The Maudlow and Sedan Formations

of the Upper Cretaceous Livingston Group

on the West Edge of the Crazy Mountains Basin,

Montana

GEOLOGICAL SURVEY BULLETIN 142 2-B

Cis

p-

The Maudlow and Sedan Formations

of the Upper Cretaceous Livingston Group

on the West Edge of the Crazy Mountains Basin,

Montana

By BETTY SKIPP and L. W. McGREW

CONTRIBUTIONS TO STRATIGRAPHY 

GEOLOGICAL SURVEY BULLETIN 142 2-B

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON . 1977
UNITED STATES DEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Skipp, Betty, 1928-

The Maudlow and Sedan Formations of the Upper Cretaceous Livingston Group on the west

edge of the Crazy Mountains Basin, Montana. (Contributions to stratigraphy)

(Geological Survey Bulletin 1422-B)

Bibliography: B66-B68.

Supt.ofDocs.no.: I 19.3:1422-B

1. Geology, Stratigraphic-Cretaceous. 2. Volcanic ash, tuff, etc.-Montana-Crazy Mountains.

3. Geology-Montana-Crazy Mountains. I. McGrew, Laura Wenger, 1921- joint author.

II. Title. III. Series. IV. Series: United States Geological Survey Bulletin 1422-B. 

QE75.B9 no. 1422-B[QE688] 557.3'08s[551.7'7] 76-608266

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington, D. C. 20402

Stock Number 024-001-02954-5
CONTENTS

Page

Abstract.................................................................................................................................... Bl

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

Livingston Group .................................................................................................................. 3

Sedan Formation............................................................................................................ 3

Lower sandstone member.................................................................................... 11

Welded tuff member .............................................................................................. 13

Parkman Sandstone equivalent........................................................................ 14

Middle sandstone member .................................................................................. 15

Mudstone member.................................................................................................. 17

Bearpaw Shale Member ...................................................................................... 18

Lennep Sandstone Member ................................................................................ 18

Age and correlation .............................................................................................. 20

Maudlow Formation...................................................................................................... 22

Member A ................................................................................................................ 26

Member B.................................................................................................................. 28

Member C................................................................................................................... 32

Member D ................................................................................................................ 35

Member E.................................................................................................................. 36

MemberF................................................................................................................... 37

Member G ................................................................................................................ 39

MemberH ................................................................................................................ 41

Age and correlation ............................................................................................... 42

Relations to Elkhorn Mountains Volcanics ........................................................... 42

Measuredsections.. .............................................................................................................. 44

References cited....................................................................................................................... 66

ILLUSTRATIONS

Page 

FIGURE 1. Geologic map showing distribution of formations in lower part of

Livingston Group, western Crazy Mountains basin, Montana .... B4

2. Geologic map showing location of type section of Sedan

Formation.................................................................................................... 6

3. Composite columnar sections showing lines of correlation of the 

Sedan and Maudlow Formations in the west-central Crazy 

Mountains basin........................................................................................ 8

4. Correlation chart of rocks of the Sedan and Maudlow Formations 

of the Livingston Group showing relationships with the type 

locality of the Livingston Group, a partial section of the 

Montana Group, and the Elkhorn Mountains Volcanics .............. 12

5. Photograph of the type section of the Sedan Formation .................... 16

6. Photograph of mudflow conglomerate unit in Sedan Formation .... 16

in
IV CONTENTS

Page

FIGURE 7. Geologic map from Klemme (1949) showing outcrop of his 

Livingston Formation and distribution of its Maudlow Conglomerate Lentil ........................................................................................ B23

8. Geologic map of the Maudlow area showing distribution of

members H through A of the Maudlow Formation .......................... 24

9. Photograph of hornblende dacite volcanic breccia of member B of

Maudlow Formation ................................................................................ 29

10. Diagram of facies and thickness relationships of member B of 

Maudlow Formation with lower sandstone member of the 

Sedan Formation ...................................................................................... 33

11. Photograph of coarse epiclastic polylithologic volcanic conglomerate at base of member C of Maudlow Formation............................ 34

12. Photomicrograph of glassy zone in welded tuff of member D of

Maudlow Formation ................................................................................ 36

13. Photograph of porphyritic dacite breccia of member F of Maudlow

Formation-.-....-...-......-. .................................................... 38

14. Diagram of facies and thickness relationships of member F of 

Maudlow Formation with middle sandstone member of Sedan 

Formation.................................................................................................... 40

TABLE

Page 

TABLE 1. Chemical and normative compositions of igneous rocks of the

Maudlow Formation and an associated sill............................................ B30
CONTRIBUTIONS TO STRATIGRAPHY

THE MAUDLOW AND SEDAN FORMATIONS

OF THE UPPER CRETACEOUS LIVINGSTON

GROUP ON THE WEST EDGE OF THE

CRAZY MOUNTAINS BASIN, MONTANA

By Betty Skipp and L. W. McGrew

ABSTRACT

The Sedan Formation and the Maudlow Formation are names adopted in this 

report for two formations in the lower part of the Livingston Group. The Sedan 

crops out for a distance of about 64 km (40 mi) from south to north along the western 

edge of the Crazy Mountains basin in Gallatin, Meagher, and Park Counties, Mont. 

The Maudlow is restricted to the Maudlow area in Gallatin County, just west of the 

Sedan outcrops.

The Sedan Formation (a new name) comprises 915-1,370 m (3,000-4,500 ft) (1,076 

m (3,528 ft) in the type section) of epiclastic volcanic sandstone, mudstone, and 

conglomerate, interbedded with volumetrically less important mudflow conglomerate, welded tuff, devitrified, silicified vitric tuff, bentonite, and lignitic coal, 

which lie above the Eagle Sandstone and below the Billman Creek Formation. The 

Sedan Formation correlates with the Cokedale and Miner Creek Formations in the 

type locality of the Livingston Group near Livingston, Mont., and is divided into 

four informally named members and two formally named members. These 

members were used in the mapping, during 1964-69, of five TVa-minute quadrangles 

and one 15-minute quadrangle. In ascending order, the members are as follows: 

lower sandstone member, welded tuff member, middle sandstone member, mudstone member, Bearpaw Shale Member, and Lennep Sandstone Member. The 

Bearpaw and Lennep remain of formation rank outside the report area. In addition, 

the Parkman Sandstone equivalent is recognized in the type section of the Sedan 

but was mapped with the welded tuff and middle sandstone members. A type 

section for the Sedan Formation and reference sections for the Bearpaw and 

Lennep Members are established along Cottonwood Creek in sees. 6,7,17, and 18, T. 

4 N., R. 8 E., Park County.

The age of the formation is Late Cretaceous, Campanian, and latest Santonian 

(approximately 83-70 m.y.).

The Maudlow Conglomerate Lentil of the Livingston Formation of McMannis 

(1955, after Klemme, 1949) is redefined and raised in stratigraphic rank to the 

Maudlow Formation. This name is proposed for the thick, 1,525-1,675+ m (5,000- 

5,500+ ft), sequence of volcaniclastic rocks of intermediate composition, including 

flows, breccias, welded tuffs, vitric, vitric-crystal and vitric-lithic tuffs, interbedded 

with epiclastic volcanic sandstone, conglomerate, mudstone, and minor lignitic 

coal, which crop out exclusively within an area of about 65 km2 (25 mi 2 ) near 

Maudlow, Mont., in northeastern Gallatin County. The formation has been

Bl
B2 CONTRIBUTIONS TO STRATIGRAPHY

subdivided into eight informal members, A through H, in ascending order. 

Lithologic correlatives of members B, D, F, and H have been recognized in places 

within the Sedan Formation and permit correlation of the volcaniclastic members 

of the Maudlow Formation with the largely epiclastic volcanic facies of the Sedan 

Formation. The Maudlow Conglomerate Lentil is approximately equivalent to 

members B and C of the Maudlow of this report.

The Maudlow Formation unconformably overlies both the Telegraph Creek 

Formation and the Virgelle Sandstone Member of the Eagle Sandstone; it is 

overlain unconformably by Tertiary rocks of the Dunbar Creek Formation and by 

Quaternary gravels. Pollen and spore assemblages, potassium-argon ages, and 

lithologic correlations suggest that the Maudlow Formation is also entirely Late 

Cretaceous in age Campanian, and probably latest Santonian in part.

The Elkhorn Mountains Volcanics and the Sedan and Maudlow Formations ure 

eroded parts of an extensive volcanic pile which may have covered as much as 

26,000 km2 (10,000 mi2 ) of southwestern Montana. In a general way, the Elkhorn 

Mountains Volcanics are the near vent facies of the pile, the Maudlow Formation is 

a remnant of the coarse reworked alluvial facies, and the Sedan and related 

formations are the fine-grained distal alluvial facies.

Potassium-argon ages on hornblende phenocrysts from rhyodacite flows and 

breccias of members B and F bracket the time of active rhyodacitic volcanism in the 

Maudlow area as being from about 75 to 79 m.y. ago, a time span which includes the 

estimated cooling time of the Butte Quartz Monzonite, a major part of the Boulder 

batholith, which is believed to be comagmatic with the volcanics.

Member D of the Maudlow Formation and the welded tuff member of the Sedan 

Formation are compositionally like welded tuffs in the lower unit of the Elkhorn 

Mountains volcanics and may indicate that those tuffs were originally very 

extensive.

INTRODUCTION

Weed (1893) named the Livingston Formation for a thick 

sequence of epiclastic volcanic sedimentary rocks in the vicinity of 

Livingston, Mont. From then until the last decade, the term 

appeared in the geologic literature attached to a wide variety of 

concepts of stratigraphy and age. Roberts (1963, p. B86-B92) 

summarized the most significant of these concepts in an important 

paper in which he limited the term "Livingston" to those rocks 

above the Eagle Sandstone and below the Fort Union Formation, 

raised the Livingston to group rank, and then subdivided the group 

into four formations, which are, in ascending order, Cokedale, 

Miner Creek, Billman Creek, and Hoppers. The type sections for 

these formations are all near Livingston, Mont., some 40 km (25 

mi) east of Bozeman. Roberts (1964a-h) used these formations in 

his mapping of eight quadrangles between Bozeman and 

Livingston.

Mapping along the east side of the Bridger Range north to 

Ringling, Mont. (fig. 1), which we completed during the period 

1964-69, has shown that the upper two of Roberts' formations, the 

Billman Creek and Hoppers, can be mapped throughout the area. 

The Sulphur Flats Sandstone Member at the base of the Miner 

Creek Formation, however, cannot be recognized a few kilometers 

north of Roberts' mapping, and it is not possible to differentiate the
MAUDLOW AND SEDAN FORMATIONS, MONTANA B3

Cokedale and Miner Creek Formations in the area east of the 

Bridger Range.

We propose here to use the name Sedan Formation for this 

general stratigraphic interval; its type section is along Cottonwood Creek in the northern Wallrock quadrangle and southern 

Sixteen NE quadrangle, Park County. The Sedan Formation is 

subdivided into four informal and two formal members (figs. 2, 3), 

which have been used singly and in combination as map units in 

parts of the area.

We also propose to raise the volcaniclastic rocks of intermediate composition in the Maudlow area (Skipp and Peterson, 

1965) to formation rank, the Maudlow Formation of the livingston 

Group. The Maudlow Formation thus defined includes the Maudlow Conglomerate Lentil of the Livingston Formation of 

McMannis (1955, p. 1410, after Klemme, 1949).

Several authors (Robinson and others, 1968; Gwinn and Mutch, 

1965) have assigned the Upper Cretaceous volcaniclastic rocks of 

the Maudlow area to the Elkhorn Mountains Volcanics 

emphasizing the presence of primary volcanic rocks. Inasmuch as 

the sequence is 50 percent or more epiclastic volcanic rocks and is 

separated geographically from the main body of the Livingston 

Group by only 8 km (5 mi), the Livingston Group assignment is 

preserved here, though the genetic relationship to the Elkhorn 

Mountains Volcanics is emphasized.

Stratigraphic correlations between the two new formations and 

the Cokedale and Miner Creek Formations in the type area of the 

Livingston Group are given. In addition, correlations are made 

with sections at Loweth and Bruno Siding that are assigned to the 

Montana Group on the northern edge of the Crazy Mountains

basm- LIVINGSTON GROUP

SEDAN FORMATION

The Sedan Formation is named for the hamlet of Sedan on the 

east flank of the Bridger Range and is the basal formation of the 

Livingston Group in this area. Sedan lies 37 km (23 mi) northnortheast of Bozeman, Mont., and about 21 km (13 mi) southsouthwest of the Cottonwood Creek area, which contains the type 

section of the Sedan Formation (figs. 1-3). The type section is 

located in sees. 6, 7,17, and 18, T. 4 N., R. 8 E., Park County, Mont., 

in the southeastern corner of the Sixteen NE quadrangle and in 

the northeastern corner of the Wallrock quadrangle. The new 

formation has been mapped for a distance of about 65 km (40 mi) 

along the western edge of the Crazy Mountains basin in the Sedan. 

(Skipp and McMannis, 1971), Hatfield Mountain (Skipp and Hepp, 

1968), Wallrock (Skipp, 1977), Sixteen, Sixteen NE, and Ringling 

quadrangles (McGrew, 1977a-c). On the southern edge of the
B4 CONTRIBUTIONS TO STRATIGRAPHY
46° 00'

KEY TO

NUMBERED 

QUADRANGLES

0MAUDLOW-15' 

SIXTEEN NE-7'/2'

(3) HATFIELD

MOUNTAIN-?^'

(4) WALLROCK-7'/2;

(5) WILSALL-7V2'

(6) SEDAN-1 5'

(7) FORT ELLIS-7V2

(DwOLFHILL-yVz

11100'

45'

10 M

o 10 KILOMETER

FIGURE 1. Simplified geologic map and index map of t

Crazy Mountains basin, Montana, showing distribution 

part of Livingston Group. Only Livingston rocks are sh

rocks are not differentiated.
B6 CONTRIBUTIONS TO STRATIGRAPHY

110°47'30" 110°45'00"

46°07'30"

R.8E.

0 1 MILE

0 0.5 1 KILOMETER

T.4JSI.

FIGURE 2. Simplified geologic map of northeast corner of Wallrock quadrangle 

and southeast corner of Sixteen NE quadrangle showing location of type section 

of Sedan Formation, distribution of members within the formation, and fossil 

localities. Geology by Betty Skipp and L. W. McGrew, 1965-66. Base from 

U. S. Geological Survey, 1951, 1:24,000.

Sedan quadrangle, the Sedan Formation becomes the combined 

Miner Creek and Cokedale Formations of Roberts (1964f, 1965, 

1972). To the northeast the formation grades into the marine and 

nonmarine formations of the Montana Group. 

The Montana Group has been used in central Montana by Gill,
MAUDLOW AND SEDAN FORMATIONS, MONTANA B7

Cobban and Schultz (1972, p. 92), by Gill and Cobban (1973), and 

by Gill and Vaughan (in McMannis and others, 1968). In the 

northern part of the Crazy Mountains basin (fig. 1), the Montana 

Group includes nonmarine rocks of the Judith River Formation. In 

the Ringling-Bruno area, Sims (1967) divided the nonmarine part 

of the sequence between the Bearpaw Shale and the Virgelle 

Sandstone Member of the Eagle Sandstone into three parts (col. 6,

EXPLANATION

Sedan Formation 

Ksle, Lennep Sandstone Member 

Ksm, mudstone member 

Ksb, Bearpaw Shale Member 

Ksmp, middle sandstone member 

Ksw, welded tuff member 

Ksl, lower sandstone member

Eagle Sandstone and upper part

of Telegraph Creek Formation

Telegraph Creek Formation

Cody Shale (part)

<J

Contact Dashed where gradational

_|_ 50 _^ 80

Inclined Vertical Overturned 

Strike and dip of beds

^ _U__ ~" D

Steep fault-Dashed where approximate. 

U and D and arrows indicate direction 

of relative movement

Thrust fault-Sawteeth on upthrown side

on 

D 

O 

W 

O

o

t

Trough line of synfonn 

50)

Line of measured section of Sedan 

Formation. Circled numbers 

correspond to unit numbers 

in described section

X D4670

Fossil locality

FIGURE 2. Continued.

fig. 4). He assigned the entire sequence to the Livingston Group. 

His basal formation of South Fork has been included in the Eagle 

Sandstone at Bruno Siding by Gill (in Gill and others, 1972), a 

correlation suggested earlier by Sims (1967, p. 27, fig. 8a), and has 

been correlated with the upper part of the Eagle Sandstone, the
B o v ° <t ®. v «° pfTop of measured section
Maudlow Formation 1624.5+ (5330+)

Member 

D

(675)

Member 

C 

241.7 

(793)

Member 

B 

159.7

(524)

Member 

A 

156.7

(514)

Telegraph Cr Fm 

(upper part)

=*=r*=*=?^4

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^»i.v-i y

< ' t * \ +L ^-^ i (

^ [ ____________

0 ,£ & A ^

® A ® A (2> 3

<' f * < '' l ^ I

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i/_u i ' "^J--1"

 a) _ C5 . _ _ K

-v-m _^_

CC> H 

b ho

IrD3324

30.5+ (100+

FIGURE 3. Composite columnar sections showing projec

measured in meters (feet) of the Upper Cretaceous Seda

ations of the Lavingston Group and associated rocks in 

Mountains basin. Cody Shale (part) through Billman C

part) measured near Cottonwood Creek in sec. 13, T. 4 N., 

and 18, T. 4 N., R. 8 E., Park and Gallatin Counties

Maudlow Formation measured in T. 4 N., R. 4 E., and R
BIO CONTRIBUTIONS TO STRATIGRAPHY

Claggett Shale, and the Parkman Sandstone in another interpretation of the same area (Gill and Cobban, 1973). Sims' formation of Hamen and formation of Bruno are local map units in the 

Hamen and Wolf Hill quadrangles (Sims, 1967), and a lithologic 

correlation with the members of the Sedan Formation is suggested 

(fig. 4).

The Sedan Formation is 915-1,372 m (3,000-4,500 ft) thick, 

1,076 m (3,528 ft) in the type section, and consists primarily of 

nonmarine epiclastic volcanic sandstone, mudstone, and conglomerate interbedded with volumetrically less important mudflow conglomerate, welded tuff, devitrified, silicified, vitric tuff, 

bentonite, and lignitic coal. McMannis (1955, p. 1407-1408) first 

recognized these beds as a unit and described in some detail "a 

lower 4,500-foot unit of tuffaceous or andesitic sandstones" of the 

Livingston Formation east of the Bridger Range. The volcanic 

detritus that composes the epiclastic facies is intermediate in 

composition and represents the mechanical breakdown of rhyodacitic, dacitic, and andesitic volcanic rocks of the same general 

composition as the primary intermediate volcanic rocks of the 

Maudlow Formation to the west and of the Elkhorn Mountains 

Volcanics farther west (Roberts, 1963, p. B87; 1972, p. C39-C47; 

Skipp and Peterson, 1965) (fig. 1).

Throughout the area of outcrop, the Sedan Formation is underlain by the upper unnamed member of the Eagle Sandstone, which 

consists of thin-bedded, slope-forming, yellowish-gray, finegrained, calcareous biotitic, quartzose, nonmarine to brackishwater sandstone. The contact between the two formations is sharp 

and disconformable along most of the western edge of outcrop but 

is gradational in the southern part of the Sedan quadrangle.

The Sedan is overlain conformably by the Billman Creek 

Formation of the Livingston Group, which is characterized by 

grayish-red and olive-gray volcanic-rich mudstone interbedded 

with lesser amounts of volcanic sandstone and conglomerate 

(Skipp and McGrew, 1972).

Within the mapped area (fig. 1), the Sedan Formation has been 

subdivided into four informally named members and two formally 

named members which are, in ascending order, lower sandstone, 

welded tuff, middle sandstone, mudstone, Bearpaw Shale and 

Lennep Sandstone (figs. 2, 3). The Bearpaw and Lennep are 

reduced from formation to member rank in the report area; they 

remain of formation rank east of the report area. In addition, the 

Parkman Sandstone equivalent is recognized within the type 

section of the Sedan but has not been mapped separately.

The Sedan Formation is essentially the same unit as "The Leaf 

Beds" of Weed (1893, p. 22), who said of this lower part of the 

Livingston sequence, "it consists of a series of sandstones, 

conglomerates and shales composed largely of angular or but 

slightly water-worn debris of volcanic eruptions and ash showers."
MAUDLOW AND SEDAN FORMATIONS, MONTANA Bll

Recent work supports these early observations and suggests that 

the Sedan Formation is analogous in mode of origin to the fine 

alluvial volcanic facies described by Parsons (1965, 1969) in the 

Absaroka volcanic field, which consists of well-bedded reworked 

volcanic sedimentary rocks, mainly volcanic conglomerate, sandstone, siltstone, and air-fall tuff, having primary dips of less than 

5° and of the same age as the vent facies rocks from which they 

were derived (Smedes and Prostka, 1972, p. C4-C7).

LOWER SANDSTONE MEMBER

The lower sandstone member ranges in thickness from 152 to 305 

m (500 to 1,000 ft) throughout the area. At Cottonwood Creek it is 

156 m (510 ft) thick and forms low ridges and moderately steep 

slopes above the Eagle Sandstone. Epiclastic volcanic sandstone, 

siltstone, mudstone, porcelanite, and silicified devitrified crystal 

lithic tuff interbedded with minor granule and pebble conglomerate and lignitic coal make up the member. The sandstone is 

dark olive gray, greenish gray, and yellowish gray; it weathers 

brown and is fine grained to conglomeratic, medium bedded, 

locally crossbedded, and calcareous. Calcareous concretions are 

present throughout. Ironstone nodules, lenses, and beds of 

magnetite-rich sandstone are common in the lower part of the 

measured section (fig. 3). Volcanic detritus ranges in composition 

from andesite to rhyodacite and, east of Flathead Pass, consists in 

part of dacite of member B of the intermediate volcanic and 

volcaniclastic rocks of the Maudlow quadrangle (Skipp and 

Peterson, 1965). Carbonaceous films on bedding planes and abundant leaf, needle, twig, and cone imprints are characteristic of this 

member. A lignitic coal bed 0.6 m (2 ft) thick is present at the base of 

the type section and at other localities. The contact with the lightcolored biotitic sandstone of the underlying Eagle Sandstone is 

abrupt and locally disconformable.

Palynomorphs identified by R. H. Tschudy (written commun., 

1971) from the lignitic coal at the base of the type section (figs. 1,2, 

3 loc. D4670) are the following:

Proteacidites 2 sp.

Araucariacites

Gleicheniidites

Plicapollis

Tricolpites reticulatus

Eucommiidites

Complexiopollis (Latipollis type)

Anemia 1

Appendicisporites

Polypodiumsporites

C3-rt (cf. C3-rtl4)

C3-sm (deep colpi)

C3-r (colpi enlarged)

P3-sm
B12 CONTRIBUTIONS TO STRATIGRAPHY

UPPER 

CRETACEOUS 

STAGES

Maestrichtian

Campanian

c

CO

1 Santon

0>

1

Q.

D

0> 

O

o>

Q. 

Q.

CD 

O

WESTERN INTERIOR 

AMMONITE 

ZONES

Discoscaphites nebrascensis

Discoscaphites roanemsis

Sphenodiscus (Coahuilites)

Baculites clinolobatus

Baculites grandis

Baculites baculus

Baculites eliasi

Baculites jenseni

Baculites reesidei

Baculites cuneatus

Baculites compressus

Didymoceras cheyennense

Exiteloceras jenneyi

Didymoceras stevensoni

Didymoceras nebrascense

Baculites scotti

Baculites gregoryensis

Baculites perplexus (late form)

Baculites gilberti

Baculites perplexus (early form)

Baculites sp (smooth)

Baculites asperiformis

Baculites mclearni

Baculites obtusus

Baculites sp (weakly ribbed)

Baculites sp (smooth)

Scaphites hippocrepis III

Scaphites hippocrepis II

Scaphites hippocrepis I

Desmoscaphites bassleri

Desmoscaphites erdmannl

Clioscaphites choteauensis

Clioscaphites vermiformis

Clioscaphites saxitonianus

Scaphites depressus

oc

Z 0

S z 1

< -1

X 5

67.4

68.5 

68.5

68.7 

69.9 

71.5

72

72.2

i

c

8 

o

c

Cl

c

c 

5

c

77.9 £ 

77.5 ilJ 

78.2

82.5

1 

ELKHORN MA 

i/IOUNTAINS / 

(Th

Upper '--rQ.

2 IV

0 

O)

c

Middle 'S

i <" member g

? 0

u.

1 M

Lower "§

member ^

Slim Sam J/Tele

Coc

2 3

UDLOW SEDANi\REA RINGLING 

is paper) AREA 

(This paper)

Billman 

Creek 

Formation 

(part)

Lennep Ss Mbr

Member \

Mudstone 

member

Member 

G

ember F (3) 74.9±1/^

Sulphur! 

FlatsSsk-jrequiv \ 

Member Middle 

E sandstone 

member

ember D (2) Weldedtuff mbr

Member C Lower

Wbr B (1) 78.9±1 \ ss

Member A mbr V 

D3324 D4670YTj pp

0

CO

0 

LL

c

CO ?CO

er .

I I I I V mbr

 " Virgelle Virgelle Ss ' 

Sandstone Member

Q. 

0

O

c 

o

O)

_c

CD

C

5) "> 0 c

graph Creek Telegraph Creek 

ormation F-| Formation

Jy Shale Cody Shale 

part) (part)

FIGURE 4. Correlation chart showing relationships of rocks of the Sedan and 

Maudlow Formations of the Livingston Group along the western edge of the 

Crazy Mountains basin with the type locality of the Livingston Group, a partial 

section of the Montana Group at the north end of the Crazy Mountains, and the 

Elkhorn Mountains Volcanics in the Elkhorn Mountains. Boundaries of stages 

of the Late Cretaceous, western interior ammonite zones, and potassium-argon 

ages are adapted from Obradovich and Cobban (1975) and Gill, Cobban, and 

Schultz (1972). Column 1 is adapted from Smedes (1966), Robinson, Klepper, and 

Obradovich (1968), Roberts (1972) and Gill and Cobban (1973). Column 4 is 

adapted from Roberts (1965, 1972). Column 5 is adapted from Gill and Vaughan 

(in McMannis and others, 1968)7Gill, Cobban, and Schultz (1972), and Gill and 

Cobban (1973). Column 6 is adapted from Sims (1967). (1), (2), and (3) are 

potassium-argon ages discussed in text; D3324 and D4670 are pollen and spore 

collections; FI is a faunal collection.
MAUDLOW AND SEDAN FORMATIONS, MONTANA B13

Tschudy stated that the assemblage is "from an interval above 

the Scaphites despressus zone and from below the Scaphites 

hippocrepis zone *** [and] probably represents the upper part of 

the Niobrara Formation."

WELDED TUFF MEMBER

The welded tuff member forms prominent ridges through the 

Sedan, Hatfield Mountain, and Wallrock quadrangles. It pinches 

out to the north in the Sixteen NE quadrangle and has not been 

traced south of the middle of the Fort Ellis quadrangle (fig. 1). The

4

LIVINGSTON

AREA

a

D

0

O

vingsto

Billman

Creek

Formation

(part)

Miner

Creek 

Formation

\ Sulphur 

) Flats

/ Ss Mbr

Cokedale

Formation

1 Sandstone

jWelded tuff

Eagle 

Sandstone

Telegraph Creek 

Formation

Cody Shale

(part)

5

LOWETH

AND

BRUNO SIDING

Q.

0

O

to

to

c

0

^

Hell

Creek

Formation

(part)

Lennep Ss

Bearpaw 

Shale

Volcanic /

eg /

 

Judith

Formation

Parkman Ss

Claggett Sh

o Upper 

.g unnamed

c member

t? Virgelle 

w Ss Mbr

Telegraph Creek 

Formation

Cody Shale

(part)

6

RINGLINGBRUNOSIDING

AREA

Hell

Creek

Formation

(part)

Lennep Ss

Bearpaw 

Shale

~<j

to

Q.

Q.

O

O

c

O

K

c

3

Formation 

of

Bruno 

Sims (1967)

Formation

of 

Hamen

Sims (1967)

X

of 

South Fork

Sims (1967)

Eagle 

Sandstone

Telegraph Creek 

Formation

Cody Shale

(part)

FIGURE 4. Continued.
B14 CONTRIBUTIONS TO STRATIGRAPHY

unit ranges in thickness from 0 to about 210 m (0 to about 700ft). It 

is thickest in the general area of Flathead Pass on the east side of 

the Bridger Range and in the Maudlow area to the west (fig. 1; 

Skipp and Peterson, 1965). In the measured section at Cotton wood 

Creek, it is 48 m (157 ft) thick.

The welded tuff member consists of pale-yellowish-green, lightgreenish-gray, grayish-red, and pale-reddish-brown welded tuff in 

three sheets (each sheet 0-92 m (0-300 ft) thick) and welded tuff 

conglomerate, separated by olive-gray to light-gray volcanic and 

quartzose sandstone, dark-colored conglomerate and mudstone, 

and dark-greenish-gray to gray porcelanite and altered vitric tuff. 

The welded tuff is fine grained and contains locally abundant (7 

percent) phenocrysts of golden-weathering biotite, crystals of 

labradorite, andesine, augite, and hypersthene, and small clasts of 

altered volcanic rock. Shards and pumice fragments are less than 1 

mm long. The glass is mostly devitrified, but thin (1-1.5 m (3-5 ft) 

thick) zones of perlitic glass are present in places near the base of 

flows along the northwestern edge of the Sedan quadrangle. 

Conglomerate composed of welded tuff pebbles and cobbles marks 

both the base and top of the unit in some places. Wood is present 

throughout. The welded tuff member lies on the eroded surface of 

lower sandstone member.

Skipp (1969) suggested that interbedded tuffs and sandstones of 

the Sulphur Flats Sandstone Member (at its type section) of the 

Miner Creek Formation (Roberts, 1963) might be a distal facies of 

the welded tuff member. However, mapping in the northern part of 

the Fort Ellis quadrangle (fig. 1) shows that the Sulphur Flats lies 

about 153 m (500 ft) above the welded tuff member and more likely 

is related to tuffaceous sandstone beds in the middle part of the 

middle sandstone member (fig. 3) (units 29 and 30 of measured 

section).

The welded tuff member and the lower sandstone member below 

it probably correlate (fig. 4) with volcanic sandstone, mudstone, 

and tuff, which have been variously assigned to the upper 

unnamed member of the Eagle Sandstone and the Claggett Shale 

(Gill and Vaughn, in McMannis and others, 1968, fig. 6; Gill and 

Cobban, 1973; this paper), to just the upper part of the Eagle 

Sandstone (Gill and others, 1972, p. 92), and to the formation of 

South Fork of Sims (1967). Palynomorphs collected from near 

Bruno Siding were identified by K. R. Newman (cited in Sims, 1967, 

p. 26-27) as "Upper Cretaceous-Campanian (probably early 

Campanian)."

PARKMAN SANDSTONE EQUIVALENT

In the western Crazy Mountains basin, the Parkman Sandstone
MAUDLOW AND SEDAN FORMATIONS, MONTANA B15

equivalent is present in the upper part of the welded tuff member 

and the lower part of the middle sandstone member and includes 

the contact between these two members. The Parkman has not 

been mapped separately in this area. In the measured section at 

Cotton wood Creek (fig. 3), the Parkman is 59 m (193 ft) thick and 

consists of light-gray and olive-gray, fine-grained, thin-bedded, 

crossbedded, mottled in places, biotitic, noncalcareous and calcareous, quartzose sandstone, interbedded with greenish-gray 

siliceous mudstone, minor orange-weathering volcanic sandstone 

and conglomeratic volcanic sandstone, and one 6-m (20-ft)-thick 

bed of biotite-rich welded tuff. The uppermost light-gray sandstone 

is characterized by contorted bedding in the measured section, 

probably a result of contemporaneous slump deformation. Leaf 

and twig imprints are common in some of the mudstone beds.

This unit is thought to be the rock-stratigraphic equivalent of the 

thin marine sandstone placed at the top of the Eagle Sandstone at 

Bruno Siding (figs. 1, 4) by Gill (Gill and others, 1972, p. 92), who 

earlier had called it Parkman Sandstone (Gill and Vaughn, in 

McMannis and others, 1968; and then again in Gill and Cobban, 

1973, p. 16).

Roberts (1972, p. C43) suggested that a thin calcareous quartzose 

sandstone 156 m (510 ft) above the base of the Cokedale Formation 

at Cokedale might be a western tongue of the Parkman Sandstone.

MIDDLE SANDSTONE MEMBER

The middle sandstone member comprises a thick (305-503 m 

(1,000-1,650 ft)) ridge-forming series of epiclastic volcanic sandstones interbedded with epiclastic volcanic conglomerate, mudflow conglomerate, and minor volcanic siltstone and mudstone 

(fig. 5). The member is 491 m (1,611 ft) thick in the type section of 

the Sedan Formation at Cottonwood Creek. The sandstones are 

light olive green, olive gray, and dark greenish gray; range from 

fine grained to conglomeratic, but are commonly medium to coarse 

grained; are calcareous; are mottled in places; and are crossbedded; 

they weather brown. They are composed largely of feldspar and 

volcanic rock fragments with subordinate hornblende, augite, 

quartz, iron oxide minerals, and clays. Carbonaceous material and 

zeolites clinoptilolite, mordenite, stilbite, and laumontite (A. J. 

Gude 3d, written commun., 1974) are common accessories, and 

wood is sparse. A mudflow conglomerate, having minor breccia 

and sandstone (figs. 2 and 6) and 54 m (177 ft) thick in the measured 

section, is locally present near the top of the middle sandstone 

member throughout the northern part of the area and south into 

the northern part of the Sedan quadrangle. It is lithologically like 

the coarse alluvial facies of member F of the Maudlow Formation.
B16 CONTRIBUTIONS TO STRATIGRAPHY

111

FIGURE 5. A view of the type section of the Sedan Formation northward across 

Cottonwood Creek. (See figure 2.) Base of the measured section is in valley 

to left. Beds dip steeply eastward. Ridge in middle of picture is top of the middle 

sandstone member. Mudstone member forms valley on right side of picture. 

Lennep Sandstone Member forms ridge on far right.

FIGURE 6. Mudflow conglomerate unit near top of middle sandstone member in 

Sedan Formation on north side of Cottonwood Creek. Outcrop forms light band 

on left side of middle ridge in figure 5.

(See section "Member F" and Skipp and Peterson, 1965.) Above the 

mudflow conglomerate, lenses of polylithologic volcanic conglomerate containing fragments as much as 0.3 m (1 ft) in diameter 

and conglomeratic sandstone make up the uppermost resistant
MAUDLOW AND SEDAN FORMATIONS, MONTANA B17

ledge of the unit (fig. 5). A similar mudflow conglomerate about 37 

m (120 ft) thick occurs in the middle of the Judith River Formation 

at Bruno Siding (Gill and Vaughn, in McMannis and others, 1968, 

fig. 6).

In the southernmost part of the area, a sandstone equivalent to 

the Sulphur Flats Sandstone Member of the Miner Creek Formation (Roberts, 1963) is present about 150 m (500 ft) above the top of 

the welded tuff member in the middle of the middle sandstone 

member, where it enters the Sedan quadrangle from the Fort Ellis 

quadrangle (Roberts, 1964f), but it can be mapped northward with 

certainty for a distance of less than 5 km (3 mi). However, a 

sequence of sandstone and tuffaceous sandstone 46 m (150 ft) 

thick, which is about 150 m (490 ft) above the base of the middle 

sandstone member at Cottonwood Creek, may correlate with the 

Sulphur Flats Sandstone Member (fig. 3).

In the northern part of the area, light-gray calcareous sandstone 

of the Parkman Sandstone equivalent is present at the base of the 

middle sandstone member (fig. 3).

Throughout that part of the area where the welded tuff member 

is present, the base of the middle sandstone member is placed at the 

top of the stratigraphically highest welded tuff bed or, in some 

places, coarse conglomerate composed of welded tuff clasts.

The part of the middle sandstone member above the Parkman 

equivalent and below the mudstone member probably is equivalent to the formation of Hamen (Sims, 1967, p. 30-33).

Silicified wood and scattered plant impressions are the only 

fossils found in the unit.

MUDSTONE MEMBER

The mudstone member is a valley-forming interval about ISO300 m (600-1,000 ft) thick, which lies below the Lennep Sandstone 

Member or locally the Bearpaw Shale Member, and above the middle sandstone member (fig. 2). At Cottonwood Creek, the member is 

274 m (899 ft) thick and consists of poorly bedded varicoloredgray, green, brown, red and yellow volcanic mudstone, much of it 

siliceous, some bentonitic or sandy, and calcareous in places. The 

mudstone is interbedded with green and gray, fine-grained to 

conglomeratic, tuffaceous, partly calcareous, epiclastic volcanic 

sandstone. Thin beds of yellowish-gray bentonite and gray-and 

white-weathering vitric tuff are common in the upper part but 

sparse in the lower part.

A prominent ridge-forming unit in the northern part of the area, 

about 46 m (150 ft) thick, of light-gray-weathering silicified crystal 

vitric tuff interbedded with dark-colored volcanic sandstone occurs
B18 CONTRIBUTIONS TO STRATIGRAPHY

near the middle of the mudstone member. This unit has been 

mapped as far south as the Wallrock quadrangle and extends 

through the entire area, though it ceases to be a ridge former in the 

southern part.

Lenses of conglomerate containing boulders as much as 1 m (3 ft) 

in diameter that consist of coarsely porphyritic augite andesite, a 

vent facies rock found in the conglomerates of member H of the 

Maudlow Formation (see section "Member H" and Skipp and 

Peterson, 1965), are present in the middle part of the mudstone 

member in the southern part of the Wallrock quadrangle and the 

northern part of the Sedan quadrangle. The conglomerates are 

interpreted to be eastward-pointing tongues of member H of the 

Maudlow Formation of the Livingston Group.

Clinoptilolite is common in the mudstones and sandstones as 

discrete orange specks. Other zeolites, mordenite and laumontite, 

are present as disseminated masses, and stilbite is common in 

veins and along fracture planes in the mudstone unit (A. J. Gude 

3d, written commun., 1974). Calcite veining is also common. 

Freshwater limestone nodules are scarce in the upper beds.

Freshwater mollusks, wood, and dinosaur bones were recovered 

from several localities within this stratigraphic inteval. A large 

carnosaurian femur found by W. J. McMannis in this unit in the 

northern part of the Sedan quadrangle is referred to cf. Gorgosaurus by Nicholas Hotton III (written commun., 1971).

Contact with the underlying middle sandstone member is 

everywhere gradational and is placed at the base of the lowest, 

thick, valley-forming mudstone sequence.

The mudstone member below the Bearpaw Shale Member and 

above the middle sandstone member probably is equivalent to the 

formation of Bruno (Sims, 1967, p. 33-34).

BEARPAW SHALE MEMBER

A unit about 76 m (250 ft) thick, above the mudstone member, is 

equivalent to the marine and brackish-water Bearpaw Shale. This 

unit is designated the Bearpaw Shale Member in the report area. 

The member has been mapped separately in the northern part of 

the area, where it consists of poorly exposed valley-forming 

mudstone (fig. 3), which is gray, green, and brown, interbedded 

with a few thin beds of gray and green sandstone and yellowishgray bentonite. The member cannot be mapped easily in the area 

south of the Sixteen NE quadrangle (fig. 1).

LENNEP SANDSTONE MEMBER

The Lennep Sandstone was named by Stone and Calvert (1910,
MAUDLOW AND SEDAN FORMATIONS, MONTANA B19

p. 746-747) for exposures at the north end of the Crazy Mountains. 

It was described as consisting "of dark-colored tuffaceous 

sandstone intercalated with dark shale" from 250 to 460 feet (76- 

140 m) thick. The outcrop was described as dark brown, and Stone 

and Calvert noted (1910, p. 747) that "fossil shells found in this 

formation are about equally fresh, brackish water, and marine; the 

fresh water and marine forms occurring in separate horizons."

In recent years, the Lennep Sandstone in the type locality has 

been determined to be a regressive marine sandstone containing 

the diagnostic Baculites cuneatus or B. compressus ammonite zone 

faunas (Gill and Vaughn, in McMannis and others, 1968; Gill and 

Cobban, 1973, p. 21).

Roberts indicated that the basal beds of the Miner Creek 

Formation correlated with the Lennep Sandstone (1963, 1965, 

1972), but mapping of the ridge formed by the Lennep equivalent 

on the western side of the Crazy Mountains (fig. 6) shows that it 

can be traced nearly continuously from near Loweth (fig. 1) 

southward to the south edge of the Sedan quadrangle, where it 

becomes the upper 120-150 m (400-500 ft) of the Miner Creek 

Formation. These beds comprise hill-forming siltstone, sandstone, 

and minor conglomerate (Roberts, 1964g, 1972; Roberts, in 

McMannis and others, 1968, fig. 3).

The Lennep is reduced in rank in the report area to the Lennep 

Sandstone Member. It is 107-152 m (350-500 ft) thick along the 

western edge of the Crazy Mountains basin and is 107 m (351 ft) 

thick at Cottonwood Creek (fig. 3). Marine beds are present in the 

northern and eastern parts of the area, but elsewhere ridgeforming brackish-water or nonmarine sandstone, conglomerate, 

minor mudstone, and zeolitized (laumontite) vitric tuff make up the 

unit. The sandstones, which are olive gray, greenish gray, light 

gray, and pale orange, commonly weather a distinctive yellowish 

brown. The brown-weathering characteristic is not as pronounced 

in the southern part of the Sedan quadrangle, where slopes are 

heavily forested, as in the remainder of the region. The sandstones 

are made up largely of fragments of volcanic rocks of intermediate 

composition; they are fine grained to conglomeratic, generally 

calcareous, poorly bedded to crossbedded; they contain much 

carbonaceous material and are commonly iron stained. Crossbedding indicates a general western source. Red-brown-weathering calcareous "cannonball" concretions are most common in the 

upper beds but locally are present throughout. Epiclastic volcanic 

pebble and cobble conglomerate is present in lenses in the southern 

part of the area, and a few pebbles are of Paleozoic limestone and 

quartzite. Altered vitric tuff and siliceous mudstone are common in
B20 CONTRIBUTIONS TO STRATIGRAPHY

the lower 30 m (100 ft) and occur locally in the upper beds.

The top of the member is marked by a thick, 30 m (100 ft), coarsegrained calcareous sandstone that is overlain by 60-90 m (200-300 

ft) of gray and brown volcanic mudstone and friable, crossbedded, 

magnetite-rich sandstone and conglomerate lenses of the lower 

part of the Billman Creek Formation.

The lower part of the Lennep Sandstone Member is marked by a 

series of thin medium- to coarse-grained sandstone lenses interbedded with siliceous mudstone and devitrified vitric tuff, which 

commonly form the lower part of a steep slope or the top of a low 

ridge above the valley-forming mudstone member.

A few dinosaur bone fragments, freshwater and brackish-water 

mollusks (W. A. Cobban, written commun., 1967), and burrows of a 

marine crustacean, Ophiomorpha (J. R. Gill, oral commun., 1971), 

were recovered from this member. The presence of Ophiomorpha 

indicates that at least part of the unit was deposited in a marine 

environment.

Palynomorphs recovered from the Lennep in the northern part of 

the Wolf Hill quadrangle were reported by K. R. Newman (in Sims, 

1967, p. 38) to be suggestive of a probable late Campanian age.

AGE AND CORRELATION

The Sedan Formation is of Late Cretaceous (latest Santonian 

and Campanian) age. Palynomorphs recovered from the lignitic 

coal at the base of the measured section (fig. 3, fossil colln. D4670) 

were identified by R. H. Tschudy (written commun., 1971) as 

representing "an interval above the Scaphites depressus zone and 

below the Scaphites hippocrepis zone *** [which correlates with] 

the upper part of the Niobrara Formation." The assemblage is 

older than any recovered from the type section of the Cokedale 

Formation (Roberts, 1972, p. C38, C42). Clioscaphites sp. (either 

vermiformis or choteauensis), identified by W. A. Cobban (written 

commun., 1971), was collected by the authors from the Telegraph 

Creek Formation below the Eagle Sandstone (figs. 2,3, fossil colln. 

Fi) in the type section of the Sedan Formation, and thus the 

stratigraphic interval of the lignitic coal at the base can be 

narrowed to above the Clioscaphites choteauensis zone and below 

the S. hippocrepis zone. From the estimated dates for the Late 

Cretaceous ammonite sequence (Obradovich and Cobban, 1975; 

Gill and Cobban, 1966, p. A35), the base of the Sedan Formation 

and the Livingston Group in this area falls at about 82-83 m.y. 

(million years) (latest Santonian). This age agrees well with the 82- 

m.y. base of the Livingston in the Livingston area, reported by 

Roberts (1972, p. C6). Roberts (1972, p. C45, C48) reported several
MAUDLOW AND SEDAN FORMATIONS, MONTANA B21

palynomorph assemblages from the Cokedale and Miner Creek 

Formations, which indicate correlation with the Blair Formation 

of Wyoming, the type Claggett Shale, the type Judith River 

Formation of Montana, and the upper part of the Pierre Shale. A 

collection from the lower part of the Lennep Sandstone in NE1^ sec. 

25, T. 4 N., R. 8 E., near Wilsall, Mont., from an original Stanton 

locality (cited in Stone and Calvert, 1910, p. 765), is reported by 

Roberts (1972, p. C47, 48) to represent the Baculites compressus or 

Baculites cuneatus zone (fig. 1, fossil colln. D3083). Roberts 

correlated this zone with the Sulphur Flats Sandstone Member at 

the base of the Miner Creek and hypothesized that the top of the 

Miner Creek is as young as Maestrichtian (1972, p. C6). Mapping 

between the type Sulphur Flats Sandstone Member and the 

Lennep Sandstone at Loweth shows that the Lennep Correlates 

instead with the upper ridge-forming beds of the Miner Creek 

Formation. The potassium-argon age of the Baculites cuneatus 

and Baculites compressus ammonite zones proposed by Obradovich and Cobban (1975, p. 36) is about 70 m.y.; thus, the 

lower part of the Livingston Group, the Sedan Formation, is 

bracketed by an 82-83-m.y. age at the base and a 70-m.y. age at the 

top; hence it was deposited within a span of about 12-13 m.y.

Some uncertainty remains regarding the assignment of beds 

above the Lennep Sandstone at Loweth, at the northwestern edge 

of the Crazy Mountains basin. J. R. Gill reported 240 m (785 ft) of 

Miner Creek Formation above the Lennep Sandstone (in Roberts, 

1972, p. C46). These beds are probably the same as the 216 m (708 ft) 

of the lower part of the Billman Creek Formation above the type 

Sedan Formation in the measured section at Cottonwood Creek 

(fig. 3), and we consider them to be lithologic correlatives of the 

Billman Creek. The criteria for the formation assignment are as 

follows: the presence of a zone of brown-weathering, spherical, 

calcareous sandstone concretions about 75 m (250 ft) above the 

Lennep Sandstone; a marker bed in the lower beds of the type 

section of the Billman Creek Formation (Roberts, 1972, p. C49); the 

preponderance of mudstone and siltstone over sandstone; and the 

lenslike nature of the sandstone beds in the section.

The top of the Eagle Sandstone appears to become progressively 

younger in the northern and eastern parts of the map area (fig. 4). 

At Bruno Siding, east of Loweth, the base of the volcanic detritus 

(the base of the Judith River Formation) is estimated to occur at 

about 78 m.y. (Obradovich and Cobban, 1975, p. 36; Gill and others, 

1972, p. 92), though the upper part of the Eagle Sandstone in this 

interpretation is dominantly nonmarine volcanic mudstone and 

shale overlain by an upper thin marine shale and sandstone. Gill
B22 CONTRIBUTIONS TO STRATIGRAPHY

and Vaughn (in McMannis and others, 1968, fig. 6) and Gill and 

Cobban (1973) earlier called the upper marine sandstone, the 

Parkman Sandstone. The interval between the top of the Virgelle 

Sandstone Member of the Eagle and the top of the marine 

sandstone (Parkman) was called the formation of South Fork by 

Sims (1967), who suggested a possible correlation with the Eagle 

Sandstone. These interpretations and suggested relations to the 

Sedan Formation are shown in the correlation chart (fig. 4).

MAUDLOW FORMATION

In the 10th census report of 1880, Waldemar Lindgren (1886, pi. 

53 and p. 736) published a map of the "Lower Sixteen-mile Creek 

Cretaceous Basin" and noted the presence of sandstone belonging 

"to the Laramie formation" and "volcanic conglomerate of dark, 

often glassy eruptives and red liparite [welded tuff]." The volcanic 

rocks, later referred to the Livingston Formation by Peale (cited 

in Weed, 1893, p. 21) form the central part of the Maudlow structural basin located between the Bridger Range to the south and the 

Big Belt Mountains to the north (fig. 1). Stone and Calvert (1910, 

pi. 7) showed the Maudlow area to be a westward extension of the 

Crazy Mountains basin, and Billingsley (1915, p. 35) described 

"the coarse andesitic conglomerate at Maudlow ***" as having 

been derived "from no other source than the lavas of the Elkhorn 

Mountains."

These general views were supported by the later work of Klemme 

(1949), who used the name Livingston Formation for the volcanic 

series, mapped the geographic extent of the outcrop, described in 

some detail a 1,220-m (4,000-ft) measured section at Maudlow, and 

proposed the name "Maudlow conglomerate lentil" (fig. 7) for a 

series of lensing "volcanic conglomerates with interbedded tuffs" 

(members B and C of the Maudlow Formation of this report) 

strikingly exposed near the town of Maudlow. The name Maudlow 

was first published in a report by McMannis (1955). The conglomerates and tuffs are undoubtedly those examined earlier by 

Billingsley. Klemme, and later, McMannis (1955, p. 1410), and 

Klepper, Weeks, and Ruppel (1957, p. 39) reiterated Billingsley's 

assertion that the sequence was derived from the Elkhorn Mountains Volcanics. Fossil plants identified by Erling Dorf for 

Klemme from the "Maudlow conglomerate lentil" suggested the 

possibility of a Paleocene age for this part of the Livingston 

Formation. The name Maudlow is redefined and raised to formation rank in this report. Certain measured sections in T. 4 N., 

R. 5 E., (p. B56) are designated the type section, because no type 

section was selected in any of the earlier reports.
MAUDLOW AND SEDAN FORMATIONS, MONTANA B23 

R.4E. R.5E. R.6E.

FIGURE 7. Simplified geologic map modified from Klemme (1949) showing outline 

of area of outcrop of his Livingston Formation (stippled) and areal distribution 

of his Maudlow Conglomerate Lentil of the Livingston Formation (patterned). 

Stratigraphic position of the lentil is discussed under members B and C (see 

sections "Member B" and "Member C").

Detailed mapping of the Upper Cretaceous volcaniclastic and 

volcanic rocks near Maudlow by Skipp and Peterson (1965) showed 

that the sequence is divisible into eight map units three of them 

largely of primary volcanic origin, the others dominantly epiclastic. These units constitute members A through H of the 

Maudlow Formation, which total about 1,677+ m (5,500+ ft) along 

the west edge of the outcrop and about 1,525+ m (5,000+ ft) along the 

eastern edge (fig. 8). Faulting in the vicinity of Maudlow has 

repeated members A, B, and C, and so earlier measurements of the 

thickness of conglomerates in that area are too large. Member D
B24 CONTRIBUTIONS TO STRATIGRAPHY

contains several fine-grained biotitic dacitic welded tuff sheets, the 

"red liparite" of Lindgren. The tuffs resemble ash-flow tuffs of the 

lower unit of the Elkhorn Mountains Volcanics (Smedes, 1966, p. 

32-33). Members B and F contain thick primary hornblendeaugite, augite-rhyodacite, and dacite flows. Facies changes and

05'00" 111°02'30'

46°07'30' -

46°00'

4 KILOMETERS

FIGURE 8. Simplified geologic map of the Maudlow area (adapted from Skipp 

and Peterson, 1965) showing distribution of members H through A of the 

Maudlow Formation of the livingston Group, location of measured sections of 

members, fossil locality D3324, and localities of samples which yielded potassium-argon ages.
MAUDLOW AND SEDAN FORMATIONS, MONTANA B25

wedging out of these units and of lithic and lithic crystal tuffs of 

members F and G from west to east across the basin indicate 

western or southwestern sources for all of the primary volcanic 

material. The remainder of the members contain much epiclastic 

material fragments of altered volcanic rocks as much as 2 m (6 ft) 

in diameter which was derived in part from the Elkhorn Mountains Volcanics to the west, though some of the material is clearly 

of local origin. The size of included boulders, the presence of 

volcanic breccias, the massive bedding, and the generally dark 

color indicate that the Maudlow Formation is an example of the 

coarse alluvial volcanic facies of Parsons (1969), which grades 

eastward into the fine alluvial facies of the Sedan Formation.

The Maudlow Formation unconformably overlies either the 

upper part of the Telegraph Creek Formation or the Virgelle Sandstone Member of the Eagle Sandstone (Skipp and Peterson, 1965; it

EXPLANATION

Quaternary and Tertiary rocks

Measured section 

x D3324 

Fossil locality

xi

K-Ar age locality

Inclined

Contact

80 |

Overturned Horizontal

Strike and dip of beds

Pre-Livingston rocks

Steep fault - Dotted where concealed. U and D 

indicate direction of relative movement

Thrust fault - Sawteeth on upthrown side 

Crestline of anticline - Dotted where concealed

Troughline of syncline Dotted where concealed 

FIGURE 8. Continued.
B26 CONTRIBUTIONS TO STRATIGRAPHY

is unconformably overlain by tuffaceous beds of the Oligocene 

Dunbar Creek Formation of the Bozeman Group, or by Quaternary gravels in the southwest part of the map area.

The eight members shown in the composite columnar diagram of 

figure 3 are described here in ascending order.

MEMBER A

Member A unconformably overlies the Telegraph Creek Formation (upper part) at the type section (measured section A) in the 

southeast corner of the area of outcrop (fig. 8). It is 157 m (514 ft) 

thick at this locality and is about 90± m (300± ft) thick elsewhere. 

On the southwest and west sides of the Maudlow area, member A 

overlies magnetite-rich sandstone of the Virgelle Sandstone 

Member of the Eagle Sandstone. Member A consists of epiclastic 

volcanic sandstone, conglomerate, and mudstone with some lithic 

tuff and a few beds of lignitic shales near the base. Conglomeratic 

sandstone and conglomerate make up almost 50 percent of the unit 

in the type section. Plant fossils are abundant in this member.

The unconformity at the base of member A is characterized by an 

abrupt and persistent change in mineral constituents. The beds 

below the unconformity, the Telegraph Creek Formation (upper 

part) or the Virgelle Sandstone Member, are composed of abundant 

quartz, some feldspar, and varying amounts of biotite, sphene, and 

magnetite in a calcite matrix, whereas the basal sandstones and 

mudstones of member A appear to be slightly reworked crystal 

tuffs with abundant plagioclase, minor quartz, moderate hornblende, and pyroxene in a clay-rich zeolite or calcite matrix. These 

lithologies are not interbedded in the Maudlow area but show 

interbedding in the lower parts of the type sections of the Sedan 

Formation and the Cokedale Formation (Roberts, 1972; Skipp and 

McGrew, 1972). A thin zone of yellow ochre clay, possibly an 

ancient soil zone, has been observed at the contact at the base of 

member A in the measured section.

The sandstones and conglomerates of member A are generally 

olive gray, greenish gray, and brownish gray, are made up of 

fragments (as much as 46 cm (18 in.) in diameter) of altered 

porphyritic rocks ranging in composition from basaltic to rhyodacitic, and are well indurated and ridge forming. Crossbedding is 

common in several places and is emphasized by heavy-mineral 

concentrations. The phenocrysts in the volcanic rock fragments 

are largely kaolinized plagioclase and augite, a little hornblende, 

some secondary opaque iron ores, and relict olivine. A few 

fragments of impure sandstone and single euhedral or subhedral 

crystals of plagioclase, augite, and hornblende are also present.
MAUDLOW AND SEDAN FORMATIONS, MONTANA B27

The sandstones are cemented with clay minerals (nontronite and 

montmorillonite), zeolite (clinoptilolite), and calcite. Finely laminated claystones, ranging in color from pale yellowish brown to 

dark greenish gray, are found interbedded with the sandstones in 

several places. They contain leaf, needle, grass, and twig impressions and silicified wood fragments. J. A. Wolfe (written 

commun., 1960, 1962) and R. A. Scott (written commun., 1961) 

identified the following flora from several localities in the map 

area:

Ferns:

Anemia elongata (Newberry) Knowlton

Dryopteris sp.

Gleichenites sp., cf. G. kurriana Heer 

Conifers:

Sequoia reichenbachi (Geinitz) Heer

Araucarites sp.

Araucarioxylon sp.

Glyptostrobus? sp. 

Dicots:

"Cissus" 1 marginata (Lesquereux) Brown

'Quotes indicate the uncertainty of generic assignments (J. A. Wolfe, written commun., 1975).

The presence of Sequoia reichenbachi, known also from the type 

locality of the Livingston, from the type Judith River Formation, 

and from the Elkhorn Mountains Volcanics, places member A in 

the Cretaceous. The fern Anemia elongata is known only from 

several Late Cretaceous formations in the Rocky Mountains (J. A. 

Wolfe, written commun., 1960).

Erling Dorf identified the following Late Cretaceous flora from 

beds "300 feet above the Eagle Sandstone Livingston Formation 

contact" (Klemme, 1949, p. 58,186) in sec. 12, T. 4 N., R. 4 E., in the 

northeastern part of the Maudlow area:

Fern:

Asplenites tenellum Knowlton 

Dicots:

"Salix" lancensis Berry

Apeibopsis ? laramiensis Knowlton

A sample of lignitic shale 19 m (63 ft) above the base of the 

Maudlow Formation, from unit 3D of measured section A (p. B64; 

fig. 8), was examined by Robert H. Tschudy (written commun., 

1964, 1968), who reported the following palynomorphs (USGS 

Paleobot. loc. No. D3324, figs. 3, 8):

Appendicisporites (App 5) 

Klukisporites (TT-rt3) 

Proteacidites 

Araucariacites 

Schizaea (M-stl) 

Gleicheniidites (Gleich 5)
B28 CONTRIBUTIONS TO STRATIGRAPHY

Minorpollis (Pa3-sml8B)

Taxodiaceaepollenites (Tax-r5)

Eucommiidites (Ca-smlS)

Complexiopollis

TO-p8

Callialasporites

CPg-rtlO?

C3 -sm7?

Sj-rt new sp

C3 -rtl4

C3 -sml6

This flora, like that reported from the base of the Sedan 

Formation, appears to be older than assemblages previously 

obtained from the Cokedale Formation (Roberts, 1972, p. C37-C39) 

and "can be no younger than early Claggett [type section] and no 

older than Telegraph Creek [type section] " (Tschudy, written 

commun., 1968). This places the assemblage in the latest Santonian to earliest Campanian age range.

MEMBER B

Member B disconformably overlies member A. This unit comprises the resistant consistently grayish red purple "andesitic" 

beds noted by Billingsley (1915, p. 35) and included by Klemme in 

his "Maudlow conglomerate lentil" in 1949. In the vicinity of 

Maudlow, the member consists of volcanic breccias (fig. 9), tuff, 

dominantly monolithologic mudflow conglomerate, and rudely 

laminated monolithologic volcanic sandstone topped by a ledgeforming massive red-purple vitric crystal tuff. Measured section B 

(p. B61; fig. 8) in this area is 159.8 m (524 ft) thick. To the south the 

member thins and consists mostly of autobrecciated and unbrecciated flows. Two chemical analyses of these flows (table 1, 

columns 1 and 2) and thin section examination show them to be 

hornblende dacites with phenocrysts of zoned plagioclase (calcic 

andesine-sodic labradorite) and hornblende as much as 5 mm long, 

accessory magnetite, minor apatite, and sparse sphene, all in an 

uneven microcrystalline mosaic of feldspar, quartz, and minor 

zeolite (clinoptilolite). In some places augite and diopsidic augite 

are important constituents of these flows and flow breccias, 

though nowhere was the augite:hornblende ratio observed to be 

greater than 1:2.

Fragments in the flow breccias are angular to rounded and are as 

much as 0.6 m (2 ft) in diameter. In places the breccias have 

incorporated a few loose cobbles and boulders of older rocks (fig. 9). 

From a distance the brecciated nature of the flows is quite striking; 

at the outcrop it is difficult to delineate (fig. 9), but it is visible in 

thin section. In some places the interstices of the flow breccias are
MAUDLOW AND SEDAN FORMATIONS, MONTANA B29

FIGURE 9. Outcrop of hornblende dacite volcanic breccia of member B of Maudlow 

Formation. Fragments and matrix are of same material. Note single rounded 

quartzite pebble (circled) caught up in the breccia. Locality is just east of 

town of Maudlow in NEJ4 sec. 13, T. 4 N., R. 4 E.

filled with a brecciated felsitic matrix of the same material as the 

boulders. In others the interstices are lined with zeolites, largely 

clinoptilolite (Anna Shepard, written commum., 1962).

The massive altered vitric crystal tuff at the top of the member 

(measured section B) near Maudlow consists of chalcedonized 

glass, much of it with relict shard outlines; 20-25 percent altered 

plagioclase and lithic fragments; and 3 percent diopsidic augite.

Member B thins to 94 m (309 ft) on the eastern side of the area 

(measured section B) and consists largely of sandstone and crystal 

lithic tuff with thin beds of conglomerate. The sandstones that 

retain the distinctive grayish-red color of the weathered dactite are 

largely monolithologic and appear to be slightly reworked crystal 

lithic tuffs.

A small remnant of member B was found in a streambed just east 

of Flathead Pass, in the lower sandstone member of the Sedan 

Formation, but it was not found farther east. Changes in lithology 

and thickness of the member (fig. 10), from vent facies in the
B30 CONTRIBUTIONS TO STRATIGRAPHY

TABLE 1. Chemical and normative compositions of igneous rocks of the Maudlow 

Formation and an associated sill

[o, looked for but not found; , not looked for]

Maudlow Formation

.......................................... 1 2

.......................................... 156295 156296

3 

157753

Sill

4 

157755

Rapid rock analysis

(Analysts: P. L. Elmore, I. H. Barlow, 

Gillison Chloe, and S. D. Bolts)

SiO2 ..................

A12 03................

Fe2 03................

FeO...................

MgQ.................

C2O 

Na2Q................

K2O 

H2 O 

TiO2..................

P2 O. .................

MnO.................

CO2...................

Sum .......

.............................. 64.6 62.0

.............................. 16.4 17.4

.............................. 3.7 3.9

................ 1.2 1.4

.............................. 1.6 2.3

5.0 5.2 

................ .. 3.5 4.2

2.1 1.7 

1.3 1.7 

.............................. .43 .46

.............................. .27 .31

.............................. .12 .14

.............................. <.05 <.05

................................. 100.0 101.0

64.6 

13.8 

1.4 

.88 

.70 

5.6 

1.2 

.58 

10.3 

.45 

.08 

.05 

<.05

100.0

53.9 

13.0 

3.1 

4.9 

9.0 

7.6 

2.4 

3.1 

2.0 

.62 

.46 

.16 

.23

100.0

CIPW norms

Orthoclase ......

Albite ..............

Magnetite ......

Ilmenite ..........

Hematite ........

Diopside ..........

Normative pla

.................................. 24.18 17.34

.................................. 12.23 10.01

.................................. 29.34 35.63

.................................. 23.07 23.63

.................................. 4.00 5.60

.................................. 3.02 4.87

.................................. .76 .91

.................................. .67 .67

.................................. 1.60 .48

gioclase.................. An48 An38

42.54 

3.34 

9.96 

27.80 

1.70 

1.62 

.91 

0 

.32

1.38 

18.35 

20.44 

15.85 

20.40 

3.27 

1.22 

1.34 

0 

14.36 

An43

Semiquantitative spectrographic analysis

[Analyst: H. W. Worthing]

Si ...................

Al ...................

Fe ...................

Mg .................

Ca ...................

Na .................

K. ....................

Ti ...................

P .....................

Mn .................

Ag...................

An .................

Major 

7.0 

1.5 

.3 

3.0 

.7 

.7 

.3 

0 

.03 

0 

0

Major 

3.0 

3.0 

3.0 

7.0 

1.5 

3.0 

.3 

0 

.15 

0 

0
MAUDLOW AND SEDAN FORMATIONS, MONTANA B31

TABLE 1. Chemical and normative compositions of igneous rocks of the Maudlow 

Formation and an associated sill Continued

Sample No ..........

Laboratory No.

Au..................

B ...................

Ba .................

Be .................

Bi ..................

Cd. .................

Ce ..................

Co..................

Cr ..................

Cs .................

Cu ..................

Dy..................

Er ..................

Eu. ....... ..........

F ....................

Ga..................

Gd..................

Ge ..................

Hf ..................

Hg ................

Ho..................

In ..................

Ir ....................

La ....... ...........

Li ..................

Lu ..................

Mo ................

Nb ................

Nd ................

Ni ..................

Os ..................

Pb ..................

Pd ..................

Pr ..................

Pt ..................

Rb ..................

Re ..................

Rh..................

Ru ..................

Sb ..................

Sc ..................

Sn ..................

Sr ..................

Sm ................

Ta ..................

Tb ..................

Te ..................

Maudlow Formation

............................................. 123

............................................. 156295 156296 157753

Semiquantitative spectrographic analysis Continued

(Analyst: H. W. Worthing)

.............................. ................ ................ 0

.............................. ................ ................ .003

.............................. ................ ................ .15

............................... .............. . ................ .0003

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ .0003

.............................. ................ ................ .0015

.............................. ................ ................ 0

.............................. ................ ................ .0007

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ .0007

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ .003

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ .0003

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ .003

.............................. ................ ................ 0

.............................. ................ ................ .0007

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ .0007

.............................. ................ ................ 0

.............................. ................ ................ .3

.............................. ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

.............................. ................ ................ 0

Sill

4

157755

0

.003

.07

.00015

0

0

0

.007

.015

0

.007

0

0

0

.0007

0

0

0

0

0

0

0

0

0

0

0

0

0

.03

0

0

0

0

0

0

0

0

0

0

.0015

0

.15

0

0

0

0
B32 CONTRIBUTIONS TO STRATIGRAPHY

TABLE 1. Chemical and normative compositions of igneous rocks of the Maudlow 

Formation and an associated sill Continued

Maudlow Formation Sill

Sample No 1234 

Laboratory No ...................................................... 156295 156296 157753 157755

Semiquantitative spectrographic analysis Continued

(Analyst: H. W. Worthing)

Th ....................................

Tl ....................................

Tm ..................................

U......................................

V......................................

W ....................................

Y......................................

Yb ....................................

Zn ....................................

Zr ....................................

............ ................ ................ 0

............ ................ ................ 0

............ ................ ................ 0

............ ................ ................ 0

............ ................ ................ .0015

............ ................' ................ 0

............ ................ ................ .003

............ ................ ................ .0003

............ ................ ................ 0

............ ................ ................ .03

0

0

0

0

.003

0

.0007

0

0

.007

1. Dacite; grayish-purple porphyritic hornblende dacite lava flow from member B on west side of Maudlow 

area center E'/2 sec. 25, T. 4 N., R. 4 E. Collected by Marjorie MacLachlan.

2. Dacite; grayish-purple porphyritic hornblende dacite lava flow from member B on southwest side of 

Maudlow area NWViNWVt sec. 1, T. 3 N., R. 4 E. Collected by Betty Skipp.

3. Altered dacitic welded tuff; grayish-black perlite of member D on southeast corner of Maudlow area 

SW'/iNW'/i sec. 26, T. 4 N., R. 5 E. Collected by Betty Skipp.

4. Shoshonite; brownish-black augite-olivine shoshonite sill in member G in southern part of Maudlow area 

SW/iSE'/i sec. 31, T. 4 N., R. 5 E. Collected by Betty Skipp.

southwest to alluvial facies (Parsons, 1969) in the east, indicate a 

western or southwestern source.

Hornblende from the dacite flow of sample No. 2 (table 1) yielded 

a potassium-argon age of 78.9±1 m.y. (J. D. Obradovich, written 

commum., 1974; referred to as sample No. 1 on figs. 4 and 8).

MEMBER C

Member C disconformably overlies member B and consists of 

polylithologic volcanic conglomerate, sandstone, and some tuff, 

which total 242 m (793 ft) on the south side of the outcrop area 

(measured section C). A thick ridge-forming conglomerate (fig. 11) 

containing some boulders of member B is present at the base of the 

member in most places. This conglomerate was included in "the 

Maudlow conglomerate lentil" (fig. 7) of Klemme (1949). Near 

Maudlow, where the basal conglomerate is repeated, it is about 60 

m (200 ft) thick. It consists of dacitic and rhyodacitic volcanic rocks 

as much as 0.9 m (3 ft) in diameter, well-rounded and loosely 

cemented by finer volcanic debris; altered shards; plagioclase; 

hornblende and augite crystals; and hematite. Many of the
MAUDLOW AND SEDAN FORMATIONS, MONTANA B33

46° 00'

Thickness in 

meters (feet)

200 n 

(600)

150 - 

(400)

100 -\

(200) 

50

EXPLANATION

vitric-crystal tuffs

Fine to coarse alluvial fades Epiclastic volcanic sandstone and conglomerate interbedded with crystal-lithic and vitric-crystal tuff of 

hornblende-augite dacite composition

FIGURE 10. Diagram of facies and thickness relationships of member B of 

Maudlow Formation in the type section and eastward to the small outcrop 

near Flathead Pass in the lower sandstone member of the Sedan Formation. 

Hachured area has been removed by erosion.

boulders have deeply oxidized banded rims as much as 2.5 cm (1 

in.) thick. Hornblende and augite porphyries make up most of the 

basal conglomerate, but a few pebbles of altered welded tuff are 

also present. No cobbles or boulders of sedimentary rocks were 

seen in any of the deposits.

Somber colors, mostly gray or olive gray, dominate the member, 

which, above the basal conglomerate, consists of sandstone, 

siltstone, claystone, conglomerate, and some thin tuff beds. The 

sandstones are fine-grained to conglomeratic, calcareous, crossbedded, volcanic microbreccias. In many places the crossbedding
B34 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 11. Outcrop of coarse epiclastic polylithologic volcanic conglomerate 

at base of member C of Maudlow Formation near Maudlow. This conglomerate 

and the breccia of figure 9 were included in the Maudlow Conglomerate Lentil 

of the Livingston Formation of Klemme (1949). Boulders and cobbles have 

weathered rinds up to 2.5 cm (1 in.) thick. Outcrop is located 0.4 km (0.25 mi) 

northeast of Maudlow in center SB1/* sec. 12, T. 4 N., R. 4 E.

is accentuated by concentrations of detrital iron oxide minerals 

and subhedral crystals of hornblende and pyroxene. Calcite, clay, 

and zeolites (clinoptilolite and stilbite) are cementing materials. 

The member is, in general, a coarse alluvial volcanic facies, but it 

is transitional into fine alluvial facies along the eastern edge of the 

outcrop.

Some parts of the fine-grained sandstone and mudstone of the 

member are highly fossiliferous; J. A. Wolfe (written commun., 

1960) identified the following flora from a locality northwest of 

Maudlow above the basal conglomerate: 

Ferns:

Dryopteris carbonensis Knowlton

Asplenites tenellum (Knowlton) Dorf 

Conifers:

Sequoia dakotensis Brown 

Dicots:

"Cinnamomum" 1 affine Lesquereux

"Ficus" impressa Knowlton?

Grewiopsis saporteana Lesquereux

"Zizyphus" cretaceus (Dawson) Bell?

'The quotes indicate the uncertainty of generic assignments (J. A. Wolfe, written commun., 1975).
MAUDLOW AND SEDAN FORMATIONS, MONTANA B35

Mr. Wolfe stated that the flora is of Late Cretaceous age.

Klemme (1949, p. 58, collns. 1, 3, and 4) listed a flora that Erling 

Dorf originally thought might be Paleocene from beds interpreted 

to belong to member C. Subsequently Dorf (oral commun., 1960) 

decided the fossils were not diagnostic.

MEMBER D

Member D disconformably overlies member C. It varies in 

thickness in the Maudlow region from about 122 m (400 ft) in the 

northeastern part to 206 m (675 ft) on the east side (measured 

section D) and 142 m (467 ft) on the west side (measured section Dx ) 

of the area of outcrop (fig. 8). The member consists of three groups 

(cooling units?) of dacitic welded tuff flows separated by thick 

volcanic sandstones and conglomerates. The welded tuffs range 

from 0 to 98 m (0-320 ft) in thickness and are extremely varied in 

color. Hydrated, fresh, or partly devitrified glass or perlite is black 

or dark brownish gray. Completely devitrified glass is light orange 

pink, pale brown, pale yellowish brown, pale reddish brown, 

grayish red, and pale green. The brown and red colors are caused 

by finely disseminated iron oxide in the chalcedony or quartz 

alteration; the pale orange results from small amounts of additional green nontronite, and the green, from a dominance of 

nontronite (or celadonite). Sandstone and conglomerate interbedded with the welded tuff sheets contain many fragments of 

welded tuff, as much as 36 cm (14 in.) in diameter, mixed with other 

volcanic debris of intermediate composition. Fossil coniferous 

wood is found in the sandstones.

The rock is difficult to classify. It contains 65 percent SiO2 ,5-10 

percent H2 O, and 3-10 percent phenocrysts plagioclase, calcic 

andesine to sodic labradorite in composition; golden-weathering 

biotite; augite and hypersthene in decreasing order of abundance; 

and altered volcanic rock fragments. Pumice and ash fragments 

are small, most less than 0.5 mm in length, and densely packed; the 

rock is poorly to moderately welded (fig. 12). In thin section the 

glass is completely isotropic and has a refractive index of 1.505± 

0.002. A chemical analysis of a perlite from member D was 

published by Robinson and Marvin (1967, p. 604) and is repeated in 

table 1 (sample No. 3). Moderately high silica content (64.6 percent) 

and the moderately calcic nature of the plagioclase phenocrysts 

suggest a dacitic composition for the magma.

Internally inconsistent potassium-argon ages on biotite (83±2 

m.y.) and plagioclase (71±7 m.y.) from a sample of the welded tuff 

(sample No. 2 of figs. 4 and 8) were reported by Robinson and 

Marvin (1967, p. 607). These numbers have not been considered on 

the correlation chart (fig. 4).

The chemical composition of these welded tuffs precludes cor-
B36 CONTRIBUTIONS TO STRATIGRAPHY

*Sifc,

SS

FIGURE 12. Photomicrograph in plain light of glassy zone in lower sheet of 

welded tuff in member D of Maudlow Formation showing small size of shards, 

moderate welding, and crystals of andesine and augite and numerous fragments 

of altered volcanic rocks. Sample comes from southeast corner of Maudlow 

Formation outcrop in NWJ/4 sec. 26, T. 4 N., R. 5 E.

relation with the rhyolite ash flows of the middle unit of the 

Elkhorn Mountains Volcanics, which are characterized by 69-71 

percent SiO2 , porphyroblastic clusters of quartz and K-feldspar, 

and plagioclase of andesine composition (Smedes, 1966, p. 22,33- 

34). The Maudlow welded tuffs are compositionally more like the 

"quartz latite ash flows" from the lower unit of the Elkhorn 

Mountains Volcanics, which contain 60.7 percent SiO2 and plagioclase phenocrysts of calcic andesine to sodic labradorite composition (Smedes, 1966, p. 22,32-33). The welded tuffs of the Maudlow 

Formation are the first recognized indication that the ash flows of 

the lower unit of the Elkhorn Mountains Volcanics may have been 

areally extensive.

MEMBER E

Member E is a series of nonresistant, well-bedded, crossbedded, 

dark-colored, generally fine-grained, calcareous, volcanic sandstones, mudstones, and tuffs, typical of the fine alluvial volcanic 

facies. Conglomerate and breccia are rarely present. The beds are 

commonly grayish green to olive gray on fresh surfaces and
MAUDLOW AND SEDAN FORMATIONS, MONTANA B37

weather various shades of brown. Brown-weathering limy concretions as much as 2 m (6 ft) in diameter are common. Crossbedded volcanic sandstone is interbedded with massive devitrified 

and silicified vitric-lithic, vitric-crystal, or lithic tuffs. The member 

is 231 m (758 ft) thick in the reference section on the east side of the 

outcrop area and commonly forms a valley between the resistant 

ridges of members D and F.

One thin section of a dusky blue-green altered vitric-lithic tuff 

consists of more than 50 percent angular to subrounded altered 

volcanic lithic fragments as much as 1 mm across. The lithic 

fragments contain fresh zoned plagioclase (andesine), iron oxide 

minerals, and a few unaltered clinopyroxene crystals in an altered 

matrix with relict shard outlines. Green sheafs of celadonite (or 

nontronite) and a little "chlorophaeite" (a mineral or mixture of 

minerals closely related to chlorite in composition) make up the 

matrix.

Silicified logs were the only fossils found.

MEMBER F

Member F is a ridge-forming thick (183-244 m (600-800 ft)) 

massive sequence of varicolored, largely gray, grayish-green, and 

grayish-red, flows, flow breccia, lithic tuff, mudflow conglomerate, 

and epiclastic volcanic conglomerate and sandstone. Porphyritic 

hornblende and hornblende-augite dacite and rhyodacite flows, 

flow breccias, and lithic tuffs make up more than 50 percent of the 

interval in the western areas of outcrop, forming a ventfacies. The 

vent facies grades eastward into the coarse alluvial facies of 

measured section F, which consists of volcanic conglomerate, 

containing boulders as much as 1.5 m (5 ft) in diameter, with 

interbedded volcanic sandstone, tuff, and clay stone.

Breccia fragments in the abundant flow breccias range in size 

from crystal fragments to angular pieces of volcanic rocks as much 

as several centimeters across (fig. 13); they are set in a felsitic or 

cryptofelsitic groundmass much the same as, though less abundant than, the groundmass of the enclosed rock fragments. The 

boundaries of the rock fragments are indistinct under the microscope, and many show incipient brecciation. Rock and crystal 

fragments make up about 50 percent of the whole. The cryptofelsitic groundmass has a low birefringence and interstitial fillings of celadonite, nontronite, quartz, and the zeolites mordenite 

and clinoptilolite (A. J. Gude 3d, written commun., 1974), in about 

that order of abundance.

Of four thin sections of the breccia from various locations, two 

had phenocrysts of only andesine plagioclase and green horn-
B38 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 13. Outcrop of porphyritic dacite breccia of member F of the Maudlow 

Formation. Large coarsely porphyritic angular fragments with large andesine 

phenocrysts are surrounded by much finer material of the same dacitic compositions. Crops out along north side of road northeast of town of Maudlow in the 

E% SWV4 sec. 7, T. 4 N., R. 5 E.
MAUDLOW AND SEDAN FORMATIONS, MONTANA B39

blende in both rock and crystal fragments and two had phenocrysts of the above plus augite showing oscillatory zoning. In 

these latter two, the hornblende is about twice as abundant as the 

augite. In places, however, augite is the dominant mafic mineral. 

Magnetite is a common accessory mineral; sphene and apatite are 

present in small amounts. Kaolinitic alteration is common in the 

feldspars in both the enclosed rock fragments and the separate 

crystals.

Lithologic correlatives of the breccias and conglomerates of 

member F are present east of Flathead Pass in the upper part of the 

middle sandstone member of the Sedan Formation (fig. 14). Also, 

the coarse volcanic conglomerate in the Judith River Formation at 

Loweth (Gill and Vaughan, in McMannis and other, 1968) is 

thought to be a distant alluvial facies of member F of the Maudlow 

Formation (fig. 4). Similar lithologies and consistent decrease in 

size of clasts across the mapped area support this interpretation.

A few silicified tree trunks were the only fossils found in the fine 

epiclastic beds of the member.

A potassium-argon age determined on hornblende phenocrysts 

from the southwestern area of outcrop is 74.9±1 m.y. (J. D. Obradovich, written commun., 1974) (sample No. 3 of figs. 4 and 8). 

Because member F is the stratigraphically highest member of the 

Maudlow Formation which contains primary volcanic rocks, this 

radiometric age indicates the time of cessation of active rhyodacitic volcanism in the Maudlow area.

MEMBER G

Member G conformably overlies member F and constitutes a 

thick (400 m (1,310 ft) in measured section G) sequence of slopeforming, moderately well bedded, epiclastic, volcanic sandstone, 

siltstone, and claystone, and lithic and crystal-lithic tuffs that 

weather a characteristic yellowish brown and support very little 

vegetation. A few of the tuffs are conglomeratic, with pebbles as 

much as 1.3 cm (0.5 in.) long. No bouldef conglomerates were seen 

in the unit. Bedding is visible in many places and ranges from 2.5 

cm (1 in.) to a meter or more (several feet) in thickness. Beds of 

coarse-grained sandstone weather out in relief. Crossbedding is 

present but is not a prominent feature. The lithologies are typical of 

a fine alluvial volcanic facies.

Two thin sections from a reworked lithic-crystal tuff from the 

upper part of the member consist of about 40 percent angular to 

rounded lithic fragments as much as 4 mm in length, 15 percent 

unaltered augite and partly altered hypersthene, 5 percent altered
B40 CONTRIBUTIONS TO STRATIGRAPHY

11 Too'

46°00'

Thickness in 

meters (feet)

A

250 n 

(800)

200 - 

(600)-

Middle sandstone mbr 

(part) of Sedan Fm 

at type section

EXPLANATION

Vent fades Porphyritic dacite and rhyodacite: breccias, flows, and lithic 

tuffs (fig. 13)

Coarse alluvial facies - Mudflow conglomerate, volcanic sandstone,and 

minor mudstone composed exclusively of member F detritus

FIGURE 14. Diagram of facies and thickness relationships of member F of 

Maudlow Formation in the Maudlow area and eastward to the tongue of 

mudflow conglomerate of member F in the middle sandstone member of the 

Sedan Formation in the type section.

plagioclase, and accessory magnetite in a matrix of clay and 

zeolite. The lithic fragments are largely devitrified vitric volcanic 

rocks.

An unbrecciated body of grayish-black shoshonite or trachybasalt, which weathers grayish-brown, crops out in the southwestern part of the area in sec. 6, T. 3 N., R. 5 E., within member G. 

A chemical analysis is given in table 1 (sample No. 4). In thin
MAUDLOW AND SEDAN FORMATIONS, MONTANA B41

section the rock contains 40 percent euhedral to subhedral phenocrysts as much as 5 mm long, consisting of augite and olivine 

altered to iddingsite and magnetite, with the augite slightly more 

abundant; 30 percent subhedral phenocrysts of altered, twinned, 

and zoned plagioclase (calcic andesine to labradorite) and accessory magnetite and apatite in a granular intergrowth of clear 

untwinned subhedral to anhedral alkali feldspar (some as crystal 

overgrowths on laths of plagioclase); subordinate subhedral plagioclase; and anhedral quartz with devitrified intersertal glass.

Merrill (1895, p. 638) reported similar rock in the Cretaceous 

about 4 km (2.5 mi) southeast of Bozeman on the east side of 

Bozeman Creek; similar rocks have been reported by Klepper, 

Weeks, and Ruppel (1957, p. 51), which they termed augite lamprophyre mafic orthoclase-clinopyroxene gabbro, and porphyritic 

olivine-augite-anorthoclase, and by Smedes (1966, p. 51-53), termed 

augite-trachybasalt in composite intrusives and sills, dikes, and 

laccoliths in the Elkhorn Mountains Volcanics.

Member G contains many plant remains. Fossil wood resembling sycamore (Platanus) is very common and was identified 

by R. A. Scott (written commun., 1961) from several stratigraphic 

positions within the member. A few freshwater clams and the 

following spores and pollen were collected from the base of the 

member in sec. 9, T. 4 N., R. 5 E., and identified by R. H. Tschudy 

(written commun., 1962):

Proteacidites P3 -rt 1A

Tricolpate pollen

Trilete fern spores TT-Sm 4?

Monolete fern spores M-Sm 3 M-Sm 4

Zlivisporis VOT-rt 1

Rugubivesiculites VOT-rugl? fragment

Tschudy concluded that the assemblage is of Late cretaceous age. 

Lithologic correlatives occur in the mudstone member of the 

Sedan Formation.

MEMBER H

Member H is a ledge of boulder and cobble conglomerate 38+ m 

(125+ ft) thick, which forms Table Mountain, the synclinal center of 

the Maudlow structural basin (fig. 8). The coarse volcanic conglomerate contains abundant cobbles and boulders as much as 2 m 

(6 ft) in diameter, of various altered volcanic rocks of intermediate 

composition, including distinctive coarsely porphyritic pyroxene 

andesite that is not recognized lower in the sequence. Boulders of 

this distinctive lithology also have been recognized in conglomerate lenses in the middle part of the mudstone member of the 

Sedan Formation and are used here for lithologic correlation of 

these two horizons.
B42 CONTRIBUTIONS TO STRATIGRAPHY

Fragments in the conglomerate range from subangular to wellrounded and characteristically make up less than 50 percent of 

the rock. The matrix contains numerous brecciated crystals of 

ferromagnesian minerals, iron oxide minerals, two varieties of 

augite, and albitized and laumontized feldspars; the matrix is 

made up of the zeolite, laumontite, and plagioclase, and clay, as 

determined by X-ray (A. J. Gude 3d, written commun., 1961).

The conglomerate on Table Mountain is overlain by a thin layer 

of leaf-bearing fine-grained brownish-weathering sandstone and 

siltstone that forms the eroded top of the formation. The Oligocene 

Dunbar Creek Formation and Quaternary gravels unconformably 

overlap the Maudlow Formation in the southern part of the area.

AGE AND CORRELATION

Palynomorphs and leaves from the Maudlow Formation all 

indicate a Cretaceous age for the formation. Palynomorphs from 

member A are of latest Santonian or early Campanian age. 

Radiometric ages calculated on hornblende from dacite breccias of 

members B and F are 78.9±1 and 74.9+1 m.y., respectively (J. D. 

Obradovich, written commun., 1974).

The upper part of the formation is dated through the lithologic 

correlation of member H with lenses of similar volcanic conglomerate in the middle part of the mudstone member of the Sedan 

Formation below the dated Lennep Sandstone Member. Thus, the 

maximum age range of the Maudlow Formation is similar to that 

of the Sedan Formation latest Santonian to Campanian 

though the present eroded outcrop of the Maudlow Formation does 

not include beds as young as those of the upper part of the Sedan 

Formation.

The igneous rocks of members B, D, F, and H all have stratigraphic counterparts in the Sedan Formation farther east and 

have been used to make the correlations shown in figure 4.

RELATIONS TO ELKHORN MOUNTAINS VOLCANICS

The Maudlow and Sedan Formations probably are time equivalents of the Elkhorn Mountains Volcanics. Radiometric ages 

determined on hornblende phenocrysts from the lower member of 

the Elkhorn Mountains Volcanics in its type locality have a mean 

of about 78 m.y. (Robinson and others, 1968; Roberts, 1972). The 

mean agrees well with the 78.9±1 m.y. radiometric age determined 

on hornblende from member B of the Maudlow Formation. The 

volcanic conglomerates and sandstones of member A and of the 

lowest part of the Sedan Formation, however, indicate that 

western volcanism of the Elkhorn Mountains type commenced
MAUDLOW AND SEDAN FORMATIONS, MONTANA B43

earlier, probably about 82-83 m.y. ago, as suggested by the 

palynomorph assemblages at the base of these eastern assemblages. The upper part of the Elkhorn Mountains Volcanics has 

been recognized as being the time equivalent of the Judith River 

Formation (Smedes, 1966, p. 27; Klepper and others, 1957, p. 38). 

Robinson, Klepper, and Obradovich (1968, p. 574) extrapolated this 

correlation to an estimated age of 72-73 m.y., and Roberts (1972, p. 

C6) indicated a possible cessation of volcanism at about 74 m.y. 

The new radiometric ages of Obradovich and Cobban (1975) 

suggest that the Baculites cuneatus zone (fig. 4) is about 70 m.y. 

old. This zone is correlated with the top of the Lennep Sandstone 

Member of the Sedan Formation, which contains several tuff beds. 

Thus, it seems probable that the last eruptions of the Elkhorn 

Mountains volcanic field may have occurred about 70 m.y. ago, at 

the same time that the uppermost beds of the Sedan and Miner 

Creek Formations were deposited.

Active rhyodacitic volcanism in the Maudlow area at the eastern 

edge of the Elkhorn Mountains volcanic field is neatly bracketed 

by radiometric ages obtained from members B and F of the 

Maudlow Formation. These ages indicate that volcanism spanned 

approximately 5 m.y., from about 75 to 79 m.y. ago. The radiometric age obtained on member F (74.9±1 m.y.) falls right in the 

middle of the estimated cooling time of the Butte Quartz Monzonite 

( ~ 76-74 m.y. ago; Tilling and others, 1968; Tilling, 1974) of the 

Boulder batholith, suggesting that at least along the eastern 

margin of the Elkhorn Mountains volcanic field, volcanism was 

contemporaneous with the emplacement of that volumetrically 

major part of the batholith. Tilling (1974, p. 1927) suggested that 

major volcanism ceased before the emplacement of the Butte mass 

and that radiometric ages obtained on adjacent volcanics may 

have been affected by reheating. The volcanic rocks of the 

Maudlow Formation are too distant from the Boulder batholith to 

have been reheated by it.

The original thickness of the Elkhorn Mountains Volcanics 

"may well have been at least 15,000 feet" (Smedes, 1966, p. 24). 

Near Maudlow, the volcanic rocks of this age have a thickness in 

excess of 1,525 m (5,000 ft). Near Wilsall, 32 km (20 mi) east of the 

Maudlow area, 1,075 m (3,528 ft) of section is present. At Livingston, 48 km (30 mi) to the southeast, and near Loweth, 40 km (25 mi) 

to the northeast, thicknesses are 884 m (2,900 ft) and 777 m (2,550 

ft), respectively. The wedging out of coarse volcanic detritus 

continues to the east across the Crazy Mountains basin (Roberts, 

1972, p. C39).

Calc-alkalic rocks dominated by dacite-rhyodacite flows and
B44 CONTRIBUTIONS TO STRATIGRAPHY

tuffs characterize the volcanic facies of the Elkhorn Mountains 

volcanic field and the Livingston Group in the Maudlow area. The 

variety of rock types present in the Elkhorn Mountains field, 

however, is not found in the Maudlow sequence. No basalt is 

present, and the extensive rhyolite ash flows of the middle member 

of the Elkhorn Mountains Volcanics are not represented. The 

Maudlow sequence is primarily rhyodacite with related andesite 

and dacite. The shoshonite sill in member G is the only departure 

from the rather uniform assemblage. The dacitic welded tuff of the 

Maudlow Formation is compositionally like welded tuffs present in 

the lower unit of the Elkhorn Mountains Volcanics and may 

indicate that those tuffs were originally very extensive areally. 

The Elkhorn Mountains Volcanics and the formations that 

compose the lower part of the Livingston Group are eroded parts of 

an extensive volcanic pile, which may have covered as much as 

26,000 km2 (10,000 mi2) of southwestern Montana (Smedes, 1966, p. 

21). In a general way, the rocks named the Elkhorn Mountains 

Volcanics are the vent facies of the pile the Maudlow Formation, 

a remnant of the coarse alluvial facies; and the Sedan, Cokedale, 

and Miner Creek Formations, the fine alluvial facies.

MEASURED SECTIONS

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members

[The six members of the Sedan were measured at the following localities in T. 4 N., R. 8 E.: Lennep Sandstone 

Member in the SV^SWVi sec. 17; Bearpaw Shale Member and mudstone member in the SWV4 sec. 17 and 

SEW sec. 18; middle sandstone member in the SEW sec. 18 and SVz sec. 7; welded tuff member in the S1A sec. 7; 

lower sandstone member in the SV^NWW sec. 6J

Thickness 

(meters) (feet)

Billman Creek Formation: 

Lower part:

83. Mudstone, gray, grayish-green, and grayish-red, 

largely calcareous; interbedded with gray sandstone, siltstone, and brown-weathering limestone 

nodules; contains fossil dinosaur bones and mollusks (Skipp and McGrew, 1972, p. 104).................... 216+ 708+

Gradational contact. 

Sedan Formation:

Lennep Sandstone Member:

82. Sandstone, volcanic, and crystal-lithic tuff, mediumlight-gray, greenish-gray, and olive-gray, ironstained, medium-grained to conglomeratic, rudely 

bedded; contains dark-brown-weathering calcareous nodules, fossil conifer wood, abundant carbonaceous material, and hexagonal columnal structures in tuffaceous sandstone ................... 5.3 17.4
MAUDLOW AND SEDAN FORMATIONS, MONTANA B45

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued Thickness 

Lennep Sandstone Member Continued (meters) (feet)

81. Sandstone, volcanic, light-olive-gray, mediumgreenish-gray, and medium-olive-gray, finegrained to conglomeratic, thin-bedded, slabby; 

some rude crossbedding; dark-purple, brownweathering friable cannonballs common; lightcolored breccia near top; greenish-gray and 

grayish-purple pebble conglomerate at base; contains intermediate volcanic cobbles as much as 

8-10 cm (3-4 in.) in diameter; much zeolite cement; forms low-angle slope .................... 6.8 22.4

80. Sandstone, volcanic, and crystal-lithic tuff, lightolive-gray to olive-gray and grayish-orange, mottled; sandstone is mostly fine grained with some 

mud-pellet conglomerate, crossbedded and slabby 

with 1.3-cm (0.5-in.) irregular slabs in upper part, 

poorly bedded in lower part; medium-dark-grayweathering calcareous cannonball nodules common throughout; some carbonized wood nuclei; 

abundant carbon; mottled, altered, zeolitic (laumontite), crystal-lithic tuff bed at top........................ 7.3 24.0

79. Sandstone, volcanic, pale-orange to olive-gray,

some mottled, iron-stained, very fine grained to 

fine-grained, poorly bedded, some thin-bedded, 

mostly noncalcareous; most weathers to moderate 

brown; abundant carbonaceous material throughout........................................................................................ 13.6 44.4

78. Partly covered interval; subcrop is very fine grained 

volcanic sandstone and mudstone, light- to darkolive-gray, noncalcareous, massive with spheroidal weathering; weathers brown; forms slope .... 10.0 32.7

77. Partly covered interval; subcrop is light- to darkolive-gray fine- to medium-grained, mostly very 

poorly bedded, generally noncalcareous brownweathering volcanic sandstone; a few mudstone 

interbeds; much carbonaceous plant material; 

a few irregular ledges of light-colored thin-bedded 

sandstone are exposed, otherwise unit forms gentle slope.............................................................................. 20.4 67.0

76. Sandstone, volcanic, greenish-gray to light- to darkolive-gray, iron-stained, mostly fine-grained, 

some medium-grained, very poorly bedded, some 

magnetite-rich; weathers moderate brown; some 

flat limy nodules; much spheroidal weathering; 

some very fine grained material breaks into small 

irregularly shaped chips; some carbonaceous 

material; forms broad top of ridge ............................ 25.0 82.0
B46 CONTRIBUTIONS TO STRATIGRAPHY

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued

Lennep Sandstone Member Continued

75. Mudstone, siliceous, and devitrified vitric tuff, darkgreenish-gray, olive-gray, and grayish-brown; 

subconchoidal fracture; blocky weathering; some 

fine-grained grayish-orange volcanic sandstone 

interbeds; 1.2 m (4 ft) of dark greenish gray 

medium- to coarse-grained volcanic sandstone at 

base; forms resistant ledge just below top of 

ridge ..................................................................................

Total Lennep Sandstone Member ..........................

Reverse fault along bedding plane; offset unknown, but

presumed small. 

Bearpaw Shale Member:

74. Mudstone, moderate to dark-greenish gray, sandy, 

massive, siliceous; probably devitrified vitriccrystal and vitric-lithic tuff in part; 0.6 m (2 ft) 

of chocolate colored bentonite 4.6 m (15 ft) above 

base; lower 1.5 m (5 ft) pale olive gray fine- to 

medium-grained calcareous volcanic sandstone, 

banded by heavy mineral concentrations along 

bedding ..............................................................................

73. Mudstone, olive-gray, greenish-gray and brownishgray, siliceous; weathers into small polygons; 

thin bentonitic claystone in center; silty in lower 

part......................................................................................

72. Sandstone, grayish-green, medium- to coarsegrained and conglomeratic, crossbedded, slightly 

calcareous; medium bedded in beds 1-46 cm (0.5- 

18 in.) thick; graded bedding common; some mottling; nearly euhedral feldspar crystals abundant; 

some reworked crystal tuff; forms ledge ..................

71. Mostly covered interval; siliceous claystone and 

(or) devitrified tuff and sandstone in float; forms 

moderate slope beneath unit 72 ..................................

70. Mostly covered interval; green iron-stained carbonaceous nodular-weathering sandstone exposed on 

hillside; contains a few mudstone interbeds and 

bentonitic clay near base...... ......- ..... ...................

69. Mudstone, brownish-gray, siliceous, zeolitic (clinoptilolitic); 31-46 cm (12-18 in.) thick greenishgray fine-grained volcanic sandstone exposed 

at base ................................................................................

Total Bearpaw Shale Member ................................

Mudstone member:

68. Partly covered interval; devitrified vitric tuff, lightgreenish-gray; bentonitic soil zones prominent; 

grades northward into altered vitric tuff ridge; 

grayish-red-purple mudstone with pale-pink zeolite (laumontite) veins exposed at base ....................

Thickness 

(meters) (feet)

18.6 61.0

107.0 350.9

11.7 38.5

10.2 33.5

8.5 28.0

24.1 79.0

20.7 68.0

6.0 19.5

81.2 266.5

10.1 33.0
MAUDLOW AND SEDAN FORMATIONS, MONTANA B47

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued Thickness 

Mudstone member Continued (meters) (feet)

67. Sandstone and mudstone, volcanic; greenish-gray 

fine-grained massive upper sandstone has hackly 

surface; underlain by olive-gray mudstone and 

sandstone containing limestone nodules 0.9-1.2 m 

(3-4 ft) long which weather reddish-brown and 

contain carbonaceous plant debris; basal 4.6 m 

(15 ft) is greenish-gray medium-grained massive 

calcareous sandstone, which contains numerous 

veins of calcium carbonate and forms a ledge ........ 17.5 57.5

66. Mudstone, volcanic, greenish-gray and brownishgray, siliceous, massive, noncalcareous; contains 

abundant clinoptilolite and calcium carbonate 

veins; basal 3 m (10 ft) is greenish-gray massive 

volcanic sandstone, which forms a ledge ................ 9.5 31.0

65. Partly covered interval; mudstone with minor sandstone, volcanic, olive-gray and brown-gray, siliceous; abundant zeolite and calcium carbonate 

in subcrop.......................................................................... 6.2 20.5

64. Mudstone and siltsone, volcanic; contain abundant 

flecks of orange clinoptilolite and mordenite; lightbrownish-gray soft sandy mudstone at top; thin 

dark-brownish-gray soft bentonitic clay in middle; 

basal 3 m (10 ft) is brownish-gray calcite-veined 

resistant volcanic mudstone and silty mudstone.. 8.2 27.0

63. Mudstone, volcanic, brownish-gray; interbedded 

with olive-gray volcanic sandstone; fine grained, 

massive, siliceous near top; contains abundant 

flecks of orange clinoptilolite; calcium carbonate 

in veins; grayish-green and dark-reddish-brown 

altered zeolitic (stilbite and laumontite) tuff in 

subcrop near top ............................................................ 8.8 29.0

62. Sandstone, volcanic, light-greenish-gray to olivegray, fine- to medium-grained, partly conglomeratic; thin veins of zeolite (stilbite); some mudstone interbeds near top; sandstone beds form 

resistant ridges between mudstone outcrops.......... 5.2 17.0

61. Mudstone, volcanic, olive-gray, partly sandy and 

silty, massive, mostly siliceous; weathers into 

small polygons; abundant orange clinoptilolite; 

some stilbite, mordenite(?), and calcite veins; interbedded with olive-gray fine-grained, argillaceous volcanic sandstone; olive-gray mediumgrained massive friable calcareous volcanic sandstone 0.9 m (3 ft) thick in lower part............................ 23.8 78.0

60. Mostly covered interval; subcrop is brownish-gray 

soft chocolate-like mudstone, interbedded with 

sandy or silty mudstone; soft mudstone may be 

bentonitic; flecks of orange zeolite common; calcite veins present in upper, more silty and si-
B48 CONTRIBUTIONS TO STRATIGRAPHY

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued Thickness 

Mudstone member Continued (meters) (feet) 

60. Mostly covered interval, etc. Continued

liceous beds; grayish-green medium grained calcareous volcanic sandstone 0.6 m (2 ft) thick 

at base................................................................................ 7.8 25.5

59. Crystal tuff, greenish-gray, fine- to medium-grained 

calcareous, 0.9 m (3 ft) thick with conspicuous 

greenish-black biotite; underlain by brownish-gray 

mudstone, and pinkish-gray and yellowish-gray 

soft slope-forming vitric tuff and bentonite; 

grayish-green medium-grained crossbedded calcareous volcanic sandstone, with zeolites on bedding surfaces, forms basal 0.9 m (3 ft); unit forms 

ridge and is a persistent marker bed in the northern part of Wallrock and in Sixteen NE and Ringling quadrangles ............................................................ 6.1 20.0

58. Mudstone and siltstone, dark-greenish-gray, 

massive; with numerous thin zeolite (stilbite) 

veins............................................................................ 6.9 22.8

Fault (dip changes from 45° east above to 90° below)

57. Partly covered interval in upper part; mudstone and 

fine-grained sandstone, volcanic, olive-gray; 

some carbonaceous material; lower part well exposed dark-olive-gray and dark-yellowish-brown 

fine- to medium-grained to conglomeratic poorly 

bedded noncalcareous volcanic sandstone; much 

iron staining with abundant slickensides and calcite veins in lower part .................................................. 10.3 33.7

56. Sandstone, volcanic, medium-olive-gray and 

greenish-gray, medium-grained to conglomeratic, 

thin- to medium- bedded; disseminated clinoptilolite; weathers to light brownish gray; graded 

bedding with some crossbedding; some calcite 

and zeolite (stilbite) veins; forms ridge .................... 2.6 8.4

55. Partly covered interval; sandstone, volcanic, darkgreenish-gray and olive-gray, medium-grained, 

poorly bedded, friable, partly calcareous; in subcrop and some outcrops; flecks of orange clinoptilolite common; a few conglomeratic lenses; some 

carbonaceous plant material; forms slope .............. 13.0 42.8

54. Mudstone, dusky-yellow-green and light-olive-gray, 

noncalcareous; contains abundant calcium carbonate veins and some brownish-gray beds with 

flecks of orange zeolite; interbedded with 

greenish-gray medium-grained to conglomeratic 

thin-bedded volcanic sandstone; a sandstone bed 

about 3.7 m (12 ft) thick is present in middle 

of unit ................................................................................ 15.2 50.0
MAUDLOW AND SEDAN FORMATIONS, MONTANA B49

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued Thickness 

Mudstone member Continued (meters) (feet)

53. Mudstone, varicolored, brownish-gray, greenishgray, grayish-red and yellowish-gray; zeolite (stilbite) zone at top; thick olive mudstone and friable 

sandstone in middle; olive-gray medium-grained 

crystal rich sandstone, 10-15 cm (4-6 in.) thick 

at base................................................................................ 4.9 16.0

52. Partly covered interval; mudstone, greenish, gray 

some siliceous, some bentonitic; specks of orange 

zeolite (clinoptilolite) common, with minor 

grayish-green and olive-gray fine-grained thinbedded crossbedded volcanic sandstone and siltstone; a thin devitrified vitric bed is present near 

the top; sandstone is more abundant in the upper 

half; a thin bentonite bed is present near the 

middle of the unit............................................................ 14.8 48.5

51. Mudstone, dark-olive-gray, massive; abundant 

orange clinoptilolite, with a thin poorly exposed 

bentonite bed at top; underlain by greenish-gray 

medium-grained punky massive volcanic sandstone crossbedded with a thin medium-light gray 

coarse-grained to conglomeratic pink-weathering 

sandstone,8-10cm(3-4in.)thickatbase................ 7.8 25.5

50. Partly covered interval; 3.0 m (10 ft) of gray and 

dark-grayish-green silty siliceous volcanic mudstone, rich in accessory orange zeolite, exposed 

at top; mudstone float throughout remainder of 

unit;forms slope.............................................................. 14.2 46.5

Total mudstone member .......................................... 192.9 632.7

Middle sandstone member:

49. Sandstone, volcanic, grayish-green and lightbrown, medium- to coarse-grained, partly conglomeratic, thin-bedded and crossbedded, noncalcareous, feldspathic; grayish-green beds common in middle of unit; weathers mostly pale 

brown; forms ridge.......................................................... 19.2 63.0

48. Partly covered interval; alternating sandstone 

ridges and mudstone intervals exposed to north 

of measured section; sandstone is volcanic, lightolive-gray, fine- to medium-grained, and weathers 

with characteristic yellowish-gray rind; finegrained hard light-orange-pink-weathering sandstone, 20 cm (8 in.) thick, forms small ridge 

at base................................................................................ 19.2 63.0

47. Covered interval; devitrified vitric tuff, white and

yellowish-gray; in float ................................................ 1.1 3.8

46. Conglomerate, grayish-green; volcanic pebbles and
B50 CONTRIBUTIONS TO STRATIGRAPHY

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued Thickness 

Middle sandstone member Continued (meters) (feet) 

46. Conglomerate, etc. Continued

cobbles, most 2.5 cm (1 in.) but some 20-25 cm

(8-10 in.) in diameter, subrounded to well-rounded,

consisting of welded tuff, rhyodacitic to andesitic

flows, and scoriaceous material; gravel is densely

packed in crossbedded sandstone matrix with

abundant orange zeolite; lenses into sandstone to

the north and south; dinosaur bone fragment in

float; forms ridge ............................................................ 10.8 35.5

45. Partly covered interval; sandstone, greenish-gray 

to olive-gray, medium-grained to conglomeratic, 

thin-bedded and crossbedded; may be some conglomerate like that of unit 46 ...................................... 19.5 64.0

Break in section: offset to SVz SE sec. 7, T. 4 N., R. 8 E. 

Mudflow conglomerate unit:

44. Mudflow conglomerate, dark-greenish-gray 

and grayish-red-purple; a few sandy ledges; 

boulders of hornblende-rich dacitic volcanic 

rocks as much as 0.3 m (1 ft) in diameter, 

set in a mudstone matrix; abundant orange 

zeolite and calcium carbonate, some of which 

is brecciated; forms steep slope .......................... 23.1 76.0

43. Mudflow conglomerate, greenish-gray, brownish-gray, and yellowish-gray; intermediate 

volcanic rock fragments, generally less than 

2.5 cm (1 in.) in diameter, some of hornblende dacite up to 0.3 m (1 ft) in diameter 

set in a mudstone matrix; some sandy conglomerate; some coarse-grained sandstone; 

one or two thin light-grayish-red-purple to 

grayish-green and gray crossbedded sandstone beds; mudstone contains abundant 

orange zeolite (clinoptilolite), calcite veins, 

and some chalcedonized wood; well exposed; 

forms broad top of ridge ........................................ 18.3 60.0

42. Sandstone, volcanic, light-olive-gray to 

yellowish-brown, fine-grained to conglomeratic, medium-bedded; crossbedded with heavy 

mineral concentrations along bedding 

planes; lenticular; interbedded with a few 

brownish-gray mudstone beds; forms ledge.... 6.5 21.5

Disconformity

41. Mudflow conglomerate, olive-gray; weathers 

brown; pebbles and cobbles of various intermediate volcanic rocks in upper part; upper 

0.6-0.9 m (2-3 ft) contain sub-rounded boulders up to 15.2 cm (6 in.) in diameter in a
MAUDLOW AND SEDAN FORMATIONS, MONTANA B51

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued Thickness 

Middle sandstone member Continued (meters) (feet) 

Mudflow conglomerate unit Continued

41. Mudflow conglomerate, etc. Continued

fine-grained, green mud matrix; orange zeolite (clinoptilolite) common; large calcite 

veins; unit grades into gray-green mudstone 

at base; forms base of resistant cliff; units 

44 through 41 resemble member F of the 

Maudlow Formation, (p. B39, this report)........ 6.1 20.0

Total mudflow conglomerate unit.................. 54.0 177.5

40. Mudstone and siltstone, olive-gray, structureless, 

siliceous; weathers brown; abundant orange zeolite and calcite veins; thin olive-gray fine-grained 

volcanic sandstone, laminated and crossbedded 

at top; forms slope...................................................... 20.4 67.0

39. Mudstone and siltstone, olive gray, noncalcareous; 

abundant calcite veins; medium-dark-gray whitespotted siliceous resistant mudstone, 1.2 m (4 ft) 

thick at top, forms ledge; olive-gray fine-grained, 

friable thin-bedded laminated volcanic sandstone 

at base; forms slope.................................................... 10.7 35.0

38. Mudstone and siltstone, olive-gray to olive-black, 

siliceous; interbedded with olive-gray finegrained thin-bedded nodular-weathering volcanic 

sandstone; crossbedded in places; some laminae; 

veins of zeolite (mordenite and clinoptilolite) common; abundant fossil wood and calcium carbonate in float; forms resistant slope............................. 14.2 46.5

37. Mudstone and siltstone, greenish-gray to olive-gray, 

siliceous; interbedded with olive-gray fineto medium-grained volcanic sandstone in beds 

about 5 cm (2 in.) thick; abundant fossil wood and 

carbonaceous material; forms resistant slope........ 5.7 18.6

36. Covered interval; creek bed-float is olive-gray

blocky-weathering siltstone and mudstone............ 25.9 85.0

35. Mostly covered interval; mudstone, olive-gray, interbedded with light-olive-gray fine-grained volcanic sandstone in lenticular beds in the lower 

part.............................................................................. 18.6 61.0

34. Sandstone, volcanic, light-greenish-gray to lightolive-gray, fine-grained, thin-bedded, crossbedded, calcareous, partly quartz-rich; contains 

zones of brown-weathering cannonball-type nodules and abundant calcium carbonate veins; 

forms small ridge which disappears to the north 

but persists to the south............................................. 18.6 61.0
B52 CONTRIBUTIONS TO STRATIGRAPHY

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued Thickness 

Middle sandstone member Continued (meters) (feet)

33. Mostly covered interval; predominantly finegrained mudstone, tuff and sandstone; pale-olive 

mudstone well exposed at base................................. 15.2 50.0

32. Mudstone and siltstone, grayish-olive, massive, calcareous; wood and carbonaceous plant debris in 

upper part; middle part is olive-gray fine-grained 

calcareous thin-bedded volcanic sandstone; lower 

part is olive-gray and brownish-gray massively 

bedded calcareous sandstone; forms slope.............. 15.2 50.0

31. Mostly covered interval; some olive-gray mudstone 

outcrop with olive-gray fine-grained volcanic 

sandstone lenses; about 6 m (20 ft) of well-exposed 

brown mudstone with concentric weathering patterns at base............................................................... 27.1 89.0

30. Sandstone, volcanic, dark-greenish-gray and lightolive-gray, very fine- to fine-grained, thin-bedded; 

breaks into irregular slabs; forms small ridge........ 17.1 56.0

29. Partly covered interval; float is greenish-gray and 

olive-gray, fine-grained calcareous volcanic sandstone; outcrop in center of interval is greenishgray and olive-gray medium- to coarse-grained 

crossbedded thin-bedded calcareous volcanic 

sandstone; forms slope.............................................. 24.4 80.0

28. Mostly covered interval; upper 3 m (10 ft) consists 

of sandstone and volcanic pebble conglomerate; 

sandstone is volcanic, greenish-gray to light olive 

gray, fine to medium grained, calcareous, and contains much organic debris; subcrop of remainder 

of unit is volcanic sandstone and mudstone........... 18.9 62.0

27. Mostly covered interval; upper 1.5 m (5 ft) well 

exposed devitrified tuff and mostly fine grained 

tuffaceous sandstone; both are light to dark olive 

gray, thick bedded, and have abundant organic 

debris; sandstone debris on remainder of slope...... 18.3 60.0

26. Sandstone, volcanic, olive-gray, conglomeratic, 

massive; contains numerous clay pebbles and 

subhedral plagioclase crystals; orange zeolite in 

irregular pods; matrix is mudstone in part, probably reworked crystal tuff; 0.6 m (2 ft) of grayishgreen thin-bedded sandstone at base............ ........ 3.0 10.0

25. Mostly covered interval; yellowish-gray volcanic 

mudstone exposed near top; weathers dark 

yellowish brown; olive-gray fine-grained volcanic
MAUDLOW AND SEDAN FORMATIONS, MONTANA B53

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued Thickness 

Middle sandstone member Continued (meters) (feet) 

25. Mostly covered interval, etc. Continued

sandstone with much carbonaceous plant material makes up subcrop on remainder of slope.......... 22.3 73.0

24. Sandstone, volcanic, grayish-olive and light-olivegray, fine-grained, thin-bedded, crossbedded, noncalcareous, resistant; interbedded with minor 

dark mudstone; 0.6-7.6 cm (0.25-3 in.) bedding 

outlined by heavy minerals including magnetite; 

forms ridge with unit 22............................................. 10.1 33.0

23. Sandstone and mudstone, volcanic, grayish-olive 

and light-olive-gray; fine-grained lenticular sandstone; light-olive-gray zeolite-rich (stilbite, laumontite and clinoptilolite) carbonaceous siltstone 

near base; unit forms shallow valley between 

units 24 and 22............................................................ 14.4 47.1

22. Sandstone, volcanic, olive-green and olive-gray, 

very fine to medium-grained, thin-bedded, crossbedded, very carbonaceous, zeolitic, noncalcareous; forms ridge with unit 24 ................................. 8.7 28.5

21. Siltstone, very fine grained sandstone and mudstone, grayish-green and olive-gray; contains abundant silicified wood; medium-dark-gray conglomeratic sandstone breccia, weathers with a 

very characteristic grayish-orange-pink rind; bed 

0.3-0.6 m (1-2 ft) thick at base forms ledge................ 15.5 51.0

20. Covered interval; float consists of grayish-green 

soft fine-grained sandstone, siltstone, and mudstone; forms saddle.................................................. 17.4 57.0

19. Sandstone, volcanic, medium-light-gray to olivegray, fine-grained, thin-bedded, crossbedded, calcareous, slightly biotitic; ledge forming in upper 

3.7 m (12 ft); lower 4.3 m (14 ft) mostly covered; 

mudstone and very fine grained sandstone in subcrop; both are volcanic, dark greenish gray, and 

siliceous (Parkman Sandstone equivalent)............ 8.2 26.8

18. Sandstone, medium-light-gray to light-olive-gray, 

mottled, fine-grained, crossbedded, contortedbedded, thin-bedded, friable, calcareous, biotitic; 

lenses of grayish-olive-green brown weathering 

siliceous mudstone; forms ledge (Parkman Sandstone equivalent)........................................................ 12.5 41.0

17. Mudstone, medium-dark-gray, siliceous; twig 

and leaf imprints (Parkman Sandstone 

equivalent).................................................................. 4.8 15.8

Total middle sandstone member.......................... 491.0 1,611.1
B54 CONTRIBUTIONS TO STRATIGRAPHY

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued 

Welded tuff member

16. Mostly covered interval; a few outcrops of welded 

tuff and pale-yellowish-green and pale-reddishbrown welded tuff, with abundant biotite in upper 

part; interbedded dark-greenish-gray and olivegray mudstone and fine-grained volcanic sandstone in lower part (Parkman Sandstone 

equivalent)..................................................................

15. Sandstone, light-gray, fine-grained, crossbedded, 

friable, calcareous, biotitic; hard greenish-gray 

limy sandstone lenses and layers; upper part is 

very fine- to fine-grained, has contorted bedding 

in thin layers, and is noncalcareous in part; forms 

ledges (Parkman Sandstone equivalent)................

14. Mostly covered interval; greenish-gray sandstone 

and conglomeratic sandstone; weathers orange at 

top; middle part is greenish-gray siliceous mudstone in subcrop; light- to medium-gray finegrained quartzose calcareous sandstone at base 

(Parkman Sandstone equivalent)..........................

Total Parkman Sandstone equivalent.................

13. Mostly covered interval; light- to dark-gray siliceous mudstone, with carbon markings; weathers 

pinkish gray and light olive gray at top; lightgreenish-gray, light-gray biotitic welded tuff in 

subcrop throughout and in outcrop at base; 

greenish-gray fine-grained calcareous volcanic 

sandstone common in interval to north...................

Total welded tuff member......................................

Break in section: offset to S1/2NW1/4 sec. 6, T. 4 N., R. 8 E. 

Lower sandstone member:

12. Partly covered interval; light olive-gray to grayisholive fine- to medium-grained noncalcareous volcanic sandstone in upper part just below welded 

tuff member; grayish olive siltstone in lower part; 

forms slope..................................................................

11. Sandstone, volcanic, olive-gray, fine- to mediumgrained, thin-bedded, calcareous; contains some 

brown-weathering elongate calcareous concretions about 0.6 m (2 ft) high and 2 m (6 ft) long in 

upper 7.5 m (24.6 ft); underlain by 0.9 m (3 ft) of 

dark gray siliceous mudstone containing abundant carbonaceous material and 3 m (10 ft) of 

greenish-gray medium-grained to conglomeratic

Thickness 

(meters) (feet)

21.6 70.9

3.9 12.9

7.9

(58.9) (203.2)

14.4 47.2

47.8 156.8

7.0 22.9
MAUDLOW AND SEDAN FORMATIONS, MONTANA B55

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued Thickness 

Lower sandstone member Continued (meters) (feet) 

11. Sandstone, volcanic, etc. Continued

massive slightly calcareous volcanic sandstone,

crossbedded and thin-bedded in basal part............. 11.5 37.6

10. Sandstone, light-olive-gray, olive-gray, and yellowish-gray, fine-grained, thin-bedded, crossbedded; 

imprints of leaves, needles, wood, and carbon 

films abundant; forms ridge..................................... 6.9 22.5

9. Partly covered interval; olive-gray and greenishgray fine- to medium-grained thin-bedded crossbedded noncalcareous to slightly calcareous volcanic sandstone; beds up to 10 cm (4 in.) thick; 

abundant organic material, leaves, wood, and so 

forth; forms steep slope below ridge......................... 21.9 72.0

8. Covered interval; slope covered by debris from units 

9, 10, and 11; probably interbedded mediumgrained volcanic sandstone and mudstone; forms 

steep slope................................................................... 18.6 61.1

7. Mudstone, olive-gray; grayish-black ironstone 

lenses in upper part; yellowish-orange and dusky 

yellow medium- to coarse-grained calcareous feldspathic volcanic sandstone cross-bedded in lower 

1.5m (5ft).................................................................... 4.9 16.2

6. Mudstone, siltstone, and sandstone, volcanic, darkgreenish-gray and light-olive-gray; has minor 

very-dark-brown to grayish-black-weathering 

ironstone nodules throughout; forms slope............. 12.0 39.2

5. Partly covered interval; volcanic, olive-gray, darkgray, olive-green, calcareous and siliceous; sandstone and siltstone have minor mudstone; a few 

grayish-black-weathering ironstone nodules; 

sandstone is mostly fine grained, some mediumgrained; upper 0.6 m (2 ft) is medium-grained darkgreenish-gray magnetite-rich sandstone which 

forms low ledge; whole unit forms slope ................ 15.2 50.0

4. Sandstone, dark-greenish-gray, medium- to coarsegrained, partly conglomeratic; some interbedded 

mudstone; 2-3-m (7-10-ft) resistant ledge at top is 

olive-gray crossbedded very calcareous and magnetite-rich sandstone; interval of greenish-gray 

mudstone next below forms rubbly slope; olivegray-weathering medium- to coarse-grained calcareous volcanic sandstone forms ledge near middle; lower part consists of a series of ledges of 

greenish-gray-weathering medium-grained to
B56 CONTRIBUTIONS TO STRATIGRAPHY

Type section of the Sedan Formation and reference sections of the Lennep 

Sandstone and Bearpaw Shale Members Continued

Sedan Formation Continued

Lower sandstone member Continued

4. Sandstone, etc. Continued

conglomeratic crossbedded calcareous volcanic 

sandstone. . ........................ ...................... ..................

3. Sandstone, olive-gray and dark-greenish-gray, 

medium- to coarse-grained, crossbedded, thin- to 

medium-bedded, feldspathic, calcareous, partly 

magnetite-rich; has some friable lenses; concentric weathering pattterns; forms series of ledges 

on steep slope; greenish-gray mudstone, with 

lenses of dark-greenish-gray medium-grained 

magnetite-rich sandstone, forms rubble-covered 

slope of upper 9 m (30 ft); lower 6 m (20 ft) is 

volcanic sandstone, which contains some brownweathering elongate limy nodules near top and 

forms ledge; imprints of leaves, needles, and 

other carbonaceous films abundant throughout 

interval..............................................................................

2. Mostly covered interval; upper part is steep slope 

covered with volcanic sandstone and conglomeratic sandstone rubble containing some organic 

debris; lower 0.6 m(2 ft) is grayish-olive mediumto coarse-grained crossbedded thin-bedded feldspathic noncalcareous volcanic sandstone ............

1. Mostly covered interval; light-olive-gray fine- to 

medium-grained friable feldspathic volcanic 

sandstone near middle; 0.6 m (2 ft) of lignitic 

coal (pollen and spore colln. D4670) overlain by 

light-gray fine- to medium-grained soft volcanic 

sandstone; weathers orange at base..........................

Total lower sandstone member ..............................

Thickness 

(meters) (feet)

21.5 70.5

15.2 50.0

15.8 52.0

5.0 16.3

155.5 510.3

Total Sedan Formation ........................................ 1,075.4 3,528.3

Conformable contact

Eagle Sandstone upper unnamed member...................................... 81.4 267.0

Type section of the Maudlow Formation

[The eight members of the Maudlow were measured at the following localities in T. 4 N., R. 5 E. (except 

member B): (H) northeast side of Table Mountain NWW sec. 16; (G) NV4 sec. 16; (F) NWW sec. 22 and 

SWW sec. 15; (E) SV4 sec. 10; (D) NEW sec. 15 and SEW sec. 10; (C) center sec. 28; (B) center sec. 

13 (T. 4 N., R. 4 E.); (A) SEW sec. 26]

Maudlow Formation: Thickness

Member H: (meters) (feet)

2. Siltstone, fine-grained sandstone, and claystone, 

olive-gray, thin-bedded, calcareous; many leaf 

impressions ...................................................................... 7.6+ 25+

1. Conglomerate and breccia, volcanic, medium-darkto light-gray; angular to subrounded fragments of 

assorted altered intermediate volcanic rocks,
MAUDLOW AND SEDAN FORMATIONS, MONTANA B57 

Type section of the Maudlow Formation Continued

Maudlow Formation Continued Thickness 

Member H Continued (meters) (feet) 

1. Conglomerate and breccia, etc. Continued

ranging from microscopic to 2 m (6 ft) in diameter, set in an abundant matrix (as much as 

50 percent) composed largely of massive zeolite 

(laumontite) and clay (montmorillonite); caps 

Table Mountain .............................................................. 30.5 100

Total member H .......................................................... 38.1+ 125+

Contact covered; probably disconformable 

Member G:

9. Claystone and some siltstone, volcanic, brownishyellow, poorly bedded; forms gen tie slope................ 73.5 241

8. Claystone, volcanic, olive-gray, light-brown, muchsilicified, partly calcareous; weathers to paleyellowish-brown and reddish-brown soil; conchoidal weathering habit; minor medium-grained 

olive-gray sandstone near base; much silicified 

sycamore wood................................................................ 66.8 219

7. Claystone, grayish-green, olive-gray, and lightbrown; subconchoidal fracture, breaks into little 

angular bits; many silicified wood and leaf imprints, largely sycamore; zeolites common; unit 

weathers into gen tie barren slope .............................. 26.5 87

6. Claystone, volcanic, dark-grayish-green, olive-gray, 

medium-gray, commonly siliceous; interbedded 

with a few ledges of siltstone; rare calcareous 

cannonball nodules with irregular rounded 

shapes; weathers to cubes and needles 0.6 cm0.3 m (0.25 in.-l ft) across; some indistinct bedding; minor silicified wood .......................................... 32.0 105

5. Claystone and siltstone, light-brown; breaks into 

small chips with subconchoidal fracture; secondary zeolites in fractures; forms slope ........................ 33.5 110

4. Sandstone, volcanic, olive-gray to yellowish-brown, 

fine- to coarse-grained, crossbedded; irregularly 

bedded in 5-15 cm (2-6 in.) beds; large ripple 

marks; much plant debris, pine needles, leaves 

carbonized wood; weathers dark yellowish brown; 

upper 23.7 m (78 ft) forms ledge. Lower 79.6 m 

(261 ft) forms partly covered slope of grayishgreen medium- to fine-grained massive unbedded 

volcanic sandstone, which weathers dark yellowish brown .......................................................................... 103.3 339

3. Sandstone, volcanic, dark-greenish-gray, olivegray, medium-grained; laminated and crossbedded with heavy mineral concentrations on bedding surfaces; forms ledge............................................ 7.9 26

2. Sandstone, volcanic, yellowish-brown, partly 

grayish-green, fine-grained to slightly conglomer-
B58 CONTRIBUTIONS TO STRATIGRAPHY

Type section of the Maudlow Formation Continued

Maudlow Formation Continued 

Member G Continued

2. Sandstone, volcanic, etc. Continued

atic; indistinct bedding; weathers yellowish 

brown; forms slope between units 1 and 3................

1. Sandstone, volcanic, light-olive-gray, mediumto coarse-grained; rude crossbedding; contains 

white-weathering feldspars; weathers light brown 

and pale yellowish brown; forms low ridge..............

Total member G ..........................................................

Base of section covered; steep slope marks contact; unit 7 of 

member F, represented by purple gray soil, may extend 3-6 m 

(10-20 ft) into unit 1 of member G. 

Member F:

7. Covered interval; float is grayish-red-purple very 

fine grained claystone ..................................................

6. Claystone, volcanic, grayish-red-purple, brownishgray, and grayish-blue-green; partly covered, like 

unit 7; much colluvium on surface of outcrop..........

5. Mudflow conglomerate, volcanic, largely vivid 

grayish-red-purple, greenish-gray, and grayishblue-green, massively bedded; boulders up to 0.6 m 

(2 ft) in diameter; much zeolitic material (mordenite and clinoptilolite); forms ledge ......................

4. Covered interval; float suggests conglomerate like 

unit3 ..................................................................................

3. Conglomeratic volcanic claystone with interbedded 

fine-grained sandstone and devitrified crystal 

vitric tuff, olive gray; boulders up to 0.3 m (1 ft) 

in diameter, both rounded and angular; not bedded; weathers dark brown; forms ledge; 1.5 m (5ft) 

of relief on disconformable surface between units 

2 and 3................................................................................

2. Tuff, crystal-lithic, medium-greenish-gray and 

grayish-red-purple; has brown limy layers and 

nodules up to 46 cm (18 in.) in diameter; bedding 

defined by color changes; fine-grained hackly 

surface; well exposed in streambed............................

1. Volcaniclastic conglomerate and breccia; boulders 

up to 1.5 m (5 ft) in diameter of grayish-redpurple, greenish-gray, and grayish-blue-green 

augite-hornblende dacite and rhyodacite; boulders mostly fresh, but some weathered; matrix 

is olive-gray and pale-yellowish-brown mediumgrained poorly bedded tuffaceous volcanic sandstone; zeolites common; some silicified conifer 

wood; forms massive cliff in places, low ridge in 

others; lower 6 m (20ft) poorly exposed ....................

Total member F ..........................................................

Disconformable abrupt contact

Thickness 

(meters) (feet)

20.1 66

35.7 117

399.3 1,310

42.3 139

34.4 113

10.7 35

10.7 35

13.1 43

18.6 61

62.5 205

192.3 631
MAUDLOW AND SEDAN FORMATIONS, MONTANA B59

Type section of the Maudlow Formation ^Continued 

Maudlow Formation Continued Thickness

Member E: (meters) (feet)

10. Sandstone, volcanic, and crystal-lithic tuff, olivegray, light-olive-gray, fine- to medium-grained, 

massively bedded, calcareous; sandstone conglomeratic in places; 3-m (lO-ft)-thick lens of 

mudflow conglomerate with porphyritic hornblende-augite rhyodacite boulders as much as 1.2 

m (4 ft) in diameter 4.6 m (15 ft) above base; 

silicified conifer wood and brown-weathering lime 

nodules common; forms slope . .. . .......... ... 17.7 58

9. Sandstone, olive-gray, yellowish-brown, tuffaceous, 

medium-grained to conglomeratic; contains volcanic pebbles as much as 0.6 cm (0.25 in.) in 

diameter; crossbedded in upper part; forms steep 

slope with ledges at top and bottom; partly 

covered .............................................................................. 38.4 126

8. Sandstone, volcanic, and tuff, grayish-green and 

light-olive-gray, fine- to medium-grained, poorly 

bedded, slightly calcareous; a few siliceous nodules; weathers olive gray, pale brown, and yellowish brown; forms slope .................................................. 24.1 79

7. Tuff and sandstone, volcanic; dark-yellowish-brown 

and medium-olive-gray tuff with interbedded 

medium-grained poorly bedded sandstone; largely covered; forms slope .................................................. 32.0 105

6. Sandstone, volcanic light-olive-gray, darkyellowish-brown, medium- to coarse-grained, calcareous; weathers olive gray; massively bedded 

with indistinct laminae; some crossbedding; interbedded with minor fine-grained sandstone; some 

conglomeratic sandstone at base; large lime nodules, as much as 1.2 m (4 ft) in diameter at 

top; locally forms cliff.................................................... 15.8 52

5. Sandstone, tuffaceous, gray, grayish-green, darkyellowish-brown, massively bedded; some 

weathers pale brown; calcareous veins common; 

limy nodules present in upper beds; forms low 

barren slope...................................................................... 48.1 158

4. Sandstone, tuffaceous, grayish-green and olivegray, indistinctly bedded, calcareous; brownweathering limy nodules as much as 2 m (6 ft) 

in largest dimension common; interbedded with 

minor tuffaceous light-brown calcareous sandstone with plant remains, which forms 1.5-m 

(S-ft)-thick ledge at base and 0.3 m (1 ft) ledge 

15m (50 ft) above base; unit forms low slope............ 26.5 87

3. Sandstone, volcanic, tuffaceous, grayish-green, 

fine-grained, massively bedded, slightly calcareous; contains large olive-gray-weathering limy 

nodules; forms slope ...................................................... 6.1 20
B60 CONTRIBUTIONS TO STRATIGRAPHY

Type section of the Maudlow Formation Continued

Maudlow Formation Continued Thickness 

Member E Continued (meters) (feet)

2. Sandstone, volcanic, light-olive-gray and grayishgreen; weathers olive gray; crossbedding emphasized by local concentrations of heavy minerals; 

forms ridge........................................................................ 4.9 16

1. Claystone, olive-gray, moderately to thin-bedded, 

siliceous; plant remains; interbedded with lightolive-gray conglomeratic volcanic sandstone; 

crossbedded with porphyritic dacite pebbles as 

much as 1.3 cm (0.5 in.) in diameter; forms poorly 

exposed slope.................................................................... 17.4 57

Total member E ...................................................... 231.0 758

Disconformable contact 

Member D:

4. Tuff, vitric, welded and unwelded, grayish-green 

(10GY5/2) and pale-reddish-brown (10R5/4); biotite and hornblende visible accessories; forms 

ridge.................................................................................... 97.5 320

3. Tuff arid tuffaceous sandstone, fine- to mediumgrained, massively to fine-bedded; interbedded 

with dark-gray and grayish-brown claystone, 

some fissile; forms slope---.--.-.---.-.----..--. 66.1 217

2. Conglomerate and conglomeratic sandstone, volcanic; largely welded tuff pebbles as much as 

10 cm (4 in.) in diameter in grayish-green or 

pale-red matrix, unconformably overlies unit 1 3.7 12

1. Tuff, vitric, welded, pale-yellowish-brown, grayishyellow-green, olive-gray, dark-gray; much accessory biotite; glassy perlitic zone 1.2 m (4 ft) 

thick occurs 1.5m (5 ft) from top of unit.................... 38.4 126

Total member D .......................................................... 205.7 675

Covered contact; probably disconformable

Member C:

10. Covered interval; may be sandstone like unit 7. 

Covered by float from biotitic welded tuff of 

member D.......................................................................... 10.7 35

9. Sandstone, volcanic, light- to medium-olive-gray, 

medium-bedded, calcareous; weathers light 

brown; beds as much as 46 cm (18 in.) thick; 

forms ledge........................................................... 3.0 10

8. Claystone, and devitrified vitric tuff, grayish-redpurple, massively bedded; cannonball concretions 

as much as 20 cm (8 in.) in diameter; weathers 

with suggestion of columnar structure .................... 4.9 16

7. Sandstone, volcanic, olive-gray, fine- to mediumgrained, crossbedded; hornblende concentrated 

on bedding surfaces; largely covered; forms saddle; no evidence of conglomerate ........................... 65.2 214
MAUDLOW AND SEDAN FORMATIONS, MONTANA B61

Type section of the Maudlow Formation Continued

Maudlow Formation Continued Thickness 

Member C Continued (meters) (feet)

6. Sandstone, tuffaceous, olive-gray, very fine grained 

to conglomeratic, medium- to thick-bedded; some 

cross laminations; calcareous in places; weathers 

light brown; 0.6 m (2 ft) interbedded with pyroxene-rich vitric crystal tuff; rare plant fragments; 

forms ridge........................................................................ 15.5 51

5. Sandstone, volcanic, gray, fine-grained, generally 

poorly bedded; small-scale crossbedding in places; 

forms slope........................................................................ 10.4 34

4. Sandstone, tuffaceous, olive-gray, fine-grained, friable; forms upper 14 m (45 ft) of unit; partly 

covered with yellowish-brown soil; lower beds are 

medium-gray and pale-reddish-brown finegrained to conglomeratic well-bedded volcanic 

sandstone; forms slope.................................................. 31.4 103

3. Sandstone, volcanic, light-gray or grayish-yellowgreen, medium- to coarse-grained, thick-bedded 

(up to 0.9 m (3 ft)), crossbedded; surfaces marked 

by concentrations of heavy minerals; much 

graded bedding; interbedded with volcanic conglomerate having andesitic boulders as large as 

15 cm (6 in.) in a tuffaceous matrix which is 

common in lower 15m (50 ft); forms ridge ................ 31.7 104

2. Sandstone, volcanic, dark-gray, conglomeratic, 

mostly massively bedded; feldspars and hornblendes visible on surface; weathers yellowish 

brown; some crossbedding .......................................... 7.9 26

1. Mostly covered; float suggests coarse-grained lithic 

tuff, volcanic sandstone, and pebble to boulder 

conglomerate with boulders as much as 15 cm 

(6 in.) in diameter, some from member B, rounded 

with weathered rinds; forms slope ............................ 61.0 200

Total member C .......................................................... 241.7 793

Disconformable contact

Member B:

7. Vitric crystal tuff, grayish-red-purple, grayish-red, 

dusky-red-purple, massive, pyroxene-rich; forms 

ridge.................................................................................... 32.6 107

6. Volcanic conglomerate and breccia in upper part, 

grayish-red-purple, massive, monolithologic; contains porphyritic hornblende-pyroxene dacite; 

conglomerate has subrounded boulders as large 

as 1.2 m (4 ft) in diameter; mostly covered; forms 

steep slope for upper 15 m (50 ft); lower part 

is moderate-red-purple banded tuffaceous volcanic sandstone; some crossbedding at base; 

forms gentle slope .......................................................... 59.7 196

5. Volcanic conglomerate and breccia, grayish-redpurple, massive, monolithologic; contains por-
B62 CONTRIBUTIONS TO STRATIGRAPHY

Type section of the Maudlow Formation Continued

Maudlow Formation Continued Thickness 

Member B Continued (meters) (feet)

5. Volcanic conglomerate, etc. Continued

phyritic hornblende-pyroxene dacite; subangular 

to rounded cobbles, and boulders as much as 

0.3 m (1 ft) in diameter; most fragments between 

1.3-2.5 cm (0.5-1 in.) in diameter................................. 12.5 41

4. Crystal-lithic tuff and sandstone, volcanic, grayishred-purple, medium- to coarse-grained, thin- to 

moderate-bedded; 0.9 m (3 ft) bed at base, other 

beds 2.5-5 cm (1-2 in.) thick; forms steep slope ...... 18.3 60

3. Volcanic breccia, grayish-red-purple; contains 

monolithologic porphyritic hornblende-pyroxene 

dacite; indistinct cobbles and boulders up to 15 

cm (6 in.); matrix same composition; some sorting; 

forms ridge........................................................................ 20.7 68

2. Lithic-crystal tuff and sandstone, volcanic, grayishred-purple, fine- to coarse-grained, rudely bedded; 

much plagioclase, some hornblende; more resistantthanunitl................................................................ 9.8 32

1. Lithic crystal tuff and sandstone, volcanic, grayishred-purple, fine-grained; forms gentle slope above 

gully; partly covered ...................................................... 6.1 20

Total member B .......................................................... 159.7 524

Contact between member A and member B covered in sec. 13;

disconformable, abrupt contact exposed in sec. 26. 

Member A:

11. Sandstone, volcanic, olive-gray, greenish-gray, 

gray, pale-brown, fine- to coarse-grained, wellbedded; float suggests some conglomerate; sandstone crossbedded with mafic minerals concentrated on bedding; secondary zeolites (clinoptilolite), carbonaceous films, and plant remains abundant; "Cissus" marginata (Lesquereux) Brown 

identified by J. A. Wolfe (written commun., 1962); 

partly covered; forms steep slope................................ 16.8 55

10. Sandstone, volcanic, olive-gray, coarse-grained, 

partly conglomeratic, blocky; feldspars weather 

white; crossbedding with mafic minerals, concentrated on bedding planes; weathers yellowish 

gray to pale yellowish brown; forms ridge .............. 7.6 25

9. Mostly covered; upper 9 m (30 ft) made up of 

medium-gray olive-gray massive tuffaceous volcanic conglomerate and sandstone; conglomerate 

has rounded boulders with diameters as large as 

5 cm (2 in.), intact framework, and some boulders 

of "spotted" Cretaceous sandstone cemented with 

analcite; middle part covered; lower 6 m (20 ft) is 

interbedded conglomerate and sandstone; sandstone is volcanic, tuffaceous, medium gray, olive
MAUDLOW AND SEDAN FORMATIONS, MONTANA B63

Type section of the Maudlow Formation Continued

Maudlow Formation Continued Thickness 

Member A Continued (meters) (feet)

9. Mostly covered, etc. Continued

gray, yellowish gray, and fine grained to conglomeratic; scattered boulders in conglomerate as 

much as 46 cm (18 in.) in diameter, most 1.3 cm 

(0.5 in.), all porphyritic volcanics; massive bedding; forms slope ............................................................ 45.7 150

8. Sandstone, volcanic, light-olive-gray, greenishgray, grayish-orange-pink, fine-grained to conglomeratic, siliceous; color changes accentuate 

bedding; beds about 46 cm (18 in.) thick; smallscale festoon crossbedding marked by heavy 

mineral concentrations present in finer grained 

beds; uppermost bed has calcareous cannonball 

concretions; thin volcanic conglomerate beds 

0.6m (2ft) thick near base, cobbles as much as 2 cm 

(0.75 in.) in diameter; some light-gray, some darkyellowish-orange siliceous interbedded mudstone 

lenses throughout unit; weathers medium dark 

gray and moderate brown; forms slope .................... 15.2 50

7. Sandstone, volcanic, light-olive-gray, medium- to 

coarse-grained, massive; weathers olive gray; 

forms slope........................................................................ 6.1 20

6. Sandstone, volcanic, and lithic-crystal tuff, moderate-olive-gray, light-olive-gray, dusky-yellowgreen, some mottled, fine-grained to conglomeratic, mostly coarse-grained, mostly massive, siliceous; some laminations; some clay layers; bedding 1.8-2.4 m (6-8 ft) thick marked by color 

changes; some plant remains; weathers yellowish 

gray and pale yellowish brown; forms ledge; 

upper 3 m (10 ft) forms small cliff................................ 13.7 45

5. Sandstone and mudstone, volcanic, pale-yellowishbrown, olive-gray; sandstone fine to coarse 

grained; much plant material; calcareous cannonballs at base; thin interbeds of black shaly material; one thin bed of yellowish-gray mostly massive pyroxene-rich tuff in middle forms steep slope 

below ledges of unit 6...................................................... 24.4 80

4. Sandstone and dark gray mudstone, volcanic; sandstone is light olive gray with scattered pods of 

grayish orange, fine grained to conglomeratic, 

mostly medium to coarse grained .............................. 4.6 15

3. Interbedded volcanic sandstone, lignitic or carbonaceous mudstone and shale: 

A. Mudstone, volcanic, grayish-brown, shaly, 

carbonaceous; plant imprints; white specks, 

altered feldspars, common ................................ .9 3

B. Sandstone, volcanic, grayish-orange, friable, 

fine- to medium-grained; limonite staining; 

much organic material...................................... .3 1
B64 CONTRIBUTIONS TO STRATIGRAPHY

Type section of the Maudlow Formation Continued

Maudlow Formation Continued Thickness 

Member A Continued (meters) (feet)

C. Mudstone, volcanic, grayish-brown, carbonaceous; white feldspars; plant fragments; 

weathers pale yellowish brown ...................... .6 2

D. Shale, pale-brown, grayish-black, lignitic; 

interbedded with very pale orange finegrained to conglomeratic volcanic sandstone; angular fragments up to 3 mm in 

diameter; much carbonaceous plant material; pollen and spore colln. D3324 .................. 1.5 5

E. Sandstone, grayish-black, lignitic; interbedded claystones, as in B ........................................ 3.1 10

6.4 21

2. Sandstone, volcanic, grayish-black, fine-grained, 

quartzose; weathers moderate yellowish brown; 

limonitic staining; forms ledge .................................. 0.6 2

1. Sandstone, volcanic, moderate-olive-gray, finegrained to conglomeratic; white feldspars stand 

out on fresh surfaces; some cannonballs 15-30 cm 

(6-72 in.) in diameter; much carbonaceous material; weathers olive gray; moderate-olive-gray 

massive volcanic claystone at base, with many 

leaf imprints and carbonaceous films; indistinct 

bedding; fractured outcrop; forms steep slope ........ 15.6 51

Total member A .......................................................... 156.7 514

Total Maudlow Formation .................................. 1,624.5+ 5,330+

Contact with underlying Telegraph Creek Formation (upper 

part) disconformable (see text).

Reference section for part (member D) of the Maudlow Formation

/NWW sec. 7, T. 4 N., R. 5 E. (fig. 8)7

Maudlow Formation (part) Thickness 

Disconformable contact with member E (meters) (feet) 

Member D:

5. Welded tuff, mottled-pale-red and grayish-yellowgreen; much black biotite; fine-grained pale-green 

lithic fragments, in matrix of biotite-rich pale-red 

tuff form upper part of unit; lower part is duskyyellow-green pale-olive fine-grained volcanic claystone and sandstone; limonitic staining; forms 

slope below unit 4 .............................................. .... ..- 15.2 50

4. Welded tuff, grayish-yellow-green, pale-reddishbrown, pale-yellowish-brown, moderate-red 

(5R4/4), fine-grained, biotitic; forms vertical 

ridge.............................................................................. 30.5 100

3. Sandstone, volcanic, olive-gray, fine- to mediumgrained, calcareous; some calcareous nodules; 

interbedded with volcanic conglomerate and 

grayish-green tuff in lower part.................................. 33.5 110
MAUDLOW AND SEDAN FORMATIONS, MONTANA B65

Reference section for part (member D) of the 

Maudlow Formation Continued

Thickness

Member D Continued (meters) (feet) 

2. Conglomerate, volcanic, olive-gray; rounded boulders of welded tuff and porphyritic intermediate 

volcanic rocks up to 8 cm (3 in.) in diameter in 

volcanic matrix; olive-green fine-grained thinbedded volcanic sandstone present in lower 1.5 m 

(5 ft) .................................................................................... 5.2 17

1. Vitric tuff, welded and unwelded, grayish-green, 

greenish-gray, light-greenish-gray, fine-grained, 

crumbly, biotitic; partly covered; forms si ope ........ 27.4 90

Total member D .......................................................... 111.8 367

Contact with member C not exposed

Reference section for part (member B) of the 

Maudlow Formation

[SW !/4 sec. 11, NW V4 sec. 14, T. 4 N., R. 5 E. (fig. 8)]

Maudlow Formation (part) Thickness 

Disconformable contact with member C (meters) (feet) 

Member B:

9. Sandstone, grayish-red-purple, partially conglomeratic, medium-bedded, crossbedded; pebbles of 

hornblende-pyroxene dacite as much as 2 cm 

(0.75 in.) in diameter form 10-20 percent of sandstone; upper 3 m (10 ft) and basal 8 m (25 ft) 

form low rounded ledges; middle covered ................ 20.1 66

8. Sandstone, volcanic, grayish-red-purple, fine- to 

medium-grained, thin-bedded, crossbedded; forms 

slope between ledges of units 7 and 9........................ 18.6 61

7. Sandstone, volcanic, and crystal tuff, grayish-redpurple, thick- and thin-bedded; 0.6 cm (0.25 in.) 

laminae of feldspar and hornblende crystals, 

some crossbedded; interbedded with minor volcanic breccia and conglomerate with fragments 

as much as 2.5 cm (1 in.) in diameter; forms 

resistant ledge with basal 5m (15 ft) cliff ................ 20.7 68

6. Sandstone, volcanic, grayish-red-purple, fine- to 

medium-grained, thin-bedded; beds 1.3-10 cm 

(0.5-4 in.) thick with irregular surfaces; some 

pockets of coarse-grained sandstone; gray sandstone bed in middle; forms slope below unit 7.......... 2.4 8

5. Conglomerate, grayish-red-purple, monolithologic, 

hornblende-pyroxene-dacite; subangular to subrounded pebbles as much as 5 cm (2 in.) in diameter in upper beds, average about 1.3 cm (0.5 in.), 

in matrix of similar composition; interbedded 

with grayish-red-purple fine-grained to conglomeratic thin sandstone; forms irregular ledges or 

hogbacks..................................................................... 11.8 38.5

4. Sandstone in upper part, volcanic, grayish-red, finegrained, laminated to thin-bedded, crossbedded;
B66 CONTRIBUTIONS TO STRATIGRAPHY

Reference section for part (member B) of the 

Maudlow Formation Continued

Maudlow Formation Continued Thickness 

Member B Continued (meters) (feet)

4. Sandstone, etc. Continued

beds 0.6-5 cm (0.25-2 in.) thick; some ripple marks; 

forms gully below conglomerate of unit 5; in lower 

part, grayish-red-purple poorly bedded lithic-crystal tuff has angular fragments of intermediate 

volcanic rocks as much as 1.3 cm (0.5 in.) in 

diameter, most 0.2 cm (0.1 in.), matrix same; 

forms rounded dark ledges .......................... ............. 5.4 17.5

3. Crystal-lithic tuff, grayish-red-purple, massive; 

fragments as much as 0.6 cm (0.25 in.) in diameter; forms low area between resistant ledges 

of units 2 and 4 ................................................................ 2.1 7

2. Sandstone, volcanic, and lithic-crystal tuff, grayishred-purple, mostly fine- to medium-grained; upper 

0.6 m (2 ft) lithic-crystal tuff with fragments as 

much as 0.6 cm (0.25 in.) in diameter, bedding 

regular; beds 0.6-5 cm (0.25-2 in.) thick in lower 

part, as much as 0.6 m (2 ft) thick in upper part; 

forms ledges...................................................................... 6.7 22

1. Crystal-lithic tuff, grayish-red-purple, fine- to 

medium-grained; feldspar crystals weather white; 

bedding irregular; graded bedding; lower 0.6 m 

(2 ft) forms resistant ledge, rest forms si ope ............ 6.4 21

Total member B .......................................................... Q . o 399

Disconformable contact with member A

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J. S. GOVERNMENT PRINTING OFFICE: 1977 - 789-025A5