Type Sections and Stratigraphy of the 

Members of the Blackleaf and Marias 

River Formations (Cretaceous) of the 

Sweetgrass Arch, Montana 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 974 

Type Sections and Stratigraphy of the 

Metnbers of the Blackleaf and Marias 

River Fortnations (Cretaceous) of the 

Sweetgrass Arch, Montana 

By W. A. COBBAN, C. E. ERDMANN, R. W. LEMKE, and E. K. MAUGHAN 

GEOLOGICAL SlTRVEY PROFESSIONAL PAPER 974 

A report of a forrnal subdivision and 

age assignments of a c lassie 

Cretaceous area 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976 
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretm~v 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Cobban, William Aubrey, 1916-

Type sections and stratigraphy of the members of the Blackleaf and Marias River Formations (Cretaceous) of the Sweetgrass arch, 

Montana. 

(Geological Survey Professional Paper 974) 

Bibliography: p. 61-63. 

Supt. of Docs. no.: I 19.16:974 

1. Geology, Stratigraphic-Cretaceous. 2. Geology, Stratigraphic-Nomenclature-Montana. I. Cobban, William Aubrey, 1916-

11. Series: United States Geological Survey Professional Paper 974. 

QE687.T94 551.7'7'09786 76-608181 

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

Washington, D.C. 20402 

Stock Number 024-001-02874-3 
CONTENTS 

Page Page 

Abstract ____ ____ __ _ _ _ _ _____ _ _ _ ________ _ _ ____ _ _ _ _ _ _ __ ___ _ __ 1 Blackleaf Formation-Continued 

Introduction________________________________________________ 2 Bootlegger ~ember ------------------------------------ 31 

Acknowledgments ___ _ _ _ ____ _ _ ____ _____ _ ___ ___ _ _ _ _ __ __ _ _ 2 ~arias River Shale ---------------------------------------- 36 

Historical summary ------------------------------------ 2 Floweree ~ember -------------------------------------- 37 

Blackleaf Formation---------------------------------------- 5 Cone ~ember _ _ _ _ _ _ ____ _ _ _ _ __ _ ___ _ _ _ _ ____ __ _ _ __ ______ __ 40 

Flood ~ember __________________________________________ 6 Ferdig ~ember ---------------------------------------- 44 

Taft Hill ~ember -------------------------------------- 15 J(evin ~ember ---------------------------------------- 50 

\Taughn ~ember -------------------------------------- 25 References cited _ ___ _ _ __ __ _ _ __ ____ __________ _ _ ______ _ _ _ ____ _ 61 

FIGURE 1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

19. 

Index ______________________________________________________ 65 

I 

ILLUSTRATIONS 

Page 

Index map of the Sweetgrass arch and surrounding area ---------------------------------------------------------- 3 

Photograph of type section of the Flood ~ember of the Blackleaf Formation ---------------------------------------- 6 

Photograph of very large sandstone concretion in the Flood ~ember------------------------------------------------ 9 

Photograph of fresh-water limestone bed near the top of the Kootenai Formation____________________________________ 10 

Lithologic column of the type section of the Flood ~ember and map showing line of section-------------------------- 11 

Photograph of the upper part of the Taft Hill ~ember of the Blackleaf Formation ---------------------------------- 18 

Lithologic column of the type section of the Taft Hill ~ember and map showing line of section ---------------------- 20 

Photograph of type section of the \Taughn ~ember of the Blackleaf Formation ______________________________________ 26 

Photograph of the upper part of the \T aughn ~ember_ _____________________________________________ --------________ 28 

Lithologic column of the type section ofthe \Taughn ~ember and map showing line of section________________________ 29 

Lithologic column of the type section of the Bootlegger ~ember of the Blackleaf Formation and map showing line of 

section----------------------------------------------------------------------------------------------------- 35 

Photograph of the Floweree ~ember of the ~arias River Shale at its type locality ---------------------------------- 38 

Lithologic column of the type section of the Floweree ~ember and map showing line of section ______________________ 39 

Photograph of the upper part of the Cone ~ember of the ~arias River Shale at its type locality ______ --- __ ----- _ _ _ _ _ _ 42 

Lithologic column of the type section of the Cone ~ember and map showing line of section ------L------------------- 43 

Lithologic column of the type section of the Ferdig ~ember of the ~arias River Shale and map showing line of section 48 

Photograph of bentonite beds in lower part of the Kevin ~ember of the ~arias River Shale__________________________ 51 

Photograph of the upper part of the Kevin ~ember overlain by the Telegraph Creek Formation---------------------- 53 

Lithologic column of the type section of the Kevin ~ember and map showing line of section__________________________ 54 

III 

TYPE SECTIONS AND STRATIGRAPHY OF THE 

MEMBERS OF THE BLACKLEAF AND MARIAS RIVER FORMATIONS 

(CRETACEOUS) OF THE SWEETGRASS ARCH, MONTANA 

By W. A. CoBBAN, C. E. ERDMANN, R. W. LEMKE, and E. K. MAUGHAN 

ABSTRACT 

Rocks of Albian-Santonian age, cropping out on the Sweetgrass 

arch in north-central Montana, consist of the Blackleaf Formation, 

186 to 253 m (610-830 ft) thick, and the overlying Marias River 

Shale, 280 to 363 m (920-1,190 ft) thick. In general, the Blackleaf 

thickens southward, and the Marias River thickens westward. Each 

formation has four named members. 

The members of the Blackleaf Formation are, from oldest to 

youngest, Flood, Taft Hill, Vaughn, and Bootlegger. Quartz 

sandstone and very dark gray shale characterize the Flood Member, 

which rests disconformably on green and red mudstones of the nonmarine Lower Cretaceous Kootenai Formation. 

The Flood Member is 42 m (138ft) thick in its type section, where 

it consists of a basal ledge-forming thin-bedded flaggy sandstone, a 

medial slope-forming dark-gray shale, and an upper thick cliffforming sandstone that contains huge sandstone concretions. Trace 

fossils, chiefly small tracks, trails, and burrows, are abundant in the 

flaggy beds and suggest a shallow-water near-shore marine environment for most of the member. On the east flank of the Sweetgrass 

arch, Inoceramus comancheanus Cragin, of late Albian age, was 

found in sandstone at the top of the member. This pelecypod is 

known from the Skull Creek Shale of the Black Hills area of South 

Dakota, the Kiowa Shale of Kansas, and the South Platte Formation 

of Colorado. 

Glauconitic sandstone and medium-gray soft bentonitic silty shale 

characterize the Taft Hill Member, which is 74 m (242ft) thick in its 

type section. The lower contact is gradational and the upper sharp 

and disconformable. The member is divisible into three units by a 

thick glauconitic sandstone separating a lower and an upper unit of 

silty bentonitic shale and thin glauconitic sandstone beds. Fossils, 

which are scarce in the member, include Inoceramus bellvuensis 

Reeside, of late Albian age. 

The Vaughn Member, 26 m (86 ft) thick in its type section, is a 

nonmarine sequence of light-colored bentonitic clay, siltstone, and 

sandstone that rests sharply on the marine Taft Hill Member. In 

general, the Vaughn Member consists of a lower unit of pale-yellow 

friable arkosic sandstone and an upper unit of bentonitic clay and 

tuffaceous siltstone and minor amounts of bentonite and sandstone. 

Minute crystals of a red zeolite (clinoptilolite) are so abundant in the 

upper clayey unit as to impart a pinkish aspect to the outcrops. The 

Vaughn Member is usually poorly vegetated and erodes into badlands. Fossil logs are present in the lower sandstone unit, and a few 

fossil leaves and fragments of reptilian bones occur in the upper 

clayey unit. 

The Bootlegger Member, a westward sandy equivalent of the 

Mowry Shale, totals 100 m (329 ft) in thickness in the type section 

and consists of thin beds of medium-gray very fine to fine-grained 

sandstone, gray siltstone, dark-gray shale, and bentonite. The 

sandstone, siltstone, and shale are usually interlaminated. At least 

12 beds of bentonite are present in the type section; the two thickest 

beds (2-3 m) contain hard lenslike masses of zeolitic tuff. Thin beds 

of black-coated chert pebbles a;re present locally, and one, just below 

the top, contains pebbles as much as 5 em in diameter in a matrix of 

coarse-grained salt-and-pepper sandstone that includes abundant 

fish bones. Many of the beds of sandstone, siltstone, and shale are 

hard, resulting in ledge-forming outcrops. Both contacts of the 

member are sharp. The Bootlegger thins westward largely by the 

lower part grading into nonmarine beds assigned to the Vaughn 

Member. Over much of the Sweetgrass arch, the Bootlegger Member 

is divisible into lower and upper hard sandy units separated by a 

medial unit of softer shale. Fish scales and bones are common in the 

upper sandy unit. Neogastroplites, an ammonite believed to be of 

latest Albian and earliest Cenomanian age, has also been found in 

the upper sandy unit. 

The Marias River Shale consists of the Floweree, Cone, Ferdig, 

and Kevin Members. The Floweree Member unconformably overlies 

the Bootlegger Member and is 19.4 m (63.5 ft) thick at its type 

section. It consists chiefly of a sequence of dark-bluish-gray shale, 

lighter gray sandy shale, and thin beds of siltstone and sandstone. 

Thin layers of chert granules or small pebbles are present locally. 

Gray septarian limestone concretions occur on the east flank of the 

Sweetgrass arch. The Floweree Member is softer and darker than the 

underlying Bootlegger Member. Trace fossils, consisting of small 

tracks, trails, and burrows, are common on the sandstone and 

siltstone layers. These sandy beds have also yielded a few impressions of inoceramids and ammonites, including Metoicoceras muelleri 

Cobban and M. mosbyense Cobban, of late Cenomanian age. A 

slightly older late Cenomanian ammonite, Calycoceras 

canitaurinum (Haas), was found in one of the septarian limestone 

concretions. These fossils occur in the lower half of the Greenhorn 

Formation of the Black Hills area. 

The Cone Member is a thin calcareous unit of latest Cenomanian 

and earliest Turonian age. A rather uniform thickness of 15-18 m 

(50-60 ft) is consistent over most of the Sweetgrass arch. Most of the 

member is dark-gray calcareous shale that weathers light bluish 

gray first and then yellowish white. The upper half of the member, 

and especially the uppermost part, contains thin beds of argillaceous 

or crystalline limestone that tend to form low ridges. Septarian limestone concretions occur at several horizons; the most conspicuous bed 

is just above the base and contains closely spaced concretions that 

weather pale lavender gray. Several beds of bentonite are present, 

and one, at the top of the lower third of the member, attains a thickness of nearly a metre. The contacts of the member are sharp and 

marked by an abrupt change from the limy shale of the Cone to the 

noncalcareous shale of the underlying Floweree and overlying Ferdig Members. A thin layer oflimonitic siltstone at the base, containing shale pebbles, soft white siltstone nodules, and fish teeth and 

bones, probably marks a disconformity. Invertebrate fossils are 

abundant and reveal the zones of Sciponoceras gracile (Shumard) 

1 
2 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

and Inoceramus labiatus lSchlotheim) which are found in the upper 

part of the Greenhorn Formation of the Black Hills area. 

The Ferdig Member, 68.6 m (224 ft) thick in its type section, is a 

sequence of gray noncalcareous shale containing thin hard sandy 

partings and gray- to yellow-weathering limestone concretions and 

dusky-red-weathering dolostone concretions. Three units make up 

the member-a lower dark-bluish-gray sandy shale containing grayand yellow-weathering calcareous concretions, and an upper darkbluish-gray shale containing gray-weathering calcareous concretions. Thin beds of bentonite are sparingly present. A very thin, but 

persistent, layer of conglomeratic sandstone in the upper part of the 

member contains polished granules and small pebbles of black, gray, 

brown, and green chert, as well as pebbles of quartz, quartzite, and 

argillite. The contact of the Ferdig with the underlying Cone 

Member is sharp and disconformable; it is marked by a thin layer of 

limonitic siltstone containing, in places, fish teeth, small pebbles of 

black chert, and larger gray and brown phosphatic pebbles. In well 

cuttings, the shale in the upper part of the Ferdig may show finely 

disseminated pyrite. The contact with the overlying Kevin Member 

is conformable. Molluscan fossils are common and reveal at least 

three zones. Prionocyclus hyatti (Stanton), of middle Turonian age, is 

found in the lower unit, characterized by dusky-red dolostone concretions. Scaphites nigricollensis Cobban, of middle late Turonian age, is 

present through much of the medial sandy unit. Scaphites corvensis 

Cobban, of slightly later Turonian age, occurs at the top of the sandy 

unit and in the overlying upper shale unit. 

The Kevin Member, 188 m (617 ft) thick in its type section, is a 

dark-gray shale that contains some thin sandy parts, numerous thin 

layers of bentonite, and many beds of calcareous concretions that 

weather gray, yellow, or dusky red. The member is divisible into 

three units on the basis of abundance of bentonite beds and composition of concretions. Numerous beds of bentonite and gray- to 

yellow-weathering limestone concretions characterize the lowest 

unit, which is 53-55 m (175-180 ft) thick. The many thin layers of 

light-gray bentonite give the outcrop a distinctive banded appearance. The medial unit, about 60 m (200ft) thick, contains numerous 

beds of orange- to dusky-red-weathering ferrocalcareous concretions 

and a few thin layers of bentonite and very fine grained sandstone. 

An important marker bed, the MacGowan Concretionary Bed, lies in 

the middle of this unit. The MacGowan Bed is a conglomeratic bed of 

concretionary dolostone that weathers light brown, orange brown, 

and dusky red, and contains polished granules and small pebbles of 

gray and black chert and larger pebbles of gray phosphatic siltstone. 

One or two thin layers of greenish-gray phosphatic pebl:lles are present in the medial unit in the interval 3-10 m above the MacGowan 

Bed. The upper unit of the Kevin Member consists of about 60 m 

(200ft) of dark-gray shale that contains many beds of yellowishgray-weathering limestone concretions and a few thin beds of bentonite and very fine grained shaly sandstone. The upper half of this 

unit is in part calcareous. The boundary between the Kevin Member 

and the overlying Telegraph Creek Formation is sharp but conformable; it is marked by a change from the dark-gray shale of the Kevin 

Member to the lighter gray shale and siltstone of the Telegraph 

Creek Formation, interlaminated with very fine grained sandstone. 

Molluscan fossils are abundant and varied in the Kevin Member. 

The lower unit, of early Coniacian age, contains a zone oflnoceramus 

erectus Meek below and a zone of Inoceramus deformis Meek above 

and is correlated with the Fort Hays Limestone Member of the Niobrara Formation of the central Great Plains. The middle unit, 

characterized by the red-weathering ferrocalcareous concretions, 

contains three faunal zones; from oldest to youngest, a zone of Inoceramus (Volviceramus) involutus Sowerby-Scaphites ventricosus 

Meek and Hayden of Coniacian age, a zone of Inoceramus stantoni 

Sokolow-Scaphites depress us Reeside of early Santonian age, and a 

zone of Inoceramus cordiformis Sowerby-Clioscaphites vermiformis 

(Meek and Haydenl of middle Santonian age. The upper unit contains the rest of the C. L'ernz iform is zone and the younger Santonian 

zones of C. choteauensis Cobban and Desmoscaphites erdmanni 

Cobban. 

INTRODUCTION 

Rocks formerly assigned to the Colorado Shale 

(Fisher, 1909, p. 36) on the Sweetgrass arch of northcentral Montana (fig. 1) are divided in this paper into 

an older Blackleaf Formation and a younger Marias 

River Shale, both formations of the Colorado Group. 

The Blackleaf Formation is assigned to the Lower Cretaceous and is subdivided into four members, from oldest to youngest: Flood Member, Taft Hill Member, 

Vaughn Member, and Bootlegger Member. The Marias 

River Shale, of Late Cretaceous age, is subdivided also 

into four units, from oldest to youngest: Floweree 

Member, Cone Member, Ferdig Member, and Kevin 

Member. Although these formations and members 

have been previously defined by the authors (Cobban 

and others, 1959), type sections, fossil lists, and the 

regional stratigraphy were not presented. The purpose 

of the present report is to complete this presentation. 

ACKNOWLEDGMENTS 

Other geologists with the U.S. Geological Survey 

who have aided in the measurement of some of the 

sections on the Kevin-Sunburst dome are B. R. Alto, 

A. F. Bateman, and W. L. Rohrer. Geologists, formerly 

with the U.S. Geological Survey, who have participated in the study of the Colorado Shale on the 

Kevin-Sunburst dome are V. K. Koskinen, J. W. 

Nordquist, J. T. Gist, G. W. Beer, and K. H. Holmes. 

HISTORICAL SUMMARY 

The earliest investigation of the rocks later assigned 

to the Colorado Group in Montana was undertaken in 

1861 by Meek and Hayden, who gave the name ('Fort \ 

B~nton Group, Formation No. 2" to the ((Dark gray 

lamina.ted clays, sometimes alternating near the upper 

part with seams and layers of soft gray and lightcolored limestone. Inoceramus problematicus, I. tenuirostratus, I. latus?, I. fragilis, Ostrea congesta, Yenilia M ortoni, P holadom ya papyracea, Ammonites M ullani, A. percarinatus, A. vespertinus, Scaphites warreni, S. larvaeformis, S. ventricosus, S. vermiformis, 

Nautilus elegans? etc. Extensively developed near Fort 

Benton on the Upper Missouri" (Meek and Hayden, 

1861, p. 419). Of these fossils, Inoceramus tenuirostratus, Veniella mortoni (as Venilia Mortoni), 

Pholadomya papyracea, Scaphites ventricosus, Clioscaphites vermiformis (asS caphites vermiformis ), and 

Cymatoceras nebrascense (as Nautilus elegans?) came 

from ((Chippewa Point*** some twenty odd miles below 
INTRODUCTION 3 

48 

EXPLANATION 

Blackleaf or Marias River 

Formation 

Oil field Gas field 

Area covered by U .S . 

Geological Survey 

topographic quadrangle 

[!] Marias Pass III Dent Bridge 

IT] Hearr Butte 0 Fairfield 

@] Sunburst ~Vaughn 

0 Shelby [!] Great Falls 

~Dutton ~ Portage 

A 

[!] Cascade 

~Carter 

~Tunis 

~ Lander Crossing 

~Valier 

10 

10 20 

L B T A 

20 30 40 50 MILES 

I I 

I I 30 40 50 60 70 80 KILOMETRES 

FIGURE I.--Sweetgrass arch and surrounding area. 

Fort Benton" (Meek and Hayden, 1862, p. 21-28. Those 

fossils are now known to be of Niobrara age. Meek and 

Hayden did not know that their chalky "Niobrara Division, Formation No. 3" underwent a facies change 

northwestward from its type locality at the mouth of 

the Niobrara River in Nebraska. Thus, inadvertently 

they included rocks of Niobrara age in the Benton 

Group which later was reduced to formation rank, the 

Benton Shale, and restricted to beds of Graneros, 

Greenhorn, and Carlile age. 
4 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Weed (1899, p. 2) applied the name Colorado Formation to the following highly generalized 565-m 

(1,850-ft) sequence on the southeastern part of the 

Sweetgrass arch, whose descriptive text has been 

taken from the columnar section sheet of the Fort Benton Folio: 

Top 

"Drab or lead-colored clay shale, carrying round or 

oval concretions of gray limestone. 

Black shale with interbedded sandstones and a 

bed of tuff. 

Red shale and sandstones in thin beds. 

Lilac-colored sandstone, red clay, and thin limestone." 

This formation rested on Weed's Dakota Formation, 

a sequence of "Red shale with limestone nodules 

capped by sandstone, and sandstone at the base." Overlying the Colorado Formation was a sandstone to 

which Weed gave the name Eagle Formation. The 

lower part of this unit probably includes the Telegraph 

Creek Formation, the basal formation of the Montana 

Group. 

Willis (1902, p. 315, 326, 327), in describing the 

geology of what is now Glacier National Park, used the 

name Benton Shale for the uppermost or "lead-colored 

clay shale" member of Weed's Colorado Formation and 

the name Dakota Sandstone for the next-to-the-top 

unit. Willis listed several plant fossils from his Dakota 

Sandstone and a dozen marine molluscan genera from 

his Benton Shale. 

The geology of the Great Falls coal field, south and 

southeast of Great Falls, was described by Fisher 

briefly in 1907 and in much more detail in 1909. He 

applied the term Colorado Shale to the upper two units 

(black and gray shales and interbedded sandstones) of 

Weed's Colorado Formation. The lower two units (red), 

as well as Weed's Dakota Formation, were assigned to 

the Kootenai Formation. The thickness of the strata 

referred to the Colorado Shale was therefore reduced to 

about 463 m (1,520 ft). Regarding the boundaries, 

Fisher (1909, p. 38) stated: "The Colorado shale rests 

with apparent conformity upon the underlying 

Kootenai, and is overlain conformably by the Eagle 

sandstone." Fisher (1907, p. 163; 1909, p. 37) gave a 

generalized section through the lower or sandy part of 

the Colorado Shale at Belt Butte. 

Stanton (1913) extended the terms Colorado Shale 

and Eagle Sandstone into the Blackfeet Indian Reservation west of the crest of the Sweetgrass arch. In the 

following year, Stebinger (1914) used the name Virgelle in place of Eagle, inasmuch as the sandstone in 

the reservation represented only the lower or Virgelle 

Sandstone Member of the Eagle Sandstone. Stebinger 

(1914, p. 62) pointed out that the Virgelle Sandstone 

Member was massive in the upper part, whereas the 

lower half was "slabby*"'* becoming shaly toward the 

base." A little later, Stebinger (1916, p. 124; 1917, 

p. 285, 287, 289) applied the name Colorado Shale as 

far west as Glacier National Park, where the formation 

then included Willis' Dakota Sandstone and Benton 

Shale. 

In the course of mapping the Disturbed belt along 

the Rocky Mountain front between Sun River and 

Birch Creek (southern boundary of Blackfeet Indian 

Reservation), Stebinger (1918, p. 154, 158-161) divided 

the Colorado Shale into a lower member, the Blackleaf 

Sandy Member, 183-213 m (600-700 ft) thick and an 

upper shale member 365 m (1,200 ft) thick. A generalized columnar section (Stebinger, 1918, p. 158) was 

given for the Blackleaf Sandy Member and basal part 

of the overlying shale member. The stratigraphic positions of five fossil collections were shown and the fossils listed (Stebinger, 1918, p. 160). Beginning about 24 

m (80 ft) above the top of the Blackleaf, Stebinger recorded a "bituminous shale and maltha limestone" sequence, about 18 m (60 ft) thick, which contained 

abundant Inoceramus labiatus (Schlotheim). This unit 

had been noted earlier by Powers and Shimer (1914, p. 

557), who noted "dark arenaceous shale" with abundant Inoceramus labiatus (Schlotheim) and Lingula sp. 

near the Sun River. Stebinger used the name Virgelle 

for the sandstone overlying the Colorado Shale and 

placed the boundary between the two formations "at 

the position where the amount of sandstone exceeds 

that of the shale in the transition beds." 

Romine (1929, p. 786-788) brought Stebinger's name 

Blackleaf to the Sweetgrass arch. Collier (1929 [1930], 

p. 70-72) presented a generalized section of the 

member in the area a few miles north of Vaughn and 

listed 39 species of invertebrate fossils, identified by J. 

B. Reeside, Jr., from 17 localities on the Sweetgrass 

arch. The stratigraphic positions of the fossils were 

given in feet below the top of the Colorado Shale. Also, 

Dobbin and Erdmann (1930) used several sandstone 

units of the Blackleaf Sandy Member in structural 

mapping on the Sweetgrass arch. 

Before 1939, the boundary between the Colorado 

Shale and the Virgelle Sandstone was placed either 

within the sandy transition beds (Stebinger, 1918, p. 

164, 165) or at the top of the transition beds (Dobbin 

and Erdmann, 1930). Erdmann and Davis (1939) 

treated the transition beds as a separate formation designated "transition zone." Later (Erdmann and 

Schwabrow, 1941, p. 280-282; Blixt, 1941, p. 334, 335), 

the transition zone was considered as the lower 

member of the Eagle Sandstone. Erdmann (1948) 

applied the name Telegraph Creek(?) Formation to the 

transition beds. Later, Cobban (1950) was able to re-
BLACKLEAF FORMATION 5 

move the query by listing fossils of Telegraph Creek 

age from the formation. 

The upper part of the Colorado Shale on the KevinSunburst dome was divided into two formations, Carlile and Niobrara, by Erdmann, Gist, Nordquist, and 

Beer (1947). A detailed columnar section of the Niobrara Formation was presented. The stratigraphic positions of key marker beds (lettered A through M) were 

indicated from near the top of the formation downward. Bed "F," near the middle of the Niobrara Formation, was described as a persistent and easily identified 

mud pellet conglomerate (later identified as conglomeratic phosphorite) that weathers orange buff and 

contains some chert pebbles. Bed "N," a thin hard layer 

of gray sandstone that contains gray and black chert 

pebbles, was used as the top of the Carlile Formation of 

Erdmann, Gist, Nordquist, and Beer (1947). In a later 

report by Erdmann (1949), on the Lothair area just 

southeast of the Kevin-Sunburst dome, the term Colorado Shale was used again. 

Cobban (1951a) showed that the Colorado Shale on 

the Sweetgrass arch was divisible into many distinctive lithologic units that could be correlated by their 

fossil content and, in places, by their lithologic features 

with the formations and members of the Colorado 

Group of the Black Hills area of South Dakota. Most of 

the guide fossils were figured (Cobban, 1951b, 1955b). 

More recent refinements in correlation have been 

made possible by new fossil discoveries (Cobban and 

others, 1956, 1958). 

Cobban, Erdmann, Lemke, and Maughan (1959), in 

recognition of a need for a more precise nomenclature 

in connection with geologic mapping on the Sweetgrass 

arch, divided the Colorado Group into an older Blackleaf Formation and a younger Marias River Shale. The 

Blackleaf Formation was assigned to the Early Cretaceous and divided into four members, from oldest to 

youngest: Flood Member, Taft Hill Glauconitic 

Member, Vaughn Bentonitic Member, and Bootlegger 

Member. The Marias River Shale was divided into four 

members, from oldest to youngest: Floweree Member, 

Cone Calcareous Member, Ferdig Shale Member, and 

Kevin Shale Member. 

Aside from Stebinger's work (1918) in the Sun 

River-Birch Creek area, little had been published concerning the Colorado Shale in the Disturbed belt along 

the Rocky Mountains front until fairly recently. Some 

of the Cretaceous outcrops along the south side of 

Glacier National Park were identified by their fossil 

content and lithologic characteristics by Cobban 

(1956). Schmidt (1963, 1966), in his studies of the 

Dearborn River area, and Mudge (1965), in his studies 

of the Sun River area, applied the nomenclature of 

Cobban, Erdmann, Lemke, and Maughan (1959) except for dropping the adjectival part of the name for the 

Taft Hill and Vaughn Members. Fox and Groff (1966) 

and Fox (1966) also applied the nomenclature of Cobban, Erdmann, Lemke, and Maughan (1959) but 

dropped the adjectival parts of all member names. In 

order to be consistent, all adjectival terms are also 

eliminated in the present study. One other important 

work is that by Cannon (1966), who presented much 

information concerning the lithology, thickness, paleocurrent patterns, and genesis of the members of the 

Blackleaf Formation. 

· BLACKLEAF FORMATION 

In defining his Blackleaf Sandy Member, Stebinger 

(1918, p. 158) stated: 

The lower 600 to 700 feet ofthe Colorado shale comprises an alternation of dark marine shales and gray sandstone in beds 20 to 75 feet 

thick , forming a unit clearly distinguishable from the remaining 

shaly portion of the Colorado. For convenience in reference and description it is here designated the Blackleaf sandy member, the 

name being taken from Blackleaf Creek, along which the beds are 

well developed. 

The exposures on Blackleaf Creek are in the Disturbed belt along the Rocky Mountains front, about 

6-11 km ( 4-7 mi) west of Blackleaf in western Teton 

County (Heart Butte quadrangle). With the exception 

of a small area near the south edge of his map, Stebinger included the Blackleaf with the Lower Cretaceous 

and Jurassic rocks as an undifferentiated cartographic 

unit. 

Without referring to any specific locality, Stebinger 

(1918, p. 158, fig. 33) presented for his Blackleaf only a 

generalized columnar section in which the base of his 

member was "bluish-gray shale" and the top was 

"coarse gray sandstone." Reference to his geologic map 

(pl. xxiv), however, indicates that the locality probably 

is about 43 km (27 mi) northwest of Choteau, in sec. 18, 

T. 26 N., R. 8 W., and sec. 13, T. 26 N., R. 9 W., Teton 

County. Notation of several fossil collections on the 

column from various places in the Disturbed belt 

seems to imply that the column might be composite, 

but evidently these collections were inserted to show 

the horizons at which fossils were found. In addition to 

the Blackleaf, the columnar section shows two overlying units of shale, the lower about 24 m (80 ft) thick 

with a 0.15-m (6-in.) pebble bed about 9 m (30 ft) 

above the base, and the upper about 24 m (80 ft) thick, 

which consist of "bituminous shale" and "maltha limestone." Stebinger noted an abundance of the pelecypod 

Inoceramus labiatus (Schlotheim) in the bituminous 

unit .. This unit, on the Sweetgrass arch, is the Cone 

Member of the Marias River Shale. In north-central 

Montana, the contact of this calcareous unit with the 

overlying noncalcareous shale is very sharp and is 
6 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

readily determined in well cuttings and on electric 

logs. This is the contact that is commonly used informally as the top of the Blackleaf by most of the petroleum geologists and scouts and to some extent formally in the literature (Collier, 1929, p. 70; Erdmann 

and Davis, 1939; Blixt, 1941, p. 337). 

The Blackleaf Formation is recognized in the area 

between the Missouri River and the international 

boundary and between the Rocky Mountains front and 

about the longitude of Fort Benton. (See fig. 1.) To 

some extent, the name has been used in southern Alberta adjacent to the Montana boundary (Russell and 

Landes, 1940, p. 24, 25). 

Throughout this area, the Blackleaf rests sharply on 

the Kootenai Formation of continental origin. At any 

one locality a disconformity is not apparent, but regional studies suggest a hiatus and that the Blackleaf 

was deposited on an uneven surface with a relief possibly amounting to about 30m (100ft). The basal metre 

of the Blackleaf has the appearance of having been 

reworked from the upper part of the Kootenai by the 

earliest transgression of the Lower Cretaceous sea. 

This initial advance may have been comparatively 

rapid, for there is no evidence that marine life had time 

to become established. 

FLOOD MEMBER 

:\/\ME AND DEFINITION 

The Flood Member, the lowest member of the 

Blackleaf Formation, is a transgressive-regressive marine unit that marks the earliest appearance of the 

Lower Cretaceous sea. The unit is the black-gray shale 

and quartz sandstone member of Cobban (1951a, 

p. 2175-2176). 

The Flood was named formally by us (Cobban and 

others, 1959, p. 2787) for exposures along the bluffs on 

the west bank of the Missouri River valley in the vicinity of Flood, a siding on the Burlington Northern Railway in the NW% sec. 34, T. 20 N., R. 3 E., Cascade 

County, Mont., 8 km (5 mi) southwest of the City of 

Great Falls. The type section is about 8 km northwest 

of Flood on the south bank of Sun River in a shallow 

reentrant (fig. 2) in the bluffs in the NW cor. NElf4 sec. 

7, T. 20 N., R. 3 E., about 7.2 km (4.5 mi) west of Great 

Falls (fig. 2A ). Most of the measurements were made 

on the west wall of the reentrant, but the upper 6 m (20 

ft) of the middle unit is better exposed across the reentrant to the southeast and was mostly measured there. 

For descriptive purposes, the type section was subdivided into three units (lower, middle, and upper). 

Since the construction in 1959 of a stretch of U.S. 

Interstate Highway 15 between Great Falls and the 

village of Vaughn, two easily accessible and more or 

FIGU RE 2.-Type section of the Flood Member in the NE V.. sec. 7, T. 

20 N., R. 3 E., Cascade County, Mont. The threefold subdivision is 

well shown. Man, holding 4.3-m (14-ft) rod, is standing on top of 

lowest subdivision. 

less completely exposed composite sections of the Flood 

Member have been studied and described from cuts 

and natural outcrops on the north side of the highway, 

each about 2.4 km (1.5 mi) north of the type section, 

between the vicinity of Emerson Junction and the 

Manchester Exit. One section, about 4.8 km (3 mi) west 

of Great Falls, in the SW14SE14 sec. 32, T. 21 N., R. 3 

E., and hereafter referred to as the "reference section," 

exposes the Flood Member nearly continuously from 

top to bottom. The second section, about 6.4 km (4 mi) 

west of Great Falls in the center NW%SE% sec. 31, T. 

21 N., R. 3 E., exposes completely the upper 11.9 m (39 

ft) of the Kootenai Formation, approximately the lower 

half of the Flood Member, and discontinuously exposes 

the remaining part of the member. The Flood Member 

~s also completely exposed about 4 km (2.5 mi) southeast of the type section in a composite section along 

Gore Hill in the NE% sec. 21, T. 20 N., R. 3 E. These 

three sections supplement one another nicely and, together with the type section, reveal the character of 

the member in full. 

THICKNESS 

On the South arch of the Sweetgrass arch, the Flood 

Member ranges in thickness from 26 to 60 m (85 to 

197 ft). At its type section the member is 42 m (138 ft) 

thick; it is 36.5 m (120 ft) thick in the reference section, 

2.4 km (1.5 mi) northeast across Sun River valley. 

From this vicinity the member thins northeastward to 
FLOOD MEMBER 7 

29.6 m (97ft) along the Missouri River, 25.7 km (16 mi) 

northeast of Great Falls, and to 25.9 m (85 ft) on Belt 

Creek, 32 km (20 mi) east of Great Falls. Westward 

from the axis of the Sweetgrass arch, the Flood 

Member thickens as it approaches its source area. Fox 

(1966, p. 61-63) measured a total thickness of 60 m 

(197 ft) on the southwestern flank of the Sweetgrass 

arch. Along Sun River near the Rocky Mountain front, 

the member attains thicknesses of 45.7-167.6 m (150--

550ft) (Mudge, 1972, p. A6). Near Drummond, in T. 11 

N., R. 12 W., Missoula County, about 169 km 

(105 mi) southwest of Great Falls, Gwinn (1961) reported thicknesses of 207-213 m (680--700 ft). Along 

the route of the Burlington Northern Railway 6.4 km 

(4 mi) southwest of East Glacier, the Flood Member is 

at least 68.6 m (225 ft) thick. 

Well cuttings from the South arch indicate thicknesses of 15-46 m (50--150 ft), whereas cuttings from the 

northern part of the Sweetgrass arch reveal thicknesses of 15-30 m (50--100 ft). Reasons for these thickness 

variations, particularly local ones, are not well understood; they may be due to erosional relief on the underlying Kootenai Formation, or they may be due to 

channeling or scour between or within the units of the 

Flood Member. 

OCTCROP DISTRIBCTION 

On the Sweetgrass arch, the Flood Member crops out 

only across the southern part of the South arch. The 

thick sandstone at the top forms conspicuous bluffs 

along the Sun River from Great Falls westward for 

several kilometres, along the west side of the Missouri 

River valley between Great Falls and Flood, along both 

banks of the Missouri River valley between Flood and 

Ulm, and on both banks of the Smith River from Ulm 

southward for many kilometres. The member is exposed at many places in the southern and eastern parts 

of the Portage quadrangle northeast of Great Falls and 

in the Belt Creek valley 24--29 km (15-18 mi) east of 

Great Falls. 

The Flood Member also is present in the Disturbed 

belt along the Rocky Mountain front, from Glacier National Park southeastward to Wolf Creek, where the 

thick sandstone at the top of the member forms prominent hogbacks wherever well exposed. Cobban ( 1956, 

p. 1001, 1002) has drawn attention to easily accessible 

outcrops along the U.S. Highway 2 and the Burlington 

Northern Railway south of the village of East Glacier, 

although recent highway construction has invalidated 

mileage details of his road log. 

GENERAL DESCRI PTIOI\' 

The strata of the Flood Member at the type section 

and in the vicinity can be separated into three individually distinctive lithologic units (fig. 2): (1) an inconspicuous lower unit of sandstone and siltstone, which 

constitutes about 16 percent of the total; (2) a middle 

unit of soft dark-gray somewhat carbonaceous shale, 

which is usually concealed in slopes and constitutes 

about 36 percent; and (3) an upper unit of cliff-making 

sandstone, which accounts for the remaining 48 percent. 

The three lithologic units are herewith described in 

a general way. Further details are given in the type 

section and in other nearby sections. 

Lower unit.-The boundary of the lower unit with 

the underlying Kootenai Formation appears to be disconformable. The upper part of the Kootenai in this 

area consists for the most part of beds of olive-gray 

siltstone, red to maroon mudstone, olive graywacke, 

and yellowish-gray sandstone of continental origin. 

The basal 1.5-2.4 m (5-8 ft) of the lower unit of the 

Flood Member has the appearance of having been reworked from this part of the Kootenai by the earliest 

transgression of the Lower Cretaceous sea. Thus, even 

at close distance, the boundary between the Kootenai 

Formation and the lower unit of the Flood Member 

may be indistinct, and the distinction between the 

lower unit and the Kootenai is not as simple as farther 

east, where the uppermost Kootenai strata are dusky 

red. Locally, the distinction between the Kootenai and 

the lower unit of the Flood is based on more uniform 

bedding and better development of fissility in the shale 

of the lower unit of the Flood and of better sorting in 

the sandstones, the prevalence of a grayish tone that 

increases in intensity upward from the boundary with 

continued mixing in of normal sediment from land, 

considerably less clastic mica in the sandstones, the 

absence of graywacke, and the appearance of thin 

laminae of reworked carbonaceous shale or coal in the 

siltstone. Moreover, the olive-gray beds here assigned 

to the Flood Member do not contain the freshwater 

"gastropod" limestone that is diagnostic of the upper 

part of the Kootenai Formation along the mountain 

front to the west. 

The lower unit at the Flood type section and at other 

nearby exposures consists of about 6. 7 m (22 ft) of 

shale, siltstone, and sandstone. Approximately the 

basal 1.5 m consists of soft olive-gray siltstone and fissile gray silty shale. This basal section commonly is 

overlain by a few feet of olive-gray siltstone with short 

irregular laminae of reworked carbonaceous debris 

and local thin films of coal. Layers of fine-grained 

quartz sandstone, less than 0.3 m thick, make resistant 

ledges in the siltstone. Approximately the upper 4.3 m 

(14ft) of the unit at the type section consists mostly of 

a light-gray silty to fine-grained generally noncalcareous sandstone that forms a ragged cliffy outcrop. The 
8 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

sandstone, which occurs as uneven lenticular layers 

1.3--51 em (0.5--20 in.) thick, is separated by films of 

dark-gray shale or gray siltstone. In places it shows 

ripple marks and a ~~torrential" or planar type of 

crossbedding that dips southwest. Nearly every bedding plane between the sandstone and shale or siltstone exhibits casts of trace fossils (trail grooves?) and 

narrow elongate trains of sand that possibly were left 

as alimentary waste of marine worms (Mudge, 1972, 

fig. 31 ). Northeastward from the type section, the 

sandstone tends to thin and be replaced by shale. In the 

eastern part of the Portage quadrangle, only a few thin 

lenses of sandstone persist in this part of the Flood 

Member. 

Middle unit.-This unit of the Flood Member consists mostly of medium-dark-gray shale about 15 m 

(50ft) thick that rests rather sharply but conformably 

on the lower unit and probably represents the culmination of the first transgression of the Lower Cretaceous 

sea. The unit is largely concealed by colluvium and 

talus in the type section but is fairly well exposed in 

the reference section approximately 0.8 km (0.5 mi) 

north of Emerson Junction (west of Great Falls in the 

NEIJ± sec. 5, T. 20 N., R. 3 E.) and in several exposures 

along Gore Hill, southeast of the type section (fig. 3). 

The basal 6 m (20 ft) of the unit includes several 

sandstone and siltstone beds up to about 0.6 m (2 ft) 

thick. Some siltstone beds contain abundant worm(?) 

castings or burrows. Sparingly present in lenticular 

sandstones representing some kind of channel fill are 

concretionary masses of compact olive-gray (brownweathering) dolostone as well as a scattering of spherical concretions of marcasite. The shallow channels in 

which the concretions are found probably are of current origin and may have a general eastward trend. 

The middle 6 m (20 ft) of the unit consists chiefly of 

gray shale that is darker, more firm, and more fissile 

than most of the shale in the Blackleaf Formation. A 

0.6-m (2-ft)-thick bed of grayish-black papery shale at 

the base of this middle part is so highly carbonaceous 

that its weathered surface resembles coal bloom. Approximately the upper 3 m (10 ft) of the unit consists 

chiefly of nodular sandstone with thin laminae and 

interbeds of dark-gray fissile shale, and it possibly represents the final stillstand of the first marine transgression. For want of an easily recognizable boundary, 

the top of the middle unit has been placed arbitrarily 

on a 13-cm-thick bed of gray silty limestone having 

cone-in-cone structure that overlies a 0.5-m (1. 7-ft)-

thick bed of calcareous siltstone containing small dark 

chert pebbles and granules. 

Upper unit.-A conspicuous cliff-forming sandstone 

about 20 m (66ft) thick at the type section constitutes 

the upper unit (fig. 2). Essentially the entire unit is 

exposed at the type section. More limited outcrops 

occur in bluffs along the Sun River valley to about 16 

km (10 mi) west of Great Falls, where the west dip of 

the Sweetgrass arch carries it under the bed of the 

river in the SWIJ± sec. 32, T. 21 N., R. 2 E. Closely 

related exposures occur in highway cuts west of Great 

Falls along the north valley wall of the Sun River, 

where in places the boundary between the Flood 

Member and the overlying Taft Hill Member is well 

revealed. 

Approximately the lower 4.6 m (15ft) of the unit at 

the type section consists of light-gray poorly sorted 

homogeneous siltstone and silty fine-grained sandstone that has a chunky or massive appearance and 

that weathers yellowish gray. Small amounts of fine 

carbonaceous material is locally intermixed. The sandstone is friable, noncalcareous, and erodes easily to 

slightly undercut the overstanding cliff. Thus, exposures are uncommon because of concealment by colluvium and talus. Small granules of dark chert are present in the basal bed. Trace fossils are common on the 

undersides of two persistent fine-grained sandstone 

beds that form narrow resistant ledges. This is the last 

appearance of these enigmatic forms in the Flood 

Member and probably results from the lack of fine mud 

that constituted their environment as the sea became 

shallower and there was greater depositional energy. 

Approximately the middle 10.6 m (35 ft) of the unit 

is the principal cliff-making part of the Flood Member. 

It consists of a fairly homogeneous firm noncalcareous 

fine-grained sandstone with individual beds, 0.9-4 m 

(3-13 ft) thick, of slightly different hardness and texture. It is light gray to very light olive gray where 

fresh and various shades of grayish and yellowish 

orange where weathered. Sorting is moderate and 

suggests incomplete reworking of fluvial sand in an 

infralittoral (subbeach) environment. Subangular 

grains of clear quartz predominate, but some beds contain as much as 10 percent chert. Small spherical concretions of marcasite are accessory. Clay galls occur in 

some upper beds. Porosity is good. Trace fossils are 

absent, and no other fossils have been found in either 

the type section or nearby sections. 

The upper part of the unit varies considerably in 

thickness and lithology within short distances of the 

type section and appears to rest on an erosion surface. 

Thicknesses range from 3 to 8 m (10 to 27ft). At least 

five different lithologies are present. More or less in 

order of deposition, they are (1) intraformational conglomerate with lumps of ferruginous sandstone, (2) 

sandstone similar to that in the middle part of the unit 

(most abundant lithology but not present at every locality), (3) medium- to coarse-grained crossbedded 

sandstone, finely conglomeratic, with chert granules, 
FLOOD MEMBER 

(4) dark-gray carbonaceous shale, in places with inter9 

beds of olive cherty sandstone, and (5) huge concretions 

of hard, resistant, calcareous sandstone. Of these five 

lithologies, the huge brown-weathering sandstone concretions of group 5 constitute the most conspicuous 

element of this part of the unit. Two horizons of concretions are present, the stratigraphic interval between 

their respective tops ranging from 3.7 to 5.5 m (12 to 18 

ft). The concretions in both horizons are similar in appearance. Generally, they occur as widely separated 

discrete masses, but, where abundant, they nearly coalesce into a continuous layer. They commonly range 

from simple tabular forms 2.7 by 2.7 by 0.3 m thick to 

huge paraelliptical masses 6.4 by 5.5 by 3 m thick 

(fig. 3). Some occupy channels. Internal structure generally is massive, but in places it is rudely concentric. 

Some of the crossbedded sandstone of group 3 alternates and interfingers with the marine sandstone of 

2 in typical scour-and-fill arrangement. These 

sandstones may have been formed in a deltaic environment with distributary streams or in a barrier bar 

island environment. If these sandstones are upper deltaic deposits, it presupposes that the sea had all but 

disappeared either by infilling or by withdrawal toward the east, thus recording a regression of the Lower 

Cretaceous sea from the southern part of the Sweetgrass arch. The only fossils found in this part of the 

unit were bits of wood and a small fragment of a rib 

bone. The top of the unit, which is the Flood- Taft Hill 

boundary, is marked in places by a disconformity of 

small-time value where marine shale rest on thin 

cherty crossbedded sandstone. 

CHARACTER OF 1-l.OOD \!EMBER l i'\ IHJ:: NORTHERN 

S\\'EETCRI\SS ARCH Ai'\ D DIST C RBED HFl.l 

A threefold division of the Flood Member suggestive 

of that shown by the outcrops in the vicinity of Great 

Falls can be noted in the subsurface in numerous tests 

for oil and gas along the crest of the Sweetgrass arch. 

For example, in the central part of the Kevin-Sunburst 

dome, T. 34 N., R. 1 W., 121-129 km (75-80 mi) north 

of Sun River, the Flood Member is about 30 m thick 

and consists of three units of sandstone separated by 

two shale units. In all probability they are paracontemporaneous homotaxial equivalents of some of the 

units of the type section, but direct correlation has not 

been established. Generally the lower sandstone, 3-7.6 

m (10-25 ft) thick, is fine grained and tightly 

cemented. Commonly, however, it is so interbedded 

with shale that it is logged as sandy shale, and in consequence, only one or two massive sandstones are recognized. On the north flank of the dome, the shale content decreases and the grain size and porosity increase. 

There, the uppermost sandstone is commonly 1.5-9 m 

FIGURE 3.-Very large sandstone concretion in upper part of the 

upper unit of Flood Member in the NV2 sec. 36, T. 21 N., R. 2 E., 

Cascade County, Mont. 

(5-30 ft) thick, fine to slightly coarse grained, and 

slightly to moderately porous. Water has been found in 

it at several localities, as in the Border- Red Coulee oil 

field, where the "1900-foot water sand" (Erdmann and 

Schwabrow, 1941, p. 318) appers to be a homologous 

equivalent of the upper unit of the Flood. 

In the Disturbed belt west of the South arch, the 

basal sandstone of the Flood Member is thin and is 

partially replaced by shale. Farther northwest from 

Badger Creek, at least as far as the St. Mary River, a 

unit offreshwater shale and limestone lies between the 

massive varicolored mud rocks typical of the Kootenai 

Formation and the black fissile shale typical of the 

Flood Member (fig. 4). These freshwater beds are well 

exposed along the Burlington Northern Railway 

tracks, 6.4 km (4 mi) southwest of East Glacier in the 

west center of SE114 sec. 35, T. 31 N., R. 13 W. (Cobban, 

1956, p. 1001, 1002). Here the beds consist of the following sequence, numbered from oldest to youngest: 

Disconformity. M Ft 

4. Shale, olive-gray; some hard layers of siltstone at top 3.6 12.0 

3. Limestone, sandy, fossiliferous; weathers brown __ .8 2.7 

USGS Mesozoic Joe. D950: 

Protelliptio douglassi (Stanton) 

U nio reesideanus Yen 

Unio f"arri Stanton 

Stantonogyra silberlingi (Stanton) 

2. Shale, olive-gray -------------------------------- 4.4 14.5 

1. Limestone, gray, massive; weathers brown; crowded 

with small poorly preserved gastropods (Reesidella 

montanaensis Stanton?). See fig. 4 ______________ 1.2 4.0 

These olive-gray freshwater shales disappear eastward 

in the subsurface somewhere between the Disturbed 

belt and the Cut Bank oil and gas field on the west 

flank of the Kevin-Sunburst dome. Wells drilled 
10 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

FIGURE 4.- Limestone bed largely formed of freshwater gastropods 

in upper part of Kootenai Formation in the SEV. sec. 35, T. 31 N. , 

R. 13 W., Glacier County, Mont. Hammerhead rests on top of green 

mudstone unit that underlies the gastropod limestone bed. Olivegray shale containing hard layers of siltstone overlies the gastropod bed. 

southwest of the Cut Bank field in the area between 

Birch Creek and the Pondera oil field generally show 

3-6m (10-20 ft) of black-gray slightly sandy shale that 

forms the base of the Flood Member and rests on massive varicolored mudstone of the Kootenai Formation. 

The question arises whether the olive-gray shales 

should be assigned to the Kootenai Formation or to the 

Blackleaf Formation. The dark color and fissility of the 

shales favor a Blackleaf assignment, whereas the 

greenish cast of the shale and the typical Kootenai 

fauna suggest a Kootenai assignment. Perhaps the ultimate assignment will depend upon which is more 

useful to the fieldman, but in this paper they are assigned to the Kootenai Formation. 

In the subsurface on the Sweetgrass arch, the lower 

contact of the Flood Member can be easily determined 

in well cuttings by the abrupt change downward from 

black-gray shale and light-gray flaggy quartzose 

sandstone to massive varicolored mudstone and lenticular fluviatile sandstone beds of the Kootenai Formation. The sandstones in the upper part of the 

Kootenai are generally some shade of greenish gray 

and consist of a great variety of colored grains in contrast to the predominance of colorless quartz grains in 

the sandstones of the Flood Member. The sharpness of 

the Kootenai-Blackleaf boundary is considered to indicate a disconformity although all exposed beds noted 

appear conformable. Erdmann and Schwabrow (1941, 

p. 284) also have described a conspicuous disconformity 

between the Kootenai and Blackleaf Formations in the 

Border- Red Coulee oil field on the north flank of the 

Kevin-Sunburst dome. 

The upper boundary of the Flood Member cannot be 

determined in well cuttings as readily as that of the 

lower. It is marked by the change downward from gray 

soft bentonitic shale and greenish-gray glauconitic 

sandstone of the Taft Hill Member to the light-gray 

quartzose sandstone and black-gray shale of the Flood 

Member. The contact usually can be determined 

within a stratigraphic range of 3-4.5 m (10-15 ft). 

AGF ASS I Cl\'MENT A:\D CORRELATION 

The upper part of the Flood Member is assigned a 

late Early Cretaceous age (late Albian). The lower part 

is undated. 

On the east flank of the South arch, 9.6 km (6 mi) 

south of Floweree in the SE'l4SWV4 sec. 19, T. 22 N., R. 

6 E. (USGS Mesozoic locality D755), Inoceramus comancheanus Cragin and what may be an undescribed 

species of Anomia were found in the sandstone at the 

top of the Flood Member. Inoceramus comancheanus, a 

marine species of late Albian age, is characteristic of 

the Skull Creek Shale of the Black Hills, the Kiowa 

Shale of Kansas, and the South Platte Formation ofthe 

Front Range of Colorado. 

The dark shale forming the greater part of the middle unit of the Flood Member in the type section may 

have formed in a marine environment, or it may have 

been deposited in a brackish-water lagoonal environment. Only trace fossils have been found in these shale 

beds. Some of these forms occur as casts of smooth 

sinuous comparatively elongate trail grooves(?) 3 mm 

wide and as much as 18 em long on the underside of 

thin beds of sandstone parted by shale or siltstone. No 

evidence of hard parts or segmentation have been 

noted, and the forms are believed to have been made by 

some species of nematode worm moving over the surface of the mud. Burrows are present but very uncommon. The thin irregular sandstones in the upper part of 

-the lower unit of the reference section are characterized by two or three other kinds of trace fossils, 

noticeably on top of the beds. A very abundant form 

consists of short unstriated sinuous trains of very fine 

sand 1-2 mm in diameter and as much as 3 em long, 

which may have been left as alimentary waste by 

worms. Other trains, less common in occurrence, are 

6-15 mm wide, as much as 4.5 em long, and commonly 

are overlain by the smaller form. Nearly all of both 

types seem to have been broken. Even more rare is the 

form resembling Ophiomorpha, but which is fairly 

common in the outcrop just south of the Great Falls 

municipal airport. Insofar as known, no other member 

of the Blackleaf Formation contains such an abundance of trace fossils. The fragile nature of the re-
FLOOD MEMBER 11 

mains, particularly in their original condition, obviously precluded much water movement or reworking 

and suggests that they originated at or just below 

wavebase in an infralittoral or upper neritic environment and that their parent organisms probably followed that environment regardless of how ephemeral 

it may have been. Thus, although of little worth for age 

determination by paleontologic methods, they are valuable intrinsic objects for local recognition of the Flood 

Member. 

The brown-weathering ripple-marked quartzose 

sandstone forming the basal part of the Flood Member 

resembles the sandstone (First Cat Creek sand, an 

economic term) found at the base of the Colorado Shale 

in central Montana as well as some of the sandstone 

beds of the Fall River Formation of the Black Hills. 

The characteristic black-gray shale middle unit of the 

Flood Member resembles closely the 76 m (250 ft) of 

black-gray shale that immediately overlies the basal 

quartzose sandstone of the Colorado Shale of central 

Montana. It is also the type of shale that is characteristic of the very dark Skull Creek Shale of the Black 

Hills. The presence of late Albian fossils in the Skull 

Creek Shale is further evidence for correlating the 

basal part of the Flood Member with the Fall River 

Formation and with at least part of the Skull Creek 

Shale. 

DESCRIPTIVE SECTIONS OF THE FLOOD MEMBER 

The type section and the reference section for the 

Flood Member of the Blackleaf Formation described 

below are based primarily upon field descriptions. Only 

a few representative specimens have been examined 

under a binocular microscope, tested for solubility, or 

analytically screened for grain size or sorting; none 

has been analyzed petrographically or mineralogically. 

Further study along these lines, however, should prove 

very interesting and worthwhile. 

In order to make the two described sections strictly 

comparable and fully complementary, the same subdivisions have been made for each section. Color descriptions follow the Rock-Color Chart of the National 

Research Council (Goddard and others, 1948). Crossreferences within the bed descriptions and to the text 

also have been used where identification of individual 

beds seems important. 

Type section of Flood Member 

[Measured thicknesses by R. W. Lemke and C. E. Erdmann, May 15, 16, 1963, on the bluffs 

along the south side of Sun River valley, 7.2 km 14.5 mil west of Great Falls. in the 

SW'.:,NW\4NE1•4 sec. 7, T. 20 N., R. 3 E .. Cascade County !Great Falls quadrangle I I fig. 51] 

Pleistocene glacial lakebed silt. 

Blackleaf Formation: 

Taft Hill Member, basal part: 

M Ft 

33. Bentonite________________________________ 0.2 0.5 

32. Shale, medium-gray, soft, slumped ________ .6 2.1 

Type section of Flood Member-Continued 

Blackleaf Formation-Continued 

Disconformity. 

Flood Member: 

Upper unit: M Ft 

31. Shale, gray; interbedded with 2-4-cm layers 

of cherty ripple-marked medium-lightgray sandstone that weathers to olive gray .9 2.9 

30. Sandstone, light-yellowish-gray, silty, finegrained, thinly and irregularly bedded; 

weathers to light brownish orange; slight 

carbonaceous wisps along parting planes 

(current bedding). This is the horizon containing concretions which are not present 

on northwest side of draw but are present 

on southeast side of draw and abundant 

on north side of Sun River valley in 

NW%NE%NE% sec. 5, T. 20 N., R. 3 E.__ 3.2 10.4 

24 .............. . ... . ·.: .·.· .. :.·.·: ............ 

... 

Q) 

c. 

c. 

~ 

LITHOLOGIC 

SYMBOLS 

Sandstone 

~ 

~ 

Siltstone 

~ 

~ 

Shale 

METRES FEET m 0 0 

20 

40 

60 

20 

Bentonite 

SEC. 7, 

T. 20 N., 

~N··.· 31 · .... ; B 

;:,:··T 

31 

0 1 MILE 

I 

0 1 KILOMETRE 

FIGURE 5.-Type section of the Flood Member of the Blackleaf Formation and map showing line of measured section, 

sec. 7, T. 20 N., R. 3 E. Numbers on the left side of the 

column are key beds in the measured section. On the map, 

the arrow points upward stratigraphically; B, base, and T, 

top of member. 
12 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Type section of Flood Member-Continued 

Blackleaf Formation-Continued 

Flood Member-Continued 

Upper unit-Continued M Ft 

29. Sandstone, medium-light-gray salt-andpepper appearing, generally massive; 

weathers yellow gray stained with limonite; consists mostly of well-rounded to 

subrounded quartz and slightly less (3:2) 

chert and a few brown grains in a matrix of 

secondary calcite ______________________ .3 1.1 

28. Sandstone, concretionary, calcareous, 

medium-light-gray (N 6); weathers dark 

yellowish brown (lOYR 4/3) to moderate 

brown. Probably nearly equal parts of secondary calcite cementing fine channel sand 

containing a small amount of ferruginous 

material. Concretion about 12 m (40 ft) 

long with maximum thickness at southeast 

end. Top flat, thinly bedded, flush with surface of underlying bed; lower part massive, 

definitely truncating bedding of underlying sandstone. Unit thins to featheredge 1.4 4.5 

27. Sandstone, grayish-yellow, fine-grained, 

very friable; bedding indistinct; thickness 

varies -------------------------------- 3.0 10.0 

26. Sandstone, grayish-yellow, medium-grained, 

well-sorted; some chert; crossbedded, with 

dip offoreset beds to the southwest. Generally massive except for about five distinct 

parting planes. Looks like a near-beach 

deposit -------------------------------- 1.6 5.1 

25. Sandstone, grayish-yellow, fine-grained, indistinctly bedded, friable, slightly calcareous------------------------------------ 1.9 6.2 

24. Sandstone, grayish-yellow, fine- to medium-grained, massive; weathers to grayish orange yellow; mostly quartz. Top surface has a thin film of medium-gray clay. 

Forms a prominent ledge overhanging 

beds below ---------------------------- 3.8 12.6 

23. Sandstone, light-yellowish-gray, indistinctly bedded, fairly friable and erodible. Upper 36 em weathers yellowish orange 

(10YR 6/6) and is thinner bedded than 

part below ____________________________ 1.4 4.7 

22. Sandstone, light-olive-gray; fine-grained, 

hard to soft; weathers pale yellowish 

brown to yellowish gray. Some flute casts 

and abundant casts of trace fossils ______ . 7 2.3 

21. Sandstone, light-gray, fine-grained, silty, 

poorly sorted; some finely macerated carbonaceous debris; nodular or chunky appearance. Trace fossils on some bedding 

surfaces. Basal part shaly siltstone ______ .9 3.2 

20. Sandstone, light-gray, silty, poorly sorted; a 

few rounded granules of dark chert; nodular or chunky. Contains occasional discrete 

masses of concretionary marcasite as large 

as 6 by 10 em. Supports a low rounded 

bench__________________________________ 1.4 4.6 

Thickness of upper unit ______________ 20.1 66.0 

Disconformity. 

Type section of Flood Member-Continued 

Blackleaf Formation-Continued 

Flood Member-Continued 

Middle unit: M 

19. Limestone, medium-dark-gray, silty; weathers light gray to pale yellowish gray; concretionary, with cone-in-cone structure __ .1 

18. Siltstone, medium-gray; highly calcareous, 

grading upward into sandy calcareous 

siltstone containing a few granules of 

polished dark chert ____________________ .5 

17. Shale, dark-gray (N 3), very fissile________ .5 

16. Sandstone, medium-gray, medium- to 

coarse-grained; thin nodular layers separated by laminae of dark-gray shale. 

Poorly exposed ------------------------ 1.8 

15. Slope concealed by colluvium and talus. 

Probably soft dark-gray fissile shale ____ 6.3 

14. Shale, grayish-black, carbonaceous, soft, fissile, papery ---------------------------- .6 

13. Shale, medium-dark-gray, poorly exposed__ 4.6 

12. Sandstone, yellowish-gray, medium-grained; 

upper surface ripple marked, ferruginous, 

weathers medium brown. Lower 2 em silty, 

containing small fragments of mineral 

charcoal (fusain) and trace fossils. A local 

asymmetric channel sand trending approximately east; maximum width about 12m 

(40 ft) with thickest section on north side .6 

11. Shale, dark-gray, silty, fissile ____________ .6 

Thickness middle unit_ _______________ 15.3 

Lower unit: 

10. Sandstone, light-gray, fine- to mediumgrained, slightly ferruginous; in wavy 

layers 12-23 em thick, alternating with 

somewhat thinner beds of siltstone. More 

indurated and better sorted than lower 

part of unit. Crossbedding, dipping southwest. Uppermost layer, 15 em thick, of 

hard medium-gray sandstone conspicuously overhangs beds below. Forms a ragged cliffy slope ________________________ 1.0 

9. Siltstone, shaly, yellowish-gray, moderately 

friable; weathers grayish orange; contains 

three or four thin more resistant beds of 

fine-grained quartz sandstone with a little 

clastic mica. Trace fossils. Forms a ragged 

cliffy slope __ __ __ _ _ ____ __ _ _ _ _ ________ _ _ 3.1 

8. Sandstone, light-olive-gray, quartzose; interbedded with about equal amounts of 

shaly siltstone with laminae marked by 

films of carbonaceous matter; trace fossils 

sparsely present, poorly preserved. Beds 

parted by short 1-cm-thick lentils of 

medium-dark-gray shaly siltstone which 

appear to represent mud-filled troughs of 

ripple marks. Depositional environment 

probably marine tidal flat ______________ 1.0 

7. Slope, concealed by colluvium ____________ 1.0 

6. Siltstone, yellowish-gray (5Y 7/2), poorly fissile; rests on light-olive-gray silty shale 

<5Y 6/1)________________________________ .6 

Thickness of lower unit ______________ 6.7 

Total measured thickness of Flood 

Ft 

.4 

1.6 

1.5 

6.0 

20.7 

2.0 

15.0 

2.0 

2.0 

50.2 

3.7 

10.0 

3.2 

3.2 

2.0 

22.1 

Member __________________________ 42.1 138.3 
FLOOD MEMBER 13 

Type section of Flood Member-Continued 

Disconformity. 

Kootenai Formation, upper part: M Ft 

5. Sandstone, arkosic, light-gray; fine- to 

medium-grained, friable, slightly to moderately calcerous; weathers light tan ____ .5 1.6 

4. Shale, sandy, light-olive-green, only slightly 

fissile__________________________________ .2 .7 

3. Sandstone, grayish-brown, very fine grained, 

vertically jointed, calcareous, wellindurated______________________________ .2 .8 

2. Shale, sandy, maroon, only slightly fissile__ .2 . 7 

1. Shale, sandy, light-olive-green, slightly fissile____________________________________ ~ 1.7 

Total measured thickness of part of 

Kootenai Formation________________ 1.7 5.5 

Reference section of Flood Member 

[Measured by C. E. Erdmann, Oct. ~10. 1965, on south-pointmg spur on north valley wall 

of Sun River. about .5.6 km 13.5 mi 1 northwest of the city of Great Falls, 0.5 km 10.3 mi I 

northeast of Emerson Junction of Burlington Northern Railroad spur track of Chicago, 

Milwaukee, and St. Paul Railway. Base of section is in the NW 4NE .~NEh sec. 5. T. 20 

N., R. 3 E.; top of section is in the SW .~SW\·4SE ·~ sec. 32, T. 21 N., R. 3 E.] 

Pleistocene deposits (very thin). 

Blackleaf Formation: 

Taft Hill Member, basal part: 

64. Shale, dark-gray, fissile, poorly exposed ___ _ 

63. Sandstone, medium-coarse, cherty, calcareous. Surface weathers dark brown _____ _ 

62. Shale, very dark gray, very fissile 

Flood Member: 

Upper unit: 

61. Sandstone, concretionary, fine-grained, calcareous, moderately gray. Concretions 

(large) are epigenetic and occupy scour 

M Ft 

0.6+ 2.0+ 

.3 1.0 

1.7 5.5 

channels in underlying sandstone 1. 7 5.6 

60. Sandstone, light-yellowish-gray, finegrained, calcareous. Irregular wavy bedding 3-6 em thick, parted by thin 6-mm 

laminae of medium-gray shale __________ 3.3 10.7 

59. Sandstone; similar to bed 56 below. Poorly 

sorted. Fine horizontal laminae. No crosslamination ____________________________ .2 .5 

58. Sandstone, grayish-orange, fine-grained, 

calcareous. Similar to upper part of bed 

53 ------------------------------------ 1.3 4.3 

57. Sandstone, light-yellowish-gray, fine- to 

coarse-grained, poorly sorted, cherty, tuffaceous, cross-laminated; thin planar 

layers dip 20° S.-20° W. Makes ledge ____ .4 1.4 

56. Sandstone, grayish-orange, fine-grained. 

Similar to upper part of bed 53 below____ .6 2.0 

55. Sandstone, light-yellowish-gray, mediumgrained, massive, hard. Makes ledge with 

small overhang ________________________ .4 1.2 

54. Sandstone, light-yellowish-gray, finegrained, calcareous; weathers grayish 

orange. Irregular wavy layers 3-6 em thick 

parted by thin (0.5 em) laminae of medium-gray shale________________________ 2.0 6.7 

53. Sandstone, as immediately above, but fresh, 

light-yellowish-gray, unweathered. Makes 

top of excavated slope __________________ 2.6 8.5 

52. Sandstone, olive-gray, medium-grained, 

poorly sorted, soft; contains irregular 

Reference section of Flood Member-Continued 

Blackleaf Formation-Continued 

Flood Member-Continued 

Upper unit-Continued 

52. Sandstone, etc.-Continued M 

masses and partings of soft medium-gray 

shale ---------------------------------- .5 

51. Sandstone, light-gray, fine-grained, hard, 

massive. Trace fossils on both base and 

top------------------------------------ .1 

50. Sandstone, light-gray, fine-grained; thin 

(3-cm) partings of medium-gray fissile 

shale at 15-cm intervals ________________ 1.0 

49. Siltstone, olive, soft, clayey ______________ .1 

48. Sandstone, light-gray, fine-grained, faintly 

calcareous; traces of carbonaceous debris; 

in small lenticular to nodular masses 3-7 

em long, commonly enclosed by thin (1 em) 

rinds of sandy gray clay. Bedding indistinct; resistant, ledge making. Upper 0.5 m 

Ft 

1.5 

.3 

3.3 

.4 

weathers grayish orange________________ 1.7 5.5 

47. Shale, medium-gray (east side of spur) ____ .8 2.5 

46. Sandstone, olive, soft, massive; irregular inclusions of shale________________________ .6 2.0 

45. Sandstone, light-yellowish-gray, poorly 

sorted, fine- to medium-grained, hard, 

faintly calcareous; a sprinkle of fine to 

coarse grains of rounded polished black 

chert, flakes of carbonaceous material, and 

clay galls; in small lenticular and nodular 

masses 2-8 em long and 2-10 em thick, 

some of which are encased or wrapped by 

rinds of light-medium-gray clay. Bedding 

very indistinct. Makes broken rubbly 

ledge. Lower 3-6 em weathered dark yellowish orange (possible diastem) .6 1.9 

Total thickness of upper unit 17.9 58.3 

Middle unit: 

44. Siltstone, olive-gray, sandy; in layers about 

15 em thick interbedded with mediumdark-gray shale and olive shale. Surface 

weathers into tough structureless mudstone---------------------------------- 2.4 8.0 

43. Shale, medium-dark-gray; thin lentils of 

olive sandstone ________________________ 1.2 4.0 

42. Shale, olive, fissile ______________________ .3 1.0 

41. Sandstone, light-olive-gray, poorly sorted, 

fine- to coarse-grained, subangular, noncalcareous, soft, friable; surface weathers 

dark yellowish orange with small internal 

splotches of dusky yellow and light olive 

brown, phasing into the external color; 

some rounded granules of gray chert; all in 

a matrix of light-gray clay which also 

marks indistinct laminae -------------- .1 .4 

40. Shale, dark-gray, fissile (as below) ________ .5 1.7 

39. Sandstone, olive-gray, fine-grained; thin 

slabby layers. Base slightly wavy (undulating), marked by 1 em of dark-yellow 

ochre__________________________________ .3 .9 

38. Shale, black, very fissile (papery). Very thin 

layer of light-gray fine-grained finely 

laminated sandstone 0.8 m above base of 

bed____________________________________ 1.4 4.5 
14 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Reference section of Flood Member-Continued 

Blackleaf Formation-Continued 

Flood Member-Continued 

Middle unit-Continued 

37. Sandstone, olive-gray, fine-grained, finely 

laminated; weathers brown. Base exhibits 

casts of worm trails and a burrow. Top 

marked by 1 em of dark-yellowish-orange 

siltstone whose surface shows trace fossils ___________________________________ _ 

36. Shale, dark-gray, fissile; laminae very thin, 

M Ft 

.1 .4 

almost papery, soft ____________________ 1.4 4.5 

35. Siltstone, light-olive-gray ________________ .1 .2 

34. Sandstone, light-olive-gray to mediumlight-gray, fine-grained, poorly sorted, 

massive, poorly stratified; weathers light 

yellowish brown to dark yellowish orange. 

Organically reworked; numerous galls and 

small irregular masses of dark-gray clay 

shale may represent burrow fillings from 

overlying bed. Oval siltstone casts of flattened tubes (worm burrows?)____________ .7 2.4 

33. Shale, dark-gray, soft, fissile. Thickens to 

1.7 m on west side of spur ______________ .8 2.8 

32. Siltstone, light-olive-gray; encloses a 12-cm 

pale-yellowish-brown bed in middle. Thin 

layers of shaly sandstone at base and top. 

Unit thickens to 0.9 m on west side of 

spur ---------------------------------- .5 1.6 

31. Shale, light-olive-gray. Similar to bed 29, 

below---------------------------------- 1.8 5.8 

30. Siltstone, pale-yellowish-brown (10YR 6/2) .1 .4 

29. Shale, light-olive-gray (5Y 6/1), poorly fissile, soft ------------------------------ .3 1.0 

28. Siltstone, light-olive-gray; in beds 12-15 em 

thick parted by 1-cm laminae of finegrained olive sandstone. Numerous trace 

fossils (worm castings or trails?) on each 

upper surface. Thin layer of siltstone in 

middle weathers reddish brown. Layer of 

reddish-weathering marcasite(?) nodules 

at top, 3--6 em thick ____________________ .5 1.6 

27. Siltstone, light-olive-gray. Contains thin 

3--6-cm-thick sandstone layers; those in 

middle show ripple-marked upper surfaces 

with relief of 2 em. Numerous trace fossils------------------------------------ .4 1.3 

26. Dolostone. Hard compact masses about 60 

em in diameter. Usually considerably shattered. Fresh surfaces medium gray, but 

weather to moderate brown (5YR 4/4) in 

rinds as much as 2 em thick. Some fractures are dusted with small calcite crystals ----------------------------------- .1 .4 

25. Siltstone, light-medium-gray, soft, clayey __ ~ ____& 

Total thickness of middle unit __________ 13.2 43.4 

Diastem(?) 

Lower unit: 

24. Sandstone, light-yellowish-brown, finegrained; in layers 9--15 em thick parted by 

laminae of medium-gray siltstone. Trace 

fossils (casts of worm trails?) on upper surfaces. Rock breaks into flat slabby handsized pieces 1-2 em thick________________ 1.2 3.8 

Reference section of Flood Member-Continued 

Blackleaf Formation-Continued 

Flood Member-Continued 

Lower unit-Continued 

23. Siltstone, light-olive-gray, sandy, soft_ ____ _ 

22. Sandstone, light-yellowish-gray, finegrained; in layers 9--15 em thick, interbedded with light-olive sandy siltstone. Upper 

surfaces show trace fossils _____________ _ 

21. Siltstone, light-olive-gray; 15-cm layer of 

light-olive-gray fine-grained massive 

sandstone in middle which is overlain by a 

6-cm layer of medium-dark-gray siltstone. 

Trace fossils on upper surface of the 

sandstone _____________________________ _ 

20. Sandstone, light-yellowish-gray, finegrained; small flakes of carbonaceous debris, faintly calcareous; in even 6-12-cm 

finely laminated layers separated by thin 

(6 mm) laminae oflight-gray siltstone with 

worm castings. Hard, resistant. Makes 

subdued ledge with slightly undulating 

top 1 ----------------------------------

19. Sandstone, light-yellowish-gray, very fine 

grained; in 6-12-cm layers interbedded 

with layers of light-olive-gray siltstone of 

about same thickness. Bedding marked by 

very thin laminae (films) of gray clay. 

Trace fossils (casts of worm trails?) present 

but fragmentary _______________________ _ 

18. Sandstone, light-olive, very fine grained, 

silty ____________________ --------------

17. Siltstone (or very fine grained sandstone), 

light-olive-gray, massive, nonfissile, 

tough _________________________________ _ 

16. Sandstone (or siltstone), light-yellowishbrown to moderate-yellowish-brown (lOYR 

5/4), fine-grained. Makes weak massive 

ledge. Contact with underlying bed gradational _________________________________ _ 

15. Claystone, bentonitic, silty; predominantly 

medium dark gray but olive at base and 

top. Breaks with subconchoidal fracture; 

M Ft 

.2 .6 

.4 1.2 

.5 1.7 

.7 2.2 

.4 1.4 

.3 .9 

.3 1.0 

.6 1.8 

becomes flaky upon weathering __________ ___l.! ____!Q 

Thickness of lower unit________________ 6.0 19.0 

Total thickness of Flood Member ______ 36.8 120.7 

Erosional disconformity. 

Kootenai Formation, upper part: 

14. Siltstone, light-olive-gray (5Y 6/1 ), thinly laminated; chippy where sandy__________________ 0.4 1.3 

13. Mudstone, medium-dark-gray, sandy, poorly fissile, hard, tough; some slight mottling by light 

olive green -------------------------------- .2 .8 

12. Siltstone, light-yellowish-gray, soft, clayey ____ .1 .5 

11. Sandstone, light-yellowish-gray, mediumgrained, noncalcareous, firm, massive; makes 

subdued ledge------------------------------ .5 1.6 

10. Claystone; light-greenish-gray when damp; 

weathers light yellowish gray ______________ .1 .3 

9. Claystone, soft; predominantly dusky red with 

some mottling (blebs) of dusky yellow green 

(5GY 5/2) ---------------------------------- 1.9 6.2 

8. Siltstone; dull medium olive gray mottled pale 
TAFT HILL MEMBER 15 

Reference section of the Flood Member-Continued 

Kootenai Formation, upper part-Continued 

8. Siltstone, etc.-Continued M Ft 

olive gray by small rounded masses of fine 

sandstone. In flat layers 9-15 em thick containing fine carbonaceous debris. Slakes down into 

thin flakes. Some manganese stain with a 

dull-purplish cast __________________________ .8 2.6 

7. Sandstone, fine-to medium-grained; cemented by 

calcareous clay; medium gray with greenish 

cast; some joint faces weather moderate brown. 

Unit is a lenticular body about 12m wide. Bedding in lower 30 em is thin and indistinct; 

upper part more resistant, with hard 6--

9-cm-thick massive layers that weather into 

large rounded blocks. Upper surface cut by 

sand-filled joint cracks 5 em wide that strike N. 

60° E. Occurrence suggests a lenticular depression fill trending N. 40° W. but not a runningwater or channel sand because of the absence of 

planar cross-lamination. Other sand bodies 

occur nearby at same horizon ______________ 1.5 5.0 

6. Mudstone, brownish-red. A wedge thinning 

rapidly to the northeast into a basin of overlying sand mass and seems to come up on other 

side. Local absence may result from scour in 

channel or it may have been squeezed out by 

load effect. Top gradational upward·__________ .2 .8 

5. Siltstone, sandy, light-olive-gray. Lower part 

clayey with nodular weathering. Upper 2.5 em 

thinly laminated. Base cut into underlying 

bed________________________________________ .5 1.5 

4. Mudstone, clayey; alternate layers of brownish 

red and light olive gray but locally is all brownish red; crossbedded ________________________ .8 2.5 

3. Mudstone, grayish-red (lOR 4/2); similar to bed 

1. Thin (6 em) layer of greenish-gray siltstone 

21 em above base. Nodular weathering very 

well developed above green layer and indicates 

stratification. Nodules are oval in cross section 

and may be 3.6 em long and 1.5 em thick ____ .7 2.3 

2. Siltstone, light-olive-gray, clayey, firm, compact, 

massive; joint faces smooth, but interior of bed 

has nodular structure. Thickness fairly uniform -------------------------------------- .4 .14 

1. Mudstone, grayish-red (lOR 4/2), silty, finely 

micaceous, firm, massive, compact. Base concealed ____ __ _ _ _ _ __ __ _ _ __ __ _____ _ _ _ ___ _ __ __ 1.2 + 4.0+ 

Total measured thickness of part of Kootenai 

Formation __ _ _ ____ __ _ _ __ _ ____ _ __ __ ____ _ _ 9.3 + 27.4 + 

TAFT HILL MEMBER 

NA.~IE AND DEFINITION 

The Taft Hill Member overlies the Flood Member 

and underlies the Vaughn Member. The Taft Hill 

Member consists chiefly of medium-gray soft bentonitic clayey to silty shale and fine-grained greenish-gray 

glauconitic sandstone; the unit is the glauconitic 

sandstone member of Cobban (1951a, p. 2177-2179). 

The member represents shelf deposits of the neritic 

zone of the first major transgression of the Lower Cretaceous sea. Cobban, Erdmann, Lemke, and Maughan 

(1959, p. 2790) named the member for exposures along 

the east and north slopes of Taft Hill, a prominent 

dissected bench extending from 2.4 to 9.6 km (1.5-6 mi) 

south of the village of Vaughn (Vaughn and Cascade 

quadrangles). The type section is a composite of measurements from four localities south of the Sun River 

between the east face of Taft Hill and a point 8 km 

(5 mi) farther east, the top being near the NI4 cor. sec. 

12, T. 20 N., R. 1 E. (Vaughn quadrangle), and the base 

at Newman Spring, in the center of sec. 9, T. 20 N., R. 2 

E. (Great Falls quadrangle). In general the beds in the 

lower half are so soft and poorly indurated that 10-20 

percent of the total thickness of the type section is concealed. However, exposures made in 1960 and 1961 

north of the Sun River during construction and rerouting of U.S. Interstate Highway 15 between Great Falls 

and Vaughn junction permitted study of excellent sections of the lower part of the member and of the underlying Flood Member. Descriptions and measurements 

from these localities, which are 5.6-8 km (3.5-5 mi) 

north and northeast of the type section of the Taft Hill 

Member, are described in this report as a composite 

reference section. Thus only about 1 m of the lower 

40 m (132ft) of the member in the Great Falls area has 

not been described from surface exposures and this 

interval was penetrated during 1960 by the U.S. Corps 

of Engineers in core hole I-10 (310x), NWI4NWI4 sec. 

19, T. 21 N., R. 2 E., near Vaughn junction. 

THICK="JESS 

The Taft Hill Member is 7 4 m (242 ft) thick in its 

type section south of the Sun River and is 76 m (249ft) 

thick north of the Sun River in the area of the reference section. It has a comparatively uniform thickness 

over the central and eastern part of the Sweetgrass 

arch, with the thickness in the Great Falls area being 

about average. On the east flank of the South arch, 

Cobban measured 81 m (266ft) in outcrop in sec. 17, T. 

22 N., R. 6 E. (Lander Crossing quadrangle), and 74 m 

(242ft) between the depths of718.7 and 792.5 m (2,358 

and 2,600 ft) in the Texas Company-Kiemele well1 in 

the SEI4NW%NEI4 sec. 26, T. 31 N., R. 13 E., Hill 

County. This line represents the east limit of the area 

in which the name may properly be applied. Farther 

east in the Bearpaw Mountains, Kerr and others 

(1957) reported 30-46 m (100-150 ft) of glauconitic 

beds in the Newcastle Sandstone equivalent in the 

Colorado Shale. Cuttings from a few tests for oil and 

gas along the crest of the South arch and high up on its 

west flank reveal thicknesses of 61-82 m (200-270 ft) 

for the Taft Hill. In the Pondera oil field (T. 27 N., R. 4 

W. ), there is a good sample thickness of 73 m (240 ft). 

In several old wells in the central and eastern part of 
16 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Kevin-Sunburst dome (Tps. 32-34 N., Rs. 1 E.-1 W.), 

where in one well the member was cored, thicknesses 

range from 70 to 72 m (230 to 235 ft). In the West 

Kevin district (T. 35 N., R. 3 W.), however, the member 

has thinned to 68.6 m (225 ft), and in one well in the 

Border-Red Coulee field (T. 37 N., R. 4 W.) to 61 m (200 

ft). Apparently the thinning to the west and north results from replacement by nonmarine sediments (discussed in description of the Vaughn Member). In tests 

drilled along the southwest margin of the Sweetgrass 

arch, the Taft Hill Member thins to 55 m (180 ft) or 

slightly less. Farther west, in the. Sun River Canyon 

area, Mudge (1972, p. A58-A60) noted that the Taft 

Hill Member thickened westwardly from 68 to 183 m 

(225--600 ft). In more recent investigations, Gwinn 

(1961) applied the name Taft Hill Member of the 

Blackleaf Formation to 273-305 m (900-1,000 ft) of 

strata in the noncontiguous Drummond area (T. 11 N., 

R. 12 W., Granite County), about 80 km (50 mi) southwest of Great Falls, where he described the lower 91 m 

(300ft) as ~~crossbedded marine sandstones." However, 

Gwinn did not mention the presence of glauconite (the 

definitive mineral at the type section), and although 

his basal unit well may be the time equivalent of some 

part of the type Taft Hill Member, further study is 

required before the name can be definitely applied in 

the Drummond area. 

OUTCROP DISTRIBUTION 

Outcrops of the Taft Hill Member on the Sweetgrass 

arch are most extensive on the southern part of the 

South arch. Some of the best exposures, including parts 

of the type and reference sections, are near Vaughn, 

where they have been mapped by Maughan (1961). 

West from Vaughn the member crops out upstream 

along the south bank of the Sun River for 8 km (5 mi) 

before the regional west dip of the South arch carries it 

below the valley floor. South of Vaughn the upper part 

of the member is exposed extensively along the east 

side of Taft Hill from the north edge of sec. 35, T. 21 N., 

R. 1 E., in the Vaughn quadrangle, south at least to a 

point 6.4 km (4 mi) west ofUlm in sec. 3, T. 19 N., R. 1 

E., in the Cascade quadrangle. Locally, the lower part 

is exposed in ravines or along roadcuts for 3.2-4.8 km 

(2-3 mi) east of Taft Hill. Northwest from Vaughn the 

member crops out fairly continuously up Muddy Creek 

for about 6.4 km (4 mi). Eastward it is exposed across 

the southern part of the Great Falls quadrangle, and 

northeastward through the Portage quadrangle to the 

Missouri River where outcrops extend into the Lander 

Crossing quadrangle and swing south along the west 

flank of the Highwood Mountains. Exposures also have 

been observed in the Sweetgrass Hills where the 

member is distinguished by a 6--15-m (20--50-ftl glauconite-bearing sandstone that probably is equivalent 

to the Second Whitlash sand of economic usage at 

Whitlash dome (Bartram and Erdmann, 1935, p. 27 4). 

Along the Rocky Mountains front west of the South 

arch, a nonglauconitic marine time-equivalent of the 

Taft Hill Member crops out in the Disturbed belt. It is 

well exposed along the western edge of the tier of townships from T. 20 N., R. 8 W., north through T. 25 N., R. 

8 W., and, thence, northwest to Birch Creek in T. 28 N., 

R. 10 W. Farther to the northwest, along the southeast 

boundary of Glacier National Park, those strata 

undergo a facies change by interfingering with and 

finally complete replacement by the nonmarine bentonitic Vaughn Member. 

Glauconitic sedimentation, probably in an environment paralleling its occurrence in the Taft Hill 

Member, also may have operated more or less contemporaneously with the deposition of the Spinney Hill 

Member of the J oli Fou Formation in west-central 

Saskatchewan (Edwards, 1960), and the Bluesky Formation at the base of the Fort St. John Group in the 

Peace River country of northern Alberta. Equivalents 

of both the Joli Fou and the Bluesky Formations occur 

at the base of the Colorado Group as that group is used 

in that report area. Indirect correlation with the Taft 

Hill is suggested by the relationship of the Joli Fou 

Formation to the Skull Creek Shale in the southern 

part of the Williston basin (Edwards, 1960, p. 150). A 

much more direct correlation may be the development 

of glauconite in sandstone that rests directly on the 

Kootenai Formation in the Northern Pump-Emil 

Guertzgen well 1, SW%NW%NE%. sec. 2, T. 31 N., R. 

19 E., Bowes field, Blaine County, north of the Bearpaw Mountains. In the absence of the Flood Member 

which has wedged out farther west, the occurrence of 

Taft Hill lithology on Kootenai is directly analogous to 

the Spinney Hill sand-Mannville Formation relationship in Saskatchewan. 

GENERAL DESCRIPTION 

As noted previously, the boundary of the Taft Hill 

Member with the underlying Flood Member is essentially transitional and has been placed arbitrarily at 

the first appearance of marine rocks following the final 

episode of deltaic or barrier-island deposition marking 

the close of the Flood Member. The contact with the 

overlying Vaughn Member appears to be a disconformity marking a change from a marine to a continental 

environment. 

Dark-gray soft bentonitic shale and greenish-gray 

glauconitic sandstone characterize the Taft Hill 

Member. The shale constitutes about 35 percent of the 

member in the type section and in the reference section, glauconitic sandstone about 25 percent, and 
TAFT HILL MEMBER 17 

nonglauconitic sandstone, siltsone, and thin concretionary limestone beds about 40 percent of the 

member. The sandstone beds generally crop out as low 

rounded ledges, whereas the shale beds form mediumgray poorly vegetated spongy slopes. Some of the concretionary limestone beds exhibit cone-in-cone structure and weather brown. 

On the basis of lithology, the member can be subdivided into three units. The lower unit consists chiefly 

of bentonitic shale, the middle unit of glauconitic sandstone, and the upper unit of mostly quartzose 

sandstone, siltstone, and bentonitic shale. The following detailed descriptions of these units are based upon 

descriptions of lithologies in the type and reference 

sections and in other exposures in the vicinity. (See 

descriptions and graphic sections that follow later.) 

Lower unit.-The lower unit is approximately 22 m 

(72 ft) thick at the type section. Shale, medium-darkgray and poorly to moderately fissile, is the predominant lithology. Siltstone and glauconitic sandstone 

constitute most of the remainder. A few thin beds of 

bentonite account for the rest. A bentonite bed 0.6 m (2 

ft) thick and about 0.3 m above the base of the unit in 

the type section makes an easily recognizable marker 

for differentiating the Taft Hill Formation from the 

underlying Flood Member because of the absence of 

bentonite beds in the underlying member. The bentonite bed is overlain by approximately 7 m (23 ft) of 

dark-gray fissile shale which closely resembles the 

shale of the middle unit of the Flood Member. Overlying the shale is a 0.6-m (2-ft)-thick glauconitic 

sandstone, which, locally, is the first appearance of 

glauconite in the Taft Hill Member. Above the 

glauconitic sandstone is about 6 m (20 ft) of soft darkgray to medium-light-gray shale and siltstone. The 

upper approximate 7.6 m (25ft) ofthe unit consists of a 

series of glauconitic sandstones and interbedded thin 

bentonites. The glauconitic sandstones are similar to 

those of the overlying middle unit, but they are more 

poorly indurated and, except for the lowest 1.5-m (5-

ft)-thick sandstone, are rarely exposed. Knowledge of 

this part of the unit is obtained from the composite 

reference section. 

Middle unit.- This unit is approximately 9 m (30ft) 

thick at the type and reference sections. Because of the 

predominant lithology, it has commonly been referred 

to as "the glauconitic sandstone." Beds immediately 1 

underlying the middle unit are not exposed in the type 

section or in other natural outcrops in the area. However, the basal part of the unit and about a 7 .6-m (25-

ft) section of the underlying beds of the middle unit are 

exposed in a highway cut about 0.6 km to the east 

(NE14SEJ.,4 sec. 19, T. 21 N., R. 2 E.), and about 0.3 m of 

shale is exposed overlying the unit in a highway cut at 

Vaughn junction (approximate center of sec. 19, T. 21 

N., R. 2 W.). In the absence of a more continuous sequence of beds, the middle unit in the type section and 

in other places has been positioned into the reference 

section of the member by aerial projection, taking into 

account the local dip of the beds. In the type section, 

the middle unit has been described as one bed (bed 14). 

In the reference section, this unit has been subdivided 

into a number of beds, more or less transitional with 

each other. Parts of the unit crop out in a number of 

places from Vaughn eastward across the southern 

parts of the Vaughn and Great Falls quadrangles. 

Farther east the glauconitic sandstone thins or grades 

laterally into shale, as indicated by its absence along 

the Missouri River in the eastern part of the Portage 

quadrangle. Southwest of the type section, the unit appears to thicken, 15 m (50 ft) of very glauconitic 

sandstone being penetrated at depths of 104--119 m 

(340-390 ft) in a well 4.8 km (3 mi) to the south in the 

NW1/4SEJ.,4 sec. 12, T. 20 N., R. 1 E. Also, glauconitic 

sandstone as thick as or thicker than in the well crops 

out farther south in the SW14 sec. 17, T. 17 N., R. 1 E., 

but the strata are much disturbed and an accurate 

measurement has not been made. 

The glauconitic sandstone is mostly olive green 

(lOY 5/2 to lOY 5/4), fine to medium grained, and in 

layers (beds) commonly 15-60 em (0.5-2 ft) thick. A 

few thin clay-ironstone concretionary masses having 

cone-in-cone structure constitute about the only variations in an otherwise essentially homogeneous unit. 

The upper part of the sandstone generally is thicker 

bedded and coarser grained than the lower part. The 

upper part also commonly exhibits a planar ("torrential") type of cross lamination, which is indicative of a 

shallow current entering quiet water of greater depth. 

Bedding surfaces in places show worm(?) castings or 

borings on ripple marks. Chunks of petrified wood, 

8--10 em long, were found near the top of the sandstone unit at one locality (NWJ.,4NWI,4 sec. 26, T. 21 N., 

R. 2 E., just east of Vaughn. The lower part of the 

sandstone unit is argillaceous and generally more 

friable. 

Samples from the middle and upper parts of the 

sandstone from the type section 2.4 km (1.5 mi) south 

of Vaughn and from an exposure at the junction of U.S. 

Highways 89 and 91 about 0.4 km east ofVaughn were 

studied for mineral composition by W. T. Pecora (Cobban, 195la, p. 2177-2178). He reported that the samples consisted of approximately 30 percent clear 

quartz, 25 percent murky chert, 10 percent glauconite, 

5 percent fresh feldspar, and 30 percent coarse calcite 

(cementing material). The quartz and chert grains are 

well rounded and well sorted. Pecora stated that the 

glauconite was an impure type formed "in a marine 
18 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

environment by replacement of material that was originally ferromagnesian, such as hornblende." 

Upper unit.-In the type section the upper unit is 

approximately 43 m (140 ft) thick. The entire unit is 

exposed at the type section, except approximately the 

basal 1.2 m, which probably is bentonitic shale. The 

thickness of the covered 1.2 m interval between the 

upper unit and the middle unit was estimated by areal 

projection between the two localities, taking into account the local dip. 

The upper unit consists predominantly of bentonitic 

shale and bentonitic siltstone, which, fOgether with 

three beds or bentonite beds, form a generally gray 

poorly vegetated spongy slope (fig. 6). A bed of brownweathering calcareous concretions, characterized by 

cone-in-cone structure, is present 3. 7 m (12 ft) above 

the base. Beds of sandstone are present here and there 

but are thickest and most numerous near the top. They 

are chiefly light olive gray, fine grained, and somewhat 

glauconitic and bentonitic. About 3 m below the top of 

the unit in the type section is a 1.1-m-thick bed of 

fine-grained sandstone that persists as a ledge former 

across the Vaughn and Great Falls quadrangles and 

into the Portage quadrangle, where it thickens to 2.4 

m. On the east face of Taft Hill, a thin bed of coarsegrained sandstone lies about 0.9 m below this ledge 

maker. Locally, south of the county road near the 

center ofthe north line of sec. 18, T. 20 N., R. 2 E., this 

coarse bed is represented by conglomerate that consists 

of pebbles as much as 5 em in diameter of gray, green, 

and reddish quartzite and black-coated gray, black, 

and brown chert. In the type section, this bed (bed 31) 

is fine to coarse grained, thin bedded to massive, and 

has undetermined green grains. It contains a few poorly preserved molds of freshwater unios and gastropods 

and fragments of large bones, probably dinosaurian. 

Possibly the bed represents a littoral deposit into 

which the freshwater elements were washed. Deposition in either shallow marine water or brackish water 

is indicated for the 3.1 m (10.2 ft) of sandstone (bed 34) 

at the top of the member by the presence of the 

brachiopod Lingula subspatulata Hall and Meek. 

AC;E i\SS!Ci'\\IH\ I A'\D C.O RRELAT!Ol\: 

Fossils are scarce in the Taft Hill Member in its type 

section west ofGreat.Falls. A small marine fauna with 

FrGURE 6.- Upper bentonitic part of upper unit Taft Hill Member on the north face of Taft Hill in the SVzSEV. sec. 1, T. 20 N., R. 1 E., 

Cascade County, Mont. Sandstone, forming ledges on upper part of slope on right side of photograph, is basal unit of Vaughn 

Member. 
TAFT HILL MEMBER 19 

Early Cretaceous (late Albian) affinities was collected 

from near the base of the member between Great Falls 

and Vaughn. The specimens that are lowest stratigraphically (USGS Mesozoic loc. D3920, D4319, and 

D4320) were collected approximately 4.2 m (14 ft) 

above the base of the Taft Hill Member. This horizon 

has yielded the following fauna: 

Anomia n. sp. 

Inoceramus bellvuensis Reeside 

P helopteria sp. 

A worm burrow 

Two other collections made by Cobban (1951a, 

p. 2179) in the Nlh sec. 27, T. 21 N., R. 2 E., and in the 

S¥2 sec. 10, T. 20 N., R. 2 E., from a horizon approximately 3 m stratigraphically higher, include the following: 

Discinoid brachiopod 

Inoceramus cf. L. caddottensis McLearn 

Inoceramus sp. 

Phelopteria cf. P. salinaensis (White) 

Anchura cf. A. quitmanensis Stanton 

Fragment of a smooth compressed ammonite 

This same zone crops out below the equivalent of the 

middle unit, the glauconitic sandstone, in a very limited area in the SE. cor. SEV!SWV! sec. 17, T. 17 N., 

R. 1 E. Collections by C. E. Erdmann and K. S. 

Soward contained the following fauna: 

USGS Mesozoic loc. D2931 (upper bed): 

Lingula cf. L. subspatulata Hall and Meek 

Ostrea anomioides Meek 

Ostrea larimerensis Reeside 

Trigonia sp. 

Cardium kansasense Meek 

Callistina? belviderensis (Cragin)? 

Cyrena? dakotensis Meek? 

USGS Mesozoic loc. D2930 (lower bed): 

Lingula cf. L. subspatulata Hall and Meek 

Phelopteria sp. 

Anomia sp. 

Trigonia sp. 

Anchura sp. 

The middle unit (glauconitic sandstone) of the Taft 

Hill Member appears not to contain any fossil mollusks; at least none have been found. But a broken 

isuroid shark tooth, generically and specifically indeterminate, and a small (10 by 12 by 27 mm) striated 

coprolite were collected from near the base of the unit 

at Vaughn junction. The explanation for the absence of 

a marine molluscan fauna in this unit and other glauconitic facies of the Taft Hill is believed to be related to 

the chemical character of the water in which they were 

laid down. The sea at that time probably was brackish, 

with a low concentration of calcium carbonate and a 

seasonally variable pH value centering around 7.5. 

Hence, sufficient CaC03 may never have accumulated 

for marine animals to make shells, or, if they did, subsequent lower concentrations or increases in acidity 

(lower pH values) might have caused the carbonate to 

go back into solution. 

Fossils consisting of the long-ranging pelecypod genera N ucula, N uculana, and Lucina were found in the 

upper part of the member in soft silty gray shale exposed in a streambank 3.2 km (2 mi) north of Vaughn 

in the NWV!SWV!NWV! sec. 7, T. 21 N., R. 2 E. (USGS 

Mesozoic loc. D529). Also, as noted previously, the 

brachiopod Lingula subspatulata Hall and Meek has 

been reported from the uppermost bed of the type 

section. 

The best dating for the lower part of the Taft Hill 

Member has been from collections made by M. R. 

Mudge at the Rocky Mountains front, on Sun River, 

about 92 km (57 mi) west of Vaughn. There, Mudge 

(1972, p. A62-A63) collected Inoceramus comancheanus Cragin and other marine pelecypods and gastropods 

of late Albian age from many levels. Both I. comancheanus Cragin and I. bellvuensis Reeside also have 

been reported by Stelck (1958, p. 3) from the Joli Fou 

Shale in northern Alberta, but, unfortunately, their 

position with respect to the Spinney Hill Member, 

which may not have been present there, is not stated. 

In the United States, these fossils indicate a correlation with at least part of the Skull Creek Shale of the 

Black Hills region and the Kiowa Shale of Kansas. 

Evidently they indicate a normal shallow-water marine environment. The strata containing I. comancheanus Cragin on Sun River have not been correlated 

with the type section, so the stratigraphic relationship 

of the fossiliferous beds to the middle unit, the glauconitic sandstone, is not known in Montana either. There 

is some reason to believe, however, that they may be 

equivalent to the lower part of the member, or even 

somewhat older, since it is possible to show that some 

of the upper part of the Taft Hill may be early late 

Albian in age. 

Cobban (1951a, p. 2177) originally considered the 

beds herein assigned to the middle unit of the Taft Hill 

Member as ~~the probable equivalent of the Newcastle 

sandstone" of the Black Hills, inasmuch as both units 

overlay black shale of Skull Creek age (late Albian). 

Mudge's recent discovery of late Albian fossils in the 

Taft Hill equivalent west of Vaughn suggested the possibility that the member as a whole represents an 

inner sublittoral facies of the upper part of the Skull 

Creek Shale. The Cyprian Sandstone Member at the 

middle of the Thermopolis Shale in the Little Rocky 

Mountains of north-central Montana also has been 

suggested as a Newcastle Sandstone equivalent by 

Knechtel (1959, p. 740). When referred to the base of 
20 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

the Colorado Group (top of Kootenai Formation), the 

base of the Cyprian Sandstone Member, which is about 

7.6 m (25 ft) thick, occupies approximately the same 

stratigraphic level as the thin littoral conglomerate 

(bed 31, type section), 5.1 m (16.6 ft) below the top of 

the Taft Hill Member. Although no direct correlation 

has been established between these two units, this correspondence of horizon is roughly suggestive of equivalence. There also is noticeable similarity in the appearance of their chert pebbles. The sandstones at the 

top of the Taft Hill, therefore, may likewise be equivalent to the Newcastle Sandstone. 

Knechtel (1959, p. 740) also pointed out that the 

Cyprian Sandstone Member ~~is considered to be equivalent to the Viking Sandstone of southern Alberta and 

Saskatchewan, Canada." The Viking Sandstone overlies the J oli Fou Formation in eastern Alberta and 

west-central Saskatchewan, and the probable correlation of the glauconitic Spinney Hill Member of the Joli 

Fou with the middle (glauconitic sandstone) and lower 

units of the Taft Hill has been suggested. According to 

Stelck (1958, p. 3), the HViking sandstone belongs to 

the base of the upper Albian substage near the top of 

the Lower Cretaceous," but in his correlation chart 

(1958, fig. 1, p. 4) it has been placed somewhat higher, 

with the Joli Fou Formation at the base of the upper 

Albian, overlapping slightly into the upper part of the 

middle substage of the Albian. 

DESCRIPTIVE SECTIONS OFT AFT HILL MEMBER 

The following descriptions of type and reference sections of the Taft Hill Member of the Blackleaf Formation are based primarily upon field descriptions. Only a 

few representative specimens have been studied petrographically and mineralogically. In order to make 

the two described sections as comparable and complementary as possible, each section has been subdivided 

into three units. Cross-reference within the bed descriptions and to the text have been used where identification of individual beds seems important. 

Type section of Taft Hill Member 

[Composite section from measurements by R. W. Lemke and E. K. Maughan at four local· 

ities south and southeast of Vaughn in the Great Falls and Vaughn quadrangles. Units 

2--6 were measured in the center of sec. 9, T. 20 N., R. 2 E.; units 7-13 in SE'4 sec. 2. T. 20 

N., R. 2 E.; unit 14 on south bank of Sun River in the SW'/4 sec. 25, T. 21 N .. R. 1 E.: and 

units 15-35 in the SW'48E1/4 sec. 1, T. 20 N., R. 1 E. (fig. 7f] 

Blackleaf Formation: 

Vaughn Member (entire member exposed at this locality but only basal bed described here): 

35. Sandstone, light-gray to buff, poorly consolidated, friable to platy. Lower third is calcareous and contains carbonized plant 

fragments; rest of unit is noncalcareous, 

slightly bentonitic near base, and weathers 

yellowish orange. Platy beds are fine to 

LITHOLOGIC 

34 SYMBOLS 

D +-' 

. 

~ 

:::J 

32 14 Cll Sandstone 

"'0 

"'0 

~ ~ -

Siltstone 

~ - Shale 

12 

27 10 ~ Q 

Calcareous 

concretions 

+-' 

+-' 

c 

:::J m c Qj Bentonite :::J 

I.. s 

23 Cll 0 

:::> 

a. 

a. 6 _J 

Carbonaceous 

~ 

shale 

19 3 METRES 

Base 0 

20 

17 

40 

R. 1 E. 60 

20 

25 

T. 0 3 MILES 

21 I 

N. I 

7436 0 4 KILOMETRES 

R. 2 E 

4 

15 

10 11 12 

34 

FIGURE 7.-Type section of the Taft Hill Member of the Blackleaf 

Formation and map showing line of measured section, sees. 2 and 

9, T. 20 N., R. 2 E., sec. 1, T. 20 N., R. 1 E., and sec. 25, T. 21 N., R. 

1 E. Numbers on the left side of the column are key beds in the 

measured section. On the map, the arrow points upward stratigraphically; B, base, and T, top of member. 
TAFT HILL MEMBER 21 

Type section of Taft Hill Member-Continued 

Blackleaf Formation-Continued 

Vaughn Member-Continued 

35. Sandstone, etc.-Continued M Ft 

medium grained, calcareous, and locally 

crossbedded; these thicken northward into 

massive crossbedded sandstone. Top of bed 

forms a fiat conspicuous bench 30-60 m 

(100-200 ft) wide ______________________ 6.2 20.3 

Taft Hill Member: 

Upper unit: 

34. Sand, light-olive-gray, glauconitic, finegrained, slightly bentonitic; weathers light 

yellowish gray; contains carbonaceous 

fragments and locally numerous small 

brachiopod (Lingula) shells. At top and 2 m 

from base are lenses of platy light-olivegray (5Y 6/1) to greenish-gray (5GY 6/1) 

glauconitic sandstone that weather 

purplish gray __________________________ 3.1 10.2 

33. Sandstone, light-gray, fine-grained, thinbedded, well-jointed; weathers light bluish 

gray; top of bed forms a ledge __________ 1.1 3.6 

32. Sand, slightly bentonitic; contains thin 

lenses of black carbonaceous materiaL___ .9 2.8 

31. Sandstone, pale-olive (lOY 6/2); fine- to 

coarse-grained, calcareous, finely stratified 

to massive, well jointed (N. 72° W.); weathers dark greenish gray. Composed largely 

of poorly sorted well-rounded to subrounded grains of clear and smoky quartz, 

dark chert, and undetermined green 

grains, as well as some black chert pebbles 

and abundant fish teeth and bones ______ 0.2 0.6 

30. Claystone, chiefly very light gray and buff, 

silty to sandy, fine-grained, slightly bentonitic; contains thin siltstone layers near 

top. A few brown-weathering calcareous 

concretions that have cone-in-cone structure are 2 m above base ________________ 4.6 15.1 

29. Siltstone, olive-brown, bentonitic, soft. 

Weathered surface is spongy and medium 

gray mottled with yellow. At base are a few 

calcareous concretions that have cone-incone structure__________________________ 2.9 9.6 

28. Shale, light-olive-gray, silty, indistinctly fissile, slightly calcareous; outcrop weathers 

to conspicuous grayish white (fig. 6) and 

can be traced for many kilometres. Locally 

contains hard thin siltstone layers and 

ovoid to round calcareous siltstone or argillaceous limestone concretions about 1 m in 

diameter; these concretions, light greenish 

buff on fresh fracture, weather to rounded 

nodules that are grayish buff mottled with 

dark brown ______ __________________ ____ 1. 7 5. 7 

27. Bentonite, greenish-yellow, impure; forms a 

light-gray spongy bench ________________ .7 2.2 

26. Siltstone, light-gray, glauconitic, moderately well bedded; weathers light bluish 

gray; consists chiefly of subangular frosted 

quartz grains with some hornblende and 

glauconite. Contains carbonaceous fragments along bedding surfaces. Few very 

Type section of Taft Hill Member-Continued 

Blackleaf Formation-Continued 

Taft Hill Member-Continued 

Upper unit-Continued 

26. Siltstone, etc.-Continued M Ft 

small crystals of red clinoptilolite in fractures at top of unit. Here and there are 

brown-weathering concretions that have 

cone-in-cone structure __________________ .3 .9 

25. Shale, dark-gray; fissile, slightly bentonitic; 

becomes lighter gray and silty upward. At 

base is a discontinuous bed of brownweathering limestone that has cone-incone structure__________________________ 1.5 5.0 

24. Shale, nearly black, bentonitic, partly fissile; 

weathers to a gray spongy slope. Includes a 

lenticular brownish-weathering bed of 

light-greenish-gray medium-grained 

slightly friable calcareous sandstone that 

contains a few grains of glauconite, hornblende, and mica ______________________ 4.1 13.5 

23. Bentonite, greenish-yellow, nearly pure; 

forms spongy bench. Scattered irregularly 

shaped gray siltstone concretions containing carbonaceous fragments. Forms a distinct spongy bench ____________________ .7 2.3 

22. Siltstone, dark-gray; weathers bluish 

gray ---------------------------------- .2 .6 

21. Sandstone, fine-grained, bentonitic, soft; interbedded with dark-gray bentonitic shale 

and in upper 1 m iron-stained bluishgray-weathering siltstone ______________ 3.4 11.1 

20. Sandstone, olive-buff, fine-grained, distinctly laminated, calcareous, bentonitic, soft; 

weathers to spongy surface marked by 

dark-gray bands, some of which are 

stained orange ________________________ 2.9 9.5 

19. Sandstone, light-gray, mostly silty and friable, massive, moderately calcareous; contains brown-weathering concretionary 

lenses of fine- to medium-grained gray calcareous sandstone through most of upper 

two-thirds of bed. Top of unit is lenticular 

bed as much as 24 em thick of dark-gray 

argillaceous limestone that weathers 

brown and has indistinct cone-in-cone 

structure _ _ __ __ _ _ _ _ ____ _ _ _ _ _ ___ _ ___ _ _ __ 3.0 9.8 

18. Shale, bentonitic; weathers to light-gray 

spongy surface. Lower part is dark gray 

except for basal 0.6 m, which is nearly all 

yellowish-green bentonite; limestone concretions occur 1.8 m and 3.7 m (6ft and 12 

ft) above base. Upper part is distinctly fissile and silty and weathers to a gray 

spongy surface; a thin bentonite bed is 5.5 

m (18ft) above base____________________ 7.9 25.8 

17. Cone-in-cone concretions, medium-darkgray, calcareous: weather brown; some as 

large as 1 m in diameter________________ .2 .6 

16. Shale, bentonitic; weathers to a light-gray 

spongy surface ------------------------ 2.4 8.0 

15. Covered -------------------------------- 1.2 4.0 

Middle unit: 

14. Sandstone, olive-green (lOY 5/2-lOY 5/4), 

fine- to medium-grained, platy and friable 
22 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Type section of Taft Hill Formation-Continued 

Blackleaf Formation-Continued 

Taft Hill Member-Continued 

Middle unit-Continued 

14. Sandstone, etc.-Continued M Ft 

to massive and well-cemented, locally 

crossbedded; mostly quartz with abundant 

glauconite, chert, and calcite; silty to shaly 

near base. Unit weathers to rough irregular surfaces. An impure greenish-yellow 

bentonite layer 18 em thick lies 2.4 m (8 ftl 

above base. Some very thin lenses of 

black-weathering siderite occur about 0.5 

m below top____________________________ 9.1 30.0 

Lower unit: 

13. Covered -------------------------------- 4.3 14.0+ 

12. Concretions, dark-brown, highly fractured; 

argillaceous and ferruginous limestone 

about 10 em thick and 30-60 em in diameter, showing prismatic structure _______ _ 

11. Covered --------------------------------

10. Sandstone, olive-green, medium-grained, 

glauconitic, friable, noncalcareous _____ _ 

9. Siltstone, bluish-gray, friable ___________ _ 

8. Siltstone, gray, clayey, partly fissile; a few 

bentonite beds 3-6 em thick ___________ _ 

7. Shale, black, fissile; a few bentonite beds 3-6 

em thick _____________________________ _ 

6. Sandstone, dusky-yellow (5Y 6/4), mediumgrained, well-sorted, porous, friable, indistinctly laminated, slightly glauconitic ___ _ 

5. Shale, grayish-yellow, sandy, bentonitic, 

poorly exposed _______________________ _ 

4. Shale, black, distinctly fissile; includes a few 

beds of bentonite about 3 em thick _____ _ 

3. Bentonite, greenish-yellow _______________ _ 

2. Shale, black, fissile _____________________ _ 

Total Taft Hill Member (rounded) ___ _ 

Flood Member (top): 

1. Sandstone, light-gray, poorly exposed; weathers tan _________________________________ _ 

.2 . 5 

.9 3.0 

1.5 5.0 

0.6 2.0 

1.4 4.5 

4.4 14.5 

.6 2.0 

.9 3.0 

6.1 20.0 

.6 2.0 

.3 1.0 -----

74 242 

3.7+ 12.0+ 

Composite reference section for approximately the lower half of the 

Taft Hill Member 

[Measured by C. E. Erdmann, Oct. 1962 and May 1967, along U.S. Interstate 15 northwest 

of Great Falls between the S•·:., sec. 36. T. 21 N., R. 2 E., and Vaughn junction (sec. 19, T. 

21 N., R. 2 E.i, Cascade County. Mont. Beds 78--82 were measured along Manchester exit 

road between sees. 26 and 25, T. 21 N., R. 2 E.; beds 77-53 were measured in the center of 

sec. 19, T. 21 N., R. 2 E.; beds 52-35 were measured in the NE14SE14 sec. 19, T. 21 N., R. 2 

E.; beds 34---19 were measured in the NE cor. NW1

4SW'4 sec. 26, T. 21 N., R. 2 E.; and beds 

18--1 were measured in the NE\4NW'4 sec. 36, T. 21 N., R. 2 E.] 

Pleistocene: 

Pre-Wisconsin. Terrace gravels along the Sun River 

consisting chiefly of argillite and quartzite from the 

Belt Supergroup; lime coated, loosely consolidated 

M Ft 

in a sandy matrix______________________________ 2.1 7.0 

Unconformity. 

Blackleaf Formation: 

Taft Hill Member: 

Upper unit: 

82. Siltstone, soft; weathers rusty ____________ .6 2.0 

81. Bentonite, light-gray, soft, weathered______ .1 .3 

80. Siltstone, medium-light-gray; weathers 

rusty ______________________ ------------ 3.1 10.0 

Composite reference section for approximately the lower half of the 

Taft Hill Member-Continued 

Blackleaf Formation-Continued 

Taft Hill Member-Continued 

Upper unit-Continued M Ft 

79. Bentonite, light-yellowish-gray____________ .1 .3 

78. Siltstone, medium-light-gray, soft, fissile; 

contains a few thin limonitic laminae. 

Base darker (N 3 ), with 0.6-cm-thick 

laminae of light-olive-gray shale at intervals of about 3 em______________________ 2.7 9.0 

Diastem(?) 

Middle unit (the glauconitic sandstone): 

77. Sandstone, olive-gray, fine-grained; thin 

0.6-cm horizontal wavy layers. Top irregular with local relief of 50 em ____________ .2 .5 

76. Sandstone, light-olive-gray, fine-grained. 

Top irregular and marked by a thin film of 

dark-gray sandy clay (graded bedding?) . 

Below this upper 15 em there is exhibited 

planar ("torrential"l-type cross-lamination 

with true dip 20° S. to 11 o E. Bedding in 

lower 10 em essentially horizontal. 

Homogeneous (nonlaminated l layers are 

here and there calcareous ______________ .2 .8 

75. Sandstone, light-olive-gray, fine-grained. 

Upper half -cross-laminated with 2-

cm-thick beds dipping 20° S. to 15° W. 

(component dip?l; lower part massive, with 

thin horizontal laminae ________________ .3 1.0 

74. Sandstone, light-olive-gray, thin, soft, friable ---------------------------------- .1 .3 

73. Sandstone, light-olive-gray, fine-grained, 

firm; consists of moderately resistant 5- to 

15-cm-thick layers. 15-cm-thick bed, in 

middle, has 0.6-cm-thick laminae in 

planar-type cross-lamination dipping 

15° s ---------------------------------- .6 2.1 

72. Sandstone, light-olive-gray, fine-grained. 

Top marked by horizontal laminae of 

dark-gray silty shale. Lower 15 em has 

0.6-cm-thick sandstone laminae in 

planar-type cross-lamination dipping 

15° s ---------------------------------- .2 .8 

71. Sandstone, light-olive-gray, fine-grained; 

weathers dusky yellowish brown to dark 

yellowish brown in scabby ferruginous 

crusts 0.3-0.6 em thick. Locally 

persistent______________________________ .1 .3 

70. Sandstone, light-olive-gray, fine-grained; in 

5-cm-thick noncalcareous layers. Softer 

than underlying bed____________________ .6 2.0 

69. Sandstone, light-olive-gray, fine-grained, 

calcareous, hard, resistant; makes a thin 

ledge on freshly cut slope. Base irregular. 

Locally persistent _____ _________________ .2 . 7 

68. Sandstone, light-olive-gray, fine-grained; in 

thin 1- to 5-cm soft friable layers. Top locally undercut__________________________ .4 1.3 

67. Shale, light-olive-gray (5Y 5/2), soft, finely 

laminated, fissile; weathers dusky yellowish olive to dark yellowish orange____ .1 .2 

66. Sandstone, olive-gray, medium-grained, 

thinly laminated, soft, glauconitic ______ .1 .4 

65. Clay ironstone, compact aphanitic to sandy, 
TAFT HILL MEMBER 

Composite reference section for approximately the lower half of the 

Taft Hill Member-Continued 

Blackleaf Formation-Continued 

Taft Hill Member-Continued 

Middle unit, etc.-Continued 

65. Clay ironstone, etc.-Continued 

hard, brittle; more or less completely 

weathered through with dark-yellowishorange concentric structures; surface 

commonly brownish black with manganese 

oxide stain and fine-textured botryoidal 

forms; contains short thin lentils of loosely 

consolidated glauconitic sand that weathers out to leave small fiat elliptical 

cavities. A broken isuroid shark tooth and 

a small (10 by 12 by 27 mm) striated copralite have been found on surface of bed. Locally persistent _______________________ _ 

64. Sandstone, light-olive-gray, fine- to 

medium-grained; in soft friable horizontal 

5-cm-thick layers _____________________ _ 

63. Sandstone; as above, as 5-cm-thick layers 

with laminae of detrital glauconite. Casts 

of trace fossils on bedding surfaces. Top 

marked by thin film of olive-gray (5Y 4/1) 

fissile shale; 15-cm-thick upper bed 

exhibits planar cross-lamination dipping 

10°-12° S. 10° W. Makes an indistinct 

ledge ----------------------------------

62. Sandstone, light-olive-gray, fine-grained, 

glauconitic; softer and more friable than 

underlying bed; bedding indistinct _____ _ 

61. Sandstone, light-olive-gray, fine-grained, 

glauconitic. Upper half soft; lower half 

firm, calcareous _______________________ _ 

60. Sandstone, as immediately above; contains 

epigenetic banded crusts 0.6-1 em thick of 

very dusky red clay ironstone in lentils 10 

em long and 3 em thick, which are especially well developed in lower part _____ _ 

59. Sandstone, light-olive-gray, fine-grained, 

glauconitic; more thinly bedded than 

above. Entire sequence makes indistinct 

ledge ----------------------------------

58. Cone-in-cone calcite, light- to medium-gray; 

occurs as radiating fibers in asymmetric 

brushlike aggregates 3-8 em long replacing yellowish-gray bentonite, some of 

which remains intergrown at point (base) 

of cones. Contains 0.6-cm-thick crusts of 

very fine fibrous yellow-brown calcite with 

silty luster ----------------------------

51. Cone-in-cone calcite; weathers dark yellowish orange. More massive than overlying 

bed, into which it grades _______________ _ 

56. Sandstone, light-olive-gray, hard, massive, 

calcareous; grades upward into finegrained bentonitic sandstone that weathers dark yellow orange. Entire sequence 

very distinct, locally persistent, making a 

valuable marker bed ___________________ _ 

55. Sandstone, light-olive-gray, glauconitic, 

fine-grained; in 3- to 20-cm-thick layers 

separated by softer more finely laminated 

partings. Slightly irregular bedding surfaces are marked by thin laminae of detriM Ft 

.1 .2 

.7 2.2 

.6 2.0 

1.0 3.2 

.2 .5 

0.2 0.5 

.2 .5 

.1 .3 

.2 .6 

.2 .7 

Composite reference section for approximately the lower 

half of the Taft Hill Member-Continued 

Blackleaf Formation-Continued 

Taft Hill Member-Continued 

Middle unit, etc.-Continued 

55. Sandstone, etc.-Continued M 

tal glauconite which show trace fossils 

(worm trails?) and other markings ______ .8 

54. Siltstone, olive-gray, soft; transitional into 

bed above______________________________ .1 

53. Bentonite, light-yellowish-gray (5Y 8/1), tuffaceous, firm, compact; weathers light yellowish orange. Breaks with fiat subconchoidal fracture. Thickens and thins ____ .1 

52. Sandstone, olive-gray, glauconitic. Lower 

part is shaly and thinly laminated with 

medium-dark-gray shaly partings weathering light olive gray __________________ .3 

51. Sandstone, olive-gray, glauconitic. Top 

makes weak ledge. Lower part is thinly 

laminated, with partings of soft mediumdark-gray fissile shale that weathers light 

olive gray------------------------------ .2 

50. Sandstone, light-olive-gray, fine- to 

medium-grained, glauconitic; upper part 

makes protruding ledge ________________ .4 

49. Sandstone, olive-gray, fine-grained, silty, 

soft____________________________________ .2 

48. Sandstone. Upper 30 em is light olive gray, 

fine grained, less silty, noncalcareous, but 

firm and resistant, and forms lowest ledge 

marker of middle unit. Lower 18 em is softer, weathers into rounded form by spalling 

corners off joint blocks. This is the basal 

bed ofthe glauconitic sandstone. Can actually be seen to wedge out to the east with 

base stepping upward __________________ .5 

Measured thickness of middle unit ____ 8.9 

Lower unit: 

47. Sandstone, light-olive-gray, glauconitic, 

fine-grained, silty, soft, friable. Stratification marked here and there by thin 

rusty-weathering bentonitic(?) laminae __ 2.2 

46. Bentonite; upper part is clayey, light olive 

gray (5Y 5/2), and weathers dark yellowish 

orange. Lower part is more bentonitic, is 

light yellowish gray, soft, and weathers 

dark yellowish orange __________________ .1 

45. Sandstone, light-olive-gray (5Y 5/2), glauconitic, fine-grained, silty, soft, friable. 

Traces of thin laminae of olive-gray shale 1.5 

44. Sandstone, olive-gray, clayey ____________ .7 

43. Bentonite, soft, silty; weathers grayish 

orange -------------------------------- .2 

42. Sandstone, olive-gray, clayey; weathers 

downward into thin scales. Contains two or 

three thin (0.6 em) bentonite beds ______ 1.1 

41. Bentonite, light-olive-gray, soft, clayey. 

Base sharp a~d distinct; top gradational .1 

40. Sandstone, pale-olive-gray; 5-cm (2-in. )-

thick layer of soft dark-olive-gray clayey 

silt at base __ __ _ _ __ _____ _ _ _ __ __ ____ _ _ __ .2 

39. Bentonite, dark-yellowish-orange, completely weathered, soft, fiakey. Base sharp; 

top gradational ------------------------ .1 

23 

Ft 

2.7 

.2 

.2 

1.0 

.8 

1.3 

.8 

1.6 

29.2 

7.3 

.3 

4.8 

2.2 

.5 

3.5 

.2 

.7 

.2 
24 TAFT HILL MEMBER 

Composite reference section for approximately the lower half of the 

Taft Hill Member-Continued 

Blackleaf Formation-Continued 

Taft Hill Member-Continued 

Lower unit-Continued M Ft 

38. Sandstone, pale-olive-gray, glauconitic .3 .9 

37. Bentonite, light-yellowish-gray to grayishorange, soft, fissile. Base sharp; top gradational__________________________________ .1 .2 

36. Sandstone, pale-olive-gray, glauconitic, 

fine-grained, silty, soft, friable, poorly consolidated. Very homogeneous; indistinct 

bedding marked by thin laminae of bentonite ---------------------------------- 0.9 3.0 

35. Interval concealed ______________________ 1.1 3.5 

34. Bentonite, very light yellowish to greenisholive-gray, soft, weathers dark yellowish 

orange; clayey, with waxy luster. Top gradational ------------------------------ .1 .3 

33. Siltstone, medium-dark-gray, soft, fissile. 

Weathered surface shows numerous thin 

translucent sheets and films of secondary 

selenite________________________________ .8 2.5 

32. Sandstone, olive-gray to moderate-olivebrown, fine- to medium-grained, cherty, 

glauconitic, soft, noncalcareous; matrix 

of dusky-yellow silt; contains thin flattened galls of medium-dark-gray siltstone 

as much as 3 em across. Top marked locally 

by a thin layer of very fragile light-brown 

Inoceramus prisms. Estimated percentage 

composition of sand fraction: subangular 

quartz, 60, with yellowish-gray dust from 

matrix; chert, dark gray to black, subangular to subround, unpolished, 25; glauconite 

(first appearance in Taft Hill Member), 

dusky yellowish green, smooth ovoidal or 

lobate grains and compound spherical 

masses ranging in diameter from 0.5 to 1.3 

rom, loosely embedded in the sandstone 

but not in the clay galls; nondetrital, authigenic (diagenetic), but with a few irregular intergranular masses as much as 2 em 

across; accessory muscovite, rare ________ .3 1.0 

31. Siltstone (mudstone), medium-dark-gray; 

contains scattered concretions of lightyellowish-gray calcite with cone-in-cone 

structure; weathers brownish gray to 

dusky red, in very complex intergrowths 

with mudstone. Individual masses as much 

as 1 m long and spaced variably at intervals of 3-15 m. Lower surface of concretions is irregular, cutting across bedding. 

Where cone-in-cone structure has not developed, base of unit may contain flat oval 

masses 5 by 20 em of massive olive-gray 

calcareous siltstone that contain fragments of Inoceramus____________________ .2 .5 

30. Siltstone, medium-dark-gray______________ .2 .5 

29. Bentonite, light-gray, soft, clayey; in local 

lenses, some of which show a thin basal 

layer of very light gray vitric tuff ______ .1 .2 

28. Siltstone, medium-dark-gray; locally shows 

thin olive laminae in upper 30 em. Middle 

and lower part characterized at vertical 

Composite reference section for approximately the lower half of the 

Taft Hill Member-Continued 

Blackleaf Formation-Continued 

Taft Hill Formation-Continued 

Lower Unit-Continued 

28. Siltstone, etc.-Continued M Ft 

intervals of 9-15 em by many short lentils 

of finely laminated fine-grained lightolive-gray sandstone____________________ 1.1 3.7 

27. Bentonite, light-yellowish-gray; weathers 

rusty at base and top __________________ .1 .2 

26. Siltstone, medium-dark-gray; contains short 

thin lentils of intraformational microconglomerate in a matrix of light-mediumgray fine-grained calcareous sandstone 

that weathers light olive gray. A more or 

less persistent layer 3-6 em thick containing fragments of Inoceramus occurs in 

middle of unit. A very thin olive-brown 

fine- to medium-grained poorly sorted 

sandstone layer at base contains fragments 

of very dark gray carbonaceous mudstone 

and impressions and fragments of Inoceramus ------------------------------ .7 2.3 

25. Siltstone, medium-dark-gray, soft, flaky; 

some laminae weathering olive gray ____ .9 3.0 

24. Vitric crystal tuff, very light gray to lightyellowish-gray, partially devitrified, firm, 

compact, porous; cutting easily, but 

slightly more resistant than enclosing 

siltstone; weathering or stained dark yellowish orange at some localities. Very fine 

grained, breaking with subconchoidal fracture; largest crystal fragments range from 

about 0.007 to 0.155 rom________________ .1 .2 

23. Siltstone, medium-dark-gray; some laminae 

weather light olive gray; very thin lightgray layer of bentonite near base________ .3 1.1 

22. Sandstone, light-gray, fine-grained, cherty; 

calcareous cement; soft and friable where 

weathered. Occurs in thin <3 em) short lentils. Locally, a thin <3 em) rustyweathering bentonite at base____________ .2 .6 

21. Siltstone, medium-dark-gray, soft, fissile; a 

few rusty laminae______________________ .4 1.2 

20. Bentonite, light-gray (grayish-white), soft, 

clayey; weathers dark yellowish orange; 

base locally irregular __________________ .4 1.2 

19. Shale, medium-dark-gray, fissile __________ 1.1 3.6 

18. Sandstone, medium-gray, fine-grained, 

clayey, calcareous. Contains a few thin 

granules of polished brownish-gray chert 

as much as 3 rom in diameter, and claystone pellets 3 by 6 em and 1 em thick. 

Flute casts on base; bedding irregular; top 

irregular, with ripple marks in places in a 

thin film of brown ferruginous sandstone 

with wavelength of 6 em and amplitude of 

1 em. Worm tubes filled with gray clay __ .1 .3 

17. Shale, medium-dark-gray; weathers rusty 

brown along sandy laminae ____________ .2 .6 

16. Sandstone, light-gray, fine-grained, noncalcareous. In short lentils ________________ .2 .7 

15. Shale, dark-gray, poorly exposed---.------- .3 1.0 

14. Shale and sandstone; in thin layers. Sand-
VAUGHN MEMBER 25 

Composite reference section for approximately the lower half of the 

Taft Hill Member-Continued 

Blackleaf Formation-Continued 

Taft Hill Formation-Continued 

Lower unit-Continued 

14. Shale and siltstone, etc.- Continued M Ft 

stone is light medium gray and fine 

grained; shale is medium dark gray .1 .3 

13. Sandstone, conglomeratic, medium-lightgray, calcareous, hard; firm and compact 

when fresh ; weathers light yellowish 

brown in places. In finely laminated lenses 

of variable thickness and width separated 

by thinner irregular partings of soft olive 

clay. Sand facies or matrix fine to medium 

grained, subangular, and with salt-andpepper appearance. Conglomeratic facies 

characterized by usually small (2 mm or 

less) rounded granules and pebbles of 

brownish and dark-gray chert; a lso characterized by subrounded granules as much as 

5 mm in size of very light gray yellowishgray to white aphanitic tuff. Percentages 

vary, but chert and tuff combined may 

make about 40 percent, and when about 

equal give the rock a very distinctive appearance. Fish scales and vertebrate bones 

are rare as accessories ________________ .1 .4 

12. Sandstone, light-gray, calcareous, hard, 

firm; makes a weak ledge ____________ __ .2 .6 

1 1. Sandstone, olive to light-yellowish-gray, fine 

grained, soft; weathers rusty brown in 

places. Occurs in thin (0.5-1 em) lentils 

separated by thin partings of dark-gray 

clay _____________ ------- ------------ .2 .8 

10. Shale, medium-dark-gray, clayey, soft, plastic. Less fissile than that just below_ _____ .4 1.4 

9. Shale, medium-dark-gray, fissile, papery; 

thin layer of fine-grained sandstone 

at top ____________ _ ------- ------------- .3 1.1 

8. Clay, dark-gray, soft. Contains thin laminae 

of fine-grained rusty sandstone ___ __ __ __ .2 .6 

7. Sandstone, fine- to medium-grained. In 

small lentils or pods -------------------- .1 .2 

6. Clay, sandy, but soft and plastic; olive-gray 

when moist. Contains here and there 

brown-weathering concretions of calcareous olive mudstone 60 em in diameter and 

13 em thick, with a layer of cone-in-cone 

structure on top _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ .1 .4 

Intramember erosional disconformity. 

5. Sandstone, medium-gray, calcareous, hard; 

weathers brownish gray; makes ledge; fine 

to medium grained, with a sprinkling of 

small granules of dark chert cemented by 

authigenic crystals of calcite. Top slightly 

irregular; rusty brown __ _ ______ ___ __ _ _ 

Thickness of partial section of Taft Hill 

Member (rounded)_ ______ _ __ ____ __ 31 

Erosional disconformity. 

Flood Member: 

4. Sandstone, yellowish-gray, medium-grained, 

subangular, weakly cemented, friable; good 

porosity and permeability; predominantly 

.5 1.7 

101 

Composite re('erence section for approximately the lower half of the 

Taft Hill Member-Continued 

Blackleaf Formation-Continued 

Flood Member-Continued 

4. Sandstone, etc.-Continued M Ft 

clear glassy quartz whose crystal faces glisten 

brightly, with about 5 percent yellowish-gray 

weathered feldspar and 5 percent dark minerals (chert) ____ _______________ ____ ______ 1.1 3.4 

3. Sandstone, yellowish-gray, fine- to mediumgrained (as above). Largely concealed by rubble _ _______________ ___________ 4.9 16.0 

2. Sandstone, yellowish-gray; as above, with a few 

small spherical concretions of orange-brown 

limonitic sandstone. Top makes resistant 

ledge --------------------------------- - -- 2.6 8.5 

1. Sandstone, medium-gray, fine-grained; in short 

lentils or nodules separated by thin partings 

of medium-gray siltstone and shale --------

Thickness of partial section of 

Flood Member (rounded) ____ ___ ______ _ 

VAUGHN MEMBER 

\:AME AND DEFINITIOI\ 

0.6+ 2.0+ 

9.0 30.0 

Strata of the Vaughn Member of the Blackleaf Formation were formerly referred to as the "red speck 

zone" (Erdmann and Schwabrow, 1941, p. 284; Blixt, 

1941, p. 337) and the "nonmarine member" (Cobban, 

1951a, p. 2180). Later, in subdividing the Colorado 

Group, the strata were called the Vaughn Bentonitic 

Member of the Blackleaf Formation by us (Cobban and 

others, 1959, p. 2790) with the intent of including rocks 

of nonmarine origin between the Taft Hill Glauconitic 

Member below and the Bootlegger Member above. As 

first measured by us (Cobban and others, 1959) and 

illustrated by Maughan (1961), the type section of the 

member included transitional beds at the base that are 

only locally present. These strata are now believed to 

be of marine origin and have been assigned to the Taft 

Hill Member in this paper. The Vaughn Member by 

definition is exclusively continental in origin. 

The Vaughn Member is named from excellent exposures north and south ofVaughn, 19 km (12 mi) west of 

Great Falls. The type section crops out along the east 

bank of an unnamed intermittent wash in the NVzNE\4 

sec. 6, T. 21 N., R. 2 E., and on a southwest-pointing 

spur in the SE\4SW\4SE\4 sec. 31, T. 22 N., R. 2 E. 

(Vaughn quadrangle) (fig. 8). This locality is somewhat 

inaccessible in that it cannot be seen or quickly 

reached from any main road. The easiest approach to it 

is over an unsurfaced road from the north and down an 

abrupt escarpment between sees. 31 and 32, T. 22 N., 

R. 2 E., to the NE. cor. sec. 6, T. 21 N., R. 2 E.; from 

there it is about a 0.8-km (0.5 mi) walk to the west. For 

greater accessibility, a composite reference section of 

the member designated in this report has been measured and described in detail 4 km (2.5 mi) to the 
26 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

FIGURE 8.- Type section of the Vaughn Member about along the center line of the NE\4 sec. 6, T. 21 N. , R. 2 E. , Cascade County, 

Mont. The sandstone cropping out in the upper middl e part of the photograph below the li ght-gray bentonitic unit bare of vegetation marks the base of the member. R. W. Lemke is standing on a limestone bed 8.5 m (28 ft) below the top of the Tall Hill 

Member. 

southwest, along U.S. Highway 91 (Interstate 15), in 

the NE% sec. 11 and SEV4 sec. 2, T. 21 N., R. 1 E. 

T HI C KNESS 

In its type section the Vaughn Member has a thickness of approximately 26 m (86 ft). As first defined 

(Cobban and others, 1959, p. 2790), the thickness given 

was 29.6 m (97ft), but this thickness included underlying probable marine strata now assigned to the Taft 

Hill Member. The thickness in the composite reference 

section, about 4 km southwest, is 26 m (86 ft). In an 

outcrop 17.7 km (11 mi) east (escarpment north of 

Blackhorse Lake Flat east of the Bootlegger Trail, 11.2 

km or 7 mi north of the city of Great Falls), the thickness has decreased to 15.9 m (52 ft). The member continues to thin to the southeast toward Belt Butte, 

where its thickness is only 7.9 m (26ft) in sec. 30, T. 19 

N., R. 7 E. The increase in thickness to the west, however, is more rapid. Schmidt (1963) assigned 114 m 

(375ft) of strata to the member in the WolfCreek area; 

south of the town of East Glacier, Vaughn lithology 

seems to have completely replaced the Taft Hill 

Member and the Bootlegger Member. 

1 Although not commonly recognized in old cable-tool 

arillers' logs, some part of the Vaughn usually can be 

identified microscopically in formation samples, and it 

serves as a general marker for the middle part of the 

Blackleaf Formation. No part of it, however, provides a 

satisfactory datum for regional structural mapping. 

Thicknesses in the subsurface on the Sweetgrass arch 

seem not to be so systematic in their range as those in 

outcrops. A number of factors, singly or in combination, probably contribute to this condition: (1) The 

internal character of the boundaries changes from 

place to place in consequence of the variable continental lithology; (2) the top may be affected by a slight 

erosional disconformity, with some variation in the 

overlying units; and (3) local absence of the diagnostic 

mineral clinoptilolite. Apparently some anomalous 

thicknesses result from disaggregation and "washing 
VAUGHN MEMBER 27 

out" of montmorillonite from the claystone. For these 

reasons the top of the member is not a good datum 

plane for regional structural mapping. 

The Vaughn is 45 m (150 ft) thick in the Brady oil 

field, 69 m (227 ft) thick near the west edge of the 

Ponder a oil field , 22 m (7 4 ft) thick at the top of the 

Kevin-Sunburst dome (sec. 15, T. 34 N., R. 1 W.), 37 m 

(120 ft) thick on the west side of the Cut Bank oil and 

gas field (sec. 13, T. 35 N., R. 6 W.), and 160m (525ft) 

thick in the Chicago Petroleum Corporation's Mabel 

Armstrong Well 1 Allotted 168 in sec. 35, T. 36 N., 

R. 8 W. The member thins eastward from the KevinSunburst dome and has not been recognized east of the 

Sweetgrass Hills. 

OUTCROP DISTRI~UTION 

On the Sweetgrass arch the Vaughn Member crops 

out only on the southern part of the South arch. Beginning with the outcrops along the Missouri River in the 

northeastern part of the Portage quadrangle, the belt 

of outcrop extends southwestward across that quadrangle to the Blackhorse Lake Flat area, 8-13 km (5-8 

mi) north of the city of Great Falls. From Blackhorse 

Lake Flat, the belt of outcrop trends slightly north of 

west and crosses the Great Falls quadrangle just below its middle. The outcrop continues west into the 

Vaughn quadrangle and up Muddy Creek to a point in 

the SElf! sec. 32, T. 22 N., R. 1 E., where it dips below 

the valley floor. Along the Sun River the member is 

exposed from a point 1.6 km (1 mi) west of Vaughn on 

up the valley for 9.6 km (6 mi) to the southern part of 

sec. 36, T. 21 N., R. 1 W., where the top disappears 

below the river. South of Vaughn the member is very 

well exposed along the east side of Taft Hill and 

southward through the Cascade quadrangle. The outcrop crosses the Missouri River at Riverdale and continues south up Bird Creek valley for 16 or 17 km. 

West of the South arch the Vaughn Member is exposed at many localities in the Disturbed belt from 3.2 

km (2 mi) northeast of Wolf Creek in sec. 31, T. 15 N., 

R. 3 W., northwest to the southeast boundary of 

Glacier National Park, where the Vaughn has completely replaced the Bootlegger Member. 

(;Fi\'LRAL DESCRIPTION 

Light-colored bentonitic clay, siltstone, and 

sandstone largely compose this member. Most of the 

rocks are soft and readily erode into poorly vegetated 

badlands that are conspicuous in the Cascade, Vaughn, 

Great Falls, and Portage quadrangles (Cobban, 1955a, 

fig. 2). 

In the type section northeast of Vaughn (fig. 8), the 

basal unit of the member consists of 4.6 m (15 ft) of 

pale-greenish-yellow medium-grained very friable arkosic sandstone. At a locality 13 km (8 mi) to the south, 

along a road ascending Taft Hill, in the SElftSWlft 

sec. 7, T. 20 N., R. 2 E., the base of this sandstone bed is 

coarse grained, contains some gray shale pebbles, and 

is channeled into the underlying fine-grained 

glauconitic sandstone of the Taft Hill Member. This 

basal sandstone bed of the Vaughn Member persists 

across the South arch eastward at least as far as the 

Missouri River, in the Lander Crossing quadrangle. 

The bed has been mapped by Dobbin and Erdmann 

(1930) as "sandstone A." A freshwater origin is probable for this bed. Fossil wood can be found on almost any 

outcrop: and locally, large logs are present. In a few 

places in the eastern part of the Portage quadrangle, 

beds of lignitic shale are at the top of the sandstone. 

The rest of the Vaughn Member consists dominantly 

of clayey beds that are light to dark gray, greenish 

gray, grayish green to olive green, greenish yellow, 

pink, or chocolate brown and generally are very bentonitic. The clayey beds are interbedded with thinner 

lenticular units of gray to green bentonitic siltstone 

and sandstone that are commonly tuffaceous and, in 

places, are orthoquartzites. Some of the beds of clay, 

siltstone, and sandstone contain small crystals of 

orange-red clinoptilolite (determined by L. G. Schultz, 

U.S. Geol. Survey). Locally, as in the small badland 

area traversed by U.S. Highway 91, about 5.6-6.4 km 

(3.5-4 mi) northwest of Vaughn, this zeolite is so 

abundant that it imparts a pinkish color to the outcrops. Bentonitic clay containing clinoptilolite forms 

the uppermost 8.5 m of the type section. About 6 km to 

the west, along Muddy Creek, black carbonaceous 

shale, which has been prospected for coal, forms the top 

of the member (fig. 9; also Cobban, 1955a, text fig. 2). 

The Vaughn Member is easily recognized in cuttings 

from wells drilled for oil and gas on the Sweetgrass 

arch. The clays are largely light to medium shades of 

gray or green, and the sandstones are generally light 

gray to white. The rocks are very bentonitic, and most 

contain small crystals of the orange-red zeolite. The 

sandstones are fine to medium grained, massive, soft, 

and clayey. They are composed mainly of subangular 

grains of clear quartz and white, gray, black, brown, 

yellow, green, and pink grains of quartz, chert, and 

other minerals, including some mica. Abundant white 

clay serves as a cement. Most wells reveal whitish fineto medium-grained sandstone 1.6-10.7 m (5-35 ft) 

thick at or near the base of the member. Wells drilled 

low on the west flank of the Sweetgrass arch, where the 

Vaughn Member is 61-91 m (200-300 ft) thick, may 

show as many as four beds of white sandstone 4.6 m (15 

ft) or more thick. Black carbonaceous shale has been 

penetrated at the top of the member in some wells in 

the Pondera and Kevin-Sunburst oil fields. 
28 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

FIGURE 9.- Upper part of Vaughn Member (light-gray bentonitic 

beds bare of vegetation) capped by layer of lignitic shale (black 

band) and overlain by lower part of Bootlegger Member 6.5 km 

(4 mi) northwest of Vaughn in the NEY4 sec. 4, T. 21 N., R. 1 E., 

Cascade County, Mont. 

The top of the Vaughn Member is readily determined in well cuttings by the abrupt change downward 

from dark-gray more or less sandy shale to lightcolored clay or, more rarely, white bentonitic 

sandstone or black lignitic shale. The lower contact is 

not so easily placed. A unit, 6 m to more than 11 m 

thick, of medium- to dark-medium-gray soft silty shale 

commonly lies between the lowest whitish bentonitic 

sandstone typical of the Vaughn Member and the uppermost greenish glauconitic sandstone typical of the 

Taft Hill Member. Interbedded with this silty shale are 

thin beds of light- to medium-gray very fine to finegrained shaly sandstone and siltstone that are slightly 

glauconitic. Locally, these beds contain tiny specks of 

the orange-red zeolite in addition to the glauconite. 

These beds are considered to represent intertonguing 

of the Vaughn and Taft Hill facies and are here assigned to the Taft Hill Member. 

ACE ASSIGNMENT AND C:ORRELATIO"i 

The fossil record from the Vaughn Member consists 

ofleaves, logs, and, rarely, fragments of reptile bones. 

The fossil plants, all from the vicinity of Vaughn, have 

been identified by R. W. Brown (Cobban, 1951a, 

p. 2180) as Pinus sp., Araucarioxylon sp., Anemia 

lremonti Knowlton, Nelumbites sp., Dryandroides? sp., 

and Tempskya knowltoni Seward. Zeller and Read 

(1956) regarded the genus Tempskya as a possible 

guide to rocks of Albian (late Early Cretaceous) age. 

Other evidence which suggests a possible late Albian 

age is the position of the Vaughn Member below the 

Bootlegger Member. The Bootlegger Member is an 

equivalent of part or all of the Mowry Shale of other 

areas. The Mowry Shale contains Neogastroplites, Inoceramus cf. I. anglicus Woods, and other mollusks 

that are considered as being probably of very late Albian age (Reeside and Cobban, 1960, p. 28-30). A recent addition to the fossil flora from the Vaughn 

Member is Tempskya grandis Read and Brown. 

Formerly the Vaughn Member was thought to be 

equivalent to the lower part of the Mowry Shale (Cobban, 1951a, p. 2180), but in light of the discovery by 

Mudge (1972, p. A62) of Skull Creek fossils in the Taft 

Hill equivalent, a correlation of the Vaughn Member 

with the Newcastle Sandstone of the Black Hills seems 

more logical. 

DESCRIPTIVE SI::CTIONS OF THE VAUGHN MI::MBER 

The following type section and reference section are 

based chiefly upon field descriptions. Only a small 

amount of mineralogical work, mainly related to identification of the minerals clinoptilolite and chamosite, 

has been done. The type section was originally measured by R. W. Lemke and E. K. Maughan in 1956, with 

minor revisions by R. W. Lemke and A. F. Bateman in 

1973. The reference section was measured by C. E. 

Erdmann in 1968 and 1969. The lithology in both sections is similar, but the stratigraphic units have been 

subdivided considerably more by Erdmann in the reference section. Also, similar strata in the two sections 

are not always described in exactly the same way, and 

therefore correlation of beds between the two sections 

is not always possible. In addition, the reference section, unlike the type section, is subdivided into three 

units (upper, middle, and lower), based upon the inferred environmental conditions that existed during 

deposition of the units. In spite of these inconsistencies, we believe that description of both sections herein 

is desirable. 

Type section of Vaughn Member 

[Measured by R. W. Lemke and E. K. Maughan , 5.6--6.1 km (3.5-2 .. 8 mil north-northeast of 

V;:tughn in the NWV.1NE1h sec. 6, T. 21 N., H. 2 E ., a nd SWV.1SEV., sec . 31, '1'. 22 N., R 2 E. , 

Ca~~.:ade County (Va ughn quadrangle ) (fig. 10)} 

Blackleaf Formation: 

Bootlegger Member (basal beds): 

21. Sandstone, gray, fine-grained, moderately friable; weathers mottled bluish gray and 

orange brown; shaly at base, becoming more 

sandy toward top; upper 18 em is greenish 

gray (5GY 6/1) that weathers pale olive (lOY 

M F't 

6/2), is platy, and contains Lingula sp __ ___ _ 2.1 6.8 

Vaughn Member: 

20. Shale, distinctly fissile, grayish-black, nonbentonitic, noncalcareous; carbonaceous, particularly in basal part. Forms a grassy slope, 

in contrast to nonvegetated beds below ---- 3.3 10.9 

19. Concretions, dark-brown, noncalcareous, fer-
VAUGHN MEMBER 

20 

19 

9 

..... .. · .. . 5 .... ·. :.·.-: .. ·.:: ...... 

. · ... ·:: . ·.·: :··.·.·.: 

R. 2 E. 

31 

METRES 

0 

20 

FEET 

0 

20 

40 

60 

LITHOLOGIC 

SYMBOLS 

~ 

~ 

Sandstone 

Siltstone 

Shale 

Calcareous 

concretions 

m 

Bentonite 

Zeolitized 

tuff 

Carbonaceous 

shale 

0 1 MILE 

0 1 Kl LOMETRE 

FIGURE 10.-Type section of the Vaughn Member of 

the Blackleaf Formation and map showing line of 

measured section, sec. 6, T. 21 N., R. 2 E., and sec. 

31, T. 22 N., R. 2 E. Numbers on the left side ofthe 

column are key beds in the measured section. On 

the map, the arrow points upward stratigraphically; B, base, and T, top of member. 

Type section of Vaughn Member-Continued 

Blackleaf Formation-Continued 

Vaughn Member-Continued 

19. Concretions, etc.-Continued M 

ruginous; weather to a very dark metallic 

I:Jrown to bluish black ____________________ .1 

18. Clay, bentonitic; contains red specks of clinoptilolite. Divisible into several subunits that 

form distinct benches (mostly with spongy 

surfaces) separated by low scarps; subunits 

range in color from light greenish gray 

through dark gray to chocolate brown. Includes 18 em of dark-gray siltstone and 1.6 m 

of alternating beds of hard tuffaceous 

siltstone and light-gray bentonitic clay. 

Forms nonvegetated slope ________________ 8.4 

17. Siltstone, medium-light-gray W 6), very hard 

and dense, tuffaceous, lenticular; weathers 

very light gray (N 8); has very fine uneven 

laminae; contains red clinoptilolite grains 

and small nodules of light-green clay. LocalFt 

.2 

27.8 

Type section of the Vaughn Member-Continued 

Blackleaf Formation-Continued 

Vaughn Member-Continued 

17. Siltstone, etc.-Continued M 

ly, upper 15 em has a distinct red appearance, owing to abundance of finely disseminated clinoptilolite ______________________ .5 

16. Clay, bentonitic, noncalcareous; contains red 

grains of clinoptilolite. Divisible into several 

subunits that form distinct benches (mostly 

with spongy surfaces), separated by low 

scarps. Subunits are light greenish gray to 

dark gray (a few are greenish gray to olive 

greenl. Includes two beds of dark-gray hard 

and dense tuffaceous siltstone 30 em or less 

thick and also a few beds of papery shale near 

top. Forms nonvegetated slopes____________ 4.9 

15. Clay, bentonitic, slightly silty, moderately 

greenish yellow (lOY 7/2) to grayish-green; 

mottled with markings of medium-gray (N 5) 

clay that may be of organic origin or fillings 

in small desiccation cracks________________ LO 

14. Sandstone, light-gray, fine-grained, dense, 

noncalcareous; weathers salmon pink; 

breaks along indistinct joints into rectangular blocks about 30 em long and 15 em wide .2 

13. Siltstone, light-gray, locally clayey__________ .6 

12. Siltstone, medium-olive-gray (5Y 5/1), tuffaceous(?), dense, and very hard; weathers yellowish gray with some light-brown-stained 

surfaces; many sharply angular quartz 

grains, some light-green grains, and a few 

black grains and small black plant fragments. Breaks into very small, sharply angular pieces-------------------------------- .3 

11. Clay, light-gray, silty, slightly bentonitic ____ .6 

10. Sandstone, gray, fine-grained, resistant, 

slightly calcareous; breaks into irregular 

blocks owing to indistinct jointing. Abundant 

fossil wood at base________________________ .2 

9. Claystone concretions, dark-brown, tabular (as 

much as 0.6 m in diameter), slightly ferruginous, very resistant, noncalcareous; 

weather to very dark metallic brown to 

metallic bluish black; some plant 

fragments________________________________ .1 

8. Clay, greenish-gray and buff, bentonitic, noncalcareous ______________________________ 1.3 

7. Ironstone concretions, dark-brown, tabular, 

slightly calcareous; weather bluish and 

brownish black with a slight metallic 

luster____________________________________ .1 

6. Sandstone, light-yellowish-gray, fine- to 

medium-grained, hard, dense, rroncalcareous; weathers medium to dark brown; welldeveloped parallel joints about 7.5 em apart 

cause sandstone to break into blocks 30--46 

em long. Abundant rushlike plant 

remains -------------------------------- .1 

5. Sandstone, pale-greenish-yellow (lOY 8/2), 

medium-grained, very friable, thinly bedded 

to massive, somewhat crossbedded, arkosic, 

porous, slightly calcareous; consists chiefly of 

subangular to subrounded quartz grains, 

abundant feldspar, and some dark grains (in29 

Ft 

1.5 

16.2 

3.2 

.8 

2.0 

1.0 

2.0 

.5 

.4 

4.4 

.2 

.3 
30 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Type section of the Vaughn Member-Continued 

Blackleaf Formation-Continued 

Vaughn Member-Continued 

15. Sandstone, etc.-Continued M Ft 

eludes chert, magnetite, and biotite); lightcolored petrified wood in logs as much as 1.8 

m long. Locally, 1.8--2.7 m (6-9ft) above base 

is a finer grained more resistant sandstone 

that is grayish brown, massive, and highly 

calcareous. Scattered ironstone concretions ------------------------------------ 4.6 15.0 

Total Vaughn Member ________________ 26.3 86.4 

Taft Hill Member (part of upper unit): 

4. Siltstone, light- to dark-gray, shaly to sandy, 

thinly bedded, noncalcareous to highly calcareous; weathers light yellowish gray (cut 

by fracture fillings that weather yellow 

orange); includes, near the top, sandstone 

lenses several feet long and as much as 0.5 m 

thick that are medium gray, fine 

grained, calcareous, dense, and weather 

orange brown. Local lenticular sandstone 

near base. Upper boundary gradational ____ 3.2 10.4 

3. Clay, gray to greenish-gray, shaly to silty, bentonitic; weathers light gray with crackly surface; scattered nodular fine-grained concretions and local platy sandstone as much as 

0.8 m (2.5 ftl thick in upper part. Lower 1.2 m 

is more bentonitic and contains a few red 

clinoptilolite grains ______________________ 5.4 17.7 

2. Limestone, yellowish-gray (5Y 8/1 ), coarsely 

crystalline, layered; weathers brown; poorly 

developed cone-in-cone structure; breaks into small irregularly shaped brown fragments ---------------------------------- .1 .3 

1. Siltstone, light-olive-gray ( 5Y 5/2) to darkgreenish-gray ( 5GY 4/1 ), dense, hard, probably glauconitic, slightly calcareous to noncalcareous; contains Lingula sp. and macerated 

fossil plants. A clayey carbonaceous shale 15 

em thick divides the siltstone into two subunits. Unit cut by well-developed parallel 

joints that strike N. 71o W ________________ .9 3.0 

Total measured thickness of part of 

Taft Hill Member (upper unit) 9.6 31.4 

Reference section of Vaughn Member 

[Measured by C. E. Erdmann, June 25--0ct. 3. 1968, and Dec. 17-21, 1969. Base of section 

starts in north center NE1f.NWY.NE1t4 sec. 11. T. 21 N., R. 1 E., just below surface of U.S. 

Highway 91 (Interstate 151. Measurements extend along northeast side of highway to 

summit of small conical hill in NE. cor. SW1/4SW%SE'I• sec. 2. T. 21 N., R. 1/E.:ithence 

northeast to southwest face of escarpment. Top of measurement in center NE14SW1/4SE''-' 

sec. 2. Vaughn quadrangle, Cascade County, Mont.] 

Blackleaf Formation: 

Bootlegger Member (basal part): 

28. Sandstone, greenish-gray (5GY 6/1 ), fine 

grained, compact, thin slabby layered, calcareous, hard, resistant; weathers olive 

brown; contains trace fossils _____________ _ 

27. Shale, medium-dark-gray, clayey, poorly fissile. Rusty laminae at intervals of about 3 em 

in lower 1.5 m. Weathers into cracked clay 

M Ft 

0.3 1.0 

slope ------------------------------------ 4.3 14.0 

Reference section of Vaughn Member-Continued 

Blackleaf Formation (basal part)-Continued 

Disconformity; erosional relief slight, about 1 m in this 

vicinity. 

Vaughn Member: 

Upper unit (lake or swamp environment): 

26. Sandstone, yellowish-gray, thin and platy 

layered, fine- to medium-grained, locally 

crossbedded. Surface of upper bed may 

show irregularities resembling oscillation 

ripple marks: wave length, 1.0 ft; 

amplitude, 0.1; strike, S. 55° E. Contains 

small carbonized logs. Usually makes a 

massive unit that supports resistant 

bench. Upper surface also may be marked 

by inclusion of small irregular reworked 

lumps of light-bluish-gray (5B 7/1) 

siltstone containing broken laminae and 

M 

thin pods of medium- to coarse-grained 

yellowish-gray, cherty sandstone, and angular fragments of brownish-gray carbonaceous shale________________________ 1.8 

25. Sandstone, yellowish-gray; in thin (5-10 em) 

layers separated by somewhat greater 

thicknesses (15-30 em) of soft yellowishgray clayey siltstone____________________ 2.6 

24. Shale, brownish-gray, silty, carbonaceous, 

soft, flaky _____________________________ _ 

23. Clay, olive-gray, silty, slightly carbonaceous. Weathered surface cracks or checks; 

unvegetated ___________________________ _ 

.2 

.6 

Measured thickness of upper unit 5.2 

Middle unit (pyroclastic clays, water laid): 

22. Mudstone, medium-gray; weathers light 

gray; contains numerous small dark granules of chamosite. Matrix firm, moderately 

resistant, weathering out of slope. Base irregular _______________________________ _ 

21. Claystone, very dark gray, nearly black; has 

small undetermined light-gray specks ___ _ 

20. Claystone; weathers pinkish gray, as in 

unit 18 --------------------------------

19. Tuff, vesicular, water laid (as in unit 17). 

Pale olive gray, with vesicles occupied by 

orange-pink clinoptilolite. Compact; tough 

18. Claystone; weathers pinkish gray (5YR 8/1) 

17. Tuff, vesicular, silty, light-greenish-olivegray; weathers very light pinkish gray. In 

16. 

15. 

14. 

13. 

thin layers. Vesicles 0.2-0.6 mm in diameter and filled with clay. Some layers are 

hard, silicified, and float from them is 

characterized by a grayish-red (lOR 4/2) to 

dark-reddish-brown (lOR 3/4) clinoptilolite 

patina, giving the pieces the appearance of 

a reconstituted igneous rock ___________ _ 

Claystone, greenish-olive-gray. Probably 

largely montmorillonite _______________ _ 

Claystone, pink; contains clinoptilolite ___ _ 

Claystone, light-olive-gray _______________ _ 

Claystone, light-olive-gray to pinkish; contains small grains of clinoptilolite. Weath-

.3 

.6 

.1 

.2 

3.0 

.7 

1.1 

.2 

.9 

ers into soft spongy unvegetated surface 

layer with expanded "popcorn" texture, 

which flows downslope when wet. Thin 

siltstone layer at top____________________ 2.4 

Ft 

6.0 

8.4 

.5 

2.0 

16.9 

1.0 

1.9 

3.6 

.5 

9.7 

2.2 

3.7 

.7 

3.1 

8.0 
BOOTLEGGER MEMBER 31 

Reference section of Vaughn Member-Continued 

Blackleaf Formation-Continued 

Vaughn Member-Continued 

Middle unit, etc.-Continued 

12. Tuff and siltstone, medium-gray and 

greenish-gray. Tuff has vesicles occupied 

M 

by light-olive-green clay________________ .1 

11. Sandstone; in rounded, flattened, oval balls. 

Hard, light gray, fine grained, clayey, with 

a few galls of greenish clay. Top of bed 

slightly undulating ____________________ .5 

10. Mudstone (claystone), light-yellowish-gray. 

Surface cracked, tough. Makes unvegetated slope _ _ ______ _ _ __________ _ _ __ __ _ _ 1. 7 

Measured thickness of middle unit ____ 11.8 

Lower unit (fluviatile): 

9. Sandstone, light-gray, fine- to mediumgrained. Upper 15 em contains small pellets of light-gray clay. Thinly laminated by 

slightly more resistant brownish-gray 

crusts that may be a variety of parting 

lamination ---------------------------- .4 

8. Sandstone, very light yellowish gray (5Y 

9/1 l; predominantly fine to medium angular to subangular quartz with some chalky 

feldspar, with about 6 or 8 percent dark 

grains, of which biotite in flakes as much 

as 0.1 mm wide are most abundant, with 

chert next, in a tight cement of very light 

gray (white) clay. Becomes light greenish 

gray (5GY 8/1) upon weathering through 

color change of cement, and friable upon its 

loss. In thin (1-24 em) massive layers, 

some of which show low-angle east-dipping 

cross-lamination. Hard and tough when 

fresh, forming a low resistant ledge______ 2.5 

7. Sandstone, clayey; light-gray; orange mottling; soft------------------------------ 1.4 

6. Sandstone, light-gray, fine- to mediumgrained; in thin alternating layers with 

softer clayey sandstone. Poorly exposed __ 2.7 

5. Sandstone, light- to medium-gray, fine- to 

medium-grained; in massive 10- to 15-cm 

layers, but strongly crossbedded where 

thinner; lower 30 em hard, brown, and ferruginous in places. Base irregular. Contains thin lentils of conglomeratic sandstone noticeably characterized by rounded 

lumps of soft grayish-yellow (5Y 8/4) clay 

ranging in size from 0.5-cm pellets to irregular fragments 7-10 em long and 2 em 

or more thick. Flattened clay galls are 

abundant on bedding surfaces. In addition, 

the matrix of coarse, cherty, subangular, 

light-olive-gray sand with clay cement 

exhibits sharp angular 1-cm pieces of firm 

compact yellowish-gray to pale-olive siltstone; subround pebbles of dark-gray chert 

~12 mm in diameter; and clear glassy 

quartz in subround grains 2 mm in diameter that break with conchoidal fracture. 

This facies in particular, which is uncommon, makes the basal conglomerate of the 

Vaughn Member ______________________ 1.2 

Ft 

.3 

1.5 

5.5 

41.7 

1.3 

8.3 

4.5 

9.0 

4.0 

Reference section of Vaughn Member-Continued 

Blackleaf Formation-Continued 

Vaughn Member-Continued 

Lower unit (fluviatile)-Continued 

Measured thickness of lower unit 

Total measured thickness of Vaughn 

M Ft 

K2 27.1 

Member (rounded) ________________ 25.0 86.0 

Disconformity; slight erosional relief. 

Taft Hill Member (part of upper unit): 

4. Sandstone, medium-yellow-orange, soft, friable, fine-grained; weathers moderate 

brown -------------------------------- .5 1.7 

3. Sandstone, medium-grained; light olive at 

top------------------------------------ .2 .7 

2. Sandstone, clayey (mudstone), soft; surface 

cracks. Poorly exposed__________________ 3.4 11.2 

1. Clay ironstone; in fragmented chippy 

mounds that have diameters of 0.5-1 m at 

intervals of 3 m or more, on thin graygreen sandstone. Ironstone surface overlain by thin light-brown recry.stallized calcite. Base concealed by alluvium ________ .2 .5 

Measured thickness of part of Taft Hill 

Member (upper unitl ______________ 4.1 13.6 

BOOTLEGGER MEMBER 

NAME AND DEFINITION 

The Bootlegger Member is the Hupper member of 

sandy shale and sandstone of marine origin" that was 

correlated with the upper part of the Mowry Shale by 

Cobban (1951a, p. 2180). The member was named by us 

(Cobban and others, 1959, p. 2791) from the exposures 

along the east-trending escarpment that is crossed by 

the Bootlegger Trail11-13 km (7-8 mi) north of Great 

Falls near the common corner ofTps. 21 and 22 N., R. 3 

E., and Tps. 21 and 22 N., R. 4 E., Cascade County 

(Great Falls quadrangle). Inasmuch as the member is 

not completely exposed at any one locality, the section 

presented here as the type is composite from measurements at several places along the escarpment westward for more than 24 km (15 mi) from the crossing of 

the Bootlegger Trail. Although parts of the section are 

not well exposed, the total thickness can be measured 

in roadcuts along U.S. Highway 91 (Interstate 15) and 

vicinity northwest of Vaughn in sees. 34 and 35, T. 22 

N., R. 1 W. Thin beds of medium-gray sandstone, gray 

siltstone, dark-gray shale, and yellowish bentonite, 

compose most of the Bootlegger Member. Much of the 

sandstone, siltstone, and shale is interlaminated. 

THICKNESS 

The composite type section is 100 m (329 ft) thick. 

However, the overall thickness of the member in sec. 3, 

T. 21 N., R. 2 E., and sec. 34, T. 22 N., R. 2 E., is 84 m 

(275 ft). This difference in thickness appears to result 
32 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

chiefly from thinning of bed 3 and also above bed 30 of 

the type section as it is traced westward. Along Muddy 

Creek, in sec. 4, T. 21 N., R. 1 E., and sec. 33, T. 22 N., 

R:l E., the Bootlegger is 72.5 m (238ft) thick. Farther 

northeast in sec. 4, T. 22 N., R. 6 E., and in sec. 16, 

T. 23 N., R. 6 E., Lemke and Maughan measured a 

total thickness of 87.2 m (286 ft). In the subsurface 

Cobban (1951a, p. 2180) noted that the member ~~thins 

westward from 320 feet on the east flank of the Sweetgrass arch to 60 feet on the west flank. This westward 

thinning is due to gradual facies change of the lower 

part into non-marine sediments." 

OUTCROP DISTRIBUTION 

On the South arch the Bootlegger Member forms a 

conspicuous escarpment that extends from Gordon on 

Muddy Creek near the center of the Vaughn quadrangle eastward across the middle of the Great Falls 

quadrangle to Blackhorse Lake Flat. From there the 

escarpment strikes northeastward across most of the 

Portage quadrangle but is obscured by overlying glacial deposits. The member crops out here and there 

northeastward at least as far as the Missouri River in 

the Lander Crossing and Carter quadrangles. In the 

Vaughn quadrangle the Bootlegger is exposed from the 

vicinity of Vaughn northwestward 17 km (10.5 mi) up 

Muddy Creek to the center of sec. 2, T. 22 N., R. 1 W., 

where it disappears below the valley floor. West of 

Vaughn the Bootlegger crops out along the Sun River 

valley as far as 2.4 km (1.5 mi) beyond the town of Sun 

River. South of Vaughn the basal part of the member 

caps Taft Hill in the Vaughn and Cascade quadrangles. 

Outcrops of the Bootlegger Member are restricted to 

the central part of the Kevin-Sunburst dome in sec. 5, 

T. 33 N., R. 1 W. (Shelby quadrangle) where the upper 

3.5----4.5 m crops out, and the very top also was exposed 

during the excavation of the stock watering pond in the 

NW%NE% sec. 32, T. 34 N., R. 1 W. These exposures 

bear very close resemblance in texture and composition to typical Bootlegger where it is crossed by U.S. 

Highway 91 about 9.5 km north of Vaughn. 

Outcrops of Bootlegger lithology are not known in 

the Disturbed belt, owing to its probable replacement 

by the Vaughn Member. 

GENERAL DESCRIPTION 

On Taft Hill south of Vaughn a 10-m-thick finegrained massive bed of sandstone forms a light-brown 

cliff at the base of the Bootlegger Member. This 

sandstone, mapped by Dobbin and Erdmann (1930) as 

~·sandstone B," is equivalent to the 17m (56ft) of sandy 

beds that form the basal part of the Bootlegger 

Member in its type section. There the basal part of the 

member consists of 8.2 m (27 ft) of light-gray fine- to 

medium-grained thin-bedded sandstone overlain by 4 

m of shaly siltstone and in turn by a fine- to mediumgrained sandstone unit of two 0.3-m-thick beds of hard 

ledge-forming sandstone separated by 1.4 m of softer 

shaly sandstone. The lower of these 0.3 m ledges 

weathers dark brown and contains Lingula, a 

brachiopod genus found in both brackish-water and 

normal marine water. Lemke and Maughan traced 

these two ledges of sandstone from Muddy Creek, in 

the center of the Vaughn quadrangle, eastward across 

the Great Falls and Portage quadrangles and into the 

Lander Crossing quadrangle where the two beds of 

sandstone dip below the Missouri River at the mouth of 

Black Coulee. They found that the interval separating 

the two sandstone ledges varied from 1.5 to 4.5 m. In 

the Portage quadrangle the lower ledge is the more 

conspicuous of the two, whereas the upper ledge is generally the more prominent in the western half of the 

Great Falls and Vaughn quadrangles. The sandy to 

shaly interval underlying these hard sandstone ledges 

and overlying the Vaughn Member thins westward 

from the type section into the Vaughn quadrangle. 

About 4 km northwest of Vaughn, the lower sandstone 

ledge lies a metre or so above the Vaughn Member. 

In the type section the upper of the two ledges of 

sandstone is overlain by 6.4 m (21 ft) of dark-gray 

shale that contains a layer of bentonite 25 em (10 in.) 

thick. Above this shale is 4.4 m (14.3 ft) of sandy 

siltstone characterized by irregular vertical and 

diagonal fractures. A thin brown-weathering bed of 

hard medium-grained sandstone forms a small ledge 

on top of the siltstone. Above the sandstone is 1.7 m 

(5.4 ft) more of shaly siltstone. This siltstone and 

sandstone sequence thins eastward and is very shaly 

where it dips under the Missouri River below the 

mouth of Black Coulee in the northwestern part of the 

Lander Crossing quadrangle. Locally, particularly 

along the escarpment north of Blackhorse Lake Flat in 

the Great Falls quadrangle, and at a few other places 

in the Portage and Vaughn quadrangles, the siltstone 

and sandstone sequence is well indurated and forms 

well-defined benches. 

The silty unit in the type section of the Bootlegger 

Member is overlain by 10.8 m (35.3 ft) of very dark 

gray shale that contains a bed of bentonite 0. 7 m (2.2 

ft) thick in the basal part. Above this shale is a unit 

about 12 m (40 ft) thick (beds 17-21 of type section) of 

thinly bedded sandstone and siltstone with a local bed 

of black-coated chert pebbles near the middle. This 

sandy unit is believed to be the equivalent of the 

sandstone along Highwood Creek (Reeves, 1929, p. 

162, bed 24). Another shaly unit about 9.5 m (31 ft) 

thick overlies these sandy beds and contains a 2-
BOOTLEGGER MEMBER 33 

m-thick bed of greenish-yellow bentonite near its base 

that forms a conspicuous light-colored poorly vegetated 

band for many miles westward along the escarpment 

from the type section. The 12-m (39-ft) thickness of 

beds above this shaly unit is largely concealed but includes two thin conglomeratic beds with black-coated 

chert pebbles as much as 5 em in diameter. The next 

7.6 m (25 ft) consists of poorly exposed beds of dark 

shale and thick layers of bentonite including one 3 m 

(10 ft) thick. The 3-m-thick bentonite contains, especially in its upper part, lenselike masses of zeolitic tuff 

and forms a conspicuous white band along the escarpment westward for more than 24 km. It is probably the 

westward extension of the Arrow Creek Member of the 

Colorado Shale of central Montana (Reeside and Cobban, 1960, p. 8, pl. 1 ). A black chert sandstone bed as 

much as 0.9 m thick just below the base of the bentonite bed forms the upland surface over several square 

miles west and northwest of the type section. The rest 

of the Bootlegger Member, about 21m (70ft), is mostly 

interbedded and interlaminated sandstone, siltstone, 

and sandy shale that contain abundant fish scales. 

Several layers of bentonite are present, and one that is 

about 15 em thick lies at the top. Just beneath this 

uppermost layer of bentonite is a brown medium- to 

coarse-grained massive bed of hard sandstone about 30 

em (1ft) thick crowded with fish bones and scales and, 

here and there, black-coated chert pebbles as much as 

5 em in diameter. This conspicuous bed of coarse sandstone is present from the western part of the Great 

Falls quadrangle westward across most of the Vaughn 

quadrangle. It can be seen readily along U.S. Highway 

91 about 9.6 km (6 mi) northwest of the intersection 

near Vaughn with U.S. Highway 89 and forms the upland surface for some distance to the north and 

northeast. 

The upper and lower contacts of the Bootlegger 

Member are sharp. Along Muddy Creek in the Vaughn 

quadrangle, gray shaly fine-grained sandstone at the 

base rests abruptly on black carbonaceous shale that 

forms the top of the Vaughn Member (fig. 9; Cobban, 

1955a, text fig. 2). At other localities gray shaly 

sandstone or darker sandy shale forming the basal bed 

of the Bootlegger rests sharply on light-gray clay or 

white crossbedded sandstone at the top of the Vaughn 

(fig. 8). The boundary between the Vaughn and Bootlegger Members can be seen clearly in a cut on U.S. 

Highway 91 about 4.8 km (3 mi) northwest of Vaughn, 

south of the center of sec. 2, T. 21 N., R. 1 E. The upper 

boundary is determined easily in most places by a 15-

cm-thick layer of bentonite that rests on the hard bed 

of coarse-grained to pebbly sandstone which contains 

abundant brown fish bones. The overlying Floweree 

Member of the Marias River Shale contains no ben tonite and only rarely fish scales or bones. That contact 

can be seen on the north, west, and south sides of the 

hill that bears the Cone triangulation station in sec. 

13, T. 22 N., R. 1 W., about 4.8 km west of U.S. Highway 91 in the Vaughn quadrangle. 

Wells drilled for oil and gas on the Sweetgrass arch 

reveal a threefold division for the Bootlegger Member 

at many localities on both the South arch and KevinSunburst dome. In general the basal unit is sandy, the 

medial unit is soft shale, and the upper unit is sandy, 

hard, and contains fish scales. 

The basal subsurface unit, about 30 m (100ft) thick, 

is chiefly gray fine-grained thin-bedded sandstone interbedded with darker sandy shale. Small black carbonaceous flakes are common. Glauconite is sparingly 

present, and pyrite occurs in a few places. One or two 

thin layers of black chert granules or small pebbles are 

present in some wells; one such layer commonly marks 

the top of this unit. 

The medial subsurface unit is dominantly dark-gray 

soft shale with a few thin beds of fine-grained 

sandstone and bentonite. Glauconite and pyrite are 

present but sparse. Granules and small pebbles of 

black chert occur locally. The thickness of this soft 

shaly unit averages about 15m. 

The upper subsurface unit, 18m to more than 30m 

thick, is characterized by thin beds of hard mediumgray sandstone and siltstone interlaminated and interbedded with darker gray silty shale. Fish scales are 

abundant. Several beds of bentonite are present. Almost all well cuttings show a few centimetres of very 

coarse sandstone or conglomerate at the top of this 

unit. The sandstone consists mostly of poorly sorted 

clear grains of quartz showing secondary crystal faces, 

subrounded to rounded colorless frosted quartz grains, 

and lesser amounts of subangular gray and black 

chert. Pebbles are chiefly gray chert that has a black 

patina. Brown fish bones are generally present. Locally 

the sandstone is somewhat an orthoquartzite. Small 

amounts of pyrite occur in the sandstone in some of the 

well cuttings. 

The boundaries of the Bootlegger Member can be 

determined as readily in the subsurface as on the outcrop. The coarse more or less conglomeratic sandstone 

layer at the top is an excellent key bed. Where it is 

absent, the top can be placed at the abrupt change from 

the dark-gray shale of the Floweree Member to the 

lighter gray fish-scale-bearing interlaminated and interbedded sandstone, siltstone, and silty shale of the 

Bootlegger. The lower contact is equally as sharp. It 

marks the change from dark-gray silty or sandy shale 

of the Bootlegger to light-colored clay, white bentonitic 

sandstone, or black carbonaceous shale at the top of the 

Vaughn Member. 
34 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

AGE ASSIGNMENT A:\'D CORRELATION 

The Bootlegger Member contains the common 

Mowry fish scales described by Cockerell (1919) as 

Holcolepis transversus, Leucichthyops vagans, and 

Erythrinolepis mowriensis. The ammonite genus 

N eogastroplites has been found in the member on Belt 

Butte 29 km (18 mi) southeast of Great Falls and at 

several localities on the Sweetgrass arch north, northwest, and west of Great Falls. This ammonite seems to 

range in age from latest Albian to early Cenomanian. 

The Bootlegger Member is correlated with the 

Mowry Shale of central Montana and the Black Hills 

region. Both units contain Neogastroplites and abundant fish scales. However, the Bootlegger Member is 

much less siliceous than the Mowry Shale, lacks the 

silvery-white weathering, and is very much sandier. It 

can be interpreted as a near-shore facies of the Mowry 

Shale. Exposures of the upper part of the Bootlegger 

Member near Carter Ferry (Carter quadrangle) 

suggest a gradation to typical Mowry Shale aspect. 

Here these beds are more siliceous and in part weather 

grayish white. 

TYPE SECTION OF THE BOOTLEGGER MEMBER 

The following type section is based almost entirely 

upon field descriptions. Only a limited amount of petrographic and mineralogic work was done. This was 

confined mostly to the identification of zeolitic tuff in 

some of the bentonite beds, which previously had been 

referred to as porcellanite. Because no great thickness 

of section is exposed at any one place, the type section 

is a composite section extending over a distance of approximately 32 km (20 mi). Therefore, the member at 

no one locality has the thickness or exactly the lithology described in the type section. 

Type section of the Bootlegger Member 

[Measured by R. W. Lemke and E. K. Maughan, along escarpment that is crossed by the 

Bootlegger Trail9.6 km (6 mi) north of Great Falls, Cascade County, Great Falls quadrangle, and near Cone triangulation station, Teton County. Vaughn quadrangle: Units 

1-10, SW'l• sec. 31, T. 22 N., R. 4 E.: units 11-30, SE14 sec. 36, T. 22 N., R. 3 E.; units 

31-38, SE'4 sec. 35, T. 22 N., R. 2 E.: units 39-52, NE~4 sec. 33. T. 22 N., R. 2 E.: units 

53-54, SE'4 sec. 29, T. 22 N., R. 2 E.; units 54-55, SE14 sec. 13, T. 22 N., R. 1 W. <fig. 11 l) 

Marias River Shale: 

Floweree Member {part of basal unit): 

56. Siltstone, dark-gray, sandy to shaly, thinly 

bedded, somewhat platy; weathers to bluish 

M Ft 

gray with yellow-brown mottling __________ 1.5 5.0 

Blackleaf Formation: 

Bootlegger Member: 

55. Bentonite, grayish-yellow; impure with small 

selenite crystals-------------------------- .2 .6 

54. Sandstone, yellowish-brown to brown (weathered), medium- to coarse-grained, resistant, 

blocky; contains abundant fish bones and 

scales. Locally contains numerous blackType section of the Bootlegger Member-Continued 

Blackleaf Formation-Continued 

Bootlegger Formation-Continued 

54. Sandstone, etc.-Continued 

coated gray chert pebbles as much as 5 em 

long. Forms upland surface over many 

square kilometres to the north ___________ _ 

53. Covered ----------------------------------

52, Sandstone, light-gray, fine-grained, platy; 

weathers grayish tan; upper 0.9 m is calcareous. Contains fish scales _________________ _ 

51. Covered ----------------------------------

50. Bentonite, yellowish-brown _______________ _ 

49. Siltstone, dark-gray, sandy, platy; alternates 

with silty brownish-gray platy sandstone. 

Contains fish scales _____________________ _ 

48. Covered ----------------------------------

4 7. Bentonite, yellowish-white _________________ _ 

46. Siltstone, dark-gray, sandy, thin-bedded, 

platy; contains fish scales _______________ _ 

45. Covered ----------------------------------

44. Siltstone, dark-gray, sandy, thin-bedded, 

platy; contains fish scales _______________ _ 

43. Bentonite; greenish-yellow and pure in lower 

part; grades upward into light-gray silty impure bentonite _________________________ _ 

42. Siltstone, dark-gr:ay, sandy, thin-bedded, 

platy; contains fish scales _______________ _ 

41. Shale, gray to tan, sandy _________________ _ 

40. Bentonite, greenish-yellow _________________ _ 

39. Siltstone, dark-gray, sandy, thin-bedded, 

platy; alternates with fine-grained mediumgray sandstone that weathers bluish gray. 

Contains fish scales _____________________ _ 

38. Siltstone, dark-gray, sandy; alternates with 

silty shale. Thin layer of impure bentonite 

about midway in unit ___________________ _ 

37. Bentonite, greenish-yellow _________________ _ 

36. Siltstone, dark-gray; weathers yellowish 

orange on fiat surfaces; shaly _____________ _ 

35. Bentonite, greenish-yellow _________________ _ 

34. Shale, brown, silty _______________________ _ 

33. Shale, silty, moderately fissile; nearly black 

except for yellow powder along bedding 

planes ----------------------------------

32. Bentonite; yellowish green at base grading 

upward into gray silty impure bentonite at 

top. About midway in interval are lenslike 

masses, commonly 1-3m long and 0.3-0.6 m 

thick, of white thin and irregularly bedded 

zeolitic tuff that breaks into angular to 

rounded splintery pieces. Bentonite is iron 

M 

.3 

3.4 

2.7 

3.2 

.2 

2.1 

1.5 

.3 

1.4 

1.7 

.2 

.9 

.6 

.5 

.1 

.6 

1.5 

1.1 

1.1 

.3 

.3 

1.5 

Ft 

1.0 

11.0 

9.0 

10.5 

.5 

7.0 

5.0 

1.0 

4.5 

5.5 

.5 

3.0 

2.0 

1.8 

.4 

2.0 

5.0 

3.5 

3.6 

.9 

1.0 

4.9 

stained and impure in upper 1.2 m ________ 3.0 10.0 

31. Shale, dark-gray to black, silty, distinctly 

fissile____________________________________ .5 1.5 

30. Sandstone, light-gray, fine- to coarse-grained, 

resistant, blocky, noncalcareous; mostly 

quartz with a few black chert grains; fairly 

numerous gray-blue fiat claystone nodules as 

much as 10 em long. Contains fish scales. 

Locally conglomeratic with very numerous 

black-coated chert pebbles 3-50 mm long. 

Unit locally as much as 0.9 m thick ______ .2 .6 

29. Covered. A bentonite bed indicated at base of 

interval__________________________________ 6.9 22.5 
Top 

28 4'· ·i:ii::i::-:::::·i:D:i· ::::::·:::::::· -~· 

23 

T. 22 N. 

T. 21 N. 

SEC. 13, 

T. 22 N., R.1 W. 

r1 

~ 55 

BOOTLEGGER MEMBER 

:-.-:-:--:- 7".-.: :-.-:-:-: ~---------

>·::::: ·.-: ~. :: ·.-.-.:. 

) :-.:-::-:-:-.7.-:-::-.-- \----------

P-----

---

---

----

..._ • • e • ,...,., • •••• 

15 :_:_:.~~-----:·-=.-=··_·.: , ___ _ 

13 II II II I 

r-- -- ---

f-----

----

5 ~~_::::::::~_:: - ------- ~ _-_-_-... ~ .-.--:-:-.-.: ~:-. 

r----------

j~~-==-:~:_:-~·:_·-~~: 

~::::·.::::.·.·-~ 

\.:...::...:·:~ ::....:..::::. 

Base 

33 

0 

34 

10 

LITHOLOGIC 

SYMBOLS 

Chert pebbles 

~ 

~ 

Sandstone 

~ 

~ 

Siltstone 

Shale 

m 

Bentonite 

~ 

~ 

Ironstone 

concretions 

Covered 

METRES 

0 

20 

20 

40 

60 

3MILES 

4 KILOMETRES 

R. 3 E. R. 4 E. 

31 

10 

11 12 

FIGURE 11.-Type section ofthe Bootlegger Member of the Blackleaf Formation and map showing line of measured 

section, sec. 31, T. 22 N., R. 4 E., sec. 36, T. 22 N., R. 3 E., sees. 29, 33, and 35, T. 22 N., R. 2 E., and sec. 13, T. 22 

N., R. 1 W. Numbers on the left side of the column are key beds in the measured section. On the map, the arrow 

points upward stratigraphically: B, base, and T, top of member. 

35 
36 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Type section of the Bootlegger Member-Continued 

Blackleaf Formation-Continued 

Bootlegger Member-Continued 

28. Sandstone, gray, blocky, resistant, calcareous; 

weathers dark gray. Conglomeratic with 

very numerous black-coated gray chert pebbles mostly about 3 mm long. Contains fish 

scales ___________________________________ _ 

27. Covered ----------------------------------

26. Bentonite; greenish yellow at base grading 

upward into bentonitic black clay _________ _ 

25. Siltstone, gray, shaly, sandy, noncalcareous; 

weathers to irregular ledge _____________ _ 

24. Shale, nearly black, distinctly fissile. Bentonitic near top ------------------------------

23. Bentonite; dark yellowish green in lower part 

grading upward into grayish-green silty impure bentonite at top. Contains tuffaceous 

lenses similar to unit 32 _________________ _ 

22. Clay, dark-gray, bentonitic _______________ _ 

21. Siltstone, thinly bedded; alternates with finegrained sandstone and grades upward into 

dark-gray fissile shale; weathers to irregular 

ledge ------------------------------------

20. Sandstone, light-gray, blocky, resistant, thinbedded, noncalcareous ___________________ _ 

19. Poorly exposed. Float indicates sandstone and 

siltstone beds ___________________________ _ 

18. Sandstone, light-gray, lenticular, resistant, 

thin-bedded, noncalcareous; weathers yellowish tan. Upper part breaks into rectangular blocks on surface. Locally contains abundant black-coated chert pebbles ___________ _ 

17. Siltstone, dark-gray, sandy, thin-bedded; alternates with medium-gray sandstone. 

Forms irregular ledge ___________________ _ 

16. Shale, silty, noncalcareous _________________ _ 

15. Sandstone, silty, irregularly bedded, noncalcareous. Contains scattered fiat dark-brown 

calcareous ironstone concretions about 30 em 

in diameter ------------------------------

14. Shale, very dark gray, distinctly fissile, noncalcareous. Ferruginous sandstone layer 

about 24 em above base. Poorly exposed ___ _ 

13. Bentonite; yellowish green in lower half grading upward into olive-green bentonitic shaly 

clay. Scattered brown ovoid-shaped ironstone 

concretions at top as much as 50 em in 

diameter --------------------------------

12. Shale, dark-gray, silty, noncalcareous _____ _ 

11. Siltstone, light- to medium-gray, shaly, friable, 

noncalcareous; orange-brown stained films 

10. Sandstone, medium-gray, medium-grained, resistant; weathers medium brown _________ _ 

9. Siltstone, bluish-gray, sandy, friable, irregularly (vertical) fractured, noncalcareous; 

stained orange-yellow in places. Lenses of 

fine-grained sandstone throughout interval; 

a 15-cm-thick thin-bedded sandstone layer at 

base. Upper half is covered east of Bootlegger 

Trail but is well exposed west of road where 

it is a thin-bedded friable noncalcareous 

light-gray sandstone containing thin films of 

carbonaceous material that weathers mottled gray and orange brown _____________ _ 

M Ft 

.4 1.3 

4.4 14.5 

.3 .9 

2.4 7.7 

4.6 15.0 

2.0 6.5 

.3 1.0 

1.6 5.3 

.2 .8 

2.4 8.0 

.3 1.0 

3.4 11.0 

2.4 7.9 

1.8 5.8 

7.5 24.7 

.7 2.2 

2.6 8.4 

1.7 5.4 

.2 .7 

4.4 14.3 

Type section of the Bootlegger Member-Continued 

Blackleaf Formation-Continued 

Bootlegger Member-Continued 

8. Shale, dark-gray, distinctly fissile, noncalcareous; yellow and orange-yellow films along 

parting planes. Bentonite bed, 25 em thick, 

1.2 m above base _______________________ _ 

7. Sandstone, light-gray; fine- to mediumgrained, resistant, noncalcareous; weathers 

light gray with orange-brown stains along 

vertical fractures; contains small wormlike 

M Ft 

6.3 20.8 

trails ---------------------------------- .3 1.0 

6. Sandstone, silty to shaly, irregularly fractured, 

noncalcareous; grades upward into silty 

shale------------------------------------ 1.4 4.5 

5. Sandstone, dark-gray, medium-grained, resistant, lenticular, calcareous, fossiliferous 

(Lingula sp.). Upper half weathers very dark 

brown and forms fairly continuous outcrops. 

Lower part is finely bedded, locally crossbedded, and weathers grayish tan ____________ .3 1.0 

4. Siltstone, shaly, noncalcareous; grades laterally into sandstone beds alternating with 

shaly siltstone; irregularly fractured and friable and forms gentle slopes; weathered surface is mottled bluish gray and orange brown 4.1 13.5 

3. Sandstone, light-gray, fine- to mediumgrained, thinly bedded (mostly less than 10 

em thick), noncalcareous; some thin films of 

dark-gray carbonaceous material; weathers 

mottled bluish gray and orange brown. 

Well-developed vertical joint pattern striking N. 70° W. Upper 30 em is more resistant 

than remainder and forms a conspicuous 

ledge with well-developed ripple marks. 

Lower part includes some thin beds of sandy 

to silty shale and is rarely exposed ________ ~ 27.0 

Total Bootlegger Member (rounded) ____ 100.0 329.0 

Vaughn Member (uppermost): 

2. Shale, bentonitic, distinctly fissile, noncalcareous; medium gray at base to dark gray at top 2.9 9.4 

1. Shale, nearly black, bentonitic; contains red 

clinoptilolite grains; distinctly fissile at top 

and noncalcareous ________________________ 4.0 13.2 

Total Vaughn Member (measured) ______ 6.9 22.6 

MARIAS RIVER SHALE 

The name Marias River Shale was given by us (Cobban and others, 1959, p. 2793) to the 275-365 m (900-

1,200 ft) of dark-gray Upper Cretaceous shale that lies 

between the Blackleaf and Telegraph Creek Formations on the Sweetgrass arch and westward into the 

Rocky Mountains front. The name is from the excellent 

exposures along the Marias River, which crosses the 

Sweetgrass arch between the South arch and the 

Kevin-Sunburst dome. Here most of the formation is 

exposed in a synclinal area, known as the Marias River 

saddle (Dobbin and Erdmann, 1955), along the boundary between Toole and Pondera Counties. The Marias 

River Shale includes all but the uppermost sandy 
FLOWEREE MEMBER 37 

transitional beds of Stebinger's (1918, p. 161) "shale 

above the Blackleaf sandy member" in the Birch 

Creek- Sun River area where he noted that "The remainder of the Colorado shale above the Blackleaf 

member forms the principal body of shale in this area 

and by its thickness alone is readily differentiated 

from the other shale units present." The Marias River 

Shale also more or less includes Willis' (1902, p. 315, 

326, 327) Benton Shale in the Glacier National Park 

area and the upper or "lead-colored clay shale" part of 

Weed's (1899) Colorado Formation. 

We (Cobban and others, 1959, p. 2793) divided the 

Marias River Shale into four members, which are from 

oldest to youngest, Floweree, Cone, Ferdig, and Kevin. 

The type sections of the Floweree and Cone Members 

are on the South arch, whereas those of the Ferdig and 

Kevin Members are on the Kevin-Sunburst dome. In 

terms of the standard stages of the Upper Cretaceous 

the Floweree Member is late Cenomanian, the Cone is 

late Cenomanian and early Turonian, the Ferdig is 

middle and late Turonian, and the Kevin is Coniacian 

and early Santonian. 

The Marias River Shale occupies the surface over 

much of the Sweetgrass arch. It crops out also around 

the Sweetgrass Hills and in the Disturbed belt. In general the formation thickens from east to west. East of 

the Kevin-Sunburst dome the thickness is about 283 m 

(930ft) in the West Utopia oil and gas field and as little 

as 277 m (910 ft) in wells drilled 16-19 km (10-12 mi) 

east of this field. Northward from the West Utopia 

field, the formation thickens to 293 m (960 ft) in the 

Sweetgrass Hills. The full thickness of the Marias 

River Shale is not known along much of the crest of the 

Sweetgrass arch inasmuch as the upper part of the 

formation has been eroded. Wells drilled in and about 

the Cut Bank oil and gas field, west of the KevinSunburst dome, show a rather uniform northwest 

thickening from 311m (1 ,020 ft) in the southern part of 

the Cut Bank field to 363 m (1,190 ft) on the Chalk 

Butte nose 16 km (10 mi) northwest of this field. A 

westward thickening is shown by wells drilled on the 

northwest flank of the South arch where the formation 

is 311- 332 m (1,020-1,090 ft) thick. 

FLOWEREE MEMBER 

NAME AND DEFINITION 

The Floweree Member was named for exposures 

about 3.2 km (2 mi) northeast of Floweree along Black 

Coulee in sees. 16 and 17, T. 23 N., R. 6 E., Chouteau 

County (Carter quadrangle). It is chiefly dark-gray 

shale and medium-gray shaly siltstone. The member 

was correlated earlier by Cobban (1951a, p. 2174) with 

the Belle Fourche Shale of the Black Hills and that 

part of the Colorado Shale of central Montana that lies 

between the Mowry Shale Member and the Mosby 

Sandstone Member. 

TH!CK N FSS 

The Floweree Member varies greatly in thickness. A 

total thickness of 19.4 m (63.5 ft) was measured by 

Lemke and Maughan at the type section. In the 

Vaughn quadrangle to the west and in the northern 

part of the Cascade quadrangle, most measured sections reveal thicknesses of 6-14 m (20-45 ft). At the 

Cone triangulation station (type section for the Cone 

Member) in sec. 13, T. 22 N., R. 1 W., the member is 

10.7 m (35ft) thick. In sec. 6, T. 21 N., R. 1 E., 9.7 km 

(6 mi) northwest ofVaughn, Maughan (1961) noted the 

member to be only 3 m (10 ft) thick. 

In the subsurface, the greatest thicknesses known 

are in the Bears Den district of the Sweetgrass Hills 

where as much as 45.7 m (150 ft) has been recorded 

(Cobban, 1951a, p. 2183). Thicknesses revealed by 

wells on the Kevin-Sunburst dome are commonly 

23-30 m (75-100 ft). On the South arch thicknesses 

greater than 15 m (50 ft) are rare. In the Pendroy area 

west of the Pondera oil field, the member thins to as 

little as 4 m. 

The member crops out at many places along the 

Rocky Mountains front. The thickness is 11.6 m (38ft) 

at the southeast edge of Glacier National Park (Cobban, 1956, p. 1003) and is about 9 m in the Sun River 

Canyon area (Mudge, 1972, p. A66) 97 km (60 mi) west 

of Great Falls. 

O UTCROP DlSTRIBUTIOi\' 

The Floweree Member is exposed on the southern 

part of the South arch, on the crest of the KevinSunburst dome, in the Sweetgrass Hills, and along the 

Rocky Mountains front. 

The largest outcrop area is on the South arch. From 

the Missouri River below Carter Ferry in the southeastern corner of the Carter quadrangle, the belt of 

outcrop (largely concealed by glacial deposits) extends 

west and slightly southwest to the Benton Lake area in 

the northeast part of the Great Falls quadrangle. From 

there it trends west across the Great Falls and Vaughn 

quadrangles to Muddy Creek where the top of the 

Floweree Member dips below the stream level 3.2 km 

southeast of Power near the center of sec. 2, T. 22 N., R. 

1 W. Southeastward from that point the outcrop follows along the west side of the valley of Muddy Creek 

and crosses the divide separating the Muddy Creek 

drainage from the Sun River drainage in sec. 8, T. 21 

N., R. 1 E. From there the outcrop extends up Sun 

River valley almost to Fort Shaw. South of the Sun 

River valley the member forms a narrow south-
38 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

trending outcrop in the eastern part of the northwest 

quarter of the Cascade quadrangle. 

On the Kevin-Sunburst dome the Floweree Member 

crops out around the apex of the dome, making a "ring" 

possibly as much as 2.4 km in width, that in large part 

is obscured by glacial deposits. The principal area of 

exposure is in sees. 28, 29, 30, and 31, T. 34 N., R. 1 W., 

in the Shelby quadrangle where measurements have 

revealed the lower 9 m. The total thickness in this area 

probably is of the order of 24.4 m (80 ft). 

West of the Sweetgrass arch the Floweree Member is 

exposed near the Missouri River 3.2 km northeast of 

Wolf Creek in sec. 31, T. 15 N., R. 3 W., and from the 

Sun River in the SW. cor. T. 22 N., R. 8 W., north toT. 

26 N., R. 8 W., and from there northwest to the southeast boundary of Glacier National Park. 

GE NERA L DESCRIPTIO N 

Dark-bluish-gray shale, lighter colored sandy shale, 

and thin beds of sandstone and siltstone (fig. 12) 

charact erize the member. Both contacts are sharp; the 

lower is marked by an abrupt change from the darkgray shale of the Floweree to either a bentonite bed or 

coarse fish-bone-bearing sandstone at the top of the 

Bootlegger Member, and the upper boundary is 

marked by an equally abrupt change from noncalcareous shale to the limy shale of the Cone Member. 

In the type section, 3.2 km northeast of Floweree on 

the east flank of the South arch, the basal part of the 

Floweree Member consists of 1.5 m of dark-gray shale 

that grades upward into 4.3 m (14 ft) of gray shaly 

siltstone. Part of the siltstone is characterized by odd 

ellipsoidal partings that give a nodularlike appearance 

to the beds. These ovoid-shaped masses weather bluish 

gray with orange-yellow rinds. A crude shaly structure 

is present although the overall appearance is massive. 

Numerous inclined fractures filled with yellowishbrown siltstone cross the beds. A 2.1-m-thick bed of 

dark-bluish-gray shale overlies the siltstone unit. It 

contains large sandy calcareous concretions that 

weather light yellowish brown. A fine- to mediumgrained layer of sandstone, less than 0.3 m thick, rests 

on the shale. Above the thin bed of sandstone is 4 m of 

shaly siltstone cut by diagonal fractures. Distantly 

FIGURE 12.- Floweree Member of the Marias River Shale at its type section in Black Coulee in the SE\.i!SW'!. NE\4 sec. 17, T. 23 N., R. 6 

E., Chouteau County, Mont. 
FLOWEREE MEMBER 39 

spaced gray sandy calcareous septarian concretions are 

present near the top of this siltstone unit. Another thin 

layer of yellowish-brown-weathering sandstone overlies the shaly siltstone. Above it is nearly 1.2 m of 

siltstone that contains numerous gray septarian limestone concretions. Dark-gray shale forms the upper 

approximate 6 m. 

The calcareous concretions noted in the type section 

are absent on the west flank of the South arch but are 

well exposed in sees. 28 and 29, T. 34 N., R. 1 W., on the 

Kevin-Sunburst dome, where at least nine beds of concretionary limestone are present. 

Local layers of chert granules and small pebbles are 

present on the crest and west flank of the South arch as 

well as on the east flank at the type section of the 

member. Well cuttings show the presence of chert 

granules and pebbles locally at other places on the 

Sweetgrass arch. None of these beds are conspicuous, 

and all seem to be an inch or less in thickness. 

AGE ASSIGNMENT AND CORRELATION 

The sandstone and siltstone layers in the Floweree 

Member contain numerous worm(?) burrows or castings and a few well-preserved worm(?) tracks. About 

3.2 km southwest of Power in the NWJ4NWJ4SWJ4 sec. 

2, T. 22 N., R. 1 W., thin hard sandy layers in the upper 

half of the member contain the following marine fossils 

(USGS Mesozoic loc. D535 ): 

Inoceramus aff. I. mesabiensis Bergquist 

Metoicoceras muelleri Cobban 

Metoicoceras mosbyense Cobban 

These fossils occur in the Mosby Sandstone Member of 

central Montana. The Mosby, which contains the ammonite Dunueganoceras albertense (Warren) of late 

Cenomanian age, is correlated by its fossil content 

with the middle part of the Greenhorn Formation of 

the Black Hills. 

At the type section of the Floweree Member, a still 

older ammonite has been found in the bed of calcareous 

septarian concretions 7.6 m (25 ft) below the top of the 

member. This ammonite, Calycoceras? canitaurinum 

(Haas), originally described as Mantelliceras 

canitaurinum (Haas, 1949, p. 9-14, pls. 1-4, text figs. 

1-4) from the lower part of the Cody Shale of Wyoming, 

occurs in the basal limestone bed of the Greenhorn 

Formation in the Black Hills area (Cobban, 1951a, 

p. 2184). 

Fossils other than worm(?) burrows or castings have 

not been found in the lower half of the Floweree 

Member. Possibly this half is equivalent to some part 

of the Belle Fourche Shale of the Black Hills. However, 

inasmuch as the Belle Fourche Shale attains thicknesses of more than 150m (500ft) and contains several 

Cenomanian ammonite zones, the probability that this 

thin undated part of the Floweree Member represents 

much of Belle Fourche time seems unlikely. 

TYPE SECTION OF THE FLOWEREE MEMBER 

The following type section is based entirely upon 

field descriptions. Measurements were made along a 

1.2-km (0.8-mi) segment of Black Coulee along the valley wall. Distinguishable beds, mostly concretionary 

beds, were traced laterally. Where not exposed, the 

beds were projected upstream taking into account the 

local dip. The thickness of the section is believed to be 

accurate to within half a metre. 

Type section of the Floweree Member 

[Section measured by R. W. Lemke and E. K. Maughan on the north side of Black Coulee 

about 3.2 km (2 mil northeast of Floweree, Chouteau County (Carter quadrangle). Base of 

section starts above creek bed of Black Coulee along a north-south section-line fence in 

SE';.!SE1t.!NE% sec. 17, T. 23 N., R. 6 E. Top of section is in NW14SE14NW14 sec. 17, T. 23 

N., R. 6 E. dig. 131] 

M Ft 

Marias River Shale: 

Cone Member (basal bed; most of member exposed 

upslope): 

12. Limestone, concretionary, argillaceous; 

weathers light grayish lavender; commonly 

15-30 em thick, 0.5-1 m in diameter, and 

spaced 1 to several metres apart; septarian 

with veins of yellow calcite; enclosed by fissile medium-brownish-gray calcareous shale 0.3 1.0 

Floweree Member: 

1L Shale, very dark gray, distinctly fissile, nonMETRES 

0 

20 

FEET 

0 

20 

40 

60 

---- -------

C------ ...... --.. 

1--···--·•"'-

c--=--=----=-- 10 (.:...::=:-~-~-:...-.::.r-o1156 

\~-~~~-~~~~~~~~: 01155 

~~ ~~~-~ ~~:_; ~~~~=-~ \------------

8 ~.::-:-·.~:-7--7::-:-::-:-::-

2 

\-=-~~-=--

------------

\·.:·..:::.·_--- ----- -----·· ········· 

SEC. 17, 

T. 23 N., 

R. 6 E. 

0 

LITHOLOGIC 

SYMBOLS 

·········· 

t ......... , 

Sandstone 

~-----~ -----

Siltstone 

Shale 

Calcareous 

concretions 

2 MILES 

I 

I 

3 KILOMETRES 

FIGURE 13.-Type section of the Floweree Member of the Marias 

River Shale and map showing line of measured section, sec.17, T. 

23 N., R. 6 E. Numbers on the left side of the column are key beds 

in the measured section; numbers on the right side are USGS 

Mesozoic collections. On the map, the arrow points upward 

stratigraphically; B, base, and T, top of member. 
40 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Type section of the Floweree Member-Continued 

Marias River Shale-Continued 

Floweree Member-Continued 

11. Shale, etc.-Continued 

calcareous; some yellow powder along bedding planes. A few beds of sandy siltstone 

less than 2 em thick. A few lenses consisting 

of chert granules and small pebbles. Poorly 

exposed and forms gentle slope. Inoceramus 

M Ft 

sp -------------------------------------- 5.7 18.8 

10. Siltstone, medium-bluish-gray, shaly to sandy; 

contains scattered light-gray argillaceous 

limestone concretions 15 em thick and 30--60 

em in diameter which are septarian with 

veins of yellow to brown calcite. Near base 

some orange-brown ironstone that weathers 

to orange-brown chips. Rarely a fossiL_____ 1.2 3.8 

USGS Mesozoic loc. D1156: 

Inoceramus sp. 

Anomia? sp. 

Pecten (Syncyclonema) sp. 

Callista? sp. 

9. Sandstone, gray, noncalcareous, thin-bedded, 

resistant, locally platy; weathers to 

yellowish-buff or orange-brown ledge; contains some worm(?) tracks. Top surface ripple 

marked__________________________________ .2 .6 

8. Siltstone.Upper one-third is sandy and weathers light bluish gray with orange-yellow 

films along fracture surfaces. Lower twothirds, which is shalier and less resistant 

than upper part, contains yellow powder 

along bedding planes and is cut by diagonal 

fractures with veinlike fillings. A finegrained sandstone about 6 em thick divides 

the two parts. Near top are a few gray calcareous septarian concretions that yielded a single specimen of Calycoceras? 

canitaurinum (Haas). USGS Mesozoic loc. 

Dl155. ---------------------------------- 4.0 13.0 

7. Sandstone, fine- to medium-grained, somewhat 

platy, lenticular; weathers grayish buff; 

forms small ledge ________________________ .2 .8 

6. Shale, dark-bluish-gray, moderately fissile; 

upper part softer and more papery than lower 

part and interbedded with thin layers of 

siltstone. Unit cut by diagonal fractures that 

have been filled with yellow and brown calcite. Calcareous sandstone concretions as 

much as 1.5 min diameter occur 0.6 m below 

top and weather buff yellow to buff brown__ 2.1 7.0 

5. Siltstone, shaly; cut by diagonal fractures that 

have been filled with brown sandy siltstone 1.5 4.8 

4. Siltstone, somewhat shaly; consists of ovoid 

masses 5---10 em long that are grayish blue 

with rinds of orange yellow. Cut by diagonal 

fractures filled with yellowish-brown sandy 

siltstone. Top 2 em is brown sandstone that 

contains very small black chert pebbles____ 1.4 4.5 

3. Siltstone, light-gray, sandy, thin-bedded; interbedded with dark-gray silty shale ______ 1.5 4.8 

2. Shale, dark-gray, noncalcareous; upper part 

papery, with yellow powder along bedding 

planes and interbedded with shaly siltstone. 

Type section of the Floweree Member-Continued 

Marias River Shale-Continued 

Floweree Member-Continued 

2. Shale, etc.-Continued M Ft 

Grades into overlying unit________________ 1.7 5.4 

Total Floweree Member (rounded) ______ 20.0 64.0 

Blackleaf Formation: 

Bootlegger Member (top): 

1. Bentonite; weathers yellowish orange; impure 

near top 

CONE MEMBER 

NAME AND DEFINITION 

.6 1.9 

The Cone Member is the so-called Greenhorn Limestone of the Sweetgrass arch. It is the calcareous shale 

and chalk marl member of Cobban (1951a, p. 2186), the 

resistant shale member of Dobbin and Erdmann 

(1930), the black lime of drillers (Erdmann and 

Schwabrow, 1941, p. 283), and equivalent probably to 

part of the rocks mapped around the Highwood Mountains east of Great Falls by Reeves (1929, p. 162, pl. 44) 

as the Mosby Sandstone Member of the Colorado 

Shale. The unit was given the formal name Cone Calcareous Member of the Marias River Shale by us (Cobban and others, 1959, p. 2794) for the excellent exposures on the slopes of the conspicuous hill on which the 

Cone triangulation station is situated, 6 km (3.7 mi) 

south of Power near the center of sec. 13, T. 22 N., R. 1 

W., Teton County (Vaughn quadrangle). The term calcareous is dropped from the formal name. The type 

section was measured on the south slope of the hill. 

Very calcareous dark-gray shale largely composes the 

member. 

THICKNESS 

A rather uniform thickness of about 15-18 m (50--60 

ft) is maintained by the Cone Member over much of the 

Sweetgrass arch. The thickness is 16.5 m (54ft) at the 

type section. Stebinger (1918, p. 162) gave 15.2 m (50 

ft) as an average for this member west of the Sweetgrass arch in the Birch Creek-Sun River area. Along 

the southeast boundary of Glacier National Park, the 

thickness appears to be of this order although a complete section has not been measured by us. In the subsurface the member thickens eastward from about 

15-18 m (50--60 ft) on the Kevin-Sunburst dome to as 

much as 24m (80 ft) in wells drilled near the Sweetgrass Hills. Westward from the Sweetgrass arch, the 

Cone thickens to about 30 m in the Sun River Canyon 

area (Mudge, 1972, p. A66). 

OUTCROP DISTRIBUTION 

The Cone Member is well exposed at many localities 

on the South arch. From the type section on Cone hill 

this member is exposed northwest up Muddy Creek 5.6 
CONE MEMBER 41 

km (3.5 mi) nearly to the center of sec. 34, T. 23 N., R. 1 

W. ; there it dips below the stream level. Eastward from 

Muddy Creek the member forms a belt 3-6 km wide 

across the north-central part of the Vaughn and Great 

Falls quadrangles and the northern part of the Portage 

quadrangle to Floweree. From the type section on Cone 

hill the member crops out southward along the west 

side of Muddy Creek valley to the Muddy Creek-Sun 

River divide in sec. 7, T. 21 N., R. 1 E. West from there 

the outcrop follows the Sun River valley about to Fort 

Shaw, where the Cone dips below the stream level. 

South of the Sun River valley the member is poorly 

exposed. The line of outcrop seems to trend south more 

or less along the eastern edge of the west third of the 

Cascade quadrangle. The outcrops continue southward 

from this quadrangle and follow Willow Creek in T. 17 

N., R. 1 E. 

On the Kevin-Sunburst dome the member is poorly 

exposed at several places near the crest of the structure. Some of the best exposures are on Raglan Butte 

in N lfz sec. 13, T. 34 N. , R. 2 W., 6 km (4 mi) southwest 

of Ferdig. Here the beds are much disturbed owing to 

glacial action. Perry (1928, p. 5) considered these rocks 

as an equivalent of the Mowry Shale. The same beds 

crop out just south of the NE. cor. sec. 13, T. 34 N., 

R. 3 W. 

The member crops out at many localities in the Disturbed belt. Powers and Shimer (1914, p. 557) recorded 

it as "dark arenaceous shale" and noted the abundance 

oflnoceramus labiatus [Mytiloides]. Stebinger (1918, p. 

158, 161-164) described the member as "bituminous 

shale and maltha limestone" and observed its occurrence at several places in the Birch Creek- Sun River 

area. Cobban (1956, p. 1002-1004) has drawn attention 

to many outcrops along the southeast boundary of 

Glacier National Park. 

GENERAL DESCRI PT!Ol'\ 

The limy character of the Cone Member readily distinguishes it from the other members of the Marias 

River Shale. Calcareous shale is the chief constituent. 

Other constituents include argillaceous limestone, 

crystalline limestone, limestone concretions, bentonite, and noncalcareous shale. 

The boundaries of the member are extraordinarily 

sharp. Both are marked by an abrupt change from the 

limy shale ofthe Cone to the noncalcareous shale of the 

underlying Floweree Member and of the overlying 

Ferdig Member. 

A thin layer, commonly less than 2 or 3 em, of soft 

rusty limonitic silt or siltstone marks the base in many 

places. It consists of a mixture of silt, limonite, and 

crystals of selenite, with here and there gray shale 

pebbles, fish teeth and bones, and soft white silty 

nodules. This contact layer is well exposed in the vicinity of the hill capped by the Cone triangulation station 

in sec. 13, T. 22 N., R. 1 W., and along the county road 

about 4.3 km (3 mi) south-southwest of Power in the 

NWlf4NW%SW% sec. 11, T. 22 N., R. 1 W. (Vaughn 

quadrangle). 

Overlying the thin limonitic basal layer is a bed of 

limy shale less than 60 em thick that contains closely 

spaced limestone concretions. Most of the concretions 

are 30-40 em thick and a metre or more in diameter. 

They are dark gray on fresh fracture but weather to a 

pale lavender gray. The fresh and weathered surfaces 

show numerous minute white specks. Most concretions 

are septarian, with calcite veins that show as many as 

three orders of crystallization. The outer wall of the 

veins consists of a thin layer of small brown calcite 

crystals. The inner and greater part of most veins consists of larger crystals of translucent white or 

yellowish-white calcite. Some of the thicker veins have 

larger crystals of barite deposited on top of the lightcolored calcite crystals. The barite is mostly translucent and pale bluish gray to pale lavender gray, but a 

few clear colorless crystals are present. These septarian concretions are remarkably widespread and apparently extend all the way from Glacier National 

Park (Cobban, 1956, p. 1003) southeastward at least as 

far as Mosby 270 km (167 mi) southeast of Great Falls 

(Cobban, 1951a, p. 2186). 

The calcareous shale, which composes most of the 

Cone Member, is very dark gray where fresh and contRins abundant minute white specks. Goodman (1951) 

believed these specks were coccoliths and rhabdospheres. 

Crystalline limestone beds are present only in the 

upper part of the member. These limestones are thin, 

shaly, silty, and irregularly bedded. They are medium 

gray where fresh and brownish gray where weathered. 

Part of the limestones is composed of pelagic 

Foraminifera and tiny calcareous prisms from macerated Mytiloides shells. Small oysters and fragments of 

Mytiloides are abundant. The limestones are hard and 

crop out as a series of small ledges in contrast to the 

slope formed by the shales of the lower part of the 

member (fig. 14). These ledge-forming beds around the 

Highwood Mountains were mistaken for the calcareous 

Mosby Sandstone Member by Reeves (1929, p. 162, pl. 

44). 

Limestones in the middle of the member are argillaceous and shaly and softer than those higher in the 

member. They are very dark gray but weather light 

blue and then orange brown. Minute white specks, 

pelagic Foraminifera, and Mytiloides mytiloides (Mantell) are numerous. 

Seven bentonite beds are present in the type section. 
42 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

FIGURE 14.-Upper part of Cone Member of Marias River Shale at its type ,;ection in the SE\4 sec. 13, T. 22 N. , R. 1 W. , Teton County, 

Mont. 

The lowest and thickest is a 0.9-m white low-swelling I Watinoc eras reesidei Warren which is known also from 

bed 4.3 m (14ft) above the base. This bed is very widely the Disturbed belt (Cobban, 1956, p. 1003, 1004). 

distributed and extends from Glacier National Park Bed 14, 7- 11 feet below the top of the member, consoutheastward at least as far as Mosby, a distance of tains Mytiloides labiatus. The overlying 2.1 m of the 

434 km (270 mi) (Cobban, 1956, p. 1002). Over this member contains fragments of Mytiloides which apgreat distance the bed seems to maintain a thickness of pear to be M. labiatus. On the Kevin-Sunburst dome 

between 0.9 and 1.5 m. the ammonite Collignoniceras woollgari (Mantell) has 

The Cone Member is readily determined in well cut- been found 1.8 m below the top of a dark-gray slightly 

tings by the change downward from dark-gray fissile ~alcareous white-speckled shale (Cobban and others, 

noncalcareous shale to massive black-gray or very 1956). This unit does not occur in the type section of 

dark gray calcareous shale containing minute white the Cone Member and may represent strata that 

specks, and the fragments of gray limestone with 'elsewhere are in the hiatus at the top of this member. 

numerous inoceramid prisms. The 0.9-m-thick bed of 'Collignoniceras woollgari is a middle Turonian ambentonite in the lower part of the member is easily monite that is a guide to rocks that are equivalent in 

recognized in cuttings. On electric logs the top of the age to the Fairport Chalky Shale Member of the CarCone Member is represented by the well-known lile Shale of the central Great Plains. 

"Greenhorn kick." 

The bluish-weathering argillaceous limestone and 

shale (unit 5) immediately overlying the 0.9-m-thick 

bed of bentonite contains numerous Mytiloides 

mytiloides (Mantell) of early Turonian age. In the central Great Plains, M. mytiloides is abundant in the 

Bridge Creek Limestone Member of the Greenhorn 

Limestone. 

The soft calcareous shale (unit 11) higher in the type 

section of the Cone Member contains the ammonite 

AGE ASS!GN'viENT AND CORRELATION 

The Cone Member contains numerous marine fossils. At the type section limestone concretions at the 

base of the member (bed 2) and at the top of the shale 

(bed 3) that immediately underlies the 0.9 m bentonite 

bed contain Sciponoceras gracile (Shumard). This 

ammonite is a guide to the latest zone of the Cenomanian Stage of the Late Cretaceous. In the central Great 

Plains, S. gracile is confined to the basal beds of the 
CONE MEMBER 43 

Bridge Creek Limestone Member of the Greenhorn 

Limestone. 

In the bentonite are tuffaceous siltstone concretions, 

not found at the type section, that commonly are fossiliferous. A collection (USGS Mesozoic loc. 25029) 

from these concretions in the NWV!SWV!SWV-t sec. 28, 

T. 21 N., R. 1 W., has been identified by J. B. Reeside, 

Jr., as follows: 

Inoceramus sp. 

Phelopteria gastrodes (Meek) 

N ymphalucina aff. N. subundata (Hall and Meek) 

Drepanochilus ruida (White) 

S ciponoceras gracile (Schumard) 

Watinoceras aff. W. coloradoense (Henderson) 

Type section of the Cone Member 

[Measured by W. A. Cobban on the south side of the hill on which is the Cone triangulation 

station 6.3 km (3.9 mil south of Power in the SEV4 sec. 13, T. 22 N., R. 1 W., Teton County 

(Vaughn quadrangle) (fig. 151] 

M 

Marias River Shale: 

Ferdig Member (lower part): 

23. Bentonite; lower part rusty brown; upper part 

gray; impure, nonswelling ~~~~~~~~~~~~~~~~ 0.5 

22. Shale, black-gray, fissile, noncalcareous; tends 

to resist weathering; contains a few thin layers of yellowish- and bluish-weathering hard 

siltstone or very fine grained sandstone with 

worm(?) tracks on the bedding surfaces. 

Basal 2-3 em limonitic and silty ~~~~~~~~~~ 8.1 

Total part of Ferdig Member measured~~ 8.6 

Disconformity 

Cone Member: 

21. Limestone, dark-gray, shaly; weathers bluish 

gray and tan; contains minute white 

specks ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 0.1 

20. Bentonite, gray, somewhat limonitic, impure .2 

19. Limestone, dark-gray, shaly; weathers bluish 

gray and tan; forms small ledge; contains 

fish bones and scales, some fragments of 

Mytiloides, and abundant pelagic Foraminifera ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .2 

18. Shale, dark-gray, calcareous. Few very thin 

bluish-weathering limestone layers. A 2-

cm-thick layer of bentonite 18 em above 

base ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .4 

17. Bentonite, limonitic, shaly, impure~~~~~~~~~~ .1 

16. Shale, limy, hard; weathers bluish ~~~~~~~~~~ .1 

15. Shale, calcareous; contains many very thin 

limestone layers that weather brownish gray 

and contain pelagic Foraminifera, worm(?) 

burrows or castings, and fragments of Mytiloides and Ostrea sp. ~~~~~~~~~~~~~~~~~~~~~~ 1.1 

14. Limestone; dark medium gray on fresh fracture, buff gray on weathering; in thin layers 

as much as 2 em thick separated by gray limy 

shale; forms ledges. Contains abundant 

pelagic Foraminifera, small oysters, and 

fragments ofMytiloides labiatus (Schlotheim). 

USGS Mesozoic loc. D559 ~~~~~~~~~~~~~~~~~ 1.2 

13. Shale, gray, limy, soft. Some thin layers as 

much as 1 em thick of medium-gray argillaceous limestone that contain fragments of 

Ft 

1.5 

26.6 

28.1 

0.3 

.7 

.5 

1.3 

.3 

.4 

3.7 

4.0 

METRES 0 

LITHOLOGIC SYMBOLS 

~ 

40 

60 

20 

-L.. --L- --'- _._ 

~__._ ......... ~__,_ 0557 

4 II II II II __ 

_,_c::>- ....1.-CP _ 0556 ........~- __ ___.,__ __ _ -~--

__,_- ........ 1--- - - 2 ~ <l) ~ a:. ~ 0555 

SEC. 13, 

T. 22 N., 

I il :~-1 -----,-, -KI_.:o:::RE 2 

~ 

Siltstone 

0 -

- Shale 

~~ Calcareous 

shale 

Limestone 

Calcareous 

~ 

concretions 

m 

Bentonite 

FIGURE 15.-Type section of the Cone Member of the Marias River 

Shale and map showing line of measured section, sec. 13, T. 22 N., 

R. 1 W. Numbers on the left side of the column are key beds in the 

measured section; numbers on the right side are USGS Mesozoic 

collections. On the map, the arrow points upward stratigraphically; B, base, and T, top of member. 

Type section of the Cone Member-Continued 

Marias River Shale-Continued 

Cone Member-Continued 

13. Shale, etc.-Continued 

Mytiloides and numerous pelagic Foraminifera ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

12. Bentonite, dusky-yellow, limonitic, impure ~~ 

11. Shale, dark-gray, very calcareous, soft, papery; 

weathers medium gray. Contains at least 

two beds of very hard and distantly spaced 

limestone concretions that are dark gray on 

fresh fracture but light bluish gray where 

weathered. The concretions range in size 

from 2 em thick and 5 em in diameter to 5 em 

thick and 30 em in diameter. Each is enclosed 

by a softer limonitic rind 1-3 em thick. The 

concretions and the shale of the unit contain 

minute white specks and the following fossils 

USGS Mesozoic loc. D558: 

Mytiloides mytiloides (Mantell) s.l . 

Watinoceras reesidei Warren 

Scaphites delicatulus Warren 

10. Limestone; weathers bluish; shaly; forms 

ledge; contains fish scales, small oysters, and 

fragments of Mytiloides mytiloides (Mantell) 

M 

.6 

.1 

4.8 

Ft 

2.0 

.3 

15.7 

s. 1. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .1 .4 

9. Shale, medium-gray, calcareous, soft ~~~~~~~~ .3 1.0 

8. Limestone, argillaceous, somewhat shaly; 

weathers gray; forms ledge. Abundant fragments of small oysters and Mytiloides mytiloides (Mantell) s. 1. ~~~~~~~~~~~~~~~~~~~~~~ .1 .3 

7. Shale, soft, calcareous; weathers light buff 

gray ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .2 

6. Bentonite, dusky-yellow, nonswelling~~~~~~~~ .2 

.7 

.7 
44 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Type section of the Cone Member-Continued 

Marias River Shale-Continued 

Cone Member-Continued M Ft 

5. Shale, dark-gray; weathers light blue; very 

limy, in part an argillaceous shaly limestone; 

contains abundant minute white specks and 

pelagic Foraminifera. A 2-cm bentonite layer 

43 em from top. Basal46 em and uppermost 

61 em moderately hard and form low bluishgray ledges. Unit crowded with Mytiloides 

mytiloides (Mantell) s. 1. USGS Mesozoic loc. 

D557 ------------------------------------ 1.4 4.5 

4. Bentonite, white, low-swelling ______________ .9 3.0 

3. Shale, dark-gray, soft, papery, slightly calcareous to noncalcareous. At very top are 

small light-gray-weathering limestone concretions 2-5 em thick and 8-10 em in diameter that contain poorly preserved and 

crushed fossils -------------------------- 3.7 12.2 

USGS Mesozoic loc. D556: 

Inoceramus sp. 

N ymphalucina sp. 

Sciponoceras gracile (Shumard) 

2. Shale, medium-brownish-gray, soft, papery, 

calcareous; basal2-3 em is limonitic and contains soft white silty nodules. Contains 

closely spaced lavender-gray-weathering 

septarian limestone concretions commonly 

0.4 m thick and 1 m in diameter and smaller 

and harder light-gray weathering nonseptarian concretions. The veins in the septarian 

concretions are made up of a thin outer wall 

of brown finely crystalline calcite and a thick 

inner part of coarsely crystalline white calcite; a few veins have still more coarsely 

crystalline colorless to pale-bluish-gray barite deposited on the white calcite. Fossils are 

rare and poorly preserved ________________ .5 1.8 

USGS Mesozoic loc. D555: 

Inoceramus sp. 

Sciponoceras gracile (Shumard) 

Ichthyodectes sp. (scales) 

Total Cone Member (rounded) 16.0 54.0 

Floweree Member (uppermost bed): 

1. Shale, dark-bluish-gray, fissile, noncalcareous; 

few thin lenses of gray siltstone and finegrained sandstone ________________________ 1.1 3.5 

FERDIG MEMBER 

NAME AND DEFI~ITION 

The name Ferdig Shale Member of the Marias River 

Shale was given by us (Cobban and others, 1959, p. 

2794) to a unit of gray firm noncalcareous shale that 

contains numerous thin hard sandy partings and concretions of gray- and yellow-weathering limestone and 

red-weathering ferruginous dolostone. The term shale 

is removed from the formal name. This member overlies the limy Cone Member and underlies the softer 

and less sandy Kevin Member. The Ferdig Member 

comprises the beds described by Cobban {1951a, p. 

2191, 2192) as the equivalent of the Carlile Shale of 

the Black Hills. The name is taken from the post office 

of Ferdig in sec. 31, T. 35 N., R. 1 W., Toole County, 

about 9. 7 km (6 mi) north of the summit of the KevinSunburst dome (Sunburst quadrangle). The type section is composite from exposures 5-8 km (3-5 mi) 

northwest of Ferdig as well as from outcrops 13 km (8 

mi) southeast of Ferdig. 

THICKNESS 

The Ferdig Member is 68.6 m (224 ft) thick in its 

type section in T. 35 N., R. 2 W., on the KevinSunburst dome. Wells drilled on both the east and west 

flanks of this dome also show thicknesses of this order. 

The member is about 70 m (230ft) thick as far east as 

the West Utopia oil and gas field 41.8 km (26 mi) east 

of the top of the Kevin-Sunburst dome. On the South 

arch the Ferdig Member thickens northwestward from 

31 m (101 ft) at its outcrop, 4.8 km (3 mi) north of the 

town of Sun River, to about 63 m (205 ft) in wells 

drilled in the Pondera oil and gas field and 67-73 m 

(220-240 ft) in wildcat wells drilled in the PendroyBirch Creek area. 

OUTCROP DISTRIBUTION 

The Ferdig Member makes a wide concentric ''ring" 

around the outcrops of the Bootlegger, Floweree, and 

Cone Members on the Kevin-Sunburst dome and 

greatly strengthens the areal geologic expression of 

the dome. In places this belt has a width of 11 km ( 7 

mi), emphasizing the difficulty of obtaining an accurate section. 

The best exposure, from the top (unit 50 of the type 

section) down to unit 15, is on the west-facing escarpment extending through sees. 1, 12, and 13, T. 35 N., R. 

2 W., where, because of the north dip, approximately 

80 percent of the member can be seen in excellent detail free of glacial cover. Another extensive exposure of 

the middle part, from unit 36 down to unit 15, is in low 

west-facing slopes along the east bank of the north fork 

of Antelope Coulee from near the SW cor. sec. 19, T. 33 

N., R. 1 E., northward through sees. 18, 7, and 6; 

thence westward into sec. 1, T. 33 N., R. 1 W.; and 

north again into sec. 36, T. 34 N., R. 1 W. Units below 

unit 15 are exposed in places as along the south and 

west sides of sec. 36, where the type section was completed. 

The Ferdig Member also is exposed in the Sweetgrass Hills. South of Kevin-Sunburst dome an inlier of 

the upper part of the Ferdig Member occurs where the 

Marias River has trenched into the Marias River saddle, the west end of the exposure being where U.S. 

Highway 91 crosses the river. 

On the South arch the Ferdig Member crops out from 
FERDIG MEMBER 45 

Fort Benton on the Missouri River westward across the 

southern part of the Tunis, Carter, and Dent Bridge 

quadrangles. The outcrop pattern swings south a little 

and crosses the northern part of the Great Falls quadrangle in the high country north of Lake Creek. The 

outcrop continues westward across the northern part of 

the Vaughn quadrangle and into the northeastern 

corner of the Fairfield quadrangle. From there it extends southeast along the west side of Muddy Creek 

valley to the Sun River-Muddy Creek divide in sec. 7, 

T. 21 N., R. 1 E., and west up the Sun River 2--3 km 

(1-2 mi) west of Fort Shaw. From the Sun River valley 

the outcrops extend south through the western part of 

the Cascade quadrangle. 

In the Disturbed belt the Ferdig Member crops out 

3.2 km (2 mi) northeast ofWolfCreek in the NW¥4 sec. 

31, T. 15 N., R. 3 W., and near the Rocky Mountains 

front from T. 20 N., R. 8 W., north through T. 28 N., R. 

8 W., and northwest to the eastern boundary of Glacier 

National Park. 

GENERAL DESCRIPTION 

On the Sweetgrass arch the Ferdig Member consists 

of a lower dark-bluish-gray shale unit that contains 

hard red-weathering concretions of ferruginous dolostone, a medial gray sandy shale that contains grayand yellow-weathering calcareous concretions, and an 

upper dark-bluish-gray shale unit with small gray calcareous concretions. 

The lower unit, about 15m (50ft) thick, beds 1-15 in 

the type section, is a rather firm dark-bluish-gray 

shale that contains a few very thin sandy partings and 

numerous beds of reddish-weathering concretions or 

thin layers of ferruginous dolostone and limestone. 

These concretions or layers are dark gray to dark olive 

gray on fresh fracture and very fine grained and hard. 

They weather from rusty brown to grayish red and 

very dusky red. They are so abundant on the outcrops 

near the crest of the Kevin-Sunburst dome that a reddish color is imparted to the hillsides. The concretions 

may be readily seen on the hill east of U.S. Highway 91 

about 0.6 km (0.4 mi) south of Four Corners in the 

NW¥4 sec. 3, T. 34 N., R. 2 W., and along the KevinOilmont road (Shelby quadrangle). On the South arch 

the concretions can be seen along both sides of the valley followed by U.S. Highway 89 and the Chicago, 

Milwaukee, St. Paul and Pacific Railway northwest of 

the town of Sun River (Vaughn quadrangle). They may 

be seen also near U.S. Highway 2 along the southeast 

boundary of Glacier National Park (Cobban, 1956, p. 

1002, mileages 7.8, 8.4; p. 1004, mileage 10.5). In the 

Glacier National Park area a bed of black, gray, and 

greenish chert pebbles occurs in the ironstone unit 

(Cobban, 1956, p. 1004). 

Bentonite is sparingly present in the lower unit. A 

gray to brownish bed 1&--46 em thick occurs 8.2 m (27 

ft) above the base of the unit on the outcrops in the Sun 

River-Power area on the South arch; this bed is present also on the Kevin-Sunburst dome. 

The medial part, about 46 m (150ft) thick in the type 

section (beds 16--42), is a resistant bluish-grayweathering shale that contains some thin layers of 

sandstone and tends to form smoothly rounded poorly 

vegetated slopes. Much of the surface is flecked by 

small thin flakes of rusty iron-stained shale that appears to result from the oxidation of laminae of pyritic 

shale or bentonite. These chips make about 25 percent 

of the surface litter for a total of about 15 m of the 

medial unit and provide a very useful widespread gross 

characteristic for the recognition of this part of the 

Ferdig. The sandstone, very fine grained and relatively 

resistant, is present chiefly as individual wavy layers 

or lentils an inch or less in thickness, though in places 

in assemblages of a few feet, that are light gray (N6) to 

light olive gray (5¥6/1) on fresh fracture and brownish 

where weathered. Upper bedding surfaces are commonly ripple marked and may show well-preserved 

tracks, trails, impressions, and burrows that appear to 

have been made by small crustaceans, annelids, and 

ammonites. The lowest of these sandstones is the thin 

layer in bed 18, which seems to represent the north 

wedge-edge of the bed capping the hill at triangulation 

station Cone. Other distinctive features of the middle 

part of the Ferdig Member are various concretionary 

lithologies that occur in no other part of the Marias 

River Shale, such as the layered concentric septarian 

limestone masses in bed 20, and also the replacement 

of bentonite by selenite in the upper part of bed 39. 

On the Kevin-Sunburst dome a thin but very persistent layer of conglomeratic sandstone marks the top of 

the medial unit of the Ferdig Member. This layer (unit 

42 of the type section and bed N of Erdmann and 

others, 194 7) consists of light-gray to light-olive-gray 

very fine to fine-grained sandstone that contains 

polished granules and small pebbles of black chert and 

some pebbles of white, gray, brown, and green chert, 

quartz, quartzite, and argillite. This pebbly layer, 

0.3-5 em thick, is irregularly bedded and commonly 

contains trails and burrows of small organisms and 

here and there a coprolite or an impression of a 

scaphite. Locally the bed is represented by light-olivegray-weathering sandy calcareous concretions that 

contain a sprinkling of pebbles on their upper surfaces. 

In the type section this pebbly bed lies 60.7 m (199 ft) 

above the base of the member and 8.2 m (27 ft) below 

the top. This bed readily can be seen west of the 

Kevin-Sunburst road &--6 km (3.2--3.8 mi) northnortheast of Kevin in theW% sec. 13, T. 35 N., R. 3 W. 
46 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

(Sunburst quadrangle). It possibly is equivalent to the 

conglomerate unit in the Bighorn (Cardium) Formation of southwestern Alberta. The Bighorn Formation 

has been recognized in wells in the Fincher Creek gas 

field 26 km (16 mi) north of the international boundary 

in sec. 24, T. 3 N., R. 29 W., 4th Meridian (Douglas, 

1952, cross-section A-C); it has not been identified, 

however, farther east in structurally higher areas on 

the north extension of the Sweetgrass arch (Spratt, 

1931, fig. 3). 

In the Disturbed belt, about 3.2 km (2 mi) eastnortheast of Wolf Creek in the NW% sec. 31, T. 15 N., 

R. 3 W., a 6-m-thick ridge-forming bed of brownishgray sandstone lies at the top of sandy beds that seem 

to be correlative with the medial unit of the Ferdig 

Member of areas farther east. The sandstone is fine 

grained, thin bedded, and ripple marked. It is capped 

by a thin layer of coarser sandstone that contains gray 

and green smooth argillite pebbles derived from the 

Belt Supergroup of Precambrian age, gray and black 

chert, and a few pebbles of light- to medium-gray 

dolomite with crinoid columnals of possible derivation 

from the Madison Limestone of Mississippian age. 

Most pebbles are 1-2 em in diameter, but some are as 

much as 5 em. The small pelecypod Cardium pauperculum Meek is abundant in the uppermost beds. This 

fossil, together with the lithologic character and thickness of the beds, suggests correlation with the Bighorn 

(Cardium) Formation of the Crowsnest River area of 

southwestern Alberta (Webb and Hertlein, 1934, fig. 2, 

p. 1394--1396). The eastward thinning of the Bighorn is 

well recognized in southern Alberta (Scruggs, 1956, p. 

25--29). It is evident that eastward thinning also prevails in Montana in beds at this horizon. The 

mechanism of the thinning is believed to be onlap of 

the strata onto the rising Sweetgrass arch; further reference is made to it in the discussion on the upper unit 

of the member. 

Thin pebbly layers characterize the medial unit at 

several other localities. Wells drilled in an area about 

40 km (25 mi) long, extending from the Brady oil field 

northeast to the Marias River, consistently penetrate a 

pebbly bed 30 m (100 ft) above the base of the Ferdig 

Member. On the north side of Sun River 2.4 km (1.5 

mi) north of Fort Shaw, a 3-5-cm-thick pebbly layer is 

24 m (78 ft) above the base of the member. 

The sandy medial unit of the Ferdig is well exposed 

along U.S. Highway 91 from 1.5 to 3 km (1-2 mi) 

northeast of Power (Vaughn quadrangle) and from the 

same highway 13 km (8 mi) north of Shelby northnorthwestward for 3.2 km in sees. 4 and 9, T. 33 N., R. 

2 W. (Shelby quadrangle). 

The upper unit, 8.2--10.7 m (27-35 ft) thick on the 

north flank of the Kevin-Sunburst dome, is chiefly 

dark-gray shale that contains a few thin beds of gray 

concretionary limestone. The top consists of a persistent layer of concretionary limestone 10-25 em (4--

10 in.) thick that is olive gray where fresh and grayish 

yellow to yellow brown where weathered. It has a conspicuous hackly fracture. This bed was mapped by 

Erdmann, Gist, Nordquist and Beer (194 7) and designated bed M. The most accessible place to see the upper 

unit of the Ferdig is just west of the Kevin-Sunburst 

road in the north center of sec. 13, T. 35 N., R. 3 W. 

Here, about 3 m below the top, fossils of late Carlile 

age Oate Turonian) were collected. Not far away, in the 

south center of sec. 12, fossils of very early Niobrara 

age (early Coniacian) were collected 5.2 m (17ft) above 

the top of the Ferdig. There appears to have been continuous deposition from upper Ferdig into lower Kevin. 

Comparison of the type section of the Ferdig Member 

with a section measured by Lemke and Maughan in 

the center of theN% sec. 15, T. 21 N., R. 1 W., indicates 

that much of the middle unit of the Ferdig is absent at 

this locality on the South arch. 

The contact of the- Ferdig Member with the Cone 

Member is very sharp. The lower boundary of the Ferdig could not be determined in the type section owing 

to poor exposures of the basal beds. On the basis of 

correlation with sections on the South arch, only a few 

feet of beds may be concealed. However, the base of the 

Ferdig on the Kevin-Sunburst dome can be seen on the 

south bank of a coulee in the SW. cor. NE%SW% sec. 5, 

T. 32 N., R. 1 E., where it is overlain by a few feet of 

beds not observed at the Cone type section. This is 

believed to result from the disconformity between the 

Cone and Ferdig Members. 

On the South arch the base of the Ferdig Member is 

commonly marked by a thin layer of soft limonitic 

siltstone 2--3 em thick which may locally contain fish 

teeth, small pebbles of black chert, and larger gray and 

brown phosphatic pebbles. The basal bed with fish 

.teeth and pebbles can be seen along the county road 8.5 

km (5.3 mi) southeast of Cascade in the SE%NE% sec. 

20, T. 17 N., R. 1 E. On the Kevin-Sunburst dome the 

boundary with the Kevin Member is drawn at the top 

of the persistent concretionary bed M of Erdmann, 

Gist, Nordquist, and Beer (1947). On the South arch 

this contact is not so easily determined and is placed at 

the change upward from nonbentonitic dark-gray 

shale to medium-olive-brown shale with numerous 

beds of bentonite. 

In well cuttings the shale forming the upper part of 

the Ferdig Member is slightly harder and darker than 

that of the Kevin Member and may contain pyrite. In 

addition the upper part of the Ferdig lacks bentonite 
FERDIG MEMBER 47 

whereas the lower part of the Kevin contains several 

beds. 

AGE ASSIGNMENT AND CORRELATION 

Three faunal zones and possibly a fourth are known 

from the Ferdig Member. The unquestioned zones are, 

from oldest to youngest, Prionocyclus hyatti, 

Scaphites nigricollensis, and Scaphites corvensis 

(Cobban and Reeside, 1952, correlation chart). The 

questionable zone is that of Collignoniceras woollgari, 

which is based on the impression of a single 

juvenile ammonite that seems assignable to C. 

woollgari (Mantell) var. regulare Haas. This impression is in a piece of ferruginous siltstone from near the 

base (bed 2) of the type section of the Ferdig Member. 

Because of the great variation within ammonite 

species, additional material is needed before the zone 

of C. woollgari can be stated definitely as present in 

the basal Ferdig beds. 

The zone of Prionocyclus hyatti can be demonstrated 

for at least the upper part of the lower or dolostonebearing part of the Ferdig Member. Poorly preserved 

juveniles of this ammonite have been found in dolostone at many localities on the South arch and KevinSunburst dome. Scaphites carlilensis Morrow, a guide 

fossil for this zone, was collected from bed 12 of the 

type section. Scaphites carlilensis and Prionocyclus 

hyatti are restricted to late Turonian rocks equivalent 

to the Blue Hill Shale Member of the Carlile Shale of 

Kansas. 

The late Turonian ammonite Scaphites nigricollensis is the next youngest species known from the Sweetgrass arch. In the type section of the Ferdig Member, S. 

nigricollensis has been found in the middle part (beds 

26-36) of the medial unit associated with Baculites 

yokoyamai Tokunaga and Shimizu. Over much of the 

western interior region between the Black Hills of 

South Dakota and the San Juan Basin of northwestern 

New Mexico, three zones of ammonites (Scaphites warreni, Scaphites ferronensis, and Prionocyclus 

wyomingensis wyomingensis) lie between the zones of 

Prionocyclus hyatti and Scaphites nigricollensis. Fossils indicative of these three zones have not been found 

on the Sweetgrass arch, and the time span of these 

zones is probably represented by a hiatus. The middle 

unit of the Ferdig is correlated with the upper part of 

the Turner Sandy Member of the Carlile Shale of the 

Black Hills. Equivalent rocks seem to be absent in 

Kansas. 

The top bed of the middle unit of the Ferdig Member 

(bed 42 of the type section) and the overlying upper 

unit of the Ferdig lie in the latest Turonian zone of 

Scaphites corvensis. This part of the Ferdig correlates 

with the Sage Breaks Member of the Carlile Shale of 

the Black Hills. 

Type section of the Ferdig Member 

[Measured by C. E. Erdmann beginning 7.4 km (4.6 mil north-northwest of Ferdig in the 

NE'4SW114 sec. 1, T. 35 N., R. 2 W., Toole County (Sunburst quadrangle). The section 

extends south through sec. 12 into the SWY4NW% sec. 13, T. 35 N., R. 2 W., and terminates in sec. 36, T. 34 N., R. 1 W. (fig. 16). The original field measurements were in feet 

and inches] 

M Ft 

Marias River Shale: 

Kevin Member (basal bed): 

51. Shale, olive-gray, flaky, noncalcareous 2.5 8.3 

Ferdig Member: 

50. Limestone, olive-gray, concretionary, hard, 

aphanitic; weathers grayish yellow to moderate yellowish brown; surface rough, 

hackly, with epigenetic prismatic structure; 

a continuous resistant layer making structural benches. Bed M of Erdmann and others 

(1947) ---------------------------------- .2 .5 

49. Shale, gray, fissile ------------------------ 4.6 15.0 

48. Limestone, dark-gray, concretionary; shattered noncontinuous discoid masses 30-40 

em in diameter __________________________ .1 .2 

47. Shale, gray, fissile, as above ________________ 1.2 4.0 

46. Shale, dark-gray, fissile; contains small discoid 

septarian concretions of dark-gray limestone 

weathering light olive gray ______________ .7 2.3 

45. Shale, dark-gray; as above, but without 

concretions ------------------------------ .6 2.0 

44. Limestone, dark-gray, concretionary, septarian, much shattered; weathers light yellowish gray. Makes small mounds with 

diameter of0.8 mat infrequent intervals, but 

horizon is locally persistent. Fossiliferous -- .1 .3 

USGS Mesozoic loc. 20912: 

Inoceramus sp. 

"Martesia'' sp. 

Scaphites sp. 

43. Shale, dark-gray; a few thin concretions of 

gray limestone about 20 em above base of 

bed. Slope heavily littered with small thin 

brown chips------------------------------ .8 2.7 

42. Sandstone, light-gray to gray, very fine- to 

fine-grained; upper surface sprinkled with 

rounded, polished granules of black chert and 

accessory gray chert, quartz, and argillite 

with thin black patina. Bed N of Erdmann, 

Gist, Nordquist, and Beer (1947) __________ .1 .2 

USGS Mesozoic locs. 20911 and D1256 

Inoceramus sp. 

Scaphites coruensis Cobban 

S. corvensis var. bighornensis Cobban 

Prionocyclus sp. 

41. Shale, brownish-olive-gray, noncalcareous, 

soft, flaky; a few thin ochreous yellow and 

brown laminae. A 1-cm-thick bed of rusty 

bentonite at base ------------------------ 1.6 5.2 

40. Shale, dark-gray, soft, fissile; scattered flat, 

oval (20 by 40 by 5 em) septarian concretions 

of gray limestone ------------------------ 2.7 8.8 

39. Bentonite, gray, soft, sandy textured; weathers 

light yellowish gray and moderate yellow to 

dark yellowish orange; in places replaced by 

caramel-brown-weathering prismatic calcite, 
48 

50 

48 

42 

39 

34 

24 

22 

17 

12 

9 

3 

BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

20912 

20911,01256 

23666 

01255 

01254 

01253 

23667 

01252 

LITHOLOGIC 

SYMBOLS 

Chert pebbles 

Ld ~ 

Sandstone 

~ 

~ 

Siltstone 

~ 

~ 

Shale 

~~~~~ Calcareous and 

I ron stone concretions 

m 

Bentonite 

20 

I MILE 

0 1 KILOMETRE 

R. 1 W. 

T 34 N h.·.·.···'3·,·········.s····':·······,t;}·.·,.·. •.. R·, .• , ..I 

>:··:·'22 

... ·:::-:····. 

I. ·:.>22 

R. 2 W. 

T. 35 N. 

Type section of the Ferdig Member-Continued 

Marias River Shale-Continued 

Ferdig Member-Continued 

39. Bentonite, etc.-Continued M Ft 

with development of incipient cone-in-cone 

and gnarly structure, and further local 

epigenetic development of selenite rosettes in 

upper part. Bed is lenticular, breaking up 

into discrete concretionary masses, but is 

persistent-------------------------------- .3 1.0 

38. Shale, gray, flaky; contains at base gray limestone concretions 1 by 2.5 by 0.3 m at intervals of 15-30 m -------------------------- .6 2.0 

37. Shale, dark-gray; contains thin stringers and 

laminae of fine-grained gray sandstone and 

ferruginous siltstone ______________________ 10.5 34.3 

36. Shale, dark-gray, fissile; 0.3-0.6-cm-thick 

layers of yellowish-gray siltstone and gray 

fine-grained sandstone at intervals of 15 em 

or less. Scattered discoid septarian concretions of gray fossiliferous concretionary limestone at base ---------------------------- 1.1 3.6 

USGS Mesozoic loc. 23666: 

Baculites sp. 

Scaphites nigricollensis Cobban 

S. nigricollensis var. meeki Cobban 

35. Shale, dark-gray, fissile; in beds 20--25 em 

thick parted by thin 0.5-cm-thick laminae of 

fine-grained gray sandstone ______________ 1.5 5.0 

34. Shale, dark-gray; contains yellowish-grayweathering septarian limestone concretions 

10--50 em in diameter and 5-20 em in thickness ------------------------------------ .9 2.9 

33. Shale, dark-gray -------------------------- .6 2.0 

32. Shale, brownish-olive-gray; contains thin 

layers of ferruginous siltstone and a 2-

cm-thick layer of bentonite near top ______ 1. 7 5. 7 

31. Shale, brownish-olive-gray; middle part contains small concretions of fossiliferous gray 

limestone _ _ __ _ _ _ _ __ __ ____ _ _ _____ _ _ _ ____ _ _ .2 .8 

USGS Mesozoic loc. D1255: 

Inoceramus sp. 

Scaphites nigricollensis Cobban 

S. pisinnus Cobban 

30. Shale, brownish-olive-gray, silty, soft, noncalcareous; contains a few very thin layers of 

light-gray fine-grained sandstone and thin 

layers of rusty-brown ferruginous siltstone. 

A few concretions of fossiliferous limestone 

at base ---------------------------------- .6 

USGS Mesozoic loc. D1254: 

Inoceramus cf. I. fragilis Hall and 

Meek 

FIGURE 16.-Type section of the Ferdig Member of the 

Marias River Shale and map showing line of measured 

section, sec. 36, T. 34 N., R. 1 W., and sees. 1, 12, and 13, T. 

35 N., R. 2 W. Numbers on the left side of the column are 

key beds in the measured section; numbers on the right 

side are USGS Mesozoic collections. On the map, the 

arrow points upward stratigraphically; B, base, and T, top 

of member. 

2.1 
FERDIG MEMBER 49 

Type section of the Ferdig Member-Continued 

Marias River Shale-Continued 

Ferdig Member-Continued 

30. Shale, etc.-Continued M 

Baculites cf. B. besairiei Collignon 

Scaphites nigricollensis Cobban 

29. Shale, brownish-olive-gray; contains scattered 

small oval limestone concretions at top and a 

thin rusty-brown siltstone layer in middle__ .5 

28. Limestone, dark-gray; weathers light gray; 

Concretionary, discrete oval septarian forms 

15 by 20 to 20 by 25 em with partitions of 

light-brown calcite. In places rests on a very 

thin sandstone. Fossiliferous ______________ .1 

USGS Mesozoic loc. D1253: 

Inoceramus sp. 

Scaphites nigricollensis Cobban 

27. Shale, brownish-olive-gray; contains rustybrown-weathering very thin resistant layers 

Ft 

1.6 

.2 

of ferruginous siltstone __________________ 2.7 8.7 

26. Limestone, dark-gray, concretionary; masses 

of shattered angular fragments which weather orange, brown, and dusky red and whose 

centers commonly remain gray. Locally 

developed-------------------------------- .2 .5 

USGS Mesozoic loc. 23667: 

Baculi tes cf. B. besairiei Collignon 

Scaphites nigricollensis Cobban 

S. nigricollensis var. meeki Cobban 

25. Shale, dark-gray; weathers bluish; contains a 

few small (diameter 15-20 em) oval septarian 

concretions of gray limestone______________ 1.6 5.1 

24. Limestone, dark-gray, concretionary, ferruginous; weathers predominantly maroon (dark 

reddish brown, lOR 3/4, to very dusky red, 

lOR 2/2) with minor crusts of moderate yellowish brown (lOYR 5/4) to dark yellowish 

orange (lOYR 6/6) concentric about the maroon. Centers commonly light-yellowish-gray 

septarian limestone. Discrete masses as 

much as 1 m in diameter making low mounds 

on 3-4m centers ________________________ .1 .4 

23. Shale, medium-dark-gray (N5), moderately fissile, noncalcareous; poorly exposed in slope__ 4.9 16.0 

22. Limestone, dark-gray, concretionary, clayey; 

weathers tan to dark yellowish orange (lOYR 

6/6). Continuous resistant layer with prismatic structure at top, locally slabby, making ledges, benches, or dip slopes. Small flat 

concretions of dark-gray ferruginous aphanitic limestone weathering orange brown rest 

on top with irregular spacing and make low 

chippy mounds __________________________ .2 .5 

21. Siltstone, dark-olive-gray, poorly fissile, noncalcareous, finely micaceous. Surface littered 

with small rusty fragments ______________ 3.4 11.0 

20. Siltstone, dark-olive-gray; as unit 21. Scattered throughout at irregular vertical and 

horizontal intervals are very finely crystalline light-olive-gray septarian limestone 

concretions weathering yellowish gray and 

having a thickness of 0.3 m and a diameter of 

1m ------------------------------------ 1.8 6.0 

Type section of the Ferdig Member-Continued 

Marias River Shale-Continued 

Ferdig Member-Continued M 

19. Limestone, olive-gray (5Y 4/1 ), concretionary, 

aphanitic; weathers grayish yellow, with 

minor mottling by moderate to dark reddish 

brown. Locally persistent ________________ .2 

18. Shale, dark-gray, noncalcareous, poorly exposed. Contains a 3-cm-thick fine-grained 

platy sandstone layer 0.5 m above base ____ 3.1 

17. Limestone, dark-gray, ferruginous, concretionary; weathers rusty brown with minor 

dusky red (maroon); flat, panlike masses 

1-1.5 min diameter. Lower 10 em stratified; 

rest concretionary with prismatic structure; 

discrete forms on about 15-m centers ______ .2 

16. Shale, dark-gray, moderately fissile, noncalcareous; weathers into thin flakes; contains a 

few thin (0.5 em) layers of gray soft fineFt 

.8 

10.2 

.5 

grained sandstone________________________ 3.7 12.0 

15. Dolostone, dark-gray, concretionary, ankeritic(?), very hard, aphanitic; weathers brown to 

dark brown; small noncontinuous muchshattered oval masses. Cores of massive 

maroon-weathering dolostone are enclosed in 

thin chippy dolostone weathering moderate 

brown ---------------------------------- .1 .2 

14. Shale, dark-gray, moderately fissile, noncalcereous ---------------------------------- 1.2 3.8 

13. Shale, dark-gray, noncalcareous, poorly fissile. 

Contains dusky-red-weathering concretions 

of hard dolostone at top and at 0.5, 1.3, and 

2.3 m above base ________________________ 3.3 10.7 

12. Dolostone, olive-gray (5Y 4/1); weathers moderate brown to very dusky red (maroon); concretionary, syngenetic, in discrete but nearly 

coalescent oval forms as much as 40 by 55 em 

in diameter supporting a small bench______ .1 .3 

USGS Mesozoic loc. D1252: 

Scaphites carlilensis Morrow 

11. Shale, medium-dark-gray, flaky, noncalcareous -------------------------------------- 1.5 5.0 

10. Shale, medium-dark-gray, noncalcareous; a 

thin bentonite in middle and at top________ .6 2.1 

9. Bentonite, yellowish-gray (5Y 8/1); weathers 

dark yellowish orange ____________________ .1 .3 

8. Shale, medium-dark-gray, noncalcareous ____ 1.8 6.0 

7. Limestone, concretionary, dark-olive-gray; ferruginous; weathers moderate brown; breaks 

down into chippy fragments in inconspicuous 

mounds 0.3 by 1 m; fossiliferous __________ .1 .3 

USGS Mesozoic loc. D1251: 

Inoceramus cf. I. fragilis Hall and 

Meek 

S caphites sp. 

6. Shale, medium-dark-gray, noncalcareous ____ 1.1 3.5 

5. Limestone, brownish-black; weathers dark yellowish brown and dusky red; concretionary; 

sandy texture. Inoceramus sp. ____________ .1 .2 

4. Shale, medium-dark-gray, flaky, noncalcareous -------------------------------------- 3.6 11.7 

3. Limestone, olive-gray; weathers yellowish 

gray; weathers rusty where locally ferrugi-
50 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Type section of the Ferdig Member-Continued 

Marias River Shale-Continued 

Ferdig Member-Continued 

3. Limestone, etc.-Continued M Ft 

nous; concretionary; fiat, discrete oval forms 

0.5--0.6 m in diameter, with radial and concentric structure developed by septae of 

white and light-brown calcite. Sparingly fossiliferous in softer silty brecciated outer 

crust. Widely spaced; usually much shattered, making inconspicuous chunky mounds .1 .3 

USGS Mesozoic loc. D1250: 

Inoceramus fragilis Hall and Meek 

Collignoniceras sp. 

I chthyodectes sp. (scales) 

2. Shale, medium-brownish-olive-gray, fissile, 

noncalcareous. A thin yellowish-orangeweathering ferruginous fossiliferous 

siltstone layer at base ____________________ .6 2.1 

USGS Mesozoic loc. D1249: 

Collignoniceras cf. C. woollgari 

(Mantell) var. regulare Haas 

1. Shale, medium-brownish-olive-gray, noncalcareous, papery __________________________ .9 3.0 

Total measured Ferdig Member 

(rounded) __________________________ 69.0 224.0 

Base concealed by alluvium 

KEVIN MEMBER 

NAME AND DEFINITION 

The Kevin (pronounced Kee-vin) Member of the 

Marias River Shale was named by us (Cobban and 

others, 1959, p. 2797) for the very good exposures a few 

kilometres north and northwest of the town of Kevin 

on the northwest side of the Kevin-Sunburst dome. The 

type section was measured in the N% T. 35 N., R. 3 W. 

(fig. 20), the same township in which Kevin is located. 

The member consists of dark-gray marine shale that 

contains some thin sandy partings, numerous thin 

layers of bentonite, and many beds of calcareous concretions weathering gray, yellow, or red; the term 

shale is removed from the formal name. This is the 

member of the Colorado Shale described by Cobban 

(1951a, p. 2193-2195) as equivalent to the Niobrara 

Formation of the Black Hills. 

THICKNESS 

In its type section on the northwest side of the 

Kevin-Sunburst dome the Kevin Member is about 

189m (620 ft) thick. It thickens westward to 213 m 

(700 ft) in the Cut Bank oil and gas field. Eastward 

from the type section, the thickness changes very little. 

It is 177m (580ft) thick in the West Utopia oil and gas 

field in T. 33 N., R. 4 E., and 171 m (560ft) thick in the 

Bears Den oil field in T. 36 N., R. 6 E. On the South 

arch the Kevin thickens westward from 203m (665ft) 

a few kilometres north of the Pondera oil field to 232 m 

(760ft) in wells drilled at the eastern edge of the Disturbed belt. 

OUTCROP DISTRIBUTION 

The Kevin Member has the greatest outcrop area of 

any of the members of the Marias River Shale. It forms 

the bedrock on the crest of the Sweetgrass arch beginning at a point 24 km (15 mi) north of Great Falls and 

extending northwest and north beyond Shelby. On the 

flanks of the Sweetgrass arch the Kevin crops out from 

Cascade northward to about one kilometre (within a 

mile) of the international boundary and southeast from 

there to Fort Benton. The member is exposed also 

around the flanks of the Sweetgrass Hills. In the Disturbed belt the Kevin crops out in a narrow 

northwest-trending belt about 16 km (10 mi) long 

north and east of Wolf Creek and a much larger area 

bordering the Rocky Mountain front from Sun River to 

the international line. 

GENERAL DESCRIPTION 

The Kevin Member can be subdivided into three 

units on the basis of the abundance of bentonite and 

the type of concretions. The lowest unit is correlated 

with the Fort Hays Limestone Member of the Niobrara 

Formation of the central Great Plains, and the middle 

and upper units are correlated with the lower and middle parts of the Smoky Hill Chalk Member of the Niobrara Formation of the central Great Plains. 

The lowest unit, 53-55 m (17~180 ft) thick, is 

characterized by many beds of bentonite, calcareous 

concretions, and concretionary limestone. It includes 

beds between I and M of Erdmann, Gist, Nordquist, 

and Beer (1947). Most of the concretions in the unit are 

composed of oval masses of limestone that are dark 

gray on fresh fracture and medium light gray to yellowish gray where weathered. They range in size from 

~30 em (2-12 in.) in thickness and 10-50 em (4-20 in.) 

in diameter. Many are highly fossiliferous. Along the 

Marias River 8 km (5 mi) south of Shelby, brownweathering ferruginous concretions, not present in the 

type section, occur in the lower part of the unit. In the 

upper part of the unit beds of hard gray aphanitic limestone consist of concretionary lenses 1~75 em (~30 

in.) thick and 1-3 m long. Some of the limestone is 

septarian with veins of brown or white calcite. Gnarly 

cone-in-cone structure characterizes a few of the beds. 

Two conspicuous beds of limestone mapped by 

Erdmann, Gist, Nordquist, and Beer (1947) were designated beds J and K. Sandstone beds, a very minor 

constituent in the unit, are thin, very fine grained, and 

shaly, and they weather brown. Some have borings and 

trails made by small marine organisms. Bentonite is 

abundant; 20 layers 2-35 em (1-14 in.) thick are pres-
KEVIN MEMBER 51 

ent in this unit in the type section. Yellowish-brownweathering prismatic calcite associated with two of the 

beds of bentonite were mapped by Erdmann, Gist, 

Nordquist, and Beer (1947) and designated beds I and 

L. The many layers of light-gray bentonite impart a 

banded appearance to the outcrops; this feature can be 

seen in figure 17. 

This lowest unit of the Kevin Member can be seen 

readily along the county road 3.2 km (2 mi) north of 

Fort Shaw (Fairfield quadrangle), along U.S. Highway 

91 about 6.4 km north-northeast of Power in the SW. 

cor. sec. 6 and NW. cor. sec. 7, T. 23 N., R. 1 E. (Dutton 

quadrangle), along the north bank of the Marias River 

where it is crossed by U.S. Highway 91 about 8 km (5 

mi) south of Shelby, just out of Shelby on Highway 91 

in SW%SW1,4 sec. 21, T. 32 N., R. 2 W., and at the type 

section of the Kevin Member 4.8 km (3 mi) northnorthwest of Kevin. Cobban (1956, p. 1004) has drawn 

attention to outcrops along the southeast side of 

Glacier National Park. 

Above the bentonitic unit with its gray limestone 

concretions and concretionary limestone is a unit of 

shale about 61 m (200 ft) thick that is characterized by 

numerous beds of reddish-weathering ferruginous concretions and concretionary limestone and dolostone. It 

includes the rocks from bed I to about midway between 

beds D and E of Erdmann, Gist, Nordquist, and Beer 

(1947; beds 58-143 of type section). The beds and concretions of limestone, commonly with gnarly cone-incone structure at the top, are dark gray to olive gray 

where fresh and yellowish gray to yellowish brown 

where weathered. The dolostone is ferruginous and occurs as concretions or concretionary layers which 

weather reddish brown to very dusky red. The beds of 

limestone commonly are 30 em thick whereas the beds 

of dolostone rarely exceed 10 em in thickness. Very 

thin layers of gray to olive-gray sandstone occur here 

and there. The sandstone is very fine grained, weathers brownish, and in places contains borings and trails 

of small marine organisms. A few thin layers of bentonite are present. 

An important marker bed, bed F of Erdmann, Gist, 

Nordquist, and Beer (1947), lies near the middle of the 

red concretion unit. This bed was first noted in geologic 

literature by Clark (1923, p. 267) as "the yellow lime 

chert conglomerate." It is a conglomeratic bed of dolostone and limestone that weathers light brown, orange 

brown, and very dusky red. It contains polished 

rounded granules and small pebbles of gray and black 

chert as much as 2 em in diameter and larger pebbles 

of light-olive-gray, medium-olive-gray, and mediumgray phosphatic siltstone as much as 5 em in diameter. 

The phosphatic pebbles are much more numerous than 

the chert pebbles. A few dark phosphatic pebbles were 

FIGURE 17.-Numerous thin beds of white bentonite in t he lower pa rt of the Kevin Member of the Marias River Shale in the SW';!, 

SW';!, sec. 18, T. 3 1 N. , R. 2 W., Toole County, Mont. 
52 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

originally small pelecypods or parts of ammonites. 

Koskinen (1951) has described this bed in further detail. The structure of many hundreds of square miles of 

the Kevin-Sunburst dome was mapped by means of 

this thin but distinctive pebbly bed. Reconnaissance 

shows that the bed is present from the Rocky Mountains front eastward across the Sweetgrass arch to the 

Bearpaw Mountains and southward from there at least 

as far as Shawmut in south-central Montana. Because 

of its widespread extent and ease of recognition, the 

bed was named by us (Cobban and others, 1959, p. 

2795) the MacGowan Concretionary Bed for exposures 

on the MacGowan lease in the SE%SW%NE'l4 sec. 4, T. 

35 N., R. 3 W., about 8.5 km (5.3 mi) north-northwest 

of Kevin in the Kevin-Sunburst oil and gas field (Sunburst quadrangle). Bed 100 (p. 58) which was measured at this locality, is the type section. Here the bed 

is 0.5 m thick and lies in the middle of the Kevin 

Member (96 m or 314 ft below the top). The interval 

from the MacGowan Concretionary Bed to the top of 

the Kevin Member may show much variation, as in the 

SE'l4NE'l4 sec. 31, T. 30 N., R. 3 E., where it is only 

about 67 m (220 ft) thick, the decrease probably being 

due to a hiatus and absence of up to 24 m (80 ft) of 

strata in the upper part of the Clioscaphites 

choteauensis zone, and perhaps another small hiatus 

in the Scaphites depressus zone. One or two inconspicuous layers of phosphatic pebbles in places lie a 

short distance above the MacGowan Concretionary 

Bed. Three metres above the MacGowan Bed at its type 

locality is a 2-cm-thick bed of rounded to subrounded 

phosphatic siltstone and sandstone pebbles. These 

pebbles are greenish gray (5GY6/1) to olive gray (5Y 4/ 

1) with darker crusts, and some have a close irregular 

light-gray reticulation. They range in size from 0.6-4 

em. On the south bank of the Marias River near the 

center of the Slfz sec. 17, T. 31 N., R. 3 W. (Valier 

quadrangle) two layers of phosphatic pebbles are present above the MacGowan Concretionary Bed. The 

lower pebbles, 5.8 m (19 ft) above the MacGowan Bed, 

consist of light-olive-gray glauconitic siltstone with an 

outer dark-greenish-gray surface (5GY 4/1) crossed by 

irregular lighter greenish-gray (5G6/1 to 5GY6/1) 

lines. These pebbles attain diameters up to 2.5 em. The 

upper pebbles, 10.4 m (34ft) above the MacGowan Bed, 

are smaller, darker, and lack the greenish cast of the 

lower bed. They are dark gray (N3) with irregular 

light-olive-gray (5Y6/1) lines on the outer surface. 

Glauconitic, phosphatic pebbles similar to those of the 

lower bed occur on the southeast flank of the South 

arch 10.8 km (6.7 mi) northeast of Carter in the 

SE14 SE% sec. 16, T. 24 N., R. 7 E. (Tunis quadrangle). 

The medial unit of the Kevin Member can be seen 

readily a few kilometres northwest of Kevin at many 

places in the NW'l4 T. 35 N., R. 3 W., and T. 36 N., R. 3 

W. (Erdmann and others, 1947). On the South arch the 

unit is well exposed 0.8-3.2 km (0.5-2.0 mi) north of 

U.S. Highway 87 from 2.4-11.3 km (1.5-7.0 mi) northeast of Carter (Carter and Tunis quadrangles). In the 

Disturbed belt easily accessible outcrops of these beds 

are along the Burlington Northern Railway tracks 

from the bridge over Two Medicine Creek (1.6 km or 1 

mi northeast of East Glacier) northeastward for 6.4 km 

(4 mi) (Marias Pass quadrangle). 

Overlying the medial red concretion unit of the 

Kevin Member is about 61 m (200 ft) of dark-gray 

shale that contains many beds of yellowish-grayweathering concretionary limestone and a few thin 

layers of shaly sandstone and bentonite. The upper 

half or more of the shale is calcareous and, in places, 

sandy. Limestone occurs as concretionary lenses 2-60 

em (1-24 in.) thick and as much as 2.7 m (9 ft) in 

diameter. The limestone is dark gray to olive gray 

where fresh and yellowish gray to yellowish orange 

where weathered. Some of the beds of limestone are 

sandy and finely laminated. A few have cone-in-cone 

structure. Sandstone is represented by thin more or 

less shaly very fine-grained beds that weather yellowish gray. A few thin beds of bentonite are present. 

Thin layers of light-gray calcite occur in abundance 

associated with one of the beds of bentonite (bed A of 

Erdmann and others, 1947; bed 178 of type section) 

about 9 m (30 ft) below the top of the Kevin Member. 

The boundary between the Kevin Member of the 

Marias River Shale and the overlying Telegraph Creek 

Formation (basal unit of the Montana Group) is very 

sharp on the west flank of the Kevin-Sunburst dome 

and can be recognized on the basis of both color and 

lithology (fig. 18). The highest bed (top of unit 187) is a 

thin persistent bentonite that commonly is obscured by 

colluvium, at which places it may be traced by reference to the resistant chips of light-gray translucent 

calcite from bed A (unit 178). The upper bentonite is 

underlain by about 1.2 m of dark-olive-gray shale that 

contains the "first white specks," which are a widely 

recognized subsurface marker for this horizon. Southward in Marias River valley, the color contrast in the 

basal part of the Telegraph Creek Formation disappears together with the shaly calcareous sandstone 

(units 180, 183, and 184), but the upper bentonite and 

bed A persist. 

The best outcrops of the upper unit of the Kevin 

Member are north and northwest of Kevin, in Tps. 35 

and 36 N., R. 3 W., and along the Marias River southwest of Shelby, in the northern part of the Valier 

quadrangle. 
KEVIN MEMBER 53 

FIGURE 18.-Upper pa rt of Kevin Member of Marias Ri ver Shale (dark) overlain by lighter gray sandy Telegraph Creek Formation 

and that, in turn, by t he cliff-forming Virgelle Sandstone in the NV2NEV.NW14 sec. 24, T. 35 N. , R. 4 W. , Toole County, Mont. 

AG E ASSIGN :vtE\:T AN D CORRELATION 

Most of the guide fossils from the Kevin Member 

have been listed by Cobban (1951a, p. 2194, 2195). 

They show that the member is of Coniacian and Santonian age and correlated with most of the Niobrara 

Formation of the central Great Plains. 

The lower unit- characterized by many layers of 

bentonite, numerous beds of gray concretionary limestone, and the brown-weathering ferruginous concretions in the Marias River valley- contains two faunas. 

The lower part of the unit with the ferruginous concretions contains a pelecypod close to if not identical to 

Inoceramus erectus Meek and the ammonites 

Scaphites preventricosus Cobban, S. preventricosus 

var. artilobus Cobban, S. mariasensis Cobban, and S . 

mariasensis var. gracillistriatus Cobban. The rest of 

the unit contains Inoceramus deform is Meek, V eniella 

goniophora Meek, Baculites mariasensis Cobban, B. 

sweetgrassensis Cobban, Scaphites preventricosus, S. 

pre ventricosus var. sweetgrassensis Cobban, S. impendicostatus Cobban, Pteroscaphites auriculatus 

(Cobban), and Actinocamax n. sp. The entire unit is 

correlated with the Fort Hays Limestone Member of 

the Niobrara Formation on the basis of the presence of 

Inoceramus deformis in this limestone. 

The middle unit, marked by the red ferruginous concretionary limestone and dolostone and by the phosphatic pebble beds, contains three faunal zones. The 

lowest embraces the rocks below the MacGowan Concretionary Bed. Guide fossils to this zone include the 

pelecypod Inoceramus (JT olviceramus) in volutus Sowerby (=Inoceramus umbonatus Meek and Hayden) and 

the ammonites Scaphites ventricosus Meek and 

Hayden and Scaphites tetonensis Cobban. The middle 

zone, which includes the MacGowan Bed and the overlying shales with the phosphatic pebble layers, contains Inoceramus stan toni Sokolow, Scaphites depressus Reeside, andScaphites binneyi Reeside. The upper 

zone contains Inoceramus (Cordiceramus) cordiformis 

Sowerby, Clioscaphites vermiformis (Meek and 

Hayden), and C. montanensis Cobban. All three zones 

indicate correlation with the lower part of the Smoky 

Hill Chalk Member of the Niobrara Formation. Cobban, Erdmann, Alto, and Clark (1958, p. 658) have 

pointed out that rocks containing the Scaphites 

depressus fauna on the Kevin-Sunburst dome are not 

kriown to exceed a thickness of 16.8 m (55ft), whereas 

in the Bighorn Basin of Wyoming the Scaphites depres-
54 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Top 

135 

133 

21664 

01242 

121 

115 

108 

100 w:;;o.~w.Eii::;~o 238 

23753 

90 0133 1 

85 20698 

81 20295 

20291 

20292 

69 

57 

55 

f-----

f-----

I- ----

- ----

----

~~:=:~:=:: 20696 

- - - -- 1-- ----

1-- _ - _ - _ -

/--- --_-

;;;-- <D-=- a;- 1---------

~ - :______--:-: _:__---:- ___:___ 

51~~=~ 

49 

44 

36 

27 

24 

18 

16 

14 

12 

2 

\..9"-- ~- I-"'---=-_-_----;;, 

1---- ---- -ovI-----<P 

= CP 

r-~ ---

-----

1----- --- f- ----

- --_ 

e=-:=- a; ==-a)- 20300 

<P 20296 

----- I='= = - = 20298 

~- ---

'-·- ---

r--- ---

f--------

1-- - -----

1--- - ---

1- ----~-

~---- --

-- ---

=-= - = - 01247 

a::>-·--· 

-- a:> - ·-

Base 

LITHOLOGIC 

SYMBOLS 

~:·:·~~·: :. ·:·J 

Chert pebbles 

F.3 Ld 

Sandstone 

Siltstone 

Shale 

IJ~~I Limestone 

Calcareous concretions 

m 

Bentonite 

~ 

~ 

I ron stone concretions 

Calcareous 

shale 

METRES 

0 

20 

20 

40 

60 

FIGURE 19.- Type section of the Kevin Member of the Marias River Shale and map showing line of measured 

section, sees. 3, 4, 12, 15, and 17, T. 35 N., R. 3 W. Numbers on the left side of the column are key beds in the 

measured section; numbers on the right side are USGS Mesozoic collections. On the map, the arrow points 

upward stratigraphically; B, base, and T, top of member. 
KEVIN MEMBER 55 

sus fauna ranges through as much as 183m (600ft) of 

the Cody Shale. The thinness of the rocks representing 

this fa una on the Kevin -Sunburst dome together with 

the presence of as many as three beds of phosphatic 

pebbles suggest breaks in deposition or at least very 

slow deposition. 

The upper unit of the Kevin Member contains the 

rest of the Clioscaphites vermiformis zone and two 

younger zones, a lower marked by Clioscaphites 

choteauensis Cobban and an upper characterized by 

the ammonites Baculites thomi Reeside, Scaphites leei 

Reeside, Clioscaphites novimexicanus (Reeside), and 

Desmoscaphites erdmanni Cobban. These zones, formerly believed to represent the youngest part of Niobrara time, are probably of middle Smoky Hill age, 

inasmuch as the upper part of the Smoky Hill Chalk 

Member is now believed to be of early Campanian age 

and to include rocks equivalent in age to the Telegraph 

Creek and Eagle Formations (Jeletzky, 1955). The 

upper zone also contains Inoceramus (Sphenoceramus) 

lundbreckensis McLearn (1929, p. 77, pl. 15, fig. 4; pl. 

16, fig. 2). This species may be the same as I. patootensiformis Seitz (1965, p. 107, pls. 20-25). 

Type section of the Kevin Member 

[Measured by C. E. Erdmann at the following localities in T. 35 N., R. 3 W., Toole County, 

Sunburst quadrangle. Many other measurements also have been made but the strata are 

best exposed at these localities. The original field measurements were in feet and inches 

(fig. 191] 

Units (beds of Erdmann 

and others, 1947) Locality 

1 (base)--12 (L) ________________________ NE%SW% sec. 12. 

12 (L)-40 (K) -------------------------- North-center sec. 15. 

40 (K)--51 (J) -------------------------- SE%SW% sec. 3. 

51 (J)-57 (!) ____________________________ NE%SW%sec. 3. 

57 (l)-81 (G) -------------------------- NW%SW%sec. 3. 

81 CG)--100 (F) _________________ _ ____ NW14SW% sec. 3 to 

R. 3 W. 

16 

I 

SEY4SEY4NE% sec. 4 

(type section 

MacGowan 

Concretionary Bed). 

10 11 

15 14 13 

2 3MILES 

I 

2 4 KILOMETRES 

FIGURE 19.-Continued. 

Type section of the Kevin Member-Continued 

Units (beds of Erdmann 

and others, 1947) Locality 

100 (F)--124 ____________________________ NE%SE%sec. 4 

(locality Scaphites 

depressus zone). 

124-156 (D)---------------------------- NEY4NE%sec. 17. 

156 (D)--187 (top) ______________________ NW%NE%sec. 17. 

M 

Telegraph Creek Formation (basal part): 

189. Sandstone, buff (lOYR 6/4), fine-grained ____ 0.3+ 

188. Shale, pale-yellowish-brown (10YR 6/2). 

Thinly laminated with fine sand__________ .4 

Marias River Shale: 

Kevin Member: 

187. Shale, dark-olive-gray, fissile; some silt or 

very fine sand. A 3-cm-thick ferruginous 

bentonite bed at top. Horizon of "first white 

specks" ----------------------~--------- 1.2 

186. Limestone, concretionary, olive-gray; weathers yellowish gray; discrete oval masses as 

much as 1 m in diameter in a nearly continuous bed. Contains Baculites sp., Ostrea 

sp., and fragments of fossil bones ________ .3 

185. Shale, dark-gray, sandy, fissile ____________ 1.8 

184. Sandstone, gray, fine-grained; in thinly laminated crossbedded layers with thin partings 

of soft sandy shale. Makes a massive comparatively resistant ledge________________ .3 

183. Shale, dark-gray, sandy, thinly laminated; 

contains a few thin layers of dark-gray 

fine-grained sandstone. Transitional into 

overlying unit -------------------------- .6 

182. Shale, dark-gray, sandy, thinly laminated; 

contains scattered limestone concretions __ 1.1 

181. Shale, dark-gray, papery; more sandy toward 

top ------------------------------------ .9 

180. Sandstone, gray, soft, fine-grained, thinly 

laminated, crossbedded; in thin 1--

5-cm-thick layers interlayed with minor 

amounts of gray sandy shale ____________ 1.6 

USGS Mesozoic loc. 20695, 60 em below top 

of bed: 

Ostrea sp. 

Baculites n. sp. aff. B. haresi Reeside 

Baculites thomi Reeside 

179. Shale, dark-gray, fissile _________________ _ 

178. Bentonite and calcite, light-gray; weathers 

yellowish gray. Bed A of Erdmann, Gist, 

Nordquist, and Beer (194 7) --------------

177. Shale, gray, flaky; thin sandy layers and 

laminae. A layer of thin discrete limestone 

concretions occurs 1.5 m above base _____ _ 

176. Shale, dark-gray, fissile. A 6-cm-thick bentonite bed at base and a very thin bed at top 

175. Shale, gray, soft, flaky. A 3-cm-thick gray 

sandy thinly laminated limestone bed at 

base ___________________________________ _ 

174. Shale, gray, firm, calcareous, fissile _______ _ 

173. Limestone, gray, sandy. Makes a thin resistant ledge ------------------------------

172. Shale, gray, firm, calcareous, flaky _______ _ 

.5 

.1 

5.3 

1.2 

5.2 

.5 

.1 

9.6 

Ft 

1+ 

1.2 

4.1 

1.0 

6.0 

1.0 

2.0 

3.8 

3.0 

5.2 

1.8 

.2 

17.5 

4.0 

17.1 

1.5 

.3 

31.5 
56 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Type section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

172. Shale, etc.-Continued M Ft 

USGS Mesozoic loc. 20299, 1.5 m above the 

base of this unit: 

Inoceramus lundbreckensis McLearn 

Pseudoperna congesta (Conrad) 

Clioscaphites novimexicanus Reeside 

Desmoscaphites erdmanni Cobban 

171. Limestone, gray, sandy. Makes thin resistant 

ledge. Thickness variable. Bed B of Erdmann, Gist, Nordquist, and Beer (1947) __ .5 1.5 

USGS Mesozoic loc. D1237: 

Inoceramus lundbreckensis McLearn 

Pseudoperna congesta (Conrad) 

Baculites thomi Reeside 

170. Shale, gray, calcareous; contains small nodular concretions and a few thin (1 em) layers 

of gray sandy limestone__________________ 5.5 18.0 

USGS Mesozoic loc. 20690, 2.4 m above 

base of this unit: 

Pseudomelania n. sp. aff. P. 

hendricksoni Henderson 

169. Shale, gray, calcareous, nonfissile __________ 1.4 4.5 

168. Limestone, gray, sandy, concretionary. Locally makes a thin ledge. Bed C of Erdmann, 

Gist, Nordquist, and Beer (1947) ________ .1 .3 

167. Shale, gray; small nodular concretions of gray 

limestone ------------------------------ 1.2 4.0 

166. Shale, sandy; thin layers of sandstone______ .3 1.0 

165. Shale, gray; numerous small (5-30 em in 

diameter) thin nodules of gray limestone__ 1.5 5.0 

164. Limestone, gray; concretions 1 m in diameter .2 . 7 

163. Shale; thin layers of sandstone as much as 3 

em thick-------------------------------- 1.1 3.7 

162. Shale, gray, sandy________________________ 2.1 7.0 

161. Shale, gray, sandy; thin layers of fine-grained 

gray sandstone-------------------------- 1.4 4.5 

160. Shale, gray, nonfissile; weathers into granular fragments __________________________ 1.5 5.0 

159. Bentonite, rusty-brown, weathered ________ .1 .2 

158. Shale, gray, nonfissile; weathers into small 

granular fragments______________________ 1.2 4.0 

157. Sandstone, gray, fine-grained; 1-cm-thick 

beds interbedded with thin layers of sandy 

shale ---------------------------------- .8 2.7 

156. Limestone, gray, concretionary, sandy, finely 

laminated; weathers yellowish gray to 

medium yellowish orange. Individual 

masses as much as 3.5 by 2 m in plan, making an almost continuous resistant layer, 

with interstices occupied by gray shale. Bed 

can be followed easily and makes a good 

marker. Fossiliferous. Bed D of Erdmann, 

Gist, Nordquist, and Beer (1947) .6 2.0 

USGS Mesozoic loc. D1246: 

Baculites asper Morton 

B. codyensis Reeside 

Clioscaphites vermiformis (Meek and 

Hayden) 

155. Shale, gray, soft, poorly exposed __________ .7 2.3 

154. Limestone, dark-gray; weathers yellowish 

gray; concretionary; thin tabular masses, 

Type section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

154. Limestone, etc.-Continued M Ft 

widely spaced. In a few places very fossiliferous ------------------------------ .1 .2 

USGS Mesozoic loc. D1245: 

Pseudoperna congesta (Conrad) 

Baculites asper Morton 

B. codyensis Reeside 

Clioscaphites montanensis Cobban 

C. vermiform is (Meek and Hayden) 

153. Shale, gray, soft. At base a 6-cm-thick bed of 

gray fine-grained soft sandstone__________ 1.3 4.2 

152. Claystone, gray, nonfissile ________________ 2.6 8.5 

151. Sandstone, gray, fine-grained, thinly laminated, and crossbedded __________________ .1 .5 

150. Shale, gray, flaky, noncalcareous __________ .5 1.7 

149. Limestone, gray, concretionary, very fine 

grained; weathers yellowish gray; flat oval 

masses 1 m in diameter, much shattered. 

Locally one is larger, weathers yellowish 

orange, and exhibits cone-in-cone structure. 

Widely separated. Fossiliferous __________ .1 .4 

USGS Mesozoic loc. 21666: 

Inoceramus sp. 

Baculites codyensis Reeside 

Clioscaphites montanensis Cobban 

C. vermiformis (Meek and Hayden) 

148. Shale, gray, flaky ________________________ .6 2.0 

14 7. Limestone, dark-gray, concretionary; weathers yellowish gray; upper surfaces rounded; 

aphanitic, hard, breaks with conchoidal 

fracture; locally makes continuous layer 

with minor development of cone-in-cone 

structure. Fossiliferous __________________ .2 .6 

USGS Mesozoic loc. D1244: 

Membranipora sp. 

Inoceramus n. sp. 

Pseudoperna congesta (Conrad) 

Baculites codyensis Reeside 

Clioscaphites montanensis Cobban 

C. uermiformis (Meek and Hayden) 

146. Shale, gray, flaky, noncalcareous. At base is a 

thin medium-olive-gray fine-grained finely 

laminated sandstone bed containing a few 

baculites ____ _ _ ________ _ _ __ ______ _ _ _ _ __ __ 1.5 5.1 

145. Limestone, concretionary; in thin, discrete 

masses densely packed with Baculites 

codyensis, replaced by white crystalline calcite ------------------------------------ .1 .2 

USGS Mesozoic loc. D1243: 

Baculites codyensis Reeside 

Clioscaphites montanensis Cobban 

144. Shale, medium-dark-gray; contains a very 

thin finely laminated layer of sandstone in 

middle and a few small dolostone concretions in lower 15 em ____________________ 1.5 4.8 

143. Dolostone, olive-gray, concretionary, hard, 

aphanitic, fairly continuous; weathers 

brown to very dusky red. Top of middle unit 

of member------------------------------ .1 .3 

142. Shale, brownish-olive-gray, soft, flaky______ .4 1.3 

141. Dolostone, olive-gray, concretionary; weath-
KEVIN MEMBER 57 

Type section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

141. Dolostone, etc.-Continued M 

ers brown to dusky red; small, infrequent 

oval masses; very fossiliferous____________ .1 

USGS Mesozoic loc. 21665: 

N uculana sp. 

Inoceramus sp. 

Baculites asper Morton 

B. codyensis Reeside 

Clioscaphites nwntanensis Cobban 

C. vermiformis (Meek and Hayden) 

140. Shale, brownish-olive-gray, soft, flaky; conFt 

.2 

tains a 3-cm-thick layer of bentonite______ 1.0 3.2 

139. Limestone, dark-gray (N3 ), aphanitic, hard; 

weathers yellowish gray (5Y 7/2). Contains 

abundant baculites and a few fragments of 

scaphites ------------------------------ .1 .2 

138. Shale, olive-gray, flaky____________________ 1.2 4.0 

137. Limestone, gray, concretionary, ferruginous; 

weathers yellowish gray ________________ .1 .3 

136. Siltstone, brownish-olive-gray, noncalcareous; weathers down into sharp granular 

fragments and chips ____________________ 2.4 7.8 

135. Dolostone, olive-gray, concretionary, aphanitic, hard; weathers reddish brown and dusky 

red. Contains lentils of gray septarian limestone with dark-brown calcite septae. A persistent layer, making a small bench. Bed E 

of Erdmann, Gist, Nordquist, and Beer 

(1947) ---------------------------------- .1 .4 

134. Shale, brownish-olive-gray, flaky, noncalcareous; some thin sandy partings ________ 4.6 15.2 

133. Limestone, brownish-olive-gray, concretionary; weathers light yellowish gray. Shattered oval masses 1-2m in diameter spaced 

at intervals of 3-6 m. Very fine grained at 

base, becoming prismatic toward top with 

some gnarly cone-in-cone structure. A fairly 

persistent bed -------------------------- .4 1.2 

132. Shale, olive-gray, flaky, noncalcareous______ 1.7 5.7 

131. Limestone, gray, concretionary, ferruginous: 

weathers brown to light yellowish brown. 

Discrete tabular masses about 1 m in 

diameter________________________________ .1 .3 

130. Shale, dark-gray, fissile __________________ .3 1.1 

129. Limestone, olive-gray, dolomitic, concretionary, aphanitic; weathers reddish brown and 

very dusky red. Tabular masses 1m or more 

in diameter ---------------------------- .1 .3 

USGS Mesozoic loc. 21664: 

Inoceramus sp. 

Baculites sp. 

Clioscaphites sp. 

128. Shale, dark-gray, flaky, noncalcareous; becomes light olive gray toward top ________ 1.5 5.0 

127. Shale, gray ------------------------------ .2 .5 

126. Limestone, gray, sandy; weathers yellowish 

brown. Coquina type; many shell fragments .1 .2 

125. Shale, dark-gray; interlaminated with thin 

brownish-gray siltstone__________________ .4 1.3 

124. Sandstone, medium-olive-gray, fine-grained, 

hard, brittle, noncalcareous, very finely 

Type section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

124. Sandstone, etc.-Continued M 

laminated; weathers medium yellowish 

brown ---------------------------------- .1 

USGS Mesozoic loc. D1242: 

Inoceramus cf. I. cordiformis Sowerby 

Baculi tes cody ens is Reeside 

Clioscaphites? sp. 

123. Shale, medium-dark-gray, silty, noncalcareous. A thin fine-grained sandstone bed at 

Ft 

.2 

base ______ _ _ __ _ _ ____ _ _ _ _ ____ __ __ _ _ ______ 1.6 5.2 

122. Shale, brownish-olive-gray.________________ 1.5 5.0 

121. Bentonite, light-gray______________________ .1 .3 

120. Shale, brownish-olive-gray; as unit 119 ____ 3.3 11.0 

119. Shale, brownish-olive-gray. At top and base 

are bentonite beds about 7 em thick ______ .5 1.8 

118. Shale, brownish-olive-gray (between 5YR 4/1 

and 5Y 4/1), noncalcareous, poorly fissile__ 1.9 6.3 

117. Limestone, light-olive-gray, concretionary, 

sandy textured, finely and evenly laminated; weathers yellowish brown. Oval 

masses 1-2m in diameter, muchjointed and 

shattered; spaced at intervals of 10 m or 

more and may not be present in local sections ---------------------------------- .3 .9 

USGS Mesozoic loc. 21663: 

Inoceramus sp. 

scaphite undet. 

116. Shale, brownish-olive-gray, silty, soft, noncalcareous, flaky ________________________ .4 1.2 

115. Bentonite, grayish-orange (10YR 7/4), granular textured, finely and smoothly laminated; 

considerable biotite. Rests on 10 em of hard 

finely crystalline concretionary limestone 

that weathers grayish to yellowish orange, 

but which may be locally absent__________ .2 .7 

114. Shale, gray, flaky, noncalcareous __________ .3 1.0 

113. Sandstone, medium-gray, calcareous; firm, 

relatively resistant, making a thin ledge. 

Very fine grained; with wavy lamination 

developed by short thin lentils of dark-gray 

noncalcareous sandstone. Locally grades 

laterally into circular masses of gray concretionary limestone 1 by 2.7 m __________ .2 .5 

112. Shale, dark-brownish-olive gray; a few thin 

layers of fine-grained platy sandstone in 

upper 1.2 m. Lower 2.1 m is dark gray (N3) 

soft, fissile, noncalcareous ________________ 3.3 11.0 

111. Shale, medium- to dark-gray, flaky, noncalcareous. A 6-cm-thick bed of bentonite at 

top ------------------------------------ .5 2.0 

110. Calcite and bentonite; grayish-orange, soft, 

friable; medium- to coarse-grained concretionary calcite and gray very micaceous 

bentonite ------------------------------ .8 2.5 

109. Shale, olive-gray (5Y 4/1), soft, flaky, noncalcareous, poorly exposed__________________ 2.0 6.5 

108. Phosphatic pebble bed. A concentration of 

smooth rounded concretions of phosphatic 

siltstone and sandstone, making a thin armored pavement-type surface over very limited areas, with here and there low mounds 
58 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Type section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

108. Phosphatic pebble bed-Continued M 

that contain small oval limestone concretions. The pebbles are greenish gray (5GY 

6/1 l to olive gray (5Y 4/1), with darker 

crusts, and some have a close, irregular 

light-gray reticulation. Size range: 5-40 

mm, with mean 12-20 mm. All are rounded 

or subrounded and a few spindle shaped; 

many are broken, apparently along desiccation cracks. Here and there a small scaphite 

among the pebbles. Accessory limestone 

concretions are of two types, both similar to 

facies of the MacGowan Concretionary Bed: 

(1) moderate yellowish-brown limestone in 

nodules 15 by 10 by 5 em, weathering dark 

yellowish orange, and containing pellets of 

light-gray phosphatic siltstone; (2) a single 

discoid mass of ferruginous limestone 25 by 

20 by 5 em, weathering very dusky red 

(lOR 2/2) with small embedded pebbles as 

above. Fossiliferous______________________ .1 

USGS Mesozoic coll. D1334: 

Inoceramus sp. 

S cap hi tes sp. 

107. Shale, olive-gray (5Y 4/1), soft, flaky, noncalcareous -------------------------------- .5 

106. Limestone, moderate-brown to grayish-red, 

concretionary, ferruginous; upper surface 

very dusky red. Thin hard resistant layer .1 

USGS Mesozoic loc. D1333: 

Inoceramus stantoni Sokolow 

Scaphites depressus Reeside var. 

stantoni Reeside 

Bacu lites as per Morton 

Baculites codyensis Reeside 

105. Shale, olive-gray, silty, soft, flaky. At base 

was found a large fossil bone, probably a rib, 

nearly 2 ft long ________________________ .6 

104. Limestone, olive-gray, concretionary, aphanitic; weathers moderate brown; uniform 

persistent layer ________________________ .1 

USGS Mesozoic loc. D1240 and Dl332: 

Inoceramus sp. 

Baculites asper Morton 

Baculites codyensis Reeside 

Scaphites depressus Reeside 

103. Shale, medium-gray, noncalcareous, silty; 

contains near top dusky-red-weathering 

dolostone concretions ____________________ .8 

102. Dolostone, dark-greenish-gray to olive-gray, 

concretionary, aphanitic; weathers very 

dusky red (maroon); in small hard discrete 

nodules as large as 8 by 15 em. Sparingly 

fossiliferous ---------------------------- .1 

101. Shale, olive-gray (5YR 4/1), noncalcareous, 

soft, poorly fissile. Thickness variable ____ 2.4 

100. MacGowan Concretionary Bed. Bed F of 

Erdmann, Gist, Nordquist, and Beer (1947). 

Type section. 

lOOc. Dolostone, brownish-gray (5YR 4/1), 

ferruginous, hard; weathers dark reddish 

Ft 

.2 

1.5 

.2 

2.0 

.2 

2.6 

.3 

8.0 

Type section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

lOOc. Dolostone, etc.-Continued 

brown (lOR 3/4) to very dusky red (lOR 2/2); 

aphanitic, breaking with subconchoidal 

fracture. Matrix contains a few small pellets 

(possibly reworked) oflight-gray phosphatic 

siltstone. Bed caps underlying limestone 

concretions and fills interstices between 

them ----------------------------------

lOOb. Limestone, brownish-olive-gray (between 5YR 4/1 and 5Y 4/1 ); weathers to 

light orange brown (between 1 OYR 6/6 and 

5YR 5/6) and shades of moderate brown 

(5YR 4/4 and 5YR 3/4) with mottling or 

splotches of moderate (lOR 4/6) to dark 

reddish brown (lOR 3/4l; concretionary, 

conglomeratic; matrix aphanatic. A few 

concretions remain predominantly gray, 

weathering dull yellowish brown. Individual forms are flat-based round-topped 

fractured oval masses, noticeably septarian, 

that vary in diameter from 30 to 50 em and 

are packed so closely as to form a continuous 

layer. Conglomeratic facies consist of erratic concretions and patches of irregularly 

shaped smoothly rounded pellets of 

yellowish-gray (5Y 8/1) to light-olive-gray 

(5Y 6/1) phosphatic mudstone as long as 2 

em, although usually smaller; cover as 

much as 10 percent or more of the outer surfaces of the concretions as crusts and are 

embedded more sparingly in the matrix. 

These pellets, with sharp color contrast to 

the weathered surface, plus an accessory 

sprinkling of 1 percent or less of smoothly 

rounded well-polished pebbles of black to 

olive-gray chert as much as 2 em in diameter, but usually much finer, are the principal diagnostic features of the bed. Entire 

unit makes a thin resistent ledge capping 

spurs or dip slopes, whose surfaces are partially covered with a thin litter of angular 

fragments ------------------------------

lOOa. Shale, dark-gray, noncalcareous, poorly 

fissile. At base a thin concretionary lightbrown- to moderate-reddish-brownweathering bed of limestone. Base of MacGowan Bed ----------------------------

99. Shale, dark-olive-gray; a few thin laminae of 

fine-grained chippy sandstone in middle and 

attop ----------------------------------

USGS Mesozoic loc. D1238: 

Inoceramus sp. 

98. Shale, olive-gray (5Y 4/1); small thin widely 

spaced septarian concretions of gray limestone ----------------------------------

97. Shale, gray, sandy, poorly exposed _______ _ 

96. Limestone, gray, concretionary; in thin flat 

discoid masses 30 em in diameter that 

weather yellowish gray; dark-greenisholive-gray hard aphanitic dolostone, with 

accessory marcasite and weathering dusky 

M Ft 

.1 .2 

.1 .5 

.2 .9 

.3 1.0 

.4 1.3 

.6 2.1 
KEVIN MEMBER 

Type section of the Kevin Me.mber-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

96. Limestone, etc.-Continued M 

red and yellowish orange ________________ .1 

USGS Mesozoic loc. 23753: 

Inoceramus (Volviceramus) involutus 

Sower by 

Pseudoperna congesta (Conrad) 

Baculites sp. 

95. Shale, gray; occasional discrete oval concretions of dark-gray limestone, 30 by 15 em, 

that weather yellowish gray. At base a thin 

light-olive-gray fine-grained soft finely 

laminated sandstone bed ________________ .9 

94. Limestone, olive-gray, concretionary, aphanitic; weathers yellowish gray; in discrete 

oval masses 30 em in diameter. Septarian 

veins of dark calcite. Breaks into angular 

fragments ------------------------------ .2 

93. Shale, gray; contains a very thin gray finegrained soft thinly laminated layer of 

sandstone ------------------------------ 1.3 

92. Limestone, dark-gray, concretionary; weathers lighter gray; small (diameter 30 em) 

oval septarian forms ____________________ .2 

91. Shale, gray, silty, soft, noncalcareous ______ 1.7 

90. Limestone, gray, concretionary, finely granular (sandy); weathers yellowish orange 

and dark yellowish orange to yellowish 

brown. Lower 15 em may exhibit rude 

cone-in-cone and prismatic structures. 

Thickness 0.3-0.5 m, diameter 1-3 by 4.5 m. 

Persistent, but occurring at large horizontal 

distances ------------------------------ .5 

89. Shale, gray; as below______________________ .3 

88. Limestone, dull-red, concretionary, sandy; 

makes persistent layer locally____________ .1 

USGS Mesozoic loc. D1331: 

Scaphites ventricosus Meek and 

Hayden 

87. Shale, gray, soft, gypsiferous (radiating 

fibrous selenite crystals). Contains a few 

sandy ferruginous limestone concretions __ 2.2 

86. Shale, dark-gray, soft, clayey; carries an occasional concretion of ferruginous limestone 

in lower 1.5 m. A very thin fine-grained 

sandstone bed at top ____________________ 2.9 

85. Limestone, gray; weathers buff to yellowish 

gray; concretionary; in fairly large isolated 

conspicuous masses making a small bench; 

flat and slabby at base, discrete oval forms 

at top; cone-in-cone structure in places____ .1 

84. Limestone, dark-gray; ferruginous, aphanitic; 

weathers very dusky red. Fossiliferous---- .1 

USGS Mesozoic loc. 20698: 

Inoceramus (Volviceramus) involutus 

Sower by 

Inoceramus (Volviceramus) undabundus 

Meek and Hayden 

83. Shale, gray, soft, clayey; contains a thin 

gray-weathering limestone bed at base 

overlain by a thin fine-grained sandstone 

Ft 

.3 

3.1 

.5 

4.3 

.5 

5.5 

1.5 

1.0 

. 3 

7.4 

9.6 

.2 

.2 

Type section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

83. Shale, etc.-Continued M 

bed. Actinocamax sp. ____________________ 1.9 

82. Shale, gray, soft; makes slope; poorly exposed 3.7 

81. Limestone, dark-gray, concretionary, ferruginous, hard, aphanitic, persistent; 

weathers very dusky red and dark yellowish 

orange. Bed G of Erdmann, Gist, Nordquist, 

and Beer (1947) ________________________ .1 

USGS Mesozoic loc. 20295: 

Inoceramus sp. 

Baculites sp. 

Scaphites ventricosus Meek and 

Hayden 

80. Shale, gray, clayey, gypsiferous; contains 

scattered thin sandstone and small oval 

limestone concretions near middle and a 

thin gray fine-grained chippy sandstone bed 

at base -------------------------------- 3.4 

79. Shale, gray, soft, flaky ____________________ .2 

78. Limestone, dark-gray; concretionary, ferruginous; weathers very dusky red; oval to 

platy shapes; fairly continuous __________ .1 

77. Shale, gray, clayey; scattered concretions of 

ferruginous limestone____________________ 1.1 

76. Limestone, dark-gray, concretionary, ferruginous, aphanitic; weathers very dusky 

red and brown; in large flat forms about 1m 

in diameter ---------------------------- .1 

USGS Mesozoic loc. 20291: 

Inoceramus (Volviceramus) 

undabundus Meek and Hayden 

Baculites asper Morton 

Scaphites ventricosus Meek and 

Hayden 

Actinocamax sp . 

75. Shale, gray, sandy, soft, flaky; contains a gray 

fine-grained thinly laminated sandstone 

layer at base---------------------------- .6 

74. Shale, dark-gray, clayey; contains a few small 

nodular concretions of ferruginous limestone weathering very dusky red ________ 1.8 

73. Limestone, dark-gray, concretionary, ferruginous; weathers very dusky red__________ .1 

72. Shale, dark-gray-------------------------- .3 

71. Limestone, dark-gray, concretionary, hard, 

aphanitic, brittle; weathers very dusky red; 

discrete masses commonly 30-50 em long, 

and smaller nodules. Fossiliferous. One concretion contained a weathered bone nearly 1 

m long and 15 em in diameter____________ .1 

USGS Mesozoic loc. 20292: 

Inoceramus (Volviceramus) involutus 

Sower by 

I. (Volviceranzus) undabundus Meek 

and Hayden 

Spondylus sp. 

Baculites asper Morton 

Scaphites uentricosus Meek and 

Hayden 

70. Shale, gray, soft, clayey __________________ 1.1 

69. Limestone, gray, concretionary; weathers yel59 

Ft 

6.2 

12.3 

.3 

11.1 

.5 

.2 

3.8 

.2 

2.0 

6.0 

.3 

1.0 

.3 

3.5 
60 BLACKLEAF AND MARIAS RIVER FORMATIONS, SWEETGRASS ARCH, MONTANA 

Type section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

69. Limestone, etc.-Continued M Ft 

lowish gray; in discrete masses with poorly 

developed cone-in-cone structure; resting on 

thin gray sandy bentonite. Makes a persistent marker. Bed H of Erdmann, Gist, 

Nordquist, and Beer (1947) ______________ .1 .3 

68. Shale, dark-gray, soft, clayey; checked surface stained by white efflorescent salt ____ 3.3 11.0 

67. Shale, dark-gray, soft, clayey; contains a few 

small oval concretions oflimestone and thin 

1-3-cm-thick gray chippy sandstones at 

intervals of 50-60 em____________________ 2.1 7.0 

66. Shale, dark-gray, soft, clayey (bentonitic); 

checked surface stained by white efflorescent salt-------------------------------- 3.9 13.0 

65. Limestone, dark-gray, concretionary; oval 

septarian masses about 30 em long spaced 

at intervals of 3m ______________________ .1 .3 

64. Shale, dark-gray, soft, poorly fissile ________ 1.1 3.8 

63. Bentonite, gray, gritty; weathers tan ______ .1 .3 

62. Shale, gray, soft, poorly exposed __________ .4 1.3 

61. Limestone, gray, concretionary, aphanitic; 

oval septarian masses about 30 em long 

with much brown and white crystalline calcite spaced at intervals of 1-6m__________ .2 .5 

60. Shale, gray, soft, poorly expo~ed __________ .5 1.5 

59. Bentonite, gray, sandy ____________________ .1 .3 

58. Shale, soft, poorly exposed ________________ 1.2 4.0 

57. Calcite, light-yellowish-gray, sandy, prismatic (fibrous); weathers tan to caramel brown; 

poorly developed cone-in-cone structure. 

Probably a calcareous replacement of a 

layer of bentonite; fairly resistant, makes a 

bench and caps small outliers. Bed I of 

Erdmann, Gist, Nordquist, and Beer (194 7l. 

Top of lower unit________________________ .1 .5 

56. Siltstone, gray, noncalcareous, firm; brittle, 

with a harsh feel; contains fossiliferous oval 

concretions of gray aphanitic limestone 

10-15 em thick and 30-50 em long in middle 

part __ __ _ _ _ _ _____ _ _ _ ___ _ _ _ _ __ _ _ _ __ _ _ _ _ _ _ 1. 7 5.5 

USGS Mesozoic loc. 20696: 

Inoceramus aff. I. corpulentus 

MeL earn 

Ostrea n. sp. 

55. Bentonite, gray \-------------------------- .1 .5 

54. Shale, gray ______________________________ 3.6 12.0 

53. Limestone, concretionary, gray; small rough 

oval masses ____________________________ .1 .2 

52. Siltstone, gray, firm; harsh feel; weathers into 

granular fragments______________________ 1.8 6.0 

51. Limestone, gray, concretionary, aphanitic; 

weathers brownish; sandy at top, with some 

gnarly cone-in-cone structure; in rounded 

fiat-topped much-fractured masses with 

septae of brown calcite; commonly 1 m in 

diameter but as much as 3 by 4 m. Makes 

ledge in main slope. Bed J of Erdmann, Gist, 

Nordquist, and Beer (1947) ______________ .3 1.0 

50. Shale, gray ______________________________ .8 2.5 

49. Sandstone, gray, fine-grained, thinly laminated; weathers brownish; some shale partType section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

49. Sandstone, etc.-Continued M Ft 

ings; bedding surfaces show worm (?) trails 

and related markings-------------------- .4 1.3 

48. Limestone, gray, concretionary, aphanitic; 

small fiat oval forms. ContainsScaphites sp. 

and Inoceramus sp. ______________________ .1 .2 

47. Shale, gray, soft, flaky; contains a few oval 

concretions of gray limestone with fragments of Scaphites sp. ------------------ 3.8 12.5 

46. Limestone, gray, concretionary, aphanitic; 

small discrete masses at 6-m intervals. Contains Inoceramus sp. and Ostrea sp. ______ .1 .3 

45. Shale, gray, soft; poorly exposed on slope. A 

3-cm-thick layer of bentonite at top ______ 1.2 4.1 

44. Bentonite, gray, sandy, weathered; at top a 

thin layer of caramel-brown secondary 

fibrous calcite; makes slight ledge ________ .2 .5 

43. Shale, gray, soft; a few hand-sized concretions 

of gray limestone; makes slope __________ .6 2.0 

42. Limestone, dark-gray, concretionary; weathers light yellowish gray; in small 30-50-

cm-thick much-fractured oval masses with 

septae of brown calcite; fossiliferous. Makes 

a rather distinct bench __________________ .2 .6 

41. Shale, gray, soft; poorly exposed on slope; contains 6-cm-thick layers of bentonite at base 

and middle______________________________ 2.5 8.4 

40. Limestone, gray, concretionary, aphanitic, 

gnarly; weathers rusty brown; veined with 

more or less white crystalline calcite; hard, 

resistant; massive discrete forms 0.5-3.0 m 

in diameter spaced on 3-5-m centers making small mounds. A conspicuous marker 

bed. Bed K of Erdmann, Gist, Nordquist, 

and Beer (1947) ________________________ .7 2.5 

39. Bentonite, sandy; weathered rusty ________ .1 .3 

38. Shale, dark-gray, soft, clayey; contains a 

3-cm-thick layer of bentonite at base______ .8 2.6 

37. Shale, dark-gray, soft, clayey ______________ 3.2 10.5 

36. Bentonite, sandy; weathers rusty __________ .1 .5 

35. Shale, gray, soft, clayey; makes slope ______ .9 3.0 

34. Limestone, gray, concretionary, fossiliferous; 

in oval discrete forms as much as 50 em long .1 .5 

USGS Mesozoic loc. 20300: 

Scaph ites sp. 

Actinocamax sp. 

33. Bentonite, gray, sandy; weathers rusty .1 .3 

32. Shale, gray, soft, clayey __________________ .3 1.0 

31. Limestone, concretionary, gray ____________ .1 .5 

30. Bentonite, gray, sandy; weathers rusty ____ .1 .5 

29. Shale, brownish-gray, clayey, bentonitic____ 1.1 3.8 

28. Limestone, gray, concretionary, aphanitic, 

hard, smooth; weathers yellowish gray; in 

discrete oval forms 30 em in diameter .2 .5 

USGS Mesozoic loc. 20296: 

Inoceramus sp. 

Ostrea sp. 

Baculites mariasensis Cobban 

Scaphites preventricosus Cobban 

Actinocamax sp. 

27. Bentonite, gray, sandy; weathers rusty .4 1.2 

26. Shale, gray, flaky; top is silicified __________ .5 1.5 
REFERENCES CITED 61 

Type section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

25. Limestone, gray, concretionary, aphanitic; 

small (diameter 30 em) smooth discrete oval 

Ft 

masses ________________________________ .2 .5 

24. Bentonite, gray __________________________ .4 1.2 

23. Shale, dark-gray, soft, flaky. A very thin 

layer of bentonite in middle______________ 1.4 4.9 

22. Bentonite, light-gray______________________ .2 .5 

21. Shale, dark-gray, soft, flaky ______________ .6 2.0 

20. Limestone, dark-gray, concretionary; weathers yellowish gray; smooth oval discrete 

forms __________________________________ .1 .5 

USGS Mesozoic loc. 20298: 

Inoceramus deformis Meek 

Pseudoperna congesta (Conrad) 

Ostrea n. sp. 

Pholadomya papyracea Meek and 

Hayden 

Veniella goniophora Meek 

Drepanochilus? sp. 

Baculites mariasensis Cobban 

B. sweetgrassensis Cobban 

Scaph ites preuen tricosus Cobban 

19. Shale, dark-gray, soft, flaky ______________ .5 1.5 

18. Bentonite, rusty-brown, weathered; stain of 

white efflorescent salt above bed__________ .1 .5 

17. Siltstone, dark-gray, soft; makes slope______ 2.7 9.0 

16. Bentonite, gray, sandy; weathers tan; upper 

part of bed littered with caramel-color secondary prismatic calcite that has some 

cone-in-cone structure __________________ .3 1.0 

15. Siltstone, dark-gray, soft; weathers into 

gumbo with stains of white efflorescent salt. 

Poorly exposed in slope __________________ 4.6 15.0 

14. Bentonite, light-brownish-gray, sandy______ .2 .5 

13. Shale, dark- olive-gray, soft, flaky, noncalcareous; contains a very thin rusty-brown 

fine-grained finely laminated sandstone bed 

near base ______________________________ 4.0 13.1 

12. Bentonite, yellowish-gray; contains much 

light-brownish-gray (5YR 4/1) prismatic 

calcite that has a tendency toward cone-incone structure and weathers moderate yellowish brown (caramel). A few small flattopped oval-shaped concretions of darkbrownish-gray aphanitic limestone. Bed L 

of Erdmann, Gist, Nordquist, and Beer 

(1947) ---------------------------------- .3 1.0 

11. Shale, dark-olive-gray, soft, noncalcareous __ .4 1.3 

10. Shale, brownish-gray, clayey, bentonitic. A 

6-cm-thick pale-yellowish-brown micaceous 

bentonite bed at base____________________ .1 .6 

9. Shale, dark-gray, soft, poorly fissile, noncalcareous -------------------------------- 1.0 3.3 

8. Siltstone, dark-gray, soft; weathers brownish .4 1.3 

7. Limestone, dark-gray, aphanitic, concretionary, hard, fetid-smelling, fossiliferous; 

weathers light yellowish gray; in discrete 

oval types (30 em in diameter) in places septariate with veins of white and brown crystalline calcite; spaced at intervals of about 

3m ------------------------------------ .1 .4 

USGS Mesozoic loc. 20289: 

Type section of the Kevin Member-Continued 

Marias River Shale-Continued 

Kevin Member-Continued 

7. Limestone, etc.-Continued 

Anomia cf. A. subquadrata Stanton 

Inoceramus sp. 

Scaphites preventricosus Cobban 

S. preventricosus var. sweetgrassensis 

Cobban 

6. Siltstone, dark-gray, poorly fissile, soft, noncalcareous; weathers into small granular 

JI Ft 

particles; some thin rusty laminae________ 2.9 9.5 

5. Limestone, dark-gray, concretionary; weathers medium olive gray to light yellowish 

gray; almost a continuous bed of small (diameter 30 em) closely packed discrete septarian masses that have rough upper surfaces and partitions of white calcite .2 .5 

USGS Mesozoic loc. Dl24 7: 

Inoceramus sp. 

Oxytoma sp. 

Tessarolax hitzii White 

4. Siltstone, medium-dark-gray, noncalcareous; 

contains small ( 10-cm-thick) nodules of gray 

clayey limestone ________________________ .5 1.5 

3. Siltstone, medium-dark-gray, noncalcareous. 

Surface littered with thin rusty-brown chips 

of clay ironstone. Unit also contains small 

1-cm-thick irregularly shaped nodules or 

concretions of soft moderate-yellow silt, 

weathering dusky yellow, that have the 

same shapes and appearance as kernels of 

buttered popcorn ________________________ 1.6 5.3 

2. Bentonite, yellowish-gray (5Y 8/1), sandy, 

highly weathered________________________ .2 .5 

1. Siltstone, dark-gray, noncalcareous, poorly 

fissile. Base of Kevin Member____________ 2.9 9.6 

Total Kevin Member (rounded) ________ 188.0 617.0 

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--1909, Geology of the Great Falls coal field, Montana: U.S. 

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Jeletzky, J. A .. 1955, Belemnitella praecursor, probably from the 

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Silurian (Primordial), Jurassic, Cretaceous, and Tertiary fossils, 

collected in Nebraska Territory*** with some remarks on the 

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---1862, Descriptions of new Cretac·: JUS fossils from Nebraska 

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shale (Cretaceous) and contemporary formations in the United 

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Page 

A 

Acknowledgments ___________ _ 2 

44 

-------------- 33 

Antelope Coulee _______ _ 

Arrow Creek Member_ 

B 

Badger Creek ____ _ 

Bearpaw Mountains _____________________ 15, 16, 52 

Bears Den district _ ___________ ___________ 37 

Bears Den oil field ____________________________ 50 

Belle Fourche Shale _ 

Belt Butte _ 

Belt Creek 

----------- 37, 39 

-------- 4, 26, 34 

------------------ 7 

Benton Lake _ 

Benton Shale 

Bighorn Basin _ 

------------------------ 37 

Bighorn Formation 

Birch Creek __ _ 

3, 4, 37 

------- 55 

------ 46 

4, 5, 10, 16, 37, 40, 44 

Bird Creek __ _ ----------- -------------- 27 

Black Coulee __ _ ---------- 39 

Black Hills ___ _ ------ 5, 19, 37, 39, 44, 47, 50 

Blackhorse Lake Flat _ 

Blackleaf Creek ___ _ 

Blackleaf Formation 

Blackleaf Sandy Member ____ _ 

------- 26, 27, 32 

5 

2, 5 

----------- 4 

Bluesky Formation_ _ ________________ 16 

Bootlegger Member _____ 2, 5, 31 

Border-Red Coulee oil field ___________________ 9, 16 

Brady oil field _____________________ 27, 46 

Bridge Creek Limestone Member 

Cardium Formation ____ _ 

Carlile Formation _ 

Carlile Shale 

Carter Ferry 

Chalk Butte nose_ 

Chert pebbles 

Cody Shale ____ _ 

Colorado Formation ____ _ 

Colorado Group 

Colorado Shale 

Cone Calcareous Member 

Cone Member_ 

c 

---------- 42 

---------- 46 

-------- 5 

42, 44, 47 

34, 37 

37 

5 

55 

4 

2, 5 

2, 50 

5, 40 

2. 5, 37, 40, 46 

Cone triangulation station __________ 37, 40 

Crowsnest River _ ___________ _ _________ 46 

Cut Bank oil and gas field _____________ 9, 27, 37, 50 

Cyprian Sandstone Member_______ 19 

D,E 

Dakota Formation 4 

Dearborn River____ ___________ ___________ 5 

Disturbed belt ______ 4, 5, 9, 16, 27, 37, 45, 46, 50, 52 

Eagle Formation 4, 55 

F 

Fairport Chalky Shale Member 

Fall River Formation _ 

------------ 42 

11 

Ferdig Member 2, 5, 44 

First Cat Creek sand _ _ ______ 11 

Flood Member ______ 2, 5, 6, 9, 11, 16 

Floweree Member ____________ 2, 5, 33,37 

Fort Benton Group, Formation No.2_ ____________ 2 

Fort St. John Group __________ 16 

INDEX 

[Italic page numbers indicate major references] 

Page 

Fort Hays Limestone Member ___________________ 50 

Fossils: 

Actinocamax sp 

Ammonites 

Ammonites mulani __ _ 

percari natus 

-------- 59, 60 

34, 39, 47, 53, 55 

2 

---------------- 2 

uespertinus _________________________ _ 

Anchura quitmanensis _____________ 19 

sp ___________________________ 19 

Anemia (remonti ____________ __________________ 28 

Anomia---------------------------------- 10, 19, 40 

subquadrata ___________________________ 61 

Araucariouxylon sp _____________________________ 28 

Baculites asper _____________ 56, 57, 58, 59 

besairiei ------------------------------ _____ 49 

codyensis __________________ 56, 57, 58 

haresi __ ___________ _ __________ 55 

mariasensis _________________________ 53, 60, 61 

sweetgrassensis ______________________ 53, 61 

thomi ________________ _ ___________ 55, 56 

yokoyamai _____________________ 4 7 

sp ___________ _ __________ 48, 55, 57, 59 

Brachiopods __________________________ 18, 19, 21, 32 

Callisto sp _______ ____________________________ 40 

Callistina beluiderensis __________________________ 19 

Calycoceras canitaurinum ___________________ 39, 40 

Cardium kansasense __________________________ 19 

pauperculum ________________ 46 

Clioscaphites choteauensis ___________________ 52, 55 

montanensis _________________________ 53, 56, 57 

nouimexicanus ________________________ 5, 56 

t'ermi(ormis ____________________________ 53 

sp _ _ _____________________________ 57 

Coccoliths ___________________ 41 

Cmyatoceras nebrascense _____________ 2 

Collingnoniceras regulare ____________________ 47,50 

woollgari ___________________________ 42, 47, 50 

sp ___ _ ____________________________ 50 

Cyrena dakotensis 

Desmoscaphites erdmanni ____ _ 

Dinosaur bones _________________ _ 

19 

------- 55, 56 

18 

Drepanochilus ruida ---------------------------- 43 

sp _ _______________ ____________ 61 

Dryandroides sp _ 28 

Dunueganocerus albertense _____ 39 

Erythrinolepis mowriensis ______________________ 34 

Fish bones ____________________________ 21, 33, 34 

Foraminifera ___ _______________ 41, 43 

Gastropods_________ _________________________ 7, 18 

Holco/epis transversus___________ _ ________ 34 

Ichthyodectes sp_______ ___________________ 44, 50 

Inoceramus anglicus _______________________ 28 

bellt'uensis ________________________________ 19 

caddottensis _________________ 19 

comanchean us 

cordi(ormis __ 

corpulentus 

de(ormis 

10, 19 

57 

------------ 60 

------------------- ----- 53, 61 

erectus ___________________________________ 53 

(ragilis __________________________ 2, 48, 49, 50 

labiatus ___ ___________ ____________________ 4 

[Mytiloides]___ _ _____________________ 41 

latus__ ___________________ 2 

lundbreckensis _____________________ _ 

mesabiensis __ 

patootenszformis 

problematic us 

stantoni ____ _ 

56 

--- 39 

55 

2 

---- 53, 58 

Page 

Inoceramus-Continued 

tenuirostratus ___________ __________________ 2 

(Cordiceramus) cordiformis __________________ 53 

(Sphenoceramus) lundbreckensis ______________ 55 

(Volt•iceramus) involutus_ _________________ 53, 59 

undabundus ----------------------- ____ 59 

sp 19, 24, 40, 43, 44, 4 7, 48, 49, 56, 57, 58, 59, 60, 61 

Leucichthyops vagans ---------------------------- 34 

Lingula ______ ---------------------------------- 32 

subspatulata ---------------------------- 18, 19 

sp _______________________________ 4, 21, 29, 36 

Lucina _____________ ---------------------------- 19 

Mantelliceras canitaurinum _____________________ 39 

Martesia sp _______ ---------------------------- 47 

Membranipora sp ___ ---------------------------- 56 

Metoicoceras mosbyense _____ ___________________ 39 

muelleri ___________________________________ 39 

Mollusks---------------------------------- 4, 19, 28 

Mytiloides ______________________________ 41, 42, 43 

labiatus -------------------------------- 42, 43 

mytiloides ___ ------------------------ 42, 43, 44 

Nautilus elegans _____________________ 2 

Nelumbites sp __ ___________________ 28 

Neogastroplites ______________ 28,34 

N ucula ________________________________________ 19 

Nuculana _____ _ __________________________ 19 

sp _________________________________________ 57 

Nymphalucina subundata _______________________ 43 

sp _ _ _ _ _ _ _ _ _ _ __ __ _ _ __ __ _ _ __ ___ _ __ __ _ ___ __ _ _ __ 44 

Ophiomorpha__ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ __ _ _ _ _ __ _ _ 10 

Ostrea anomioides ------~------------------ 19 

congesta __ 2 

larimerensis __________ -----~ __ ------------~~ 19 

sp __________________ 43, 55, 60 

Oysters ________________________________________ 41 

Oxytoma sp 

Pecten syncyclomema sp _ 

Pelecypods ___ _ 

Phelopteria gastrodes 

salinaensis 

61 

40 

--- 5, 19, 46 

43 

------------------- 19 

sp ------------------------- ------ 19 

Pholadomya papyracea 2, 61 

Pinus sp ___ _ ______________________ 28 

Prionocyclus hyatti -------------------~---------- 47 

wyomingensis wyomingensis ______ 4 7 

sp --------------------------~--------- 47 

Protelliptio douglassi ___________ 9 

Pseudomelania hendricksoni _____________________ 56 

Pseudoperna congesta ____________________ 56, 59, 60 

Pteroscaphites auriculatus ____________________ 53 

Reesidella montanaensis ---------~ ------------~ 9 

Rhabdospheres ----------------~ ________________ 41 

Scaphites artilobus _________________________ 53 

bighornensis 47 

binneyi ____ ------------------------------- 53 

carlilensis ________________ _ ________ 47, 49 

corvensis ------------~-- ____________________ 47 

delicatulus ____________ _ _______ 43 

depressus --------------------~ 52, 53, 58 

(erronensis __ _ ------------------------ 47 

gracillistriatus ------------------------------ 53 

larvae(ormis _________________________ 2 

leei -------------------------~ ------~-~-~-- 55 

mariasensis _________________ ___________ 53 

meeki _____________________________ 48, 49 

nigricollensis _____________________ 47, 48, 49 

pendicostatus ___ 53 

pisznnus ___________________ --------~~- 48 

65 
66 

Page 

Scaphites -Continued 

pret•entru~osus ____ ____________ 53, 60, 61 

stantoni _____________ _________________ 58 

sweetgrassensis _______ _ 53, 61 

tetonensis --------------- 53 

ventricosus _____ _ _______________ 2, 53, 59 

vermiform is_______________ 2, 55, 56, 57 

warreni _________________________ 2, 47 

sp ______________________________ 47, 49, 58,60 

Sciponoceras gracile _____________________ 42, 43, 44 

Shark tooth_________ _______________________ 19, 23 

Spondylus sp ___ _ _____________________________ 59 

Stantonogyra silberlingz 

Tempskya grandis __________ _______________ 28 

knowltoni ____________________ 28 

Tessaro/ax hitzii ___ _ 

Trigonia sp 

Unio farri ______________ _ 

reesideanus 

Unios _______ _ 

61 

19 

9 

-------------- 9 

18 

Veniella goniophora _ _______________ 53,61 

mortoni __________________________________ 2 

Venilia mortoni__ _________________________ 2 

Watinoceras co/oradoense 43 

reesidei ____ _ _________ 42, 43 

worm burrows _______ 8, 10, 14, 17, 19, 23, 24, 39, 43 

G,H 

Glacier National Park __ 4, 7, 16, 27. 37, 40, 45, 51 

Gore Hill _______________ _ -------------------- 6. 8 

Greenhorn Formation _________ _ ---- 39 

Greenhorn Limestone __________ _ ------ 40, 42 

Highwood Creek _____________________ _ -------- 32 

Highwood Mountains 

Historical summary __ 

Introduction _______ _ 

Joli Fou Formation 

I,J, K 

16, 40, 41 

2 

2 

16 

INDEX 

Joli Fou Shale _ 

Kevin Member _ 

Kevin Shale Member _______ _ 

Kevin-Sunburst dome 

Page 

------------- 19 

------ 2, 46, 50 

---------- 5 

5, 27, 32, 33, 36, 37, 40, 44, 46, 50, 52, 53 

Kiowa Shale _______________________________ 10, 19 

Kootenai Formation _____________ 4, 6, 7, 10, 13, 14 

L 

Lake Creek ----------- 45 

M 

MacGowan Concretionary Bed _______________ 52. 53 

Madison Limestone 46 

Mannville Formation_______ _ __________ 16 

Marias River Shale ________ _____________ 2, 5, 36 

Missouri River _______________ _ ___ 6, 16, 27, 32 

Montana Group ___ _____________ 4 

Mosby Sandstone Member ________ _ _____ 37, 39, 40 

Mowry Shale__________ 28, 31, 34, 37, 41 

Muddy Creek __ 16, 27, 32, 33, 37, 40, 45 

N 

Newcastle Sandstone _____________________ 15, 19, 28 

Newman Spring _______________ 15 

Niobrara Division, Formation No.3 _____ ________ 3 

Niobrara Formation 

Niobrara River _____________ _ 

p 

5, 50, 52 

---- 3 

Phosphatic pebbles____ ______________ 51, 53, 55 

Fincher Creek gas field ______ _ _______ 46 

Pondera oil field_____ _ ___ 10, 15, 27. 37, 44, 50 

R 

Raglan Butte___ _____________ _____________ 41 

References cited ______________________________ 61 

Page 

s 

St. Mary River_____________________ _ _____ 9 

Sage Breaks Member ____________________________ 47 

San Juan Basin _ _ _______________________ 47 

Second Whitlash sand ___________________________ 16 

Skull Creek ------------------------------------ 28 

Skull Creek Shale_______________ 10, 16, 19 

Smith River ___________ ________________________ 7 

Smoky Hill Chalk Member ________________ 50, 53, 55 

South arch ______________ 6, 15, 16, 33, 36, 37, 44, 46 

South Platte Formation____________ _ ____ 10 

Spinney Hill Member _______________________ 16, 19 

Stebinger, Eugene, quoted________ _______________ 5 

Sun River 4, 5, 6, 8, 9, 15, 16, 27, 32, 37, 40, 45, 46, 50 

Sweetgrass arch __ 2, 4, 6, 9, 15, 27, 33, 36, 40, 50, 52 

Sweetgrass Hills ____ ____________ 16, 27, 37, 44, 50 

T 

Taft Hill Glauconitic Member___ _______________ 5 

Taft Hill __________________________________ 27,32 

Taft Hill Member __________________ 2, 13, 15, 30, 31 

Telegraph Creek Formation _______________ 4, 52, 55 

Thermopolis Shale ______________________________ 19 

Turner Sandy Member ___ _ ____________ 47 

Two Medicine Creek _____________ 52 

v 

Vaughn Bentonitic Member ___________________ 5, 25 

Vaughn Member _________________ _ __ 2, 16, 25, 33 

Viking Sandstone _______________________________ 20 

Virgelle Sandstone Member__ ____________ ______ 4 

w 

West Utopia oil and gas field ___________ _ 37, 44, 50 

Whitlash dome _________ _ --------- 16 

Williston basin___________ _____________ _ ______ 16 

Willow Creek _______ _ _______________ 41 

WolfCreek _________________ 7, 26, 27, 38, 45, 46,50