Depositional Environments for 

Strata of Composite Section of 

Frontier Formation, Madison Range, 

Southwestern Montana 

U.S. GEOLOGICAL SURVEY BULLETIN 1949 
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Depositional Environments for 

Strata of Composite Section of 

Frontier Formation, Madison Range, 

Southwestern Montana 

By R.G. TYSDAL 

U.S. GEOLOGICAL SURVEY BULLETIN 1949 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

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purposes only and does not imply endorsement by the U.S. Government. 

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Library of Congress Cataloging-in-Publication Data 

Tysdal, Russell G. 

Depositional environments for strata of composite section of Frontier Formation, 

Madison Range, southwestern Montana I by R.G. Tysdal. 

p. em. - (U.S. Geological Survey bulletin ; 1949) 

Includes bibliographical references (p. ). 

Supt. of Docs. no.: I 19.3:1949 

1. Geology, Stratigraphic-Cretaceous. 2. Geology-Montana-Madison 

Range. 3. Frontier Formation. I. Title. II. Series. 

QE75.B9 no. 1949 

[QE685] 

557.3 s-dc20 90-19939 

[551.7'7'09786] CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Purpose of study 3 

Previous work 3 

Lincoln Mountain measured section 3 

Lower delta front facies 4 

Sandstone and siltstone 4 

Interpretation 7 

Delta plain facies 7 

Tuff 7 

Interpretation 7 

Mudstone and siltstone 8 

Interpretation 8 

Clayey sandstone 8 

Interpretation 8 

Finely laminated sandstone 8 

Interpretation 8 

Limestone coquina 9 

Interpretation 9 

Upper delta front facies 9 

Rhythmically interbedded sandstone and siltstone or mudstone 9 

Interpretation 9 

Contact relationships of facies in Lincoln Mountain measured section 9 

Big Sky measured section 10 

Interbedded delta front and prodelta facies 10 

Rhythmically interbedded sandstone and siltstone or mudstone 10 

Interpretation 13 

Hummocky crossbedded sandstone 13 

Interpretation 13 

Bioturbated muddy fine-grained sandstone 15 

Interpretation 15 

Delta front facies 15 

Tabular and trough crossbedded salt-and-pepper sandstone 15 

Interpretation 15 

Delta plain facies 15 

Bioturbated salt-and-pepper sandstone 15 

Interpretation 16 

Carbonaceous mudstone 16 

Interpretation 16 

Clayey siltstone and mudstone 16 

Interpretation 16 

Tuff and bentonite 16 

Interpretation 17 

Laminated and burrowed sandstone 17 

Interpretation 17 

Contents Ill 
Claystone 18 

Interpretation 18 

Conglomerate, sandstone, and lenticular-bedded siltstone 18 

Interpretation 20 

Contact relationships of facies in Big Sky measured section 20 

Conclusions 20 

References cited 21 

FIGURES 

1. Map of southwestern Montana showing geographic features and location of 

measured sections of the Frontier Formation in the Madison Range 2 

2. Sequence of Cretaceous formations directly above and below Frontier 

Formation in Madison Range 2 

3. Generalized graphic measured section of lower part of composite section of 

Frontier Formation, measured at Lincoln Mountain 4 

4. Generalized graphic measured section of upper part of composite section of 

Frontier Formation, measured near Big Sky 5 

5. Schematic diagram of delta showing environments interpreted from strata of 

composite measured section 6 

6. Classification of sedimentary environments 7 

7-11. Photographs showing: 

7. Rhythmically interbedded strata overlain and underlain by hummocky 

cross-laminated sandstone 11 

8. Rhythmically interbedded sandstone, siltstone, and mudstone 11 

9. Thick sandstone beds in rhythmically interbedded strata 12 

10. Ball-and-pillow structure in rhythmically interbedded strata 12 

11. Hummocky cross-laminated sandstone 14 

12. Sequence of sedimentary structures in hummocky crossbedded strata 14 

13-14. Photographs showing: 

13. Ball-and-pillow structure in tuff bed of delta plain strata 17 

14. Tidal channel sandstone and mudstone in upper part of delta plain 

strata 19 

IV Contents 
Depositional Environments for Strata of 

Composite Section of Frontier Formation, 

Madison Range, Southwestern Montana 

By R.G. Tysdal 

ABSTRACT 

The Frontier Formation was measured in two partial 

sections in the Madison Range in order to obtain nearly 

complete exposures for stratigraphic and sedimentologic 

interpretation. The two sections form a composite section for 

the Frontier. The lower section of the Frontier, measured at 

Lincoln Mountain where it is 102m (336ft) thick, is a tripartite 

succession. The lower part consists mainly of sandstone and 

bioturbated sandstone deposited in a lower shoreface 

environment of a delta front facies. These strata are 

conformable with the underlying Mowry Shale. The middle 

part is composed ofthin interbedded strata laid down in a delta 

plain setting. Tuff of the middle part forms a marker unit of 

resistant outcrops and likely was deposited subaerially. 

Carbonaceous mudstone forms marsh deposits that grade into 

sandstone interpreted as deposits of a sandflat marginal to a 

lagoon or bay. At one horizon in the middle part, a gently 

dipping, planar sandstone that contains a few percent of 

well-rounded quartz grains crops out and is interpreted as an 

overwash-fan deposit. Limestone coquina composed of 

brackish-water bivalves caps the middle part and is interpreted 

as a deposit of a bay or lagoon. The upper part of the sequence 

present at Lincoln Mountain is composed of rhythmically 

interbedded mudstone, silty mudstone, and sandstone 

deposited in a delta front environment, possibly a mouth-bar 

deposit. An unconformity, which represents much of the 

Cenomanian as well as early Turonian time, exists between the 

delta front deposits and those of the underlying delta plain. 

The upper section of the Frontier was measured near the 

hamlet of Big Sky, where it is about 140m (459ft) thick. Strata 

deposited in interbedded prodelta and delta front facies make 

up most of the section. Delta front clastic rocks include several 

units of hummocky cross-stratified sandstone, some of which 

contain abundant trace fossils, and are interpreted as storm 

deposits. Thinly interbedded siltstone, fine-grained sandstone, 

and mudstone are interpreted as deposits of a prodelta slope 

that formed directly seaward of the lower shoreface of a delta 

Manuscript approved for publication, June 26, 1990. 

front. Some of the individual sandstone beds within the 

prodelta strata show hummocky crossbeds, interpreted as 

single-event distal storm deposits. Delta front sandstones of the 

foreshore and backshore record upward shallowing 

environments. 

A delta plain facies forms the upper 25 m (82 ft) of the 

measured section near Big Sky. The strata consist of carbonaceous to coaly mudstone, siltstone and claystone, and tuff. 

These rocks include abundant fresh and brackish water 

megafossils and sedimentary structures indicative of delta 

plain lakes and brackish water swamps. The tuffs range from 

well indurated, silicified beds to deeply weathered bentonitic 

strata. Some of the tuffs were deposited within water, showing 

ball-and-pillow structures, whereas other beds may have been 

deposited subaerially. Uppermost strata of the delta plain 

sequence constitute an estuarine tidal channel deposit that 

consists, in ascending order, of pebble conglomerate, finegrained sandstone that contains siltstone to mudstone interbeds, and lenticular bedded mudstone and clayey siltstone. 

These strata are erosionally based in a mudstone interpreted as 

a marsh deposit. An unconformity marks the contact of the 

Frontier Formation with the overlying Cody Shale. 

INTRODUCTION 

The Madison Range lies in the Rocky Mountain 

foreland of southwestern Montana (fig. 1) and contains as 

much as 4,600 m (15,000 ft) of Phanerozoic sedimentary 

strata, which overlie Archean metamorphic rocks. Geologic 

studies in the Madison Range were undertaken during 

evaluation of much of the range for inclusion in the National 

Wilderness System. An understanding of the Cretaceous 

strata was a prerequisite to deciphering structural features of 

the range. Detailed measurement of Cretaceous sections 

permitted correlation of formations throughout the map 

area. Study of the Frontier Formation led to recognition 

ofdepositional changes, time constraints, and, ultimately, 

depositional environments of the strata. 

Introduction 
Cretaceous rocks, virtually all clastic rocks, make up 

about half of the Phanerozoic sequence in the Madison 

Range (fig. 2). Sediments of the Frontier were derived from 

terrane undergoing folding and thrusting in the Sevier 

orogenic belt in the hinterland to the west. These sediments 

were deposited in the western part of the Western Interior 

Seaway in a deltaic setting, which existed in the region of 

the Madison Range (McGookey and others, 1972). 

The Frontier Formation in the Madison Range 

generally is poorly exposed, and nowhere is the formation 

exposed in one continuous sequence. The formation is about 

217 m (713 ft) thick in the composite measured section 

presented here. The lower part of the composite section is 

exposed at Lincoln Mountain, about 8 km (5 mi) west of 

Montana State Highway 191 (fig. 1 ). The upper part of the 

composite section is exposed in a lengthy roadcut about 3 

km (2 mi) west of the village of Big Sky. Exposures 

Tobacco 

Root 

Mountains 

111" 

BS•o Big Sky 

20 MILES I 

I 

30 KILOMETERS 

Bridger 

Range 

0 Livingston 

Beartooth 

Mountains 

Figure 1. Index map of southwestern Montana showing 

mountain ranges and locations of measured sections referred 

to in text. Abbreviations for locations are as follows: LM, 

Lincoln Mountain; BS, Big Sky; ME, Mount Everts. The Lincoln 

Mountain locality is site of measured section shown in figure 3, 

and the Big Sky locality is site of measured section shown in 

figure 4. 

SYSTEM SERIES FORMATION 

Telegraph Creek Formation 

Upper Cody Shale 

Frontier Formation 

CRETACEOUS Mowry Shale -?-

Muddy Sandstone 

Lower Thermopolis Shale 

Kootenai Formation 

Figure 2. Sequence of Cretaceous formations 

present directly above and below Frontier 

Formation in the Madison Range. Queried 

boundary of Lower and Upper Cretaceous rocks is 

based on work of Cobban and Kennedy (1989). 

showing the contact of the two parts of the composite 

section were not found during mapping in the range. 

The two measured sections are interpreted as 

comprising a composite section of the Frontier on the basis 

of their megafauna! content (Tysdal and others, 1990). 

Megafossils from the top of the Lincoln Mountain section 

are of the early middle Turonian Collignoniceras woollgari 

faunal zone. Megafossils obtained from near the middle of 

the Big Sky section are fragmented and not abundant, but 

they are interpreted to represent the late middle Turonian 

Prionocyclus hyatti faunal zone (W.A. Cobban, in Tysdal 

and others, 1990). W.A. Cobban (oral commun., 1989) said 

a possibility exists that the faunal remains could be of forms 

that are from the older C. woollgari zone, but this is not the 

favored interpretation. If the latter possibility is true, then 

the two sections could contain some time-equivalent strata, 

except for uppermost strata of the Big Sky section from 

which fossils of the P. hyatti zone were obtained. The 

sedimentological interpretations presented here would still 

pertain, but the lateral relationships of any time-equivalent 

strata, which would be of different depositional 

environments, are unknown. The two sections do not share 

a key bed or like sequences. 

Many of the data for this report were obtained from 

the two measured sections that are graphically displayed in 

figures 3 and 4. Environments represented in the composite 

measured section are interpreted within a deltaic framework 

present in the region at the time of deposition, even though 

the composite section by itself does not permit the 

reconstruction of the lateral relationships over long 

distances. Figure 5 shows a schematic deltaic setting 

illustrating the depositional enviroments used for 

interpretation of the strata present in the composite measured section. 

The graphic measured sections of figures 3 and 4 

were generated from the Stratigraphic Report Graphic, a 

stratigraphic applications computer program developed by 

the U.S. Geological Survey and Petroleum Information 

Corporation of Denver, Colo. (Petroleum Information 

Corporation, 1984; Dyman and others, 1985). The program 

2 Depositional Environments, Frontier Formation, Madison Range, Montana 
is proprietary to Petroleum Information Corporation, but it 

is available by contract to the U.S. Geological Survey for a 

variety of research applications. 

Megafossils discussed in this report were identified 

by W.A. Cobban and, except where specifically noted, their 

names were published previously in Tysdal and others 

(1990). 

Purpose of Study 

The Frontier Formation in the Madison Range affords 

a key composite measured section for regional stratigraphic 

correlation. The Madison Range composite section is about 

80 km (50 mi) southwest of outcrops near Livingston, 

Mont.; 55 km (35 mi) west of outcrops at Mount Everts in 

the northernmost part of Yellowstone National Park; and 

200 km (125 mi) northwest of outcrops in the Jackson Hole 

area of northwestern Wyoming, south of the Park (fig. 1). 

Frontier strata have been eroded from most of the region 

between the Madison Range and these other areas, although, 

locally, Frontier strata are concealed beneath younger 

sedimentary and volcanic rocks. Strata of the Big Sky 

measured section are younger than those of sections in some 

areas to the east, facies differ from some of those present in 

other areas, and the strata are distant from those of other 

areas. Frontier strata in the Greenhorn Range, about 55 km 

(35 mi) west of the Madison Range, are much thicker and 

were deposited in marginal marine and nonmarine 

environments (Tysdal and others, 1990). The purpose of this 

report is to describe the strata of the composite measured 

section and to interpret their depositional environments. 

In this report, "lithologic unit" is used as a nongenetic 

term to describe a specific kind of rock (for example, 

sandstone, limestone, tuff); or to describe interbedded rocks 

of different kinds (for example, interbedded sandstone and 

mudstone) that, combined, form a distinct rock unit. The 

term has virtually the same meaning as the informal 

lithostratigraphic unit called a lithosome (Bates and 

Jackson, 1987, p. 384), which has fallen into disuse in 

recent years. The Lincoln Mountain section (fig. 3) and the 

Big Sky section (fig. 4) each is composed of lithologic units 

that are numbered sequentially from bottom to top. 

Depositional environments are interpreted for each of the 

lithologic units, which are placed within a deltaic 

framework facies (for example, delta plain facies) as shown 

in figure 6. Contact relationships for the facies are discussed 

for each of the measured sections. 

Previous Work 

The Frontier Formation in the Madison Range was 

first distinguished from other Cretaceous strata by Tysdal 

and Simons ( 1985) during mapping in the central and 

southern part of the range. The lower contact of the Frontier 

was placed at the base of the first sandstone unit above 

dark-gray mudstone and silty mudstone assigned to the 

Mowry Shale (Tysdal and others, 1989a, b). A graphic 

composite section and description of rock units that make 

up the formation were presented in Tysdal and others 

(1990). 

Many studies in the Madison Range were mapping 

projects in which Frontier strata were not distinguished 

from most other Cretaceous rocks. Frontier strata are part of 

a unit mapped as "Colorado and Montana formations" 

(Peale, 1896), Colorado Group (Becraft and others, 1970; 

Garihan and others, 1983), and an unnamed unit (Swanson, 

1950). Along the western flank of the central part of the 

Madison Range, Hadley (1969, 1980) used the name 

Frontier Formation for a thick succession of strata that 

included the Frontier Formation as used here, but also he 

included several formations that underlie and overlie the 

Frontier. In the east-central part of the Madison Range, 

much of the previous work constituted fairly detailed thesis 

studies. Hall (1961), Lauer (1967), Ray (1967), Rose 

(1967), Bolm (1969), Kehew (1971), and Walsh (1971) 

used Cretaceous strata undivided or undifferentiated for 

rocks that included part or all of the Frontier Formation. 

Some of the lower Frontier strata at Lincoln Mountain 

were assigned to the Mowry Shale by Wilson (1970) and 

Vuke (1982, 1984). Schwartz (1972) assigned the same 

lower Frontier strata to his Blackleaf Formation unit D 

which he equated with the Mowry. Austin and Stoeve; 

( 1950) assigned the same Frontier strata to the Aspen(?) 

Formation, a name used in western Wyoming and eastern 

Idaho. These assignments of lower Frontier strata to other 

formations were made, at least in part, to include porcellanitic tuff (fig. 3, lithologic units 13, 14) that lies about 33m 

(108ft) above the base of the Frontier as mapped by Tysdal 

and Simons (1985) and Tysdal (1990) and as defined in 

some detail by Tysdal and others (1989a, b; 1990). Pastelcolored porcellanitic and bentonitic strata are common in 

the lower part of the Mowry Shale, 84 m (274ft) beneath 

the base of the Frontier at Lincoln Mountain. The above 

thesis authors probably thought that the porcellanitic tuff of 

units 13 and 14 of the Frontier represented additional 

volcanic material of the Mowry. The pastel strata were 

assigned to the Vaughn Member of the Mowry Shale by 

Tysdal and others (1989b ). 

LINCOLN MOUNTAIN 

MEASURED SECTION 

Delta front and delta plain facies are represented in 

strata that make up the section of the lower part of the 

Frontier measured at Lincoln Mountain. The strata are 

Lincoln Mountain Measured Section 3 
EXPLANATION 

DESCRIPTION 

Granitic rocks 

Conglomerate 

Sandstone, massive 

Sandstone, laminated 

Sandstone, rippled 

Sandstone, crossbedded 

Limestone 

Siltstone 

Shale 

Mudstone 

Tuff 

Bentonite 

Covered 

Figure 3. Graphic log of lower part of Frontier 

Formation, showing facies interpreted for sequences of 

lithologic units. Arrow to right of each lithologic unit 

number points to middle of unit. The section was 

measured in gently dipping strata on Lincoln Mountain, 

NW1/4NW1/4 sec. 7, T. 9 S., R. 4 E., Lincoln Mountain 

7.5-minute quadrangle. 

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

Lower Delta Front Facies 

Sandstone and Siltstone 

LITHOLOGIC 

UNIT 

NUMBER 

LITHOLOGY 

3H-...... T :~~===Upper 

delta 

front 

Delta 

plain 

facies 

~-~=Lower 

delta 

front 

-1]212~~ facies 

~~~j_ 

transitional with the underlying Mowry Shale, but the top of 

the uppermost strata of the section is a present-day erosional 

surface. The lithologic unit numbers referenced in the text 

concerning the measured section at Lincoln Mountain 

correspond to those in figure 3 and to the those presented in 

Tysdal and others (1990). 

This facies, about 13 m (43 ft) thick, consists of 

interbedded sandstone and lesser siltstone and mudstone 

4 Depositional Environments, Frontier Formation, Madison Range, Montana 
LITHOLOGIC 

UNIT 

NUMBER 

FEET METERS LITHOLOGY 

0 0 -----..----

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

275 

300 

325 

350 

375 

400 

Delta 

plain 

facies 

Delta 

front 

facies 

Interbedded 

delta front and 

prodelta facies 

10 

20 

30 

52 

51 

50 

49 

48 

47 

46 

45 

43~-~~ 4140 

39 

38~=.;._.,., 

37 

3635 ~~==-

34 

33 

32 

31 

30 

29 28 

27 ..F==-..,-,--, 

EXPlANATION 

liTHOLOGY DESCRIPTION 

Granitic rocks 

Conglomerate 

Sandstone, massive 

Sandstone, laminated 

Sandstone, rippled 

Sandstone, crossbedded 

Limestone 

Siltstone 

Shale 

Mudstone 

Tuff 

Bentonite 

Covered 

Figure 4. Graphic log of upper part of Frontier 

Formation, showing facies interpreted for sequences of 

lithologic units. Arrow to right of each lithologic unit 

number points to middle of unit. The section was 

measured in gently dipping strata 3.7-5.6 km (2.3-3.5 

mi) west of the Post Office in the village of Big Sky, 

Mont., in roadcuts in the SE1/4 sec. 28 and SW1/4SW1/4 

sec. 27, T. 6 S., R. 3 E., thence southeastward from the 

road along the ridge crest in the NW1/4 sec. 34, T. 7 S., 

R. 3 E., Gallatin Peak 7.5-minute quadrangle. 

Lincoln Mountain Measured Section 5 
Upland 

14------D E L T A 

Washover fan 

River 

mouth 

bar 

Figure 5. Schematic diagram of a delta showing depositional environments that are interpreted to be represented 

by strata in the composite measured section of the Frontier Formation in the Madison Range. 

6 Depositional Environments, Frontier Formation, Madison Range, Montana 
DEPOSITIONAL FACIES ENVIRONMENT 

Lake or pond 

Lagoon Delta Plain Backshore 

Dune 

Foreshore 

Upper shoreface Delta Front 

Lower shoreface 

Offshore Prodelta 

Figure 6. Diagram showing the facies classification 

scheme used in this report and the corresponding 

depositional environments interpreted for strata of the 

Frontier Formation in the Madison Range. 

(lithologic units 1-8). The units are generally 1-2m (3-7ft) 

thick and record an abrupt change in grain size from the 

mudstone and clayey siltstone that characterize the 

underlying Mowry Shale. The sandstone is greenish gray 

and is composed of quartz, feldspar, chert, rock fragments, 

mica, and locally glauconite. Sorting ranges from good to 

poor, with some strata (fig. 3, lithologic units 4, 5, and 7) 

consisting of grains that range from very fine to fine and, 

locally, medium size, along with clay. Beds are commonly 

thin, planar, and finely laminated. Sandstone of units 1 and 

3 is thin to medium bedded and displays both plane beds 

and low-angle crossbeds. Dark-brown to gray carbonaceous 

mudstone and clayey siltstone, like that in the upper part of 

the underlying Mowry Shale, occur locally (unit 2) in the 

lower part of the sequence. Siltstone and clayey siltstone 

units are thinly bedded and finely laminated to rippled. 

Ophiomorpha burrows are abundant in lithologic unit 

5 and so are fragments of megafossils and plants. Bioturbation has disrupted most of the laminae of units 4, 5, and 

7. The following megafossils, which are not age-diagnostic, 

were collected from unit 1 by Tysdal and others (1990): 

Oxytoma sp., Camptonectes sp., Aphrodina sp., Goniomya 

sp., Lingula subspatulata Hall and Meek, a juvenile ammonite of indeterminate species, and fish bones. Unit 6 yielded 

a palynomorph assemblage of mixed marine and nonmarine 

fossils (Tysdal and others, 1990). This palynomorph 

assemblage, and those from the underlying Mowry Shale 

that were previously assigned to the Albian by Tysdal and 

others (1989a, b), are considered to be early Cenomanian in 

age. The early Cenomanian age is based on a reinterpretation of molluscan megafossil data of the Cretaceous 

Western Interior (Cobban and Kennedy, 1989); assignments 

for palynomorphs assemblages are based on correlation 

with the Cretaceous molluscan fossil zones. 

Interpretation 

Lithologic units 1-8 form a sequence deposited in 

shallower water than that of the mudstone and clayey 

siltstone in the underlying Mowry Shale. Intertonguing of 

lithic units of differing grain size reveals a gradual upward 

increase in grain· size from underlying Mowry strata and 

reflects a gradual overall increase in wave energy. The 

sequence displays physical sedimentological features 

characteristic of lower shoreface deposits, as described by 

Reinson (1984) and Leckie and Walker (1982). The lower 

shoreface is a low energy environment where waves first 

begin to affect bottom sediments. Sands are generally fine to 

very fine grained and contain interlayered silt and clay. 

Laminae are planar but commonly are obliterated by bioturbation. The abundant fossil fragments of differing sizes 

indicates little reworking by waves. 

Megafossils present in the lowermost unit are marine 

forms. The palynomorphs of unit 6 are a mixed assemblage 

of marine and nonmarine forms. This grouping of mixed 

forms is most likely to occur in a nearshore marine setting 

where nonmarine palynomorphs constitute an admixture, 

assuming the marine fossils are not recycled. The horizontal 

boxwork pattern displayed by Ophiomorpha (lithologic unit 

5)-the burrow of a suspension-feeding animal-also is 

characteristic of the lower shoreface. Ophiomorpha has two 

general patterns of occurrence: steeply inclined shafts, 

which predominate in higher energy environments (such as 

the upper shoreface or foreshore), and boxwork mazes, 

which characterize lower energy environments (such as the 

lower shoreface) (Frey and others, 1978). 

Delta Plain Facies 

This facies totals about 56 m (184 ft) thick and 

contains thin interbedded lithologic units deposited in 

several closely related depositional environments of a delta 

plain. 

Tuff 

Pale-yellowish-green to medium-gray indurated tuff 

(fig. 3, lithologic units 9, 11, 13, and 14) forms prominent 

outcrops in the middle part of the Lincoln Mountain section. 

Units 13 and 14 together form a marker unit that is about 20 

m (66ft) thick at Lincoln Mountain and is widespread in the 

central part of the Madison Range. The tuff is glassy 

(porcellanitic) and breaks with a conchoidal fracture. It is 

well indurated, although not welded, and contains well 

preserved shards and some nearly complete bubble 

structures. Plagioclase phenocrysts are widely dispersed 

within the tuff; biotite is a minor constituent. None of the 

phenocrysts or shards, some of which are elongate, show 

preferred orientation. 

Interpretation 

Tuffs of units 13 and 14 are interpreted as airfall 

deposits laid down subaerially on a- delta plain. The lack of 

preferred orientation of the constituents of the tuff, and 

Lincoln Mountain Measured Section 7 
preservation of delicate elongate shards and nearly complete 

bubbles within glass, indicate that the constitutents did not 

undergo sorting by water. The tuffs of units 9 and 11 were 

not examined in thin section; thus, their internal fabric is 

unknown, and it is uncertain if the tuffs were deposited 

subaerially or in water. None of the units show balland-pillow structures, which would indicate deposition on a 

muddy, water-saturated substrate. 

Mudstone and Siltstone 

Dark-gray to olive-brown mudstone, clayey siltstone, 

and shale make up most of the strata of lithologic units 10, 

15, 16, 20, and 22 (fig. 3). Thin interbeds of bentonite are 

present in units 10, 15, and 16. Each of the five units is 1-2 

m (3--7 ft) thick and is poorly exposed. The rocks contain 

comminuted carbonaceous debris and generally lack 

sedimentary structures that are visible in the outcrop or hand 

specimens. Clayey siltstones contain thin interbeds of finely 

laminated bentonite (weathered tuft). Unit 16 contains a 

freshwater gastropod, Carinorbis sp., and upper units are 

closely associated with strata that contain brackish water 

fossils (see next section), but no fossils were recovered from 

these other units. 

Interpretation 

The mudstones and clayey siltstones were laid down 

where nonmarine and brackish water environments merge 

and are interpreted as deposits of the lower part of a marsh 

that fringed a restricted bay or a lagoon. The interlayered 

bentonites of units 10, 15, and 16 are subaerial tuffs, and 

unit 16 contains a nonmarine fauna. The upper two litho· 

logic units (20 and 22) of the lithofacies are interbedded 

with strata that contain a brackish water fauna and are 

thought to be deposits of a restricted bay. The siltstone of 

unit 16 is thoroughly burrowed. Preservation of the fine 

layers of bentonite in some of the lower rock units at 

Lincoln Mountain suggests a quiet depositional 

environment that lacked reworking by burrowing organisms 

or plants. This lack of reworking could indicate deposition 

in an anoxic environment, such as a pond, or perhaps 

deposition of the volcanic ash caused temporarily 

unfavorable living conditions for infauna. 

Clayey Sandstone 

Sandstone strata included here range from clay-rich to 

clay-poor and are 1-1.5 m (3-5ft) thick. Lithologic unit 23 

(fig. 3) is a clay-rich sandstone that is dark gray, fine 

grained, clayey, carbonaceous, and rubbly weathering. 

Clay-poor sandstone units 18 and 21 are medium and light 

gray, fine to medium grained, and chert- and quartz-rich. A 

resistant sandstone bed about 1 m (3 ft) thick (unit 19) forms 

the most distinctive lithologic unit and includes both clayey 

and fairly "clean" sand. Most of the bed is very dark gray to 

black sandstone that has a matrix of carbonaceous mudstone. The dark sandstone contains abundant brackish water 

fossils in a near-coquinoid assemblage of Veloritina? sp., a 

gastropod; and Corbula, aff. C. subtrigonalis Meek and 

Hayden, and Melania sp., which are bivalves. The mediumto light-gray sandstone of the upper few centimeters of unit 

19 is slightly calcareous and displays planar to wavy 

laminations. No megafossils were found in the upper part of 

unit 19, but nearly vertical burrows of the trace fossil 

Skolithos are present. The animal that produced the trace 

fossil Diplocraterion caused mixing of clay into the 

sandstone. 

Interpretation 

The fairly clean sandstones of units 18, 21, and the 

upper few centimeters of unit 19 are interpreted as intertidal 

deposits laid down in the foreshore to shoreface of a lagoon 

or a bay. Sandstones with a claystone matrix of high organic 

content (carbonaceous) (unit 23 and lower part of unit 19) 

and the brackish water faunal assemblage of unit 19 both are 

suggestive of the shallow part of a lagoon or bay, similar to 

modem lagoons described, for example, by Shepard and 

Moore (1955), Newman and Munsart (1968), and Warme 

(1971 ). The change in composition from a mixed sandstone 

and mudstone in the lower part of unit 19 to a fairly clean 

sandstone with vertically oriented trace fossils in the upper 

part indicates a change in energy conditions. The lower part 

represents quiet water deposition relative to the upper part 

that likely was directly affected by waves and (or) currents. 

The change is abrupt and is consistent with a change from 

subtidal or lower intertidal to intertidal shoreline (sandflat) 

deposit of a lagoon or bay. The sandy fossil-bearing strata 

also could have been deposited in a tidal channel that 

coursed its way through intertidal deposits (Howard and 

Frey, 1975; Thorbjamarson and others, 1985), although this 

seems less likely. 

Finely Laminated Sandstone 

Finely laminated sandstone of lithologic unit 17 (fig. 

3) is about 10 m (33 ft) thick and is composed of 

light-greenish-gray sandstone that is fine grained and poorly 

sorted. Beds are 1-3 em thick, dip 1-2°, are planar and 

finely laminated. The sandstone is composed of quartz, 

chert, feldspar, rock fragments, and glass shards. Some of 

the quartz grains are well rounded. 

Interpretation 

The strata are interpreted as a storm washover-fan 

deposit on the basis of bedding style, primary sedimentary 

structures, composition, and stratigraphic position. Thin, 

flat beds that commonly dip 1-4 o toward a fan terminus are 

8 Depositional Environments, Frontier Formation, Madison Range, Montana 
the dominant bedding style of washover fans (Hayes, 1967; 

Schwartz, 1981; Barwis and Hayes, 1985). The strata lie 

within a regressive sequence. Because washover fans occur 

at high elevations in a barrier sequence, their preservation 

potential is greatest in a regressive succession where they 

can be buried beneath prograding back-barrier sediments. 

Strata associated with unit 17 are nonmarine, downsection a 

few meters, and a bay or lagoonal sandstone containing 

brackish-water fossils a few meters upsection. Lateral 

exposures are not sufficient to observe intertonguing of the 

different stratigraphic units. 

Limestone Coquina 

The delta plain facies is capped by a prominent bed 

(fig. 3, lithologic unit 25) of light-gray limestone coquina 

composed of the brackish water mollusks Leptesthes sp., 

Veloritina sp., and Corbula sp. in a matrix of micrite. 

Disarticulated bivalves of the nonmarine mollusk Protoelliptio hamili (McLeam) were reported by Wilson (1970) 

from this unit. Residue of the limestone contains abundant 

charophytes (W.A. Bryant, USGS, oral commun., 1988). 

The bed is about 1 m (3 ft) thick and forms a marker unit. 

Interpretation 

The coquina of unit 25 is interpreted as a brackishwater limestone deposited along the margin of a restricted 

bay or perhaps along a tidal stream. Charophytes, green 

algae of which modem species reside only in nonmarine 

habitats, mixed with the brackish-water mollusks, indicate 

that fresh water carried materials to the depositional site. 

The fossils have been reworked into an assemblage of 

fragmented debris by wave or current action. Brackishwater assemblages of fragmented megafossils are common 

in the back-barrier bays of the Mississippi delta, where they 

are concentrated by storm processes (Kosters, 1989). 

Upper Delta Front Facies 

Rhythmically Interbedded Sandstone and 

Siltstone or Mudstone 

Strata of this facies (fig. 3, lithologic units 27-35) 

consist of rhythmically interbedded sandstone and mudstone about 16 m (52 ft) thick. (The 16-m- (52-ft-) thick 

covered interval of unit 26 is not assigned to a facies.) The 

overall sequence coarsens upward, however. Mudstone 

comprises a greater volume of strata of the lower part of the 

sequence composed of units 27-35 and sandstone the 

greater volume of the upper part. Mudstone ranges from 

light gray to dark olive gray, is carbonaceous, thin bedded, 

and finely laminated. The mudstone is sandy locally, but in 

the lower part of the sequence some strata are fissile, 

micaceous, and shal y. 

Sandstone ranges from olive-green to gray-green or 

gray-brown and is composed of quartz, chert, feldspar, and 

minor glauconite. The grain size commonly is fine to 

medium, although lithologic unit 35 locally is coarse 

grained and contains well-rounded chert pebbles in layers as 

much as 5 em (2 in.) thick. Most beds are only a few 

centimeters thick and show planar lamination, but a few 

beds in the upper part are as much as 50 em (20 in.) thick. 

Some sandstone beds are lenticular, others are sharp-based 

and eroded into underlying sandstone to form composite or 

amalgamated beds. The 1-m- (3 ft) thick sandstone of unit 

28, in the basal part of the sequence, grades from massive in 

the lower part; through plane-bedded in the middle; to finely 

laminated, wavy cross-laminated paper-thin layers in the 

upper part. Tool marks (flute and groove casts), oscillatory 

ripple marks, and horizontal burrows indicative of the 

Cruizana ichnofacies are common on bedding planes of 

sandstone in the upper third of the sequence of units 27-35. 

The only fossils recovered from units 27-35 are of the 

middle Turonian mollusk Collignoniceras woollgari (Mantell), and some associated fish vertebrae (Tysdal, 1989; 

Tysdal and others, 1990). The fossils were found on 

bedding planes of the uppermost sandstone, which also 

contained tool marks. 

Interpretation 

Strata that make up the uppermost part of the Lincoln 

Mountain section are interpreted as delta-front sandstones, 

possibly distributary mouth-bar deposits, laid down directly 

shoreward of a prodelta slope. Rhythmic interbedding of 

fine-grained sandstone and mudstone or siltstone are typical 

features for this environment, as are graded sandstone beds 

(unit 28) (Allen, 1965). Most beds lack the grading 

displayed by unit 28, but both the grading and tool marks, 

displayed on several beds, would be expected in a setting 

where sediment was deposited rapidly from a turbid plume 

of river water debouching onto a delta front. Oscillatory 

ripple marks and trace fossils of the Cruizana ichnofacies 

both are features common to low wave-energy environments. 

Contact Relationships of Facies in 

Lincoln Mountain Measured Section 

The basal contact of the lower delta front facies of the 

Frontier Formation at Lincoln Mountain clearly is 

conformable with strata of the underlying Mowry Shale. 

Strata of the two formations are gradational, and the change 

Lincoln Mountain Measured Section 9 
in nomenclature was determined on the basis of a fairly 

abrupt influx in silt and sand content from mudstone of the 

Mowry to sandstone assigned to the Frontier (Tysdal and 

others, 1990). 

The contact of the lower delta front facies and the 

overlying delta plain facies is a hiatus, based on faunal 

content and interpretation of sedimentary environments. 

The upsection change in facies-from offshore strata in the 

upper part of Mowry Shale, to delta front strata (lower 

shoreface environment) of the Frontier, to delta plain strata 

of the Frontier-is a regressive sequence. Absence of upper 

shoreface and foreshore strata between the lower Frontier 

and delta plain sequences suggests that strata of these 

environments were deposited and later eroded. This could 

have been accomplished by a drop in sea level and erosion 

of the upper shoreface and foreshore deposits. Strata of the 

delta plain would have shifted seaward. With this 

interpretation, strata of the delta plain facies are closely 

related genetically and in time to the underlying Frontier 

and Mowry strata. 

The contact of the regressive delta plain facies with 

the overlying upper delta front facies at Lincoln Mountain is 

not exposed. However, the marked contrast in depositional 

environments-brackish-water deposits of the uppermost 

delta plain facies to marine deposits of the delta front 

facies-almost certainly requires that a environmental 

change exist within the 16-m- (52-ft-) thick covered interval 

(unit 26) between the two facies. Further, the delta front 

facies is of middle Turonian age, in contrast to the early 

Cenomanian age of the regressive strata of the underlying 

part of the Frontier. The major hiatus between the two facies 

corresponds to the lacuna of the early Cenomanian-middle 

Turonian unconformity reported by Merewether and Cobban (1986) in western Wyoming and northward into central 

Montana (near Townsend, fig. 1). 

BIG SKY MEASURED SECTION 

Deltaic strata of prodelta, delta front, and delta plain 

facies crop out in the section of the upper part of the 

Frontier, measured near Big Sky. A covered interval and a 

thick granitic sill lie at the base of the section. Rocks 

beneath the sill, including strata correlatable with the lower 

section at Lincoln Mountain, are not exposed. The lithologic 

unit numbers referenced in the text concerning the section 

measured near Big Sky correspond to those in figure 4 and 

to those presented in Tysdal and others (1990). 

Interbedded Delta Front and 

Prodelta Facies 

Rocks of the lower part of the delta front and upper 

part of the prodelta facies form repeated lithologies within a 

progradational (regressive) marine sequence. The facies is 

87 m (285 ft) thick near Big Sky, making up the major part 

of the measured section. 

Rhythmically Interbedded Sandstone and 

Siltstone or Mudstone 

This recurring assemblage of rocks (fig. 4, lithologic 

units 7, 9, 11, 13, 14, 15, 20, 21, 23, and 24) consists of 

rhythmically interbedded siltstone or mudstone and lesser 

amounts of fine-grained sandstone (figs. 7 and 8). Two thin 

beds of bentonite (lithologic units 10 and 12) occur within 

the rhythmically interbedded sequence. The rhythmically 

interbedded strata are 1-7m (3-23ft) thick. Lithologic units 

7, 9, 11, and 20 contain thin-bedded finely laminated 

siltstone and mudstone; sandstone is absent. Beds 

commonly range between 1 em (0.4 in.) and about 10 em (4 

in.) thick, but a few are as much as 20 em (8 in.) thick (fig. 

9). Some of the sandstone fills erosional scours into the 

underlying siltstone or mudstone, forming sandstone lenses 

a few centimeters to tens of centimeters across. Pale-colored 

fine-grained sandstone traced laterally locally intertongues 

with dark siltstone, such that the sandstone is really a 

composite or amalgamated bed. Similar relationships were 

described by Swift and others (1987) in Cretaceous strata of 

central Utah. 

The thickness and abundance of sandstone beds 

decrease upward; above unit 19, the lower few meters of 

these units contain more than 50 percent sandstone. Some 

sandstone beds only a centimeter or two thick are hummocky cross-stratified, and the clayey siltstone that overlies 

each sandstone bed reflects this microtopography. In other 

sandstone beds, hummocks grade laterally and upward into 

plane-bedded sandstone, which is overlain by siltstone and 

(or) mudstone. The base of each sandstone bed commonly is 

sharp, overlying mudstone. 

A few groove casts were observed at the base of 

sandstone beds and graded sandstone beds occur locally. 

Horizontal burrows were found on a few beds, and vertical 

burrows occur in hummocky sandstone beds in a few places, 

but neither type of burrow is common. Chondrites burrows 

are present locally in mudstones that overlie beds of 

hummocky cross-laminated strata but also are not common. 

Ball-and-pillow structures (fig. 10) form a zone about 

50 em (20 in.) thick in the lower part of unit 21 of the 

rhythmically interbedded strata. These ellipsoidal pillowshaped structures formed from a bed of fine-grained 

sandstone that became segmented and foundered into the 

underlying silty mudstone. In some cases, the segmentation 

was not completed and some of the pillows are still 

connected. The pillows are fairly evenly spaced and range 

from 20--30 em (8-12 in.) thick and from 100--125 em 

(39-49 in.) wide; the length of the structures could not be 

determined. The laminae within each pillow are concave 

upward, parallel to the outline of the pillow and parallel to 

1 0 Depositional Environments, Frontier Formation, Madison Range, Montana 
Figure 7. Rhythmically interbedded sandstone, siltstone, and mudstone of lithologic units 23 and 24 (dark gray) between 

light-gray hummocky cross-laminated sandstone of lithologic units 22 and 25. Outcrop is about 25 m (82ft) high roadcut in Big 

Sky measured section, SW1/4SW1f4 sec. 27, T. 6 S., R. 3 E., Gallatin Peak 7.5-minute quadrangle. 

Figure 8. Rhythmically interbedded sandstone, siltstone, and mudstone of lithologic unit 24. Hammer handle 

is 30 em (12 in.) long. Outcrop is in roadcut of Big Sky measured section, SW1/4SW1/4 sec. 27, T. 6 S., R. 3 E., 

Gallatin Peak 7.5-minute quadrangle. 

the flow structure displayed by the surrounding mudstone. 

The upper part of each pillow has been eroded: nearly 

vertical laminae in the outer part of each pillow, as well as 

the vertical flow structure of the mudstone between the 

pillows, is overlain by undeformed, nearly horizontally 

bedded silty mudstone or fine-grained sandstone. 

Big Sky Measured Section 11 
Figure 9. Thick sandstone beds, some of which contain hummocky crossbeds, in rhythmically interbedded strata of 

lithologic unit 21. Hammer handle is 30 em (12 in.) long. Outcrop is about 3 m (10ft) high, in roadcut of Big Sky measured 

section, SW1

/4SW1/4 sec. 27, T. 6 S., R. 3 E., Gallatin Peak 7.5-minute quadrangle. 

Figure 10. Ball-and-pillow structures in lower part of lithologic unit 21 of rhythmically interbedded strata. Note upturning 

of sandstone along margins of pillows, opposite both ends of hammer head. Hammer handle is 30 em (12 in.) long. Outcrop 

is in roadcut of Big Sky measured section, SE1/4SE1/4 sec. 27, T. 6 S., R. 3 E., Gallatin Peak 7.5-minute quadrangle. 

12 Depositional Environments, frontier formation, Madison Range, Montana 
Interpretation 

The depositional environment of the rhythmically 

interbedded sandstone and siltstone or mudstone is 

interpreted to be a prodelta slope, where it is transitional 

with the lower shoreface environment of the delta front 

facies. Hummocky cross-stratification (unit 21) in finegrained sandstone is a storm wave-generated feature; hence, 

the feature formed above storm wave base but below 

fair-weather wave base (Harms and others, 1982). The few 

graded beds and tool marks found may have been produced 

by storm processes as well. 

Ball-and-pillow structures are characteristic of deltafront facies (Howard and Lohrengel, 1969; Hubert and 

others, 1972). The structures form during soft sediment 

deformation when a dense sand layer is deposited on top of 

a less dense, loosely packed water-saturated layer of mud. 

Gravitational instability causes the less dense watersaturated mud to flow upward, disrupting the sand layer into 

segments that were deformed into pillow shapes (Allen, 

1982, p. 363 ). 

Eroded tops of the ball-and-pillow structures in the 

Big Sky section indicate that the structures developed 

shortly after deposition of the sandstone bed from which 

they formed, and prior to burial beneath the overlying layer 

of strata. Sandstone of the ball-and-pillow structures was 

examined for hummocky crossbedding, which would 

indicate rapid deposition of the sand. Rapid deposition 

would have facilitated flowage and formation of the balland-pillow structures (Chan and Dott, 1986). No hummocky 

crossbedding was found, although hummocky sandstone 

lies directly upsection. 

Hummocky Crossbedded Sandstone 

. Sandstone beds of several lithologic units (fig. 4, 

umts 2, 3, 4, 5, 6, 16, 17, 18, 22, and lower part of unit 25) 

range from 2 to 8 m (6.5 to 26 ft) thick in the measured 

section of the upper part of the Frontier and all except units 

2 and 3 (described and discussed later) show features typical 

of hummocky cross-stratification as described by Harms 

and others ( 197 5): ( 1) cross bed sets display lower bounding 

surfaces that are erosional, commonly sloping at angles of 

less than 15°; (2) overlying laminae are parallel to set 

boundaries, or nearly so; (3) laminae above the erosional 

base systematically thicken laterally in a set, such that their 

traces on a vertical surface are fan-like; and (4) the direction 

of dip of erosional set boundaries are scattered. At Big Sky 

individual fan-like structures locally can be seen to fill 

depressions; laminae are nearly parallel to the erosional 

surface at the base of a crossbed set, gradually fill the 

ero~ional scour and decrease in dip until a horizontal dip is 

attamed upward, and then build upward to form a hummock 

from which the laminae dip outward. 

Laminae in the hummocky beds range from less than 

1 mm to 1 em (0.04 to 0.4 in.) thick (fig. 11) and are 

alternately cream colored and pale green. Each light-colored 

lamina is composed of well-sorted fine-grained quartzofeldspathic sand, mica, and minor clay; each pale-green lamina 

is composed of clay and minor silt and mica. The change 

from one sediment size to the other is not gradational, rather 

it is fairly abrupt. Bases of hummocky sandstone sequences 

are erosive into underlying strata. Where a base is exposed 

over several tens of meters, contact of the hummocky 

sandstone sequence cuts into the underlying silty mudstone 

to a depth of less than 1 m (3 ft). A few thin carbonaceous 

shale lenses also are present within the unit. These 

characteristics of hummocky crossbeds generally 

correspond with those observed by Bourgeois (1980), 

Hunter and Clifton (1982), and the model of Dott and 

Bourgeois (1982, 1983) that contains the four bedding types 

shown in figure 12. 

The lowermost hummocky crossbedded sandstone 

(unit 4) of the Big Sky section is underlain by massive to 

finely laminated sandstone that is faintly rippled, very fine 

grained sandstone (unit 3) and a similar lithology (unit 2) 

that contains some tiny mudchips. It is uncertain if units 2 

and 3 are genetically related to the hummocky beds, but 

they display the same alternating buff and pale-green colors, 

and the same alternating compositional makeup, as the 

cross-laminated sandstone. These strata may represent two 

additional bedding types that Walker and others (1983) 

believed should be a part of the depositional model for 

hummocky crossbeds (fig. 12). 

Bioturbated hummocky crossbedded sandstones 

(units 6 and 18) are poorly sorted and contain abundant 

organic debris. Most sedimentary structures have been 

destroyed, although a few hummocky crossbeds are 

preserved. Some of the hummocky sandstones intertongue 

with mudstone, showing that the cross stratification is an 

amalgamation of hummocky strata. Fossil debris is common 

in unit 18 and includes fish scales and fragments of 

Cyprimeria?; Inoceramus sp., possibly /. flaccidus White; 

an oyster fragment, possibly Pycnodonte; an ammonite, 

possibly Scaphites arradiensis Moreman; and the longranging brachiopod Lingula subspatula Hall. Ophiomorpha 

is the only trace fossil that was identified. 

Interpretation 

Hummocky crossbedding commonly is attributed to 

oscillatory motion of storm waves (Harms and others, 1975; 

Bourgeois, 1980). The hummocky crossbedded sandstones 

of the Frontier are interpreted as deposits of the lower 

shoreface, where they were not reworked by fair-weather 

waves. Sandstone sequences with hummocky crossbedding 

are sharp based (erosional); whereas, the uppermost part of 

each sequence is gradational into overlying siltstone, or 

siltstone or mudstone strata. These data and the intense 

bioturbation in the uppermost part of sandstone sequences 

(units 8 and 19) suggest that the hummocky cross-stratified 

sandstones were deposited rapidly into an environment 

Big Sky Measured Section 13 
Figure 11. Hummocky cross-laminated sandstone of lithologic unit 6. Hammer head is 20 em (8 in.) long. 

Outcrop is in roadcut of Big Sky measured section, NE1/4SE1/4 sec. 28, T. 6 S., R. 3 E., Gallatin Peak 7.5-minute 

quadrangle. 

M 

X 

F 

_H 

p 

B 

Mudstone 

Cross laminae 

Flat laminae 

TOP 

Model of Dott and Bourgeois (1982, 1983) 

Hummocky zone ______________ ---i 

Parallel laminae 

Base, massive 

or graded 

Walker and others ( 1983) proposed that this 

part be added to model 

BOTIOM 

Figure 12. Sequence of sedimentary structures in hummocky 

crossbedded strata as determined by models of Dott and 

Bourgeois (1982, 1983) and Walker and others (1983). 

normal for siltstone or mudstone-that is, hummocky 

sandstone is a storm deposit that accumulated seaward of 

where fair-weather sand deposition would have taken place. 

This interpretation agrees with the concept of storm 

generated hummocky beds espoused by Walker and others 

(1983), and Aigner (1985). Aigner interpreted proximal 

(near-shore) storm beds as thick bedded and coarse grained, 

and distal equivalents as mud-dominated and thin, oneevent beds. Proximal deposits should be more abundant and 

thicker than distal storm deposits, and they should show 

evidence of repeated reworking or amalgamation of several 

events and ecologic communities. 

The thin hummocky sandstone beds in the prodelta 

strata of unit 21 represent single events. Strata above the 

single-event hummocky sandstone beds are siltstone or 

mudstone beds that lack hummocky sands and may have 

been deposited still farther offshore. (Alternatively, a 

cessation of storms or decrease in storm strength could 

produce the same sequence of siltstone or mudstone.) 

Trace fossils present in the hummocky crossbeds are 

consistent with a lower shoreface depositional environment. 

In general, Ophiomorpha with vertical components ranges 

throughout the shoreface environment and into the 

foreshore-shoreface transition zone, and are indicative of 

relatively high levels of wave or current energy and shifting 

particulate substrates (Frey and Howard, 1985). In the Big 

. Sky section, burrows of Ophiomorpha are present in 

virtually all of the hummocky cross-stratified sandstone 

units. Most of these probably were formed below fairweather wave base where they were not reworked by waves. 

Upper shoreface and paralic sediments are absent 

from the hummocky strata in the Big Sky section and most 

likely never were deposited after'the storm deposits were 

laid down--only lower shoreface sands ever existed. The 

existence of "thin" hummocky sandstone beds interbedded 

within the prodelta strata (described in the previous section) 

is a strong argument for this interpretation. Further, the 

occurrence of mudstone layers within the upper part of the 

hummocky sequences argues for a depositional environment of the lowermost lower shoreface of the delta front. 

This interpretation contrasts with that of some 

workers (for example, Weise, 1980; Bourgeois, 1980), 

wherein coarsening-upward clastic sequences within deltas 

represent truncated sequences-that is, offshore and lower 

shoreface strata are present but upper shoreface, beach, and 

14 Depositional Environments, Frontier Formation, Madison Range, Montana 
delta plain strata have been eroded. In this interpretation, 

upper shoreface and foreshore strata were reworked, 

destroyed, during marine transgressions that followed progradation. In the Big Sky section, I found no conglomerates, 

lag deposits of any kind, or other evidence of erosion or 

transgression at the top of the hummocky cross beds or in the 

overlying prodelta strata, to support the concept of a 

truncated sequence. 

Bioturbated Muddy Fine-Grained Sandstone 

Bioturbated muddy fine-grained sandstone to siltstone (fig. 4, lithologic units 8 and 19) directly overlies 

hummocky crossbedded sandstone in a few places. The 

units are 1.5-3 m (5-10 ft) thick. The strata are rubbly 

weathering and lack laterally continuous laminae, although 

discontinuous shale or mudstone layers a few millimeters 

thick are present locally. The sandstone and siltstone is 

thoroughly bioturbated. Unit 19 definitely is gradational 

through 10-15 em (4--6 in.) with underlying hummocky 

sandstone; mudstone or siltstone has been churned by 

infauna into the uppermost part of the sandstone. No 

specific trace fossils were identified from this unit. 

Interpretation 

These rocks are interpreted as deposits of the 

lowermost shoreface of the delta front facies or uppermost 

prodelta facies, deposited seaward of the bioturbated hummocky cross-stratified beds that lie directly beneath it. The 

thorough bioturbation reflects total, repeated reworking of 

sediments by infauna rather than absence of organisms; 

intense bioturbation also indicates slow, continuous rates of 

deposition (Frey and Howard, 1985). 

Delta Front Facies 

Tabular and Trough Crossbedded 

Salt-and-Pepper Sandstone 

The upper 3-4m (10-13 ft) of lithologic unit 25 (fig. 

4) is pale-yellowish-gray, fine- to medium-grained, and 

locally coarse-grained and pebbly quartzose cherty 

sandstone. Tabular and trough crossbeds are abundant and 

are transitional into hummocky crossbedded sandstone that 

is present locally in the lower part of the unit. A few thin 

shale lenses are present in the upper strata. Strata of unit 25 

are transitional into the sandstone of unit 26, which appears 

massive where freshly exposed in roadcuts, but displays 

gently dipping planar laminated beds where weathered. The 

sandstone of unit 26 is pale brown to light gray but weathers 

very light gray to white in an irregular zone 0.1-2.0 m 

(0.3-7 ft) thick in its uppermost part. It is fine to medium 

grained and composed primarily of quartz, chert, and 

feldspar. Quartz grains range from subangular to well 

rounded. No heavy minerals, crossbeds, or burrow 

structures were observed in this unit. 

Interpretation 

The tabular and trough crossbeds and the wide range 

of grain sizes in the upper part of unit 25 are common in 

upper shoreface deposits (Reinson, 1984 ), or in the 

lowermost part of the foreshore (Frey and Howard, 1988). 

The better sorting and very low angle dip of thin, planar 

beds of unit 26 correspond to features typical of foreshore 

deposits as described, for example, by Harms and others 

(1975), Reinson (1984), and Frey and Howard (1988). The 

sandstone contains grains of a common size grading, but the 

mix of well-rounded to subangular grains reflects different 

sorting histories. The few percent of well-rounded quartz 

grains suggests either reworking of existing sandstone or an 

admix of windblown grains from dunes or other wind-sorted 

deposits onshore. 

Delta Plain Facies 

Strata of delta plain origin comprise a sequence of 

thin lithologic units, commonly a few centimeters to about 

1 m (3 ft) thick. The units include medium-gray "saltand-pepper" sandstone, dark-gray mudstone and siltstone; 

coaly and carbonaceous mudstone; and yellowish-green 

tuff, some of it weathered to mudstone that is light gray or 

reddish orange. The contact of the delta plain sequence with 

the underlying marine delta front deposits is marked by a 

general color contrast from light-gray sandstone of the delta 

front strata to the dark colors of much of the delta plain 

sequence. 

Bioturbated Salt-and-Pepper Sandstone 

Bioturbated sandstone of lithologic unit 27 (fig. 4) is 

about 0.5 m (1.5 ft) thick and is gradational with the 

sandstone of the underlying unit 26, but unit 27 differs in 

that it mainly is composed of quartz, with only a few grains 

of chert and feldspar. Most of the quartz grains are angular 

to subangular, but 15-20 percent of them are well rounded, 

in contrast to a much smaller percentage of rounded grains 

in unit 26. Some of the well-rounded grains are frosted. In 

addition, sandstone of unit 27 contains some clay, giving it 

a medium-gray color, and is thoroughly bioturbated by plant 

roots. Unit 27 also contains abundant tubular fractures filled 

with a white noncalcareous material. A thin section shows a 

rootlet-like structure composed of carbonaceous material 

(charcoal?) that defines the two walls of the tubular 

structure; the structure was not disturbed by compaction. 

The thin section also shows isolated grains of the same 

carbonaceous material. 

Big Sky Measured Section 15 
Interpretation 

The sandstone of unit 27 represents a backshore 

deposit, landward of the uppermost reach of waves of the 

foreshore environment. The association of angular to subangular and rounded grains, tubular root-like structures in 

vertical (growth) position, bioturbation by plants, and finely 

comminuted carbonaceous debris suggests deposition where 

the sand was stabilized by vegetation. Retallack (1988) 

described root traces in growth position as being one of the 

most diagnostic features of paleosols. The well-rounded 

quartz grains, particularly the frosted ones, are suggestive of 

a wind-blown origin, but the mix with angular grains 

indicates the preserved sandstone is not a dune. 

Carbonaceous Mudstone 

Dark-gray carbonaceous mudstone and coaly 

carbonaceous mudstone (fig. 4, lithologic units 31, 35, 37, 

40, 45) occur at several horizons in the delta plain strata of 

the Big Sky section. Lithologic unit 37 also contains a large 

percentage of clayey siltstone and is shown as siltstone in 

figure 4. The rest of the units are thin bedded and display 

very fine laminations. Burrowing is evident locally. A 

mudstone believed to part of unit 35, but on the opposite 

side of the road from the measured section and not traceable 

across the road, yielded the freshwater clams Protelliptio n. 

sp. and Plesielliptio n. sp., and the freshwater snail Cairnarbis sp. The coaly carbonaceous mudstone of unit 37, 

which gradationally overlies carbonaceous clayey, crossbedded, siltstone of the unit, yielded crushed unionid 

bivalves. The fossils of both assemblages are disarticulated 

and abraded. 

Interpretation 

The coaly carbonaceous mudstone of unit 37 

represents a heavily vegetated environment with little 

coarse clastic input. The carbonaceous material in the 

mudstone is not rooted but is debris that likely was 

transported into the depositional site. The unionid bivalves 

indicate the depositional site was a coastal freshwater 

swamp. The ripple crossbedded carbonaceous muddy siltstone of unit 37 could represent an overbank deposit or even 

a small delta where a stream debouched into the freshwater 

swamp. 

The very fine laminations of carbonaceous strata of 

lithologic units 31, 35, 40, and 45 record deposition in quiet 

water where only occasional wave and current action are 

characteristic, reducing conditions are common, and 

burrowing organisms exist. Fragmental accumulations of 

mollusks, which churned through sediment mixing sand and 

silt into adjoining clays, probably reflects wave action along 

a lake shore. The silty and sandy strata are not typical of 

lake sediments (Krinitzky and Smith, 1969) but represent a 

lacustrine delta where a stream entered a lake. These strata 

are much like those described and pictured by Coleman 

( 1966), Krinitzsky and Smith (1969), and Krinitzsky (1970) 

from lake deposits of the Mississippi River delta plain, 

Louisiana. 

The Plesielliptio sp. fauna of unit 35 indicate a 

freshwater lake environment. Shells of the fauna are disarticulated, but they are not severely abraded. The fauna could 

have lived in a paleoecological environment different from 

that indicated by the carbonaceous mudstone in which they 

were preserved, having been transported from a fluvial 

environment into a lake. Hanley (1976) found mollusks of 

the genera Plesielliptio sp. to occupy a fluvatile habitat near 

freshwater lakes and transported specimens were found in 

lakeshore sediments. Hanley and Flores (1987) found nontransported specimens of Plesielliptio were most common 

in very shallow water, near-shore lake settings, although the 

fauna also were found offshore in small lakes. 

Clayey Siltstone and Mudstone 

These strata (fig. 4, lithologic units 33, 43, 48, and 50) 

are dark-gray clayey siltstone, black carbonaceous mudstone, and coaly mudstone. They contain abundant detrital 

plant debris. The mudstone appears massive. Unit 43 

contains very small crystals of pyrite and dispersed 1-2 mm 

(0.04-0.08 in.) fragments of calcareous shell debris. Unit 33 

consists of coaly mudstone that gradationally overlies siltstone containing small-scale ripple crossbeds. The coaly 

mudstone of unit 33 and the siltstone of unit 43 both yielded 

disarticulated, abraded shells of Brachiodontes sp., Anomia 

sp., and Corbula sp., which are brackish water genera. 

Interpretation 

The sedimentological and compositional features of 

these strata conform to those of poorly drained swamp 

deposits as described and pictured by Coleman (1966), 

Krinitzsky and Smith ( 1969) and Krinitzsky ( 1970). The 

carbonaceous material in the rock units is not rooted but is 

debris that likely was transported into the depositional site 

during floods. The faunal assemblage of units 33 and 43 are 

brackish water forms. The abraded and disarticulated fossils 

of both units could have been transported into their 

depositional sites or simply reworked within them. 

Tuff and Bentonite 

Tuff and bentonite are interstratified with the delta 

plain sequence and form beds 0.2-2.0 m (0. 7-6.6 ft) thick. 

Tuff beds (fig. 4, lithologic units 30, 32, 34, 39, 43, and 49) 

are pale-greenish-gray to pale-yellowish-green. Some of the 

tuff beds are hard, due to silicification, and contain very 

small plagioclase phenocrysts. At least one of the beds is 

graded and contains lithophysae. Other tuffaceous beds are 

16 Depositional Environments, Frontier Formation, Madison Range, Montana 
deeply weathered and are more properly called bentonite 

(fig. 4, lithologic units 28, 29, 31, 36, 38, and 44). The 

bentonite ranges from very light gray to reddish orange. 

X-ray diffraction examination of two of the bentonites 

shows presence of montmorillonite clay. All colors of the 

tuff and bentonite contrast strongly with the dark grays of 

the carbonaceous clayey sandstone, siltstone, and mudstone 

of the sequence. Contacts of the tuff and bentonite beds with 

overlying and underlying strata are sharp. No root structures 

were observed in any of the tuff or bentonite beds, and no 

bioturbation-mixing of the tuff or bentonite with underlying 

or overlying dark-gray mudstone or siltstone was observed. 

Interpretation 

The characteristics displayed by the tuff and bentonite 

are consistent with their airfall deposition into shallow 

water lakes or swamps that lacked reworking by waves, and 

(or) on land that lay adjacent to the water bodies. Only one 

of the tuff beds in the Big Sky measured section showed 

convincing evidence of reworking by water. A 0.5 m (1.6 ft) 

thick bed of tuff about 13 km (8 mi) west-southwest of the 

Big Sky section definitely was deposited in water, as 

indicated by ball-and-pillow structures (fig. 13). Inundation 

of land adjacent to the water bodies during floods, or a 

relative rise in sea level, could account for accumulation of 

overlying deltaic sediments. The deep weathering of tuff to 

form bentonite may reflect oxidation shortly after the time 

of deposition, but such effects also_ could have taken place 

recently. No soil textures indicative of weathering shortly 

after deposition were observed. 

Laminated and Burrowed Sandstone 

Sandstone of lithologic unit 47 (fig. 4) is light gray, 

fine to medium grained, and composed of well-rounded and 

angular to subangular grains of quartz, some feldspar 

(chiefly potassium feldspar), and chert. In one location the 

sandstone is poorly sorted, contains wispy mudstone flasers, 

and is thoroughly burrowed. The main burrow structure is of 

the trace fossil Ophiomorpha irregulaire, which is filled 

almost entirely of carbonaceous mudstone. Also observed 

were a few burrows of Teichichnus, another horizontal trace 

fossil formed by a deposit-feeding animal. 

In another location the sandstone is well sorted and 

displays parallel laminations. Rip-up mud clasts a few 

millimeters to about a centimeter across are associated with 

similar sized fragments of clams. Fragments of Crassostrea 

sp., an oyster, and Corbula sp., another bivalve, were found 

at this location. U-shaped burrows of the trace fossil 

Diplocraterion, a vertically oriented dwelling structure, 

were the only trace fossils found in these strata. 

Interpretation 

The depositional environment interpreted for this unit 

is a bay or lagoon, or partially enclosed distributary river 

Figure 13. .Pillow structure in tu~ bed of delta plain strata in upper part of Frontier Formation. Outcrop is at head of Muddy Creek, 

13 km (8 mr) southwest of the Brg Sky measured section. Hammer handle is 30 em (12 in.) long. 

Big Sky Measured Section 17 
mouth, such as at the seaward edge of the delta plain of the 

Niger delta where barrier spits restrict entrance to the rivers 

and form small estuaries that undergo tidal reversal (Allen, 

1965). The low diversity of trace fossils, the horizontal 

versus vertical forms, the contrasting abundances of trace 

fossils, and the strata known to directly overlie and underlie 

the sandstone unit support this interpretation. An open 

marine setting likely would have produced a sandstone that 

differed little in the character of its ichnofauna from one 

place to another. 

The sandstone of unit 47 contains contrasting types of 

trace fossils at two locations less than 0.8 km (0.5 mi) apart. 

The restriction of specific types of trace fossils to different 

areas of the same rock unit reflects contrasting 

environments. The U-shaped traces of Diplocraterion are 

indicative of a moderate to high energy environment. The 

traces are truncated by laminae of mudstone and carbonaceous debris indicating that the lamination of the sandstone 

records successive depositional events. The sediment and 

trace fauna are indicative of the Skolithos ichnofacies, 

which is characterized by moderate to high energy 

conditions and shifting sediments (Ekdale and others, 

1984 ). In contrast, the area of the sandstone that is 

thoroughly bioturbated by the horizontal burrows of Ophiomorpha and Teichichnus indicates a moderate to low 

energy environment of the Cruizana ichnofacies. This mix 

of trace fauna representing the Skolithos and Cruizana 

ichnofacies at a locality is suggestive of a brackish water 

environment (Howard and Frey, 1985; Ekdale and others, 

1984). 

A tidal sand-flat in a protected back-barrier 

environment provides a likely depositional model for the 

sandstone and the trace fauna. The well-laminated, least 

burrowed area of the sandstone was deposited closest to the 

low tideline where it was subjected to continuous current 

and wave action. The extensively burrowed sandstone, with 

its mud flasers, is characteristic of a middle tidal flat (mixed 

flat) (Due and Tye, 1987), which is commonly inhabited by 

abundant deposit-feeding organisms that rework the 

substrate and obscure the original bedding (Ekdale and 

others, 1984). Such an environmental setting characteristically contains only a few species that can adapt to the 

extreme variation of environmental conditions, although 

such species may occur in high densities (Ekdale and others, 

1984; Benyon and others, 1988). The well-rounded quartz 

grains in the sandstone indicate deposition proximate to a 

beach or a dunes, either (or both) of which subsequently was 

eroded. Due and Tye (1987) reported that back-barrier sand 

flats of Kiawah Island, South Carolina, periodically 

received washover material, including shell debris, which 

later was reworked. The stratigraphic position of unit 47, 

within a delta plain sequence of lagoonal, swamp, and 

marsh deposits, is an appropriate setting for washover 

deposits (Hennessy and Zarillo, 1987). 

Claystone 

Claystone (fig. 4, lithologic unit 51) is pale gray 

green, massive, hard, and appears homogeneous in hand 

specimen. It lacks laminations or evidence of primary soil 

structures. Thin section examination shows a few widely 

dispersed silt grains, minor comminuted and dispersed 

organic debris, and bioturbation, perhaps by both plants and 

animals. The mottled, churned appearance visible in thin 

section suggests burrowing by animals. Later cross-cutting 

tubular structures probably are rootlets. The rootlets are 

filled with claystone of slightly different composition from 

that of the claystone outside the rootlets. 

Interpretation 

The claystone is interpreted to be a marsh deposit, 

displaying characteristics of modem marsh sediment 

described by Edwards and Frey (1977), Basan and Frey 

( 1977), and Kosters ( 1989). These authors recognized three 

classes of marsh: low, transitional, and high. Unit 51 has the 

characteristics of the low marsh sediment. It is highly clayey 

silt to sandy mud that is essentially homogeneous, having 

been thoroughly reworked by animals and plant roots. 

Laminations are rare. 

Conglomerate, Sandstone, and 

Lenticular-Bedded Siltstone 

A ledge of conglomerate, sandstone, and lenticularbedded siltstone to mudstone forms a prominent finingupward sequence about 6.5 m (21 ft) thick at the top of the 

Big Sky measured section (fig. 14). The conglomerate 

forms a discontinous bed (fig. 4, lithologic unit 52) at the 

base of the sequence and is composed of well-rounded 

pebbles 0.5-6 em (0.2-2.4 in.) in diameter in a sandstone 

matrix. The matrix is pale-brown, calcareous, fine- to 

coarse-grained sandstone that is mainly composed of 

angular to well-rounded quartz grains in mudstone. No shell 

fragments were found in the conglomerate. The pebbles 

consist of dark-gray chert (70 percent), light- to dark-gray 

quartzite (25 per cent), and other compositions (5 per cent). 

Some of the chert contains silicified pelmatazoan debris and 

is probably from the Mississippian Madison Group, but the 

major part of the chert likely is from the Permian Phosphoria (or Shedhorn) Formation. Pebbles from these formations 

also are present in pre-Frontier Mesozoic strata of the 

Madison Range. The pebbly sandstone is lenticular, ranges 

up to 15 em ( 6 in.) thick, and extends for several meters 

along the strike of the outcrop. The base is erosional. 

Sandstone (lithologic unit 53) directly overlies the 

conglomerate. The sandstone unit is about 2.8 m (9 ft) thick, 

is ripple laminated and trough crossbedded, and contains 

interlayered discontinuous lenses of mudstone. Sandstone 

18 Depositional Environments, Frontier Formation, Madison Range, Montana 
Figure 14. Tidal estuarine deposit of lithologic units 52 (covered), 53, and 54, underlain by mudstone of lithologic unit 51 

(covered). Outcrop is in uppermost part of Big Sky measured section, along ridge crest in SW1/4NE1/4 sec. 28, T. 6 S., R. 3 E., 

Gallatin Peak 7.5-minute quadrangle. 

beds are about 10-30 em (4 -- 12 in.) thick, and the base of 

each commonly is erosional. The sandstone is fine grained 

and is composed of quartz, feldspar, and minor biotite; 

finely comminuted organic debris is common, but no shell 

material was found. A few horizontal burrows are present 

locally where the sandstone contains a large amount of mud. 

In the lower part of unit 53, mudstone makes up about 25 

percent of the rock. Ripple cross-laminated sandstone layers 

0.5-2 em (1.3-5 in.) thick are separated by mudstone layers 

that are about 0.5 em (0.4 in.) thick. Some of the mud layers 

are drapes: they are present only at the toe of a cross lamina, 

decreasing in thickness and pinching out upward in the 

crossbed. Within areas of trough crossbeds, mainly in the 

upper part of unit 53, medium- to dark-gray mudstone 

alternates with sandstone. The mudstone content of the 

cross-laminated sandstone increases in the upper part of the 

trough, and in some places grades into horizontally 

laminated mudstone and silty mudstone that forms 

discontinuous lenses between trough cross-stratified 

sandstone beds. The lenses are as much as 15 em (6 in.) 

thick and pinch and swell, both along and normal to strike. 

Some mudstone layers extend for several meters along 

strike, although most extend only a meter or two; their 

extent was not determined normal to strike owing to limited 

exposures. The lenses occur at irregular stratigraphic 

intervals and are overlain by sandstone beds that have 

erosional bases. Unit 53 is gradational into unit 54. 

Strata (unit 54) in the upper part of the ledge-forming 

sequence are 3.0 m (1 0 ft) thick and are composed of 

lenticular bedded mudstone and clayey siltstone. Locally 

discontinuous, isolated lenses of cross-laminated finegrained sandstone, the record of ripples that migrated across 

a mud substrate, are present within the mudstone. The 

sandstone layers are 0.1-1 em (0.04-0.4 in.) thick, and 

mudstone layers range up to 3 em ( 1.2 in.) thick. The 

contact of sandstone and mudstone layers is sharp. The unit 

has been burrowed, but no identifiable forms were found. 

Molds of a few burrows, as much as I em (0.4 in.) across 

Big Sky Measured Section 19 
and 5 em (2 in.) long, were found weathered out on the 

ground surface and also were unidentifiable. 

Upper strata of the lenticular-bedded unit yielded the 

following megafossils, reported previously by Tysdal and 

others (1990): Inoceramus lamarcki stumckei Heinz, and/. 

flaccidus White, which are clams; Rhynchostreon suborbiculatum (Lamarck), an oyster; Scaphites carlilensis 

Morrow, and Baculites sp., which are ammonites. The 

fossils are marine and marginal marine forms and constitute 

a late-middle Turonian assemblage. None of the clams, 

which constitute most of the specimens, were in growth 

position and all were disarticulated, consisting of large 

pieces of individual valves. 

Interpretation 

The sequence formed by lithologic units 52-54 is 

interpreted as a tidal estuarine deposit because it has 

characteristics of the subtidal part of a point bar and because 

it contains a mixed assemblage of open marine and marginal 

marine fauna. It has characteristics of modem back-barrier 

tidal streams of the microtidal Niger delta. The upward 

changing sequence of intermittent lag gravel, interbedded 

cross-laminated sandstone and mudstone lenses, and 

overlying interbeds of siltstone and mudstone are 

characteristic of tidal estuarine deposits described by 

Howard and Frey (1985) in modem deposits of Georgia 

tidal estuaries that have rivers flowing into their headwaters. 

Interbedded sandstone and mudstone constitute the 

most prominent feature of unit 53 and record contrasting 

depositional conditions. Rippled and trough cross-laminated 

sandstone reflect stronger current conditions than the 

laminated mudstone that contains horizontal trace fossils, 

which reflect weak currents. The laminated mudstone likely 

was deposited in the uppermost part of an estuary, or in a 

tidal stream that flowed into an estuary. In such a location, 

fine sediment settles out when water flow is inhibited by 

slack tides (Dorjes and Howard, 1975) and clay-silt floccules suspended in the brackish waters are deposited, 

forming cohesive muds that are difficult to erode (Allen, 

1965). Thin clay beds occur throughout the cross-laminated 

sand of tidal channel deposits (Oomkens, 1974), and grainsize contrasts between the layers of tidal creeks are much 

more marked than in any other layered deposit in the Niger 

delta (Allen, 1965). 

The upward fining of grain size of the Big Sky strata 

is reflected in the lenticular beds of unit 54, which indicate 

decreasing transport power. The marine and marginal 

marine megafossils of these strata is an argument in support 

of a subtidal origin. These strata represent subtidal 

accretionary deposits. 

None of the cross-laminated strata of unit 53 show 

rhythmically interlayering reflective of inequal diurnal 

tides, as reported by Visser (1980) in a mesotidal Dutch 

tidal flat. There, thin mud laminae form a couplet of two 

mud layers separated by a thin sandstone layer, each couplet 

separated by a thick sand layer called a tidal bundle. Such 

tidal bedding patterns have not been reported from 

microtidal environments, the tidal range commonly 

considered applicable to deposits of the Cretaceous Western 

Interior Sea. No herringbone crossbedding, indicative of 

reversing current flow, was found either. 

Contact Relationships of Facies in 

Big Sky Measured Section 

The relationship of the upper delta front strata at the 

top of the Lincoln Mountain measured section to the 

interbedded lower delta front and upper prodelta rocks of 

the Big Sky measured section is not known because the 

contact of the two sequences has not been observed at either 

location or elsewhere in the range. The lower Frontier strata 

are concealed in the area of Big Sky, and upper Frontier 

strata that are the time-equivalent of the Big Sky section 

have been eroded from the general area of Lincoln 

Mountain. 

An unconformity occurs at the top of the Frontier in 

the Madison Range. Uppermost delta plain rocks of the 

Frontier are overlain by offshore (prodelta) mudstone of the 

Cody Shale, which contains late Coniacian megafossils. 

Several Western Interior molluscan fossil zones are unrepresented between the late-middle Turonian fauna at the top 

of the Frontier and the late Coniacian fauna of the basal 

Cody. The faunal and stratigraphic break correspond to the 

time of an unconformity recognized_ by Merewether and 

Cobban (1986) throughout northwestern Wyoming and 

adjacent southwestern and south-central Montana. 

CONCLUSIONS 

The section for the Frontier Formation in the Madison 

Rangt: consists of two measured sequences that, combined, 

form a composite section. The lower part of the composite 

section, at Lincoln Mountain, consists of a regressive 

sequence of delta front strata that are overlain by delta plain 

strata. These rocks early Cenomanian rocks are separated by 

an unconformity from middle Turonian delta front deposits 

in the uppermost part of the Lincoln Mountain section. The 

upper part of the composite measured section, near Big Sky, 

consists of a regressive sequence of late-middle Turonian 

lower delta front hummocky crossbedded sandstone and 

intertonguing prodelta strata. This sequence is overlain by 

sandstone of the upper delta front (shoreface) and lower 

delta plain facies. The uppermost 25 m (82 ft) of the Big 

Sky section of the Frontier is interpreted as a delta plain 

sequence composed mainly of mudstone and siltstone. 

20 Depositional Environments, Frontier Formation, Madison Range, Montana 
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References Cited 23 

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