Sandstone, 

Lombard Limestone, and Conover Ranch Formation 

(fig. 1). Exposures of the Kibbey, Lombard, and Conover Ranch in the Snowcrest Range are the type area 

for the group. The Snowcrest Range Group overlies 

the Mission Canyon Limestone of the Madison 

Group east of the Tendoy Mountains and, in the 

Tendoy Mountains, the McKenzie Canyon Limestone of the Tendoy Group. The Snowcrest Range 

Group is ordinarily overlain by the Quadrant Sandstone, but, at Arasta Creek, the group is overlain by a 

thin finger of the Ranchester Limestone Member of 

the Amsden Formation, which, in turn, is overlain by 

the Quadrant Sandstone. 

Lombard Limestone 

The Lombard Limestone consists of a geographically restricted lower unit of poorly fossiliferous, thin 

to thick indistinctly bedded lime-mudstone and 

packstone and an upper unit of fossiliferous, thin- to 

thick-bedded skeletal lime-mudstone, wackestone, 

and packstone with thin interbeds and partings of 

CENTRAL MONTANA SOUTHWEST MONTANA EAST-CENTRAL IDAHO 

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SCOTT ( 1935) MAUGHAN AND THIS PAPER SKIPP, HOGAN, AND 

ROBERTS (1967) THIS PAPER OTHERS (1979) 

QUADRANT QUADRANT 

FORMATION FORMATION 

SNAKY CANYON SNAKY CANYON a.. DEVILS POCKET :::> FORMATION FORMATION FORMATION 0 QUADRANT SANDSTONE a: 

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FORMATION z (.)C) FORMATION FORMATION (/') KI3BEY :=: 

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1-

1 The ages of Maughan and Roberts (1967) have been refined by abundant new data (Davis, 1983, Wardlaw; unpublished data) 

that indicate the Tyler Formation and units below are Mississippian. 

Figure 1. Nomenclature applied to Mississippian and Pennsylvanian rocks in central Montana, southwest Montana, and 

east-centralldaho. 

82 Mississippian and Pennsylvanian Stratigraphy, Montana and Idaho 
silty limestone, siltstone, and shale. The Lombard 

Limestone ranges in thickness from 49.7 m to as 

much as approximately 130 m. 

Geographic Distribution: The Lombard Limestone 

crops out throughout most of southwest Montana including the Tendoy Mountains, Pioneer Mountains, 

Blacktail Mountains, Snowcrest Range, Ruby Range, 

Tobacco Root Range, northwestern part of the Gravelly Range, Elkhorn Mountains, Horseshoe Hills, 

and Bridger Range (fig. 2). It is tentatively identified 

at Targhee Pass. The lower unit is well developed in 

MONTANA 

the Tendoy Mountains, Blacktail Mountains, Pioneer 

Mountains, Ruby Mountains, and Snowcrest Range. 

Age: The Lombard Limestone ranges in age from 

late Meramecian (Mamet Foraminifer Zone 15, 

Sando and Bamber Coral Zone IV) to late Chesterian 

(Mamet Foraminifer Zone 19). 

Regional Relations: The Lombard Limestone is 

equivalent to the Otter and Heath Formations of the 

Big Snowy Group and to part of the Tyler Formation 

of the Amsden Group in central Montana. It is also 

equivalent to the South Creek and Surrett Canyon 

Formations in east-central Idaho. 

HELENA 

LOMBARD STATION 

e BUTTE 

IDAHO 0 

I 

0 

50 Ml 

I 

50 KM 

Figure 2. location map showing areas in southwest Montana and adjacent Idaho cited in report. 

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WYOMING 

New Mississippian-Pennsylvanian Stratigraphic: Units, Montana and Idaho 83 
Conover Ranch Formation 

The Conover Ranch Formation is a heterogeneous 

unit consisting largely of pale-reddish-brown to palereddish-purple mudstone and lesser units of thinbedded marine limestone and calcareous sandstone 

and siltstone. Cherty carbonate conglomerate, 

phosphatic claystone, mottled or nodular calcareous 

mudstone, and recrystallized grainstone are locally 

present. The Conover Ranch Formation ranges in 

thickness from 0 to as much as 33m. 

Geographic Distribution: The Conover Ranch Formation crops out discontinuously in the Tendoy 

Mountains, Blacktail Mountains, Snowcrest Range, 

northwestern part of the Gravelly Range, ElkhornBoulder Mountains, Tobacco Root Range, and part 

of the Pioneer Mountains (fig. 2). 

Age: The Conover Ranch Formation is commonly 

late Chesterian (Mamet Foraminifer Zone 19, Sando 

and Bamber Coral Zone VI, and part of the 

Rhipidomella nevadensis Brachiopod Zone). The unit 

is younger to the northeast and is as young as early 

Morrowan (Early Pennsylvanian) in outcrops along 

the Jefferson River in southwest Montana; therefore, 

it includes the Mississippian-Pennsylvanian boundary in these outcrops. 

Regional Relations: The Conover Ranch Formation is equivalent to part of the Tyler Formation, to 

part of the Alaska Bench Limestone of the Amsden 

Group in central Montana, and to part of the Bluebird Mountain Formation in east-central Idaho. It is 

laterally equivalent to, and a facies of, the uppermost 

part of the Lombard Limestone and the lowermost 

part of the Quadrant Sandstone. 

Railroad Canyon Formation 

The Railroad Canyon Formation is predominantly 

thin- to medium-bedded dark shale, lime-mudstone, 

and clayey calcareous siltstone. Conglomeratic grainstone is locally present. Limestone containing thin 

chert stringers and nodules is dominant in the upper 

part. 

Geographic Distribution: The Railroad Canyon 

Formation is found only in the Beaverhead Range, 

where it is a part of the Medicine Lodge Thrust Plate 

(fig. 2). 

Age: The Railroad Canyon Formation is Chesterian (Late Mississippian). Brachiopod and conodont 

faunas, though sparse and generally abraded, indicate 

a Sando and Bamber Coral Zone V, or younger, age. 

Conodonts from the overlying Bluebird Mountain 

Formation indicate a late Chesterian (Mamet Foraminifer Zone 19) age. 

Regional Relations: The Railroad Canyon Formation is equivalent to the upper unit of the Lombard 

Limestone and the Conover Ranch Formation of the 

Snowcrest Range Group and to the Surrett Canyon 

and Arco Hills Formations of the foredeep carbonate 

bank sequence of Idaho (Skipp, Sando, and Hall, 

1979). 

MEASURED SECTIONS 

Type Section of the Lombard Limestone (fig. 3) 

Section located in S1h sec. 7, T. 4 N., R. 3 E., 

Toston 7.5-minute quadrangle, access by road 1 km 

north from Lombard Station, Broadwater County, 

Mont. The Lombard Limestone is underlain by the 

Kibbey Sandstone and overlain by the Tyler 

Formation. 

Thickness 

!!J:!1m. teet 

11. Shale. Fissile, papery, grayish-black - - 0.6 2.0 

10. Claystone. 15~ to 30~ mud-whisps and 

blebs with sparite grains (5 percent), 

and spiriferid brachiopod impressions. Light olive gray weathering 

dusky yellow - - - - - - - - - - - - - - - - - - - 1.4 4.6 

9. Wackestone, cliff-forming, medium 

bedded. Algal coated grains and filaments ( 40 percent), poorly washed 

biosparite, chert stringers in top 7 m. 

Silicified grains weather in relief. 

Dark olive gray. Siphonophyllia 

marker beds with corals in life position ( 40.6 em thick) begin 7 m below 

top. Thin interbeds of olive-black bituminous shale and calcareous siltstone. Siltstone has fine (0.3 mm 

thick) brachiopod spines and debris 

with platy horizontal orientation and 

minor, fine-grained dolomite rhombs 18.0 59.0 

8. Concealed, limestone as float ------- 18.5 61.0 

7. Lime-mudstone, thick-bedded. Knobbly texture, grayish-orange clay fills 

space between 3.5 em diameter 

micrite cobblelike grains produced 

by bioturbation. Injlatia sp. (brachiopod) molds. Olive gray weathering 

light olive gray - - - - - - - - - - - - - - - - - 1.2 4.0 

6. Shale, covered, dark-greenish-gray --- 0.2 0.5 

5. Coal and coaly claystone, underlain by 

clay, laterally continuous in several 

adits north along river --------- -- 3.1 10.0 

4. Concealed, underlain by argillaceous 

limestone - - - - - - - - - - - - - - - - - - - - - 2.4 8.0 

3. Wackestone. Sandy, knobby texture on 

weathered surface. Rugose corals and 

Diaphragmus? sp. Medium gray - - - 1.7 5.4 

84 Mississippian and Pennsylvanian Stratigraphy, Montana and Idaho 
PENNSYLVANIAN 

0 

MISSISSIPPIAN 

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UNCONFORMITY 

Figure 3. Columnar sections of the Snowcrest Range Group and Railroad Canyon Formation showing 

relationships to the Scott Peak Formation in Idaho and the Amsden Formation and Group in Montana. 

New Mississippian-Pennsylvanian Stratigraphic Units, Montana and Idaho BS 
Lombard Limestone-Continued 

2. Wackestone and packstone, thick-bedded. Abundant brachiopods in coquina zone. Medium gray. Conodont 

Thickness 

meters kJ!1 

sample USGS 0557-PC --------- 1.1 3.6 

1. Wackestone, medium-bedded. Ostracodes and abraded brachiopod 

debris in sparse biomicrite. Olive 

gray weathering yellowish gray. Conodont sample USGS 0558-PC--- 1.5 4.8 

Total thickness of Lombard 

Limestone------------- 49.7 162.9 

Type Section of Conover Ranch Formation 

(fig. 3) 

Section located 122 m uphill from northwest bank 

of Sheep Creek, Conover Ranch, Blacktail Mountains, NE1

/4 NW 1

/4, sec. 18, T. 9 S., R. 8 W., Gallagher 

Mountain 7.5-minute quadrangle, Beaverhead County, Mont. The Conover Ranch Formation is underlain by the Lombard Limestone and overlain by 

the Quadrant Sandstone. 

18. Lime-mudstone, wackestone, and calcareous siltstone, thin-bedded, nodular. Skeletal, sparse biomicrite with 

ostracodes, brachiopod spines, and 

indeterminate fossiliferous debris. 

Edges of some grains pyritized. Light 

gray. Conodont sample USGS 

Thickness 

meters kJ!1 

0524-PC --------------------- 1.2 4.0 

1 7. Shale, poorly exposed, medium-olivegreen to light-brown ---- - -------- 1.2 4.0 

16. Lime-mudstone, thin-bedded. Skeletal, 

brachiopod fragments, similar to 18. 

Light gray --------------------- 0.6 2.0 

15. Wackestone and calcareous shale, thinbedded. Laminations (25-50 mm 

thick) of alternating micrite and biomicrite with preferred horizontal 

orientation of shell debris. Minor 

sparfilled fractures. Light yellowish 

gray -------------------------- 1.5 5.0 

14. Shale, siltstone, and wackestone, thinbedded. Complete Rhipidome/la 

nevadensis specimens weather from 

unit. Wackestone contains Mamet 

Foraminifer Zone 19 foraminifers. 

Light yellowish brown to light gray-- 4.9 16.0 

13. Limestone, thin-bedded. Recrystallized 

biosparite containing brachiopod 

shells and bryozoan debris. Medium 

Conover Ranch Formation-Continued 

Thickness 

meters ff!et 

gray. Conodont sample USGS 

0525-PC --------------------- 0.6 2.0 

12. Claystone, slumped and poorly exposed. Calcareous, weathered, forming soil. Pale red purple. Thin wackestone interbed, light gray -- ------ 3.1 10.0 

11. Mudstone, thin-bedded. Sandy 

(quartz), poorly consolidated, containing silty lime-mudstone. Pale 

reddish brown. Conodont sample 

USGS 0526-PC---------------- 1.2 4.0 

10. Conglomerate and grainstone, medium-bedded. Conglomerate calcareous, abundant crinoid fragments 

with dark-brown and medium-gray 

chert grains. Medium gray ----- --- 0.4 1.2 

9. Siltstone, thin-bedded. Poorly consolidated, sandy calcareous, minor calcareous sandstone and limestone 

(sparite) interbeds. Pale reddish 

brown------------------------ 4.9 16.0 

8. Shale, with fine-grained sandstone at 

top, medium-olive-green --------- 1.8 6.0 

7. Sandstone, poorly exposed, thin-bedded. Calcareous, poorly to moderately well sorted, subangular to subrounded grains of quartz, chert and 

crinoids, with interbedded phosphatic mudstone. Moderate brown. Conodont sample USGS 0527-PC --- 1.2 4.0 

6. Shale, poorly exposed, moderatebrown --------------------------- 2.1 7.0 

5. Conglomerate, massive-bedded. Limestone pebble, containing clasts 

(0.1-2.5 em) of skeletal lime-mudstone with crinoid columnals and indeterminate hash, silty lime-mudstone, and minor chert. Light to 

medium gray------------------- 1.5 5.0 

4. Grainstone, thin-bedded. Well sorted, 

subround to subangu1ar carbonate 

grains with some quartz. Light yellowish brown - - - - - - - - - - - - - - - - - - 0. 6 2. 0 

3. Conglomerate, thick-bedded. Limestone pebble, containing subround to 

well-rounded clasts up to 35.5 em in 

width, consisting of medium-gray 

lime-mudstone clasts and minor 

moderate-yellow-brown chert clasts. 

Medium gray ------------------ 1.4 4.5 

2. Wackestone, medium-bedded. Brachiopods (Diaphragmus? sp.) weather in 

relief. Light gray---------------- 1.5 5.0 

1. Claystone, poorly exposed, calcareous, 

greenish-gray ----- ---- --------- 0. 9 3.0 

Total thickness of Conover 

Ranch Formation ------- 30.6 100.7 

86 Mississippian and Pennsylvanian Stratigraphy, Montana and Idaho 
r;.:pe Section of Railroad Canyon Formation Railroad Canyon Formation-Continued 

( •g. 3) Thickness 

meters 

Section located in NE% sec. 31 and SE1/4 sec. 30, R. 

feet 

27 E., T. 17 S., Bannock Pass 7.5-minute quadrangle low and light gray--------------- 0.6 2.0 

northwest of Railroad Canyon, Lemhi County, Ida- 22. Lime-mudstone and siltstone, thin-bedho. The Railroad Canyon Formation is underlain by ded, fissile, platy, and flaggy. Ballthe Scott Peak Formation and overlain by the Blue- and-pillow compaction fabric to bedbird Mountain Formation. ding. Disseminated clay whisps and 

dark fine grains, microspar cement. 

Thickness Lime-mudstone is medium dark 

meters fm gray weathering pale red to yellowish 

30. Concealed, underlain mainly by thin-to gray-------------------------- 12.8 42.0 

medium-bedded limestone and 21. Float, same as above - - - - - - - - - - - - - - 17.3 57.0 

sandy siltstone having low angle tab- 20. Siltstone, very fine sandstone, poorly 

ular cross-laminations - - - - - - - - - - - 16.6 54.7 exposed. Calcareous, microspar ce29. Wackestone, thin-bedded. Biomicrite ment with 25-35 percent angular to 

with broken and abraded brachio- subangular quartz grains with 30 perpods. Medium dark gray weathering cent blebs and clasts. Several generamedium gray. Conodont sample tions of anastomosing veinlets of 

USGS D504-PC---------------- 2.6 8.8 dark amorphous material and calcite. 

28. Covered - - - - - - - - - - - - - - - - - - - - - - - - 9.1 30.0 Dark gray with very light gray frac2 7. Wackestone, poorly exposed. A bun- tures of sparite on surface, weatherdant brachiopod fragments weather- ing light olive gray --------- ----- 4.0 13.0 

ing out. Medium dark gray and gray- 19. Lime-mudstone, thin-bedded, flaggy. 

ish brown weathering grayish red, Silty, angular to sub-angular, medimedium gray, and yellowish orange- 6.5 21.5 urn sorted (50-20J.1) quartz grains (20 

26. Wackestone, thin-bedded. Sparry bi- percent) in micrite matrix. Dissemiomicrite with Orbiculoidea sp. brach- nated dark grains. Medium dark gray 

iopods abundant along bedding weathering light olive gray - - - - - - - - 1.7 5.4 

planes. Medium gray weathering me- 18. Grainstone (top of bed), mudchip, bidium dark gray ------- ---------- 6.3 20.9 oclastic conglomerate (base), thick25. Packstone and wackestone, medium- bedded. Poorly sorted, very coarse 

bedded. Fetid on fresh break, abun- grained, subangular micritic grains, 

dant brachiopod debris and co- silty micritic, phosphatic, and brachnodonts in very thin beds, preferen- iopod (spiriferid) grains and clasts. 

tial orientation ofbioclasts. Dark Clay-silt hematite cement (very 

gray weathering medium gray. Co- dusky purple). Mudchip conglomernodont sample USGS D505-PC --- 11.9 39.3 ate (base) has oblate clasts (chips) up 

24. Packstone and siltstone, rhythmically to 3.2 x 0.4 em of dark gray silty, 

thin- to medium-bedded, dark-gray calcareous claystone. Platy fabric 

chert lenses (2 to 15 mm long) in with fractures normal to bedding. 

middle of unit. Silty dismicrite with Second grain mode (0.5-1.0 mm) of 

graded laminations (0.25 to 0. 75 em subround, more spherical micritic 

thick), quartz silt and fine sand well grains. Some micritic clasts have 

rounded. Disseminated kerogen (?) quartz silt grains, other micritic 

(fetid smell in hand sample, amor- clasts are pelloidal. Very dusky purphous blebs in thin section) and ple weathering dark greenish gray to 

abundant light-gray oval grayish olive. Conodont sample 

Orbicu/oidea brachiopods on bed- USGS D507-PC---------------- 3.2 10.2 

ding surfaces. Unit has increasing 17. Shale, fissile to papery, brownish gray 

abundance of 2-9 em thick beds of and grayish back - - - - - - - - - - - - - - - - 2.0 6.6 

siltstone near base. Limestone is me- 16. Wackestone, thin-bedded. Silty sparse 

dium dark gray weathering medium biomicrite with brachiopod debris, 

gray, siltstone is brownish gray and subangular quartz silt ( 15 perweathering yellowish gray. Conodont cent). Medium light gray weathering 

sample USGS D506-PC --------- 18.4 60.4 grayish orange - - - - - - - - - - - - - - - - - 2.0 6.6 

23. Packstone and conglomerate, thin-bed- 15. Lime-mudstone, massive, finely lamided, lenticular. Phosphatic gravel nated. Pelloidal ghosts, churned texgrains and micrite clasts. Dusky yel- ture, void space ( 5 percent) filled 

New Mississippian-Pennsylvanian Stratigraphic Units, Montana and Idaho 87 
Railroad Canyon Formation-Continued Railroad Canyon Formation-Continued 

Thickness Thickness 

meters feet meters feet 

with single calcite crystals (0.25-1.0 

mm). Brachiopod biomicritic packstone near base. Fragmental brachiopods, disseminated subangular 

quartz silt grains (15 percent), echinoid plates (10 percent). Light brownish gray to pale brown, weathering 

medium gray------------------- 9.2 

14. Claystone, flaggy, with papery shale interbeds. Slightly silty and calcareous, 

disseminated hematite grains (5 percent), and lenticular olive-gray limemudstone. Claystone is olive gray 

weathering olive gray, shale is 

brownish gray to grayish black----- 3. 7 

13. Conglomerate. Heterogeneous grain 

types: Lime-mudstone, phosphate, 

intraclastic claystone, and brachiopod fragments; very poorly sorted 

with platy clasts as much as 6 mm in 

length, average less than 3 mm in 

length. Generally well rounded 

grains, but with some oblate laths 

subparallel to bedding; sparite matrix. Medium gray to medium dark 

gray weathering pale yellowish 

brown ----------------------- - 3.3 

12. Calcareous siltstone, thin-bedded to 

flaggy. Quartz silt (20-40Jl) with 

hematitic clay matrix, calcite cement. Pale yellowish brown weathering pale red to grayish orange - - - - - 1. 6 

11. Calcareous siltstone. Friable, pale-red 

fossil fragments, Inflatia and Antiquatonia ---------------------- 0.6 

1 0. Lime-mudstone, massive bedded. Skeletal. Medium dark gray weathering 

pale yellowish brown ------------ 0.9 

9. Claystone and interbedded shale, laminated, platy and flaggy. Hairlike 

veins of spar (0.5 mm), clay whisps 

as long as 1.1 mm, filamentous-spicular sparite grains. Dark olive gray 

weathering light olive gray and pale 

red--------------------------- 3.0 

8. Calcareous shale and interbedded limemudstone, flaggy to papery. Claystone lamellae form wavy, gradational partings. Dark gray, olive gray, 

and medium gray, weathering light 

olive gray to yellowish gray - - - - - - - 21.9 

7. Lime-mudstone, laminated, thick-bedded. Skeletal, abundant brachiopod 

impressions, brachiopod spines ( 15 

percent) in micrite matrix, clay 

whisps. Moderate to dark yellowish 

brown weathering yellowish gray--- 1.0 

6. Shale and claystone, thin and papery at 

base and top, conchoidal fracture 

pattern in middle. In part, laminated 

and calcareous, 5 percent quartz 

grains, 20-11 OJ! in size. Shale is 

black to dark gray, claystone is dark 

gray to brownish black----------- 12.4 40.8 

30.0 5. Siltstone, poorly exposed. Calcareous, 

disseminated hematite, brecciated. 

Grayish red purple weathering light 

brown ----------------------- - 2.0 6. 7 

4. Shale, dark gray, like unit 6 --------- 7.5 24.6 

3. Claystone, poorly exposed. Wavy laminated, in part low-angle planar lami12.0 nated, 15 percent fine quartz silt 

grains, disseminated opaques 5 percent. Grayish red purple weathering 

pale red purple to grayish orange - - - 18.4 60.4 

2. Claystone, as float. Weakly calcareous, 

in part laminated. Grayish red purple weathering pale red purple----- 2.1 7.0 

1. Siltstone, float. Angular to subround 

quartz grains (40 percent) in calcareous, hematite clay matrix, well laminated. Yellowish brown weathering 

grayish orange ----------------- 3.1 10.0 

11.0 Total thickness of Railroad 

5.2 

Canyon Formation 205.7 676.3 

CONCLUSIONS 

The sedimentary succession and age relations (fig. 

3) represent a transgressive-regressive sedimentary 

2.1 package that forms the Snowcrest Range Group. The 

transgression was in the northeast direction and is 

represented by Kibbey Sandstone and Lombard 

3.2 Limestone deposition. The Conover Ranch Formation generally represents regressive sedimentation. 

The Snowcrest Range Group is largely Mississippian 

and equivalent to the Amsden Formation in western 

Wyoming, which shows diachronous transgressive 

deposition. The Snowcrest Range Group is also 

9.6 equivalent to the Big Snowy Group and only the 

Tyler Formation of the Amsden Group of central 

Montana. 

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88 Mississippian and Pennsylvanian Stratigraphy, Montana and Idaho 
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Hayden, F. V., 1872, Preliminary report of the United 

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New Mississippian-Pennsylvanian Stratigraphic Units, Montana and Idaho 89