McGowan Creek Formation, 

New Name for Lower Mississippian 

Flysch Sequence in 

East-Central Idaho

GEOLOGICAL SURVEY BULLETIN 1405-E

McGowan Creek Formation, 

New Name for Lower Mississippian 

Flysch Sequence in 

East-Central Idaho

By CHARLES A. SANDBERG

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1405-E

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975
UNITED STATES DEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data 

Sandberg, Charles Albert, 1929-

McGowan Creek Formation, new name for Lower Mississippi-an Flysch sequence in eastcentral Idaho.

(Contributions to stratigraphy) (Geological Survey Bulletin 140S-E) 

Bibliography: p.

Supt.ofDocs.no.: I19.3:1405-C

1. Geology, Stratigraphic Mississippian. 2. Flysch Idaho. 3. Geology Idaho. 

I. Title. II. Series. III. Series: United States Geological Survey Bulletin 1405-E. 

QE7S.B9 no. 1405-E [QE672] 557.3'08s [551.7'5l'0979632] 75-619272

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

Washington, D. C. 20402 

Stock Number 024-001-02726-7
CONTENTS

Page

Abstract........................................................................................................................... El

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

Stratigraphy.................................................................................................................... 3

Age.................................................................................................................................. 7

Correlation...................................................................................................................... 9

References cited............................................................................................................... 10

ILLUSTRATION

Page

FIGURE 1. Index map showing area of McGowan Creek Formation and distribution 

of Sappington Member of Three Forks Formation, and areas of Copper 

Basin Formation and Milligen Formation................................................. E2

TABLE

Page

TABLE 1. Representative Kinderhookian conodont faunas from McGowan Creek 

Formation and Drummond Mine Limestone of Paull and others (LCO1 only)........................................................................................................ E7

CONTRIBUTIONS TO STRATIGRAPHY

McGOWAN CREEK FORMATION, NEW NAME

FOR LOWER MISSISSIPPIAN FLYSCH

SEQUENCE IN EAST-CENTRAL IDAHO

By CHARLES A. SANDBERG

ABSTRACT

The argillite sequence of Early Mississippian age in east-central Idaho that had been erroneously correlated with the Devonian Milligen Formation in central Idaho is here named 

the McGowan Creek Formation from exposures south of McGowan Creek on the west side of 

the Lost River Range. The McGowan Creek is dated and zoned on the basis of conodont 

faunas and its type section is described. It comprises a thick lower member, predominantly 

grayish-black argillite, as much as 1,000 metres (3,280 feet) thick, and a thin upper member, 

predominantly pinkish-gray-weathering calcareous siltstone, as much as 150 metres (492 

feet) thick. Regionally, the McGowan Creek forms a westward-thickening wedge of flysch 

sediments that were deposited on the east side of the Antler foreland basin. Westward it correlates with the lower part of the Mississippian Copper Basin Formation near the axis of the 

foreland basin. Eastward it grades into the Lodgepole Limestone and lower part of the Mission Canyon Limestone on the cratonic platform in southwestern Montana.

INTRODUCTION

The argillitic flysch sequence of Early Mississippian age lying between the Three Forks Formation of Late Devonian age and the Middle 

Canyon Formation of Late Mississippian age in the Lost River and 

Lemhi Ranges and Beaverhead Mountains of east-central Idaho is here 

named the McGowan Creek Formation. Heretofore, this argillite sequence had been erroneously called the Milligen Formation on the basis 

of its correlation by Ross (1934, p. 967) with the type Milligen of the Wood 

River area in central Idaho, which Ross (1934, p. 972; 1962, p. 384) considered to be Mississippian. Ross' correlation appeared to be confirmed 

when the argillite sequence in the Lost River Range was found to contain 

Early Mississippian conodonts at its base by Sandberg, Mapel, and 

Huddle (1967). Subsequently, the usage of Milligen Formation for the argillite sequence of Early Mississippian age became firmly entrenched in 

geologic literature (for example, Mamet and others, 1971; Paull and 

others, 1972). Recently, however, the type Milligen Formation of the 

Wood River area was shown to be a relatively deep water transitional sequence of exclusively Devonian age that had an eastern provenance 

(Sandberg and others, 1975). Thus that name no longer is applicable to

El
E2 CONTRIBUTIONS TO STRATIGRAPHY

the exclusively Lower Mississippian western-derived flysch sequence that 

is here named the McGowan Creek Formation.

The discrete areas in which the McGowan Creek and Milligen Formations are recognized are shown in figure 1. The type locality of the Mil114 113°

45°

44C

Arco° 

SNAKE RIVER PLAIN

0 5 10 15 20 25 MILES

I I I l

0 10 20 30 40 KILOMETRES

FIGURE 1. Index map showing area of McGowan Creek Formation (light stippling) 

and distribution of directly underlying Sappington Member of Three Forks Formation, and areas of Copper Basin Formation (dark stippling) and Milligen Formation 

(medium stippling). Ki, Cretaceous intrusive rocks of Idaho batholith; pCr, Precambrian sedimentary and metamorphic rocks. Localities mentioned in text: CH, 

Chamberlain Creek; DH, Donkey Hills; FCR, Fish Creek Reservoir; FS, Freighter 

Spring; GV, Grandview Canyon; HM, Hawley Mountain; HU, Hurst Canyon; LC, 

Long Canyon; LCD, Lower Cedar Creek; LCO, Little Copper Creek; LK, Lake Canyon; 

MC, Middle Canyon; MCG, McGowan Creek (type section of McGowan Creek Formation); MG, Milligen Gulch (type locality of Milligen Formation); RC, Railroad Canyon.
McGOWAN CREEK FORMATION E3

ligen is Milligen Gulch. In the intervening area of the Pioneer Mountains, the Mississippian Copper Basin Formation contains in its lower 

part western equivalents of the McGowan Creek Formation. One of these, 

the Drummond Mine Limestone of Paull and others (1972) is demonstrated to be equivalent to the lower part of the McGowan Creek on the 

basis of similar conodont faunas. Figure 1 also shows the area where the 

Devonian part of the Sappington Member of the Three Forks Formation 

directly underlies the McGowan Creek and, to the west, the area where the 

Sappington is absent and the underlying Devonian Trident Member of 

the Three Forks directly underlies the McGowan Creek. Recognition of 

die paleogeology of the pre-McGowan Creek unconformity provides important information on the direction of sediment transport for the basal 

beds of the McGowan Creek. These beds contain largely eastern-derived 

sediments, whereas higher beds of the McGowan Creek contain predominantly western-derived flysch sediments that were shed from die Antler 

Highlands, which were composed in part of the Milligen Formation.

STRATIGRAPHY

The McGowan Creek Formation is here named for its exposures on the 

ridge crest south of McGowan Creek on the west side of the Lost River 

Range in the Doublespring quadrangle, where it was mapped as Milligen Formation by Mapel, Read, and Smith (1965). The following description of the type section of the McGowan Creek Formation is modified from their measured section.

Type section of McGowan Creek Formation

[Section measured by Mapel, Read, and Smith (1965) along ridge crest south of McGowan Creek in NK sec. 

28 and EK sec. 29, T. 12 N., R. 21 E. (unsurveyed), Custer County, Idaho (Doublespring 15-minute 

quadrangle). Lithologic descriptions modified by Sandberg on basis of reconnaissance measurements 

and sampling along crest of next ridge to south in NK sec. 33 and SEK sec. 28]

Middle Canyon Formation (Upper Mississippian). Thickness 

Gradational contact. ^metres'> "**'> 

McGowan Creek Formation {Lower Mississippian): 

Upper member:

Siltstone, calcareous, dark-gray and pale-yellowish-brown, thin-bedded; 

interbedded with dark-gray silty micritic limestone. Weathers pinkish 

gray, light gray, and pale yellowish brown............................................. 143 470

Gradational contact. 

Lower member:

Argillite, silty argillite, and siltite, carbonaceous, noncalcareous to slightly 

calcareous, grayish-black, thin-bedded, containing abundant terrestrial 

plant stems and debris and U-shaped worm trails. Interbeds, 0.3-15 

m (1-50 ft) thick, of medium-gray to medium-dark-gray very fine to 

fine-grained sandstone and quartzite, in part gritty to finely conglomeratic, throughout sequence, and a few thin interbeds, 0.3 m (1 ft) 

thick, of dark-gray silty micritic limestone near top. Weathers dark 

gray, grayish black, and light gray........................................................... 968 3,175

Total McGowan Creek Formation .......................................................1,111 3,645

Unconformity.

Trident Member of Three Forks Formation (Upper Devonian).
E4 CONTRIBUTIONS TO STRATIGRAPHY

The lower member of the McGowan Creek Formation consists predominantly of interbedded grayish-black noncalcareous carbonaceous argillite, silty argillite, and siltite containing abundant macerated plant 

debris, crushed plant stems, as much as 2.5 centimetres (1 inch) wide, and 

common U-shaped worm trails. At Lower Cedar Creek (fig. 1), these lithologies characterize the basal 3 m (10 ft) of the formation, but the next 

higher interval, 3-8 m (10-26 ft) above the base, is mostly dark-yellowishorange limonitic calcareous siltstone and subordinate sandstone that contain broken plates of phosphorite and abundant conodonts (Sandberg 

and others, 1967). Interbedded with the calcareous siltstone are several 

thin medium-dark-gray silty encrinite beds that contain silicified ostracodes, horn corals, and brachiopods and lithologically and faunally resemble the basal part of the Lower Mississippian Lodgepole Limestone in 

southwestern Montana. These encrinite beds also contain abundant 

conodonts as many as 10,000/kg that represent largely an indigenous 

fauna of the Kinderhookian Lower Siphonodella crenulata Zone and 

partly a reworked fauna of the very late Devonian Upper Polygnathusstyriacus Zone. The same dark-yellowish-orange siltstone unit, without encrinite interbeds, forms the base of the McGowan Creek at Freighter 

Spring (fig. 1; Sartenaer and Sandberg, 1974, fig. 2). At Lower Cedar 

Creek, a slightly higher interval, 10-13 m (33-43 ft) above the base of the 

formation, comprises deeper water silty micritic limestone containing an 

indigenous conodont fauna of the Kinderhookian Upper S. crenulata 

Zone. Similar micritic limestone forms the faulted base of the McGowan 

Creek Formation at Donkey Hills (fig. 1) and a large part of the Drummond Mine Limestone of Paull and others (1972) at Little Copper Creek 

in the Pioneer Mountains (fig. 1).

In the northern part of the Lost River Range, thin to thick beds of lightgray- to medium-gray-weathering siltstone and fine-grained sandstone 

and quartzite, some of which are locally conglomeratic, are interbedded 

throughout the lower part of the McGowan Creek Formation. These 

interbeds are generally more resistant and form ledges, whereas the carbonaceous argillite and siltite are less resistant and form slopes. The 

ledge-forming interbeds are interpreted as eastward-thinning turbidite 

deposits. In the same area, thin interbeds of dark-gray silty micritic limestone are common near the top of the lower member of the McGowan 

Creek. At Donkey Hills (fig. 1), one of these interbeds yielded an Osagean 

conodont fauna.

The lower member of the McGowan Creek Formation is present at all 

ten localities where the formation has been studied in the Lost River and 

Lemhi Ranges, southern Beaverhead Mountains, and just to the west of 

the Lost River Range at Grandview Canyon (fig. 1). It also is present at 

Railroad Canyon (fig. 1) in the northern Beaverhead Mountains (Ruppel, 

1968) and at Lake Canyon (fig. 1) in the adjacent part of the Tendoy 

Range (M'Gonigle, 1965). At Chamberlain Creek, Long Canyon, and
McGOWAN CREEK FORMATION E5

Middle Canyon (fig. 1), the lower member of the McGowan Creek is 

less than 84 m (275 ft) thick and includes a basal limestone bed as much as 

55 m (180 ft) thick. In the southern part of the Lost River Range (fig. 1), 

the lower member thickens northward from about 60 m (197 ft) at Hurst 

Canyon to 104 m (341 ft) at Hawley Mountain and westward to about 150 

m (492 ft) at Lower Cedar Creek. Northwest of Lower Cedar Creek, the 

lower member forms a clastic wedge that thickens abruptly westward 

toward the axis of the depositional basin and attains a maximum thickness of about 1,000 m (3,280 ft) in the area between McGowan Creek and 

Grandview Canyon (fig. 1).

The McGowan Creek Formation rests unconformably on the Three 

Forks Formation, which is solely of Late Devonian (Famennian) age in 

the area where it is overlain by the McGowan Creek (fig. 1). The type 

Three Forks Formation at Logan, in southwestern Montana, consists, in 

ascending order, of the Upper Devonian Logan Gulch and Trident Members and the Upper Devonian and Lower Mississippian Sappington 

Member (Sandberg, 1965; Sandberg and Klapper, 1967). Westward from 

Logan, however, the Sappington is truncated beneath the Lodgepole 

Limestone so that only basal unit 1 of the Sappington, which is of very 

late Devonian (late Famennian) age, is preserved in Idaho (Sandberg and 

others, 1972, fig. 2). The McGowan Creek rests unconformably on unit 1 

of the Sappington in the southern Lemhi Range and southern Beaverhead Mountains (fig. 1). In this area, unit 1 consists mainly of grayishblack chertified and silicified siltstone, as much as 23 m (75 ft) thick, and 

resembles the lower member of the McGowan Creek. Unit 1 had been misidentified as Milligen Formation (McGowan Creek Formation of this 

report) at Long Canyon (fig. 1) by Mapel and Sandberg (1968). Unit 1 of 

the Sappington, however, contains a basal lag sandstone and interbedded 

large limestone concretions, such as that illustrated as a pod of dolomite 

in the Milligen Formation by Mapel and Sandberg (1968, fig. 6). Both the 

lag sandstone and concretions at Long Canyon yielded indigenous conodont faunas of the very late Devonian Upper Polygnathus styriacus Zone.

In the area where the Sappington Member has been eroded, mainly in 

the Lost River Range (fig. 1), the McGowan Creek Formation rests unconformably on the underlying Trident Member of the Three Forks Formation (Sandberg and others, 1967). The unconformable contact is well 

exposed at Lower Cedar Creek and Freighter Spring (Sartenaer and Sandberg, 1974, fig. 2). Evidence of the truncation and reworking of the Sappington and of the eastern source of sediments in the basal beds of the McGowan Creek is provided by the reworked conodonts of the Upper 

Polygnathus styriacus Zone found in these basal beds at Lower Cedar 

Creek. There, in the interval 3-8 m (10-26 ft) above the base of the 

McGowan Creek, conodonts reworked from the Sappington comprise 

30-42 percent of the total counted conodont fauna. That the eastern 

source of sediments abruptly diminished is demonstrated by the fact that
E6 CONTRIBUTIONS TO STRATIGRAPHY

at 13 m (43 ft) above the base of the McGowan Creek at the same locality, 

conodonts reworked from the Sappington comprise only 0.6 percent of 

the total counted conodont fauna.

The member of the Three Forks Formation that underlies the isolated 

patch of McGowan Creek Formation in the northern Beaverhead Mountains and northern Tendoy Range (fig. 1) has not been positively identified because of poor exposures and faulting. At Railroad Canyon, which 

the author has not visited, Ruppel (1968) did not differentiate the Milligen (McGowan Creek of this report) and Three Forks Formations. At 

Lake Canyon (fig. 1), the highest identified member of the Three Forks is 

the Logan Gulch, which is separated from the McGowan Creek by a bedding-plane thrust and fault breccia containing fragments of chertified 

and silicif ied grayish-black siltstone that probably were derived from unit 

1 of the Sappington Member. The intervening Trident Member in this 

area apparently ha't been removed by erosion prior to Sappington deposition on the ancestral Southern Beaverhead Mountains uplift (Sandberg 

and others, 1972, fig. 2; Sandberg and others, 1975).

The upper member of the McGowan Creek Formation consists mainly 

of dark-gray and pale-yellowish-brown calcareous siltstone interbedded 

with subordinate silty micritic limestone. It is distinguished from the 

lower member by having a characteristic pinkish-gray weathering color, a 

slightly higher carbonate content, and a lower organic-carbon content. 

The contact between the two members is gradational, however, even 

where they are well exposed, as in the type section and on the two ridges to 

the south of the type section. Mamet and others (1971, p. 22) suggested the 

existence of a hiatus between the lower and upper members of the Milligen Formation (McGowan Creek of this report) but stated that there was 

no supporting physical evidence. The two members were mapped separately, mainly on the basis of different colored float, in the Doublespring quadrangle (Mapel and others, 1965), but they were mapped together in the Hawley Mountain quadrangle (Mapel and Shropshire, 

1973). The upper member has been recognized only near Hawley Mountain and to the north and west of Hawley Mountain, in the area where the 

McGowan Creek is thickest. The upper member ranges in thickness from 

about 60 m (197 ft) to 150 m (492 ft). Its upper contact with the Middle 

Canyon Formation of Late Mississippian (Meramecian) age is gradational in the type section of the McGowan Creek Formation (Mapel and 

others, 1965). There and elsewhere in the Lost River Range, the upper 

member of the McGowan Creek is regarded as transitional lithologically 

and depositionally between the lower member and the Middle Canyon 

Formation. East of the Lost River Range, where the upper member is 

absent, the contact between the lower member of the McGowan Creek and 

the Middle Canyon is sharp and may represent an unconformity. The 

abrupt lithologic change across this contact at Middle Canyon in the 

Lemhi Range (fig. 1) is shown graphically by Huh (1967, fig. 5).
McGOWAN CREEK FORMATION

AGE

E7

The McGowan Creek Formation is dated as Early Mississippian primarily on the basis of conodont collections from thin limestones at the 

base and top of the lower member. The ages range from middle Kinderhookian at the base to early Osagean at the top. The exact position of the 

Kinderhookian-Osagean boundary within the lower member is not 

known, however, because of the scarcity of limestone interbeds that yield 

datable conodont faunas in the thick sequence of argillite. Representative conodont faunas from three Kinderhookian zones near the base of the 

lower member of the McGowan Creek at Lower Cedar Creek (collections 

LCD-16, 19, and 22) and at Donkey Hills (collection DH-16) and from a 

horizon high in the equivalent Drummond Mine Limestone of Paull and 

others (1972) at Little Copper Creek (collection LCO-1) in the Pioneer 

Mountains (fig. 1) are listed in table 1.

The locality of the three collections from Lower Cedar Creek is at an 

altitude of 7,920 feet in the NEMSW1^ sec. 2 (unsurveyed), T. 7 N., R. 24 E., 

Custer County, in the Mackay 15-minute quadrangle. The Lower Cedar

TABLE 1. Representative Kinderhookian conodont faunas from McGowan Creek Formation and Drummond Mine Limestone of Paull and others (LCO-1 only)

Lower 

Siphonodella crenulata 

Zone

Metres above base of

............. LCD-16

............. 4

LCD-19

7

Upper 

Siphonodella 

crenulata 

Zone

LCD-22 

13

Siphonodella isostichaS. cooperi 

Zone

DH-16 

20±

LCO-1

700±

Number of identified conodont specimens

leptus. .................................

Polygnathus communis

Pseudopolygnathus

Total indigenous 

specimens...... ..............

............. 12

............. 1

............. 6

............. 11

............. 12

............. 2

............. 20

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

............. 8

4

............. 1

7

............. 19

2

............. 114

21 

1 

1 

2 

10 

5

16 

10

8

"5 

"'4

'5 

12

"l2

'"7 

39 

1

159

'32

268 

39

22 

6

"'4 

297

1

18

687

4 

1

'"9 

3

37 

2

7 

""2

36

8

109

4

'"5

8 

1

27 

2

2 

62

111

Specimens reworked 

from Upper Polygnathus 

styriacus Zone.................... 50 114
E8 CONTRIBUTIONS TO STRATIGRAPHY

Creek locality was illustrated by Sandberg, Mapel, and Huddle (1967, fig. 

2), who gave directions for reaching it. The locality of collection DH-16 

from the Donkey Hills is at an altitude of 7,200 feet on a small spur on the 

north side of Dry Creek in the SEHNWM sec. 16, T. 10 N., R. 25 E., Custer 

County, in the Hawley Mountain 15-minute quadrangle. The locality of 

collection LCO-1 from the Drummond Mine Limestone of Paull and 

others (1972) is on the ridge crest between North and South Forks of Little 

Copper Creek at an altitude of 9,750 feet in the SEHSEMSEH sec. 23, T. 4 N., 

R. 21 E., Blaine County, in the Muldoon Canyon 15-minute quadrangle. 

It is believed to come from approximately the 5,536-foot level in the 

graphic section of Paull and others (1972, fig. 8), which was measured 

about 1.6 km (1 mi) to the north.

The faunas from the lower 20 m (66 ft) of the McGowan Creek Formation provide a succession of three Kinderhookian conodont zones the 

Lower Siphonodella crenulata, Upper S. crenulata, and S. isosticha-S. 

cooperi Zones (table 1). The faunas from the Lower S. crenulata Zone (collections LCD-16, 19), which represent approximately middle Kinderhookian time, are identical to faunas reported from the same zone at the 

base of the Lodgepole Limestone in Montana (Sandberg and Klapper, 

1967, table 3).

The succeeding Upper Siphonodella crenulata Zone, which apparently represents a short time span, is defined primarily by the overlap 

of the upper part of the range of S. crenulata with the lower part of the 

range of Gnathodus delicatus. The only other North American locality 

from which the Upper S. crenulata Zone has been reported to date is 

Crowsnest Pass, Alberta, in the lower part of the Mississippian Banff Formation (Macqueen and Sandberg, 1970, p. 51, 54). S. crenulata is apparently a relatively deepwater species that is rarely found in the Upper 

Mississippi Valley area. There, the short time span of the Lower S. crenulata Zone probably is included in the lower part of the S. isosticha-S. 

cooperi Zone.

The highest Kinderhookian zone the Siphonodella isosticha-S. 

cooperi Zone represents a long time span in North America. It extends 

from above the last occurrence of S. crenulata to the highest range of S. 

isosticha at the top of the Kinderhookian. In east-central and central 

Idaho, the S. isosticha-S. cooperi Zone is characterized by the occurrence 

of S. isosticha and S. obsoleta as the only siphonodellids, with Gnathodus delicatus and G. punctatus.

The Siphonodella isosticha-S. cooperi Zone is recognized in the 

Copper Basin Formation and in the upper part of the Drummond Mine 

Limestone of Paull and others (1972) at Little Copper Creek (table 1, collection LCO-1). It also has been found in the upper part of the Drummond Mine at a nearby locality and in the lower and middle parts of the 

Drummond Mine at Fish Creek Reservoir (fig. 1) and five other localities 

in the Pioneer Mountains. Thus the S. isosticha-S. cooperi Zone apparently encompasses the entire Drummond Mine Limestone and is repre-
McGOWAN CREEK FORMATION E9

sented by at least 800 m (2,624 ft) of flysch sediments in the Pioneer Mountains.

The Siphonodella isosticha-S. cooperi Zone has been recognized in the 

McGowan Creek Formation as low as 20± m (66± ft) above the base at 

Donkey Hills (table 1, collection DH-16). In consideration of the large 

thickness of rocks representing this zone in the Pioneer Mountains, the 

zone may encompass at least two-thirds of the thick lower member of the 

McGowan Creek in the northern Lost River Range.

The only post-Kinderhookian conodont fauna from the lower member 

of the McGowan Creek Formation is in collection DH-126-9a, obtained 

from a sample collected by W. J. Mapel, from a limestone bed in the top 2 

m (6 ft) of the lower member at the same locality as collection DH-16 in the 

Donkey Hills. Although many conodonts are silt coated, broken, and 

poorly preserved, the collection includes recognizable fragments of the 

form-species Hindeodella segaformis. In the Northern Rocky Mountains 

region, H. segaformis has its lowest occurrence in an early Osagean fauna 

from the lower part of the Lower and Upper Mississippian Brazer Dolomite in the Crawford Mountains, northern Utah. Thus the top of the 

lower member of the McGowan Creek is at least as young as early 

Osagean.

The upper member of the McGowan Creek Formation is presumed to 

be entirely of Osagean age mostly on the basis of its gradational contact 

and hence presumed continuous deposition with the lower member, 

which contains Osagean conodonts at the top. This presumption is 

strengthened by a single conodont collection, DH-126a-4, obtained from a 

sample collected by W. J. Mapel, from 28 m (92 ft) above the base of the 

upper member in the Donkey Hills, near the locality of collection DH-16. 

The fauna of collection DH-126a-4, although most of the conodonts are 

poorly preserved and heavily silt coated, includes Polygnathus communis communis. The presence of this subspecies is a certain indication 

of an Osagean age> as the subspecies does not range as high as the top of 

the Osagean.

The age of the Middle Canyon Formation, which overlies the McGowan Creek Formation, is clearly Meramecian on the basis of foraminifers reported by Mamet and others (1971, p. 24-25) and a conodont collection obtained from a sample collected by W. J. Mapel, from 1.5-2 m (5-7 

ft) above the base in the Donkey Hills. This collection, DH-240-5, contains an association of Cavusgnathus sp. with Gnathodus texanus. This 

association has its lowest occurrence in the upper part of the Meramecian 

St. Louis Limestone in the Upper Mississippi Valley area.

CORRELATION

The McGowan Creek Formation is correlative on the basis of conodont 

faunas to all of the Lodgepole Limestone and the lower part of the Mission Canyon Limestone in southwestern Montana. Gradational and 

intertonguing relations between the McGowan Creek and Lodgepole
E10 CONTRIBUTIONS TO STRATIGRAPHY

have been observed in the Beaverhead Mountains (Sandberg and others, 

1967, p. C130; Ruppel, 1968). The McGowan Creek is correlative to the 

lower part of the Copper Basin Formation in the Pioneer Mountains. It is 

equated on the basis of conodont faunas to all of the Drummond Mine 

Limestone of Paull and others (1972) and may be partly equivalent to 

their overlying nonfossiliferous Scorpion Mountain Formation. The 

deeper water silty micritic limestone that yielded conodont collections 

LCD-22 and DH-16 at Lower Cedar Creek and Donkey Hills had a western source, judging by the greatly diminished content (0-0.6 percent) of 

eastern-derived reworked Devonian conodonts (table 1). This limestone 

probably represents an eastward-thinning tongue of the Drummond 

Mine Limestone that extends into the lower member of the McGowan 

Creek Formation.

REFERENCES CITED

Huh, O. K., 1967, The Mississippian System across the Wasatch line, east-central Idaho, 

extreme southwestern Montana, in Montana Geol. Soc. guidebook, 18th Ann. Field 

Conf.: p. 31-62, 13 figs.

Macqueen, R. W., and Sandberg, C. A., 1970, Stratigraphy, age, and interregional correlation of the Exshaw Formation, Alberta Rocky Mountains: Canadian Petroleum 

Geology Bull., v. 18, no. 1, p. 32-66.

Mamet, B. L., Skipp, Betty, Sando, W. J., and Mapel, W. J., 1971, Biostratigraphy 

of Upper Mississippian and associated Carboniferous rocks in south-central Idaho: 

Am. Assoc. Petroleum Geologists Bull., v. 55, no. 1, p. 20-33.

Mapel, W. J., Read, W. H., and Smith, R. K., 1965, Geologic map and sections of 

the Doublespring quadrangle, Custer and Lemhi Counties, Idaho: U.S. Geol. Survey 

Geol. Quad. Map GQ-464.

Mapel, W. J., and Sandberg, C. A., 1968, Devonian paleotectonics in east-central Idaho 

and southwestern Montana, in Geological Survey research 1968: U.S. Geol. Survey 

Prof. Paper 600-D, p. D115-D125.

Mapel, W. J., and Shropshire, K. L., 1973, Preliminary geologic map and section of 

the Hawley Mountain quadrangle, Custer, Butte, and Lemhi Counties, Idaho: U.S. 

Geol. Survey Misc. Field Studies Map MF-546.

M'Gonigle, J. W., 1965, Structure of the Maiden Peak area, Montana-Idaho: Pennsylvania 

State Univ. unpub. Ph.D. thesis, 146 p.

Paull, R. A., Wolbrink, M. A., Volkmann, R. G., and Grover, R. L., 1972, Stratigraphy 

of Copper Basin Group, Pioneer Mountains, south-central Idaho: Am. Assoc. Petroleum Geologists Bull., v. 56, no. 8, p. 1370-1401.

Ross, C. P., 1934, Correlation and interpretation of Paleozoic stratigraphy in southcentral Idaho: Geol. Soc. America Bull., v. 45, no. 5, p. 937-1000.

 1962, Upper Paleozoic rocks in central Idaho: Am. Assoc. Petroleum Geologists 

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Ruppel, E. T., 1968, Geologic map of the Leadore quadrangle, Lemhi County, Idaho: 

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(Devonian), Wood River area, Idaho: U.S. Geol. Survey Jour. Research, v. 3, no. 

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McGOWAN CREEK FORMATION Ell

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research 1967: U.S. Geol. Survey Prof. Paper 575-C, p. C127-C131.

Sandberg, C. A., Streel, Maurice, and Scott, R. A., 1972, Comparison between conodont 

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Famennian rhynchonellid genus Megalopterorhynchus from Lost River Range, 

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.S. GOVERNMENT PRINTING OFFICE: 1975-677-340/23