Twin Creek Limestone 

(Jurassic) in the 

Western Interior of the 

United States 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 

Twin Creek Limestone 

(Jurassic) in the 

Western Interior of the 

United States 

By RALPH V\1. IMLAY 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 

A description of the stratigraphic and 

faunal succession in the Twin Creek 

Limestone and regional comparisons 

with contemporary formations 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEW ART L. UDALL, Secretar.y 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

Library of Congress catalog-card No. GS 616--2.97 

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

Washington, D.C. 20402 - Price $1.25 (paper cover) 
CONTENTS 

Page Page 

Abstract __________________________________________ _ 1 Paleontology of the Twin Creek Limestone, etc.-Con. 

Introduction ______________________________________ _ 2 Some faunal zones in the western interior-Con. 

Stratigraphy of the Twin Creek Limestone ____________ _ 2 Megasphaeroceras zone_______________________ 59 

Stratigraphic summary _________________________ _ 2 Parachondroceras andrewsi zone _________ --_--- 59 

Gypsum Spring Member ________________________ _ 17 Paracephalites sawtoothensis zone ________ -- ___ - 59 

Sliderock Member _____________________________ _ 22 Warrenoceras codyense zone___________________ 60 

Rich Member _________________________________ _ 30 Gowericeras and Kepplerites zones _____ ----_--- 60 

Boundary Ridge Member _______________________ _ 36 Comparisons with other faunas___________________ 60 

VVatton Canyon Member _______________________ _ 41 Central and southern Utah___________________ 60 

Leeds Creek Member __________________________ _ 45 Montana, VVyoming, and South Dakota_______ 60 

Giraffe Creek Member __________________________ _ 50 VVestern interior of Canada__________________ 61 

Paleogeography of the western interior region during Cook Inlet region, Alaska____________________ 62 

deposition of the Twin Creek Limestone ________ _ 53 Geographic distribution__________________________ 62 

Paleontology of the Twin Creek Limestone and some Systematic descriptions_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 7 5 

contemporary formations _____________________ _ 57 Class Pelecypoda_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 7 5 

Biologic analysis _______________________________ _ 57 Class Cephalopoda__________________________ 88 

Some faunal zones in the western interior region ___ _ 59 References__________________________________________ 99 

Chondroceras-Stemmatoceras zone _____________ _ 59 Index_____________________________________________ 103 

ILLUSTRATIONS 

PLATES 

-221-6910-66--2 

[Plates 1-16 follow index] 

1. Gervillia?, Grammatodon, Idonearca, Modiolus (Musculus), Plicatula, 

Goniomya, M actromya?, and Pinna. 

2. Camptonectes, Opis (Trigonopis), Ctenostreon, and Corbula. 

3, 4. Gryphaea. 

5. Astarte, Prorokia, Quenstedtia, Pleuromya, Pholadomya, M yopholas, Cercomya, and Platymya. 

6. Chondroceras, Spiroceras, and Stephanoceras. 

7. Stemmatoceras and Stephanoceras. 

8-10. Stemmatoceras. 

11. Sohlites. 

12. Normannites? and Parachondroceras. 

13. M egasphaeroceras and Parachondroceras. 

14. M egasphaeroceras and Sohlites. 

15. Eocephalites. 

16. Megasphaeroceras, Stemmatoceras, and Eocephalites. 

III 
IV CONTEN'DS 

Page 

FIGURES 1. Index map of western Wyoming, southeastern Idaho, and northern Utah______________________________ 5 

2-8. Columnar sections from2. Willow Creek, Idaho, to Lower Slide Lake, Wyo _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 6 

3. Big Elk Mountain, Idaho, to Red Grade, Wyo_________________________________________________ 7 

4. Stump Creek, Idaho, to South Piney Creek, Wyo_ ____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 8 

5. Preuss Creek, Idaho, to La Barge Creek, Wyo_________________________________________________ 9 

6. Mosquito Pass to Leeds Creek, Wyo__________________________________________________________ 10 

7. Monks Hollow, Utah, to Fontenelle Gap, Wyo_________________________________________________ 11 

8. Burr Fork to Steinaker Draw, Utah__________________________________________________________ 12 

9. Stratigraphic distribution of Bajocian and Callovian pelecypods in the Twin Creek Limestone and contemporary formations in the western interior of the United States _______ ---------------------------- 14 

10. Correlation of Middle and lower Upper Jurassic formations in the western interior of the United States____ 20 

11. Stratigraphic distribution of Bajocian ammonites in the western interior of the United States_____________ 28 

12. Correlation of some Middle and early Late Jurassic faunas in North America__________________________ 29 

13. Regional distribution of Bajocian marine rocks in North America_____________________________________ 54 

14. Regional distribution of Bathonian and Bathonian(?) marine rocks in North America___________________ 55 

15. Regional distribution of Callovian marine rocks in North America____________________________________ 56 

16-18. Index maps of Jurassic fossillocalities16. Twin Creek Limestone in southeastern Idaho and western Wyoming_____________________________ 63 

17. Twin Creek Limestone in northern Utah_____________________________________________________ 64 

18. Montana and northern Wyoming outside of the area of the Twin Creek Limestone________________ 65 

TABLES 

Page 

TABLES 1. Generalized stratigraphic section and characteristics of the Twin Creek Limestone______________________ 3 

2. Stratigraphic thickness of the members of the Twin Creek Limestone in Idaho, Wyoming, and Utah_______ 13 

3-5. Geographic distribution of marine Jurassic megafossils3. Sliderock Member of the Twin Creek Limestone_______________________________________________ 24 

4. Lower part of the Twin Creek Limestone______________________________________________________ 27 

5. Rich Member of the Twin Creek Limestone___________________________________________________ 32 

6. Geographic distribution of marine Bajocian ammonites in Montana and northwestern Wyoming north of area of Twin Creek Limestone__________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 34 

7-10. Geographic distribution of marine Jurassic megafossils7. Boundary Ridge Member of the Twin Creek Limestone_________________________________________ 39 

8. Watton Canyon Member of the Twin Creek Limestone_________________________________________ 43 

9. Leeds Creek Member of the Twin Creek Limestone_____________________________________________ 49 

10. Giraffe Creek Member of the Twin Creek Limestone_________________________________ --·------ 52 

11. Biological relationships and relative abundance of Bajocian and Callovian pelecypods in the Twin Creek 

Limestone___________________________________________________________________________________ 58 

12. Biological relationships and relative abundance of Bajocian cephalopods in the western interior region_____ 58 
TWIN CREEK LIMESTONE (JURASSIC) IN THE WESTERN INTERIOR OF THE UNITED STATES 

By RALPH w. IMLAY 

ABSTRACT 

The Twin Creek Limestone crops out in an area of extensive 

thrust faulting along the Wyoming-Idaho border and in northcentral Utah, extending from the southern end ·of the Teton 

Mountains southward to the south end of the central Wasatch 

Range. It was deposited in the northern end of a northwardtrending trough whose axis lay west of the westernmost 

Jurassic rocks now exposed. The position of the axis is indicated by westward thickening ·of the formation from about 

665 feet in northwestern Wyoming to about 2,720 feet near 

Idaho Falls and by a similar westward thickening in northern 

Utah from 440 feet in the Uinta ~ountains to about 2,850 feet 

near Salt Lake City. Facies and thickness changes in the formation near the Snake River indicate that the northern end 

of the trough curves toward the ~wrthwest. 

The Twin Creek Limestone is characterized by thick sequences 

of gray shaly limestone that weathers into long splinters and 

that forms conspicuous barren sloJ)es. Besides this shaly limestone, the formation contains one sandy member at the top, two 

red-bed members in the lower third, and one cliff-forming limestone at the top of this lower third. The members of the Twin 

Creek Limestone from bottom to top are named Gypsum Spring, 

Sliderock, Rich, Boundary Ridge, Watton Canyon, Leeds Creek, 

and Giraffe Creek. 

The Gypsum Spring Member thickens irregularly westward 

from 12 to 400 feet. It consists mostly of soft red to yellow 

siltstone and claystone that is interbedded with brecciated, or 

vuggy, or chert-bearing limestone. In Wyoming a basal unit o.f 

brecciated limestone passes eastward into thick masses of 

gypsum. Chert-bearing limestone thickens westward from a 

few feet in Wyoming to thick cliff-forming units in Idaho. The 

topmost bed in many s~ctions is a green tuff. Both upper and 

lower contacts of the Gypsum Spring member are sharp. Fossils 

are scarce and consist mostly of crinoid and echinoid fragments. 

The member was deposited in very shallow warm marine waters 

and lagoons in a sea that was transgressing slowly eastward. 

Its age is probably middle or early late Bajocian. 

The Sliderock Member thickens westward from 20 to 285 feet 

and consists mostly of grayish-black medium to thin-bedded 

limestone. Its basal beds in Wyoming are oolitic, in Idaho are 

sandy, glauconitic, crossbedded, and pebbly, and in Utah are 

sandy and oolitic. It grades upward within several feet into 

soft shaly limestone at the base of the Rich Member and generally forms a low ridge between adjoining members. The 

member was deposited in a shallow warm sea that either 

terminated a short distance east of the Twin Creek trough or 

that passed eastward into lagoons and basins in which were deposited red beds and gypsum. The association of the ammonite 

genera Spiroceras, Megasphaeroceras, Stemmatoceras, and 

Step.hanooeras indie.ates an early late Bajocian age. The 

pelecypod Gryphaea planocmwema fraterna Imlay is characteristic of the middle and upper parts of the member. 

The Rich Member thickens westward from 40 to 500 feet and 

consists mostly of gray shaly splintery limestone that is very 

soft basally. It becomes more clayey northward. Throughout 

the extent of the member its upper few feet grades upward 

either into hard sandy limestone or into soft red siltstone of 

the overlying member. It was deposited as soft calcareous mud 

in a shallow sea that trangressed eastward far beyond the Twin 

Creek trough into central Utah, much of Montana, northern 

Wyoming, and southernmost Canada. The age of the Rich Member is probably late Bajocian. The member is characterized by 

the pele-cypods Gryphaea planoconvema Whitfield and Gervillia 

montanaensis Meek and by the new ammonite genera Sohlites 

and Parachondroceras. It marks the top of the range of 

Prorokia fontenellensis Imlay, n. sp., Goniomya montanaensis 

Meek, and Thracia weedi Stanton, and the first appearance of 

many other pelecypod species. 

The Boundary Ridge Member thickens westward from 30 to 

285 feet and consists of soft red, green, and yellow siltstone that 

is interbedded with silty to sandy or oolitic limestone. The 

member changes eastward into soft gypsiferous red siltstone 

and claystone and westward into cliff-forming oolitic to dense 

limestone that is interbedded with some red siltstone. It is 

overlain sharply by cliff-forming limestone at the base of the 

Watton Canyon Member. It was deposited in a shallow warm 

retreating sea and in lagoons. The stratigraphic position of the 

member and regional correlations indicate that its age is probably Bathonian. 

The Watton Canyon Member thickens westward from 60 to 

400 feet. It consists mostly of gray dense compact brittle 

medium- to thin-bedded limestone that forms prominent cliffs 

and ridges. It is characterized by a conspicuous rectangular 

fracture pattern and by even bedding. The basal bed is generally massive and oolitic. Some oolitic beds occur throughout. 

Silty or sandy beds are uncommon. The upper part of the member is transitional into the soft shaly limestone of the overlying Leeds Creek Member. The Watton Canyon Member is 

of early Callovian age and was deposited in a shallow sea that 

trangressed eastward to the Black Hills area, northward across 

most of Montana, and southward probably as far as central and 

southwestern Utah. It contains My.opholas hardyi Imlay near 

the middle and Gryphaea cf. G. nebrascensis Meek and Hayden 

near the top. It passes eastward in the Jackson Hole area, 

Wyoming, into beds that contain Oadoceras and Warrenoceras. 

The Leeds Creek Member thickens westward from about 260 

to 1,600 feet and consists mostly of soft dense light-gray shaly 

limestone that weathers into slender splinters. It contains 

some beds or units of oolitic silty or sandy ripple-marked limestone and becomes more clayey northeastward in Idaho and 

Wyoming and southward in Utah. It is the least resistant of 

all the members of the Twin Creek Limestone and commonly 

forms valleys. The member grades upward into harder silty to 

sandy limestone of the overlying member. It was deposited 

1 
2 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

rapidly as soft calcareous mud in a shallow sea that deepened 

westward and that was probably somewhat deeper than the 

sea during Watton Canyon time. The sandy and silty material 

was derived mostly from the east or southeast. The age of 

the member is early Callovian, as shown by the presence of 

Gryphaea nebrascen.sis Meek and Hayden. 

The Giraffe Creek Member thickens westward and southward 

from 25 to 295 feet and generally forms low cliffs or ridges. It 

consists mostly of yellowish-, greenish-, pinkish-gray silty to 

sandy ripple-marked thin-bedded limestone and sandstone but 

includes some thicker beds of oolitic sandy limestone. It becomes more sandy and glauconitic westward, and grades upward 

within 10 feet, or les,s, into soft red siltstone at the base of the 

Preuss Sandstone. The Giraffe Creek Member was deposited in 

a shallow marine to littoral environment in a sea that was retreating westward. Its sandy material was derived from the 

west. The stratigraphic position and regional correlations indicate an early Callovian age for the member. 

The Twin Greek Limestone is not very fossiliferous, and most 

of the fossils are poorly preserved. It has a more varied fauna 

than the Oarmel Iformation or the Arapien Shale of Utah and a 

less varied fauna than equlivalent beds in Montana and northern 

Wyoming. The Carmel Formation has a greater abundance of 

Ostrea but lacks the pelecypod g·enera Gryphaea, Opis, Prorokia, 

Protocardia, Corb1tla, and Platyrnya, and lacks ammonites except at the northwest end of the San Rafael Swell. Equivalent 

beds north of the Twin Creek trough contain more ammonites, 

more Myas, and a greater abundance of fossils. Factors influencing the growth of marine organisms, such as depth of 

water, salinity, bottom conditions, and food supply, apparently 

became more favorable northward. 

Pelecypods are more common than any other megafossils in 

the Twin Creek Limestone. They are represented by 1,575 

specimens distributed among 43 genera and subgenera and 50 

species. The common genera in order of decreasing abundance 

include Gryphae'a, Carnptonectes, Ostrea, Pronoella, and Pleurornya. Much less common genera in order of decreasing abundance include Astarte, Tr·igonia, Vaugonia, Gratnrnatodon, Pinna, 

Myophorella, Modiolus, Lirna., Plicatula, Quenstedtia, Prorokia, 

~"41aotron~ya ?, Ctenostreon, Thracia, PholadCYmya, GoniCYmya, Protocardia, and Isognornon. Genera represented by less than three 

specimens include GerviUia, Mytilus, Lopha, JJJyopholas, Platym.ya, H onwrnya, n·igonopis and N ucula. New species and subspecies include Gryphaea planoconvema traterna Imlay, Prorokia 

tontenellen.sis Imlay, and Platyrnya rockymontana Imlay. 

Cephalopods are represented in the Twin Creek Limestone by 

143 specimens of which 5 are from the Rich Member; the rest 

are from the Sliderock Member. Genera present in the Sliderock Member include Step·hanooeras, Sternrnatoceras, Norrna.nnites?, Eocephalites, n. gen., JJiegasphaerocera8, and Spiroceras. 

Genera present in the Rich Member include Stephanoceras ?, Sohlites, n. gen., and Parachondroceras, n. gen. New species include 

Stemrnatocera.s arcicostum Imlay, Sohlites, spinosus Imlay, 

Parachondroceras andrewsi Imlay, P. filicostatutn Imlay, and 

FJocephalites prirnu8 Imlay. 

INTRODUCTION 

This study of the stratigraphy and paleontology of 

the Twin Creek Limestone is based on the publications 

of many geologists, on fieldwork by the writer in 1944-

47, 1949-51, and 1960-64, and on 197 fossil collections 

made by 47 geologists since 1892. In addition, 28 collections of Bajocian ammonites from other formations 

in the western interior region are included in the study 

because they supplement the data furnished by the Bajocian ammonites from the Twin Creek Limestone and 

because the total number of specimens involved is not 

very large. Ammonites of Callovian age are not described herein because only one nondescript specimen of 

that age has been found in the Twin Creek Limestone. 

Most of the collections from that formation consist of 

pelecypods of late Bajocian age, but some are of early 

Callovian age. The stratigraphic and biologic data 

furnished by study of 1the pelecypods are supplemented, 

as with the ammonites, by examination of fossil collections from other formations. 

Special thanks are due to W. W. Rubey for help in 

locating many complete unfaulted sections of the Twin 

Creek Limestone in western Wyoming and southeastern 

Idaho. Other people who have assisted in locating sections in Idaho include L. S. Gardner and M. H. Staatz, 

in Wyoming include A. J. Eardley and H. R. Wanless, 

in Utah include A. A. Baker, M. D. Crittenden, A. E. 

Granger, J. W. Huddle, and T. E. Mullens. 

STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 

STRATIGRAPHIC SUMMARY 

The Twin Creek Limestone was defined by Veatch 

(1907, p. 56, chart opposite p. 50) for exposures of marine limestone, shale, and sandstone along and near Twin 

Creek between Sage and Nugget in western Wyoming. 

The term has subsequently been used over a large area in 

southeastern Idaho (Mansfield, 1920, p. 53; 1927, p. 97, 

98; 1952, p. 36-38), in western Wyoming (Imlay 1950a, 

1952b, p. 965; 1953a, Horberg, 1938, p. 11, 19, 20; Dobrovolny, 1941, p. 430-433; Wanless and others, 1955, p. 

49, 50; Rubey, 1958, Oriel, 1963), and in north-central 

Utah (Thomas and Kruger, 1946, p.1275-77; Baker and 

others, 1947; Imlay, 1953a; Granger, 1953, p. 4, 10, 11; 

Richardson, 1941, p. 30; Eardley, 1944, p. 834, 836). 

The formation was divided by I1nlay (1950a) into seven 

members that from bottom to top were called members 

A-G. Of these, member A was renamed the Gypsum 

Spring Member by Oriel ( 1963), and the others are 

herein assigned new names (table 1) . 
TABLE I.-Generalized stratigraphic section and characteristics of the Twin Creek Limestone 

Member 

Giraffe Creek _______ _ 

Leeds Creek ________ _ 

Watton Canyon _____ _ 

Lithologic features 

Mostly gray silty to sandy ripple-marked thin-bedded limestone and sandstone. Some 

thicker beds of oolitic sandy 

limestone. Becomes sandier 

and glauconitic westward. 

Forms ridges. 

Mostly light-gray soft dense 

shaly splintery limestone. 

Some oolitic silty or sandy 

ripplemarked limestone. Be- comes clayey northeastward in 

Wyoming and southward in 

Utah. Forms valleys. 

Mostly gray dense compact 

brittle even-bedded mediumto thin-bedded limestone. 

Basal bed generally massive 

and oolitic. Forms prominent cliffs and ridges. 

Boundary Ridge _____ -I Red, green, and yellow soft 

siltstone interbedded with 

silty to sandy or oolitic limestone. Changes eastward 

into red siltstone and westward into limestone. 

Rich _______________ _J Medium-gray shaly limestone, 

very soft basally; contains some thin beds near top. 

Becomes clayey and fossiliferous eastward. Forms 

ravines. 

Sliderock ____________ I Grayish-black medium- to thinbedded limestone. Basal 

beds in W yarning are oolitic; 

in Idaho, sandy cross-bedded 

and pebbly; in Utah, sandy 

and oolitic. Sandiness in- creases westward. Forms 

low ridges. 

Gypsum Spring _____ -I Mostly red soft siltstone and 

brecciated vuggy or chertbearing limestone. Basal 

brecciated limestone passes 

eastward into thick gypsum mass. Chert-bearing limestone thickens westward. 

Forms ravines. 

Thickness (feet) 

25-295. Thick- ens westward 

and southward irregularly. 

260-1,600. 

Thickens 

westward. 

60-400. Thick- ens westward. 

30-285. Thick- ens westward 

irregularly. 

40-500. Thick- ens westward 

irregularly. 

20-285. Thick- ens westward. 

12-400. Thick- ens irregularly toward 

west and 

southwest. 

Contacts 

Grades upward 

within a few feet 

into soft red beds 

at base of Preuss 

Sandstone. 

Grades upward 

evenly into silty 

to sandy thinbedded limestone. 

Grades upward 

evenly in to shaly 

limestone of the 

Leeds Creek 

Member. 

Overlain sharply by 

cliff-forming limestone of Watton 

canyon Member. 

Grades upward 

within several 

feet into harder 

sandy limestone or softer red siltstone. 

Grades upward 

within several 

feet into soft shaly 

limestones of the 

Rich Member. 

Contacts sharp _____ _ 

Characteristic fossils 

Coquinas of crinoid, echinoid, and 

Camptonectes fragments. Ostrea 

present locally. 

Gryphaea. nebrascensis, Platymya 

rockymontana, 

and Cadoceras? 

sp. Megafossils uncommon. 

Gryphaea sp. juv. 

cf. G. nebrascensis near top. Myopholas hardyi 

Imlay near middle. Megafossils uncommon. Astarte ( Coelastarte) livingstonensis Imlay found near top. Megafossils rare. 

Gryphaea planoconvexa, Gervillia 

montanensis, Sohlites spinosus, and 

Parachondroceras 

spp. 

Gryphaea planocon- vexa fraterna Imlay n. var., Megasphaeroceras spp., 

Spiroceras sp., 

Stemmatoceras 

spp., and 

Stephanoceras 

spp. Fossils com- mon. Crinoid and echinnoid fragments. 

Fossils rare. 

Age 

Callovian ______ _ 

Early Callovian_ 

_____ do ________ _ 

Probably Bathonian. 

Late Bajocian __ _ 

_____ do ________ _ 

Probably middle or late 

Bajocian. 

Origin 

Shallow marine to 

littoral. Sea re- gressive westward. 

Sand derived from 

west. Much wave action. 

Shallow marine. 

Soft calcareous 

mud formed rapidly. Sand and 

silt derived mostly 

from east. 

Shallow sea transgressive eastward. 

Shallower than 

during deposition 

of Leeds Creek 

Member. 

Shallow marine to 

littoral or lagoonal. 

Sea regressive 

westward. Climate probably hot 

and dry. 

Shallow sea transgressive eastward. 

Bottom probably 

muddier and 

deeper than during Sliderock 

time. 

Shallow warm sea transgressive eastward. Sand and 

pebbles derived 

from west. 

Shallow marine to 

lagoonal. Sea 

transgressive eastward. Climate 

probably hot and 

dry. 

~ ~ 

0 

t::::l 

c:l 

c 

~ 0 

z 

~ 
4 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

The Twin Creek Limestone was deposited in the 

northern part of a trough that extended frmn southwestern Utah northeastward into central Utah and then 

northward through north-central Utah into southeastern Idaho (Peterson, 1954, p. 474; 1957a, p. 401-403). 

The northernmost part of the trough near the Snake 

River in Idaho curves toward the northwest as shown 

by facies and thickness changes in the Twin Creek Limestone and perhaps by present structural trends. The 

axis of the trough lay at least as far west as the westernInost outcrops of Jurassic rocks as shown by westward 

thickening of those rocks. 

The limestone and the associated beds deposited in the 

deeper part of this trough during Middle and early 

Late Jurassic time are simila,r lithologically throughout 

its extent and conceivably eould have been called by a 

single name. Nevertheless, the term Carmel Formation 

is used in the southern part, of the trough because the 

Jurassic rocks are continuous with those at Mount Ca,rmel and in other areas to the east and northeast in 

southen1 Utah. The term Twelvemile Canyon Member 

of'the Arapien Shale is used for marine beds in the central part of the trough that differ from the Carmel Formation by being thicker, shalier, less gypsiferous, and 

grayer and that are separated from the Carmel Formation geographically by areas with few Jurassic outcrops. 

The northenunost exposures of the Twelvemile Canyon 

Mmnber northeast of Nephi, Utah, are only 15 miles 

southwest of lithologically smiliar exposures of the 

Twin Creek Limestone south and southeast of Thistle. 

Overall, the Twin Creek Limestone differs by being divisible into well-defined mappable members and by its . 

upper part being more calcareous, less shaly, and less 

red. The actual southen1 limit of the formation appears to have been chosen arbitrarily, however, in an 

area that has not yet been studied in detail. 

The Twin Creek Limestone crops out in an area of ex~ 

tensive thrust faulting along the Idaho-Wyoming border and in north -central u~tah, extending from the 

sou1then1 end of the Teton Mountains west of Jackson, 

Wyo., southward to near the south end of the Wasatch 

Range a fe\Y miles south of Thistle, Utah (figs. 1-8). 

The westernmost outcrops are a few miles east of Salt 

Lake City, Utah, and Idaho Falls, Idaho, and the easternmost outerops in Wyoming are about 5 miles east of 

a north-south line drawn through Kemmerer. The 

Twin Creek Limestone also occurs in the Uinta Mountains, Utah, as far east as Lake Fork. Its extent from 

north to south is about 170 miles and from east to west 

about 60 miles. 

Within this area the formation thickens southwestward from about 665 feet in northwesten1 Wyoming to 

about 2,720 feet at Willow Creek near Idaho Falls. In 

northern Utah it thickens westward frmn about 440 feet 

at Lake Fork in the Uinta Mountains to about 2,850 feet 

near Burr Fork in the Wasatch Range (table 2). 

The Twin Creek Limestone consists mainly of medium- to light-gray limestone, most of which is shaly 

and weathers into long splinters. The formation also 

contains one sandy member at the top, two persistent 

red-bed members in the lower third, and one cliffforming limestone member at the top of this lower third. 

These make possible the division of the Twin Creek 

Limestone into seven mappable mem:bers. 

The term Twin Creek Limestone is not used east of the 

Darby-Absaroka line of thrusting because the equivalent 

beds east of the faulted area, such as those exposed at 

Lower Slide Lake on the Gros Ventre River, Wyo., are 

much thinner, much less calcareous, and much more fossiliferous than the typical Twin Creek Limestone, and 

may more properly be classified according to the terminology used in north-central Wyoming. Nevertheless, 

the Jurassic sequence at Lower Slide Lake contains units 

that are similar litholo~ically and stratigraphically 

to the lower six members of the Twin Creek LimeStone. 

The presence of these units and particularly of the fossils (fig. 9) that they contain aids greatly in correlations 

and age determinations of members of the Twin Creek 

Limestone. 

The type locality of !the Twin Creek Limestone on 

Twin Creek (fig. 1, loc. 29,) includes only the lower four 

of the seven members present in the formation. The 

nearest exposed complete section of the formation is on 

Leeds Creek about 8 miles due north of the type locality 

in the same fault block. On Leeds Creek the lower four 

members are not as well exposed as at the type locality, 

but the upper three members are excellently exposed. 

The section at Leeds Creek (fig. 1, lock 28) , therefore, 

supplements the type section and may be considered as 

a standard of reference for 1the upper part of the formation. Both sections are described below. 
41° 

BINGHAM 

Blackfoot 

0 

BOX ELDER 

STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 

Dubois 

0 

'13 

FREMONT 

NAMES OF SECTIONS 

FIGURE 1.-Index map of western Wyoming, southeastern Idaho, and northern Utah showing looations of columnar 

sections illustrated in figures 2-8 and some other sections or places discussed in text. Locations of sections are 

given in more detail in table 2. 

5 
6 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

w 

z 

0 

fen 

w 

::i?: 

....1 

::.:: 

w 

w 

c:: 

(.) 

z 

::: 

fWEST EAST 

4 

Willow Creek 

(north side) 

Sees. 17-19, T. 1 N., 

R. 40 E., 

Bingham Co., Idaho 

PREUSS SANDSTONE 

GIRAFFE CREEK 

MEMBER 

WATTON CANYON 

MEMBER 

BOUNDARY RIDGE 

MEMBER 

RICH MEMBER 

GYPSUM SPRING 

MEMBER 

NUGGET SANDSTONE 

5 

Fall Creek 

(west side) 

Sec. 18, T. 1 N., 

R. 43 E., 

Bonneville Co., Idaho 

6 

Mosquito Pass 

Nih sec. 34, T. 41 N., 

R. 118 W., 

Teton Co., Wyo. 

7 

Lower Slide Lake 

Sec. 4, T. 42 N., 

R. 114 W., 

Teton Co., Wyo. 

20993 

"Upper Sundance" Formation 

20958, 20962, 20967 G. p. 

H-lf='""'='=f--,;G-:-::yp::::s::-:u m::-;;-S·p ring Formation 

G. n. = Gryphaea nebrascensis Meek and Hayden 

G. p. =G. planoconvexa Whitfield 

G. f. =G. planoconvexafraterna Imlay, n. subsp. 

28507 =Mesozoic fossil locality 

X =Glauconitic 

0 =Oolitic 

FEET 

COLOR SYMBOLS 0 

LIGHT GRAY 

200 

MEDIUM GRAY 

DARK GRAY 

TO BLACK 400 

GREENISH GRAY 

YELLOW TO 

~ 600 BROWNISH GRAY 

RED. MAROON. 

PURPLE u 800 

FIGURE 2.-Columnar sections from Willow Creek, Idaho, to Lower Slide Lake, Wyo. Explanation of lithologic 

symbols shown in figure 3. 
LLJ 

z 

0 

1-

m 

LLJ 

2 

....1 

~ 

LLJ 

LLJ 

0::: 

() 

z 

$: 

1-

WEST 8 9 

Big Elk Mountain Cabin Creek 

(north side) (north side) 

Sec. 6, T. 2 S., R. 45 E., Sec. 17, T. 38 N., R. 116 W., 

Bonneville Co., Idaho Teton Co., Wyo. 

10 

Mumford Creek 

(north side) 

Sec. 32, T. 38 N., 

R. 115 W., 

Lincoln Co., Wyo. 

11 

Hoback Canyon 

Sees. 31 and 32, T. 39 N., 

R. 114 W., 

sec. 6, T. 38 N., R. 114 W., 

Teton 

and Sublette Cos., Wyo. 

12 

Green River Lakes 

T. 39 N., R. 108 W. 

and 109 W., 

Sublette Co., Wyo. 

(after Richmond, 

1945) 

PREUSS SANDSTONE --~ 1 : :I -·~ .. ·~ Sandstone ~ 

GIRAFFE CREEK 

MEMBER 

LEEDS CREEK 

MEMBER 

WATTON CANYON 

MEMBER 

BOUNDARY Rl DGE 

MEMBER 

RICH MEMBER 

SLIDEROCK MEMBER 

GYPSUM SPRING MEMBER 

NUGGET SANDSTONE 

X =Glauconitic 

0 =Oolitic 

C =Chert bands 

V =Vesicular 

18181 =Mesozoic fossil locality 

COLOR SYMBOLS 

LIGHT GRAY 

MEDIUM GRAY 

DARK GRAY 

GREENISH GRAY 

YELLOWISH TO 

BROWNISH GRAY 

RED, MAROON, 

PURPLE 

FEET 

0 

200 

400 

600 

800 

D . 

. Conglomerate 

~ 

~ 

Sandstone 

~ 

~ 

LITHOLOGY 

~ Limestone 

Shaly 

~ limestone 

LLJ.:1:J 

~ 

Shaly sandstone Sandy limestone 

~ 

~ 

Siltstone 

~ 

~ 

Shale 

Brecciated limestone 

rJIIjjJ 

Gypsum 

13 

Red Grade 

Sees. 12 and 13, 

T. 5 N., R:6 W., 

Fremont Co., Wyo. 

(after Love 

and others, 1945) 

_;_:_:_;1 X 

EAST 

"Lower Sundance'' 

Formation 

Gypsum Spring Formation 

0 ~ Jackson' 13 

'-ldahoFa~lsl 0 _j--, L..., 8 I 11 r 12 

I D 1H 0 9 10 , 

1- ----j I \___ l I ) ' Soda1 Springs I 0 .., ""' t--<J Aftot . I 

{~~er0 I t!!__!O~ IN_<j 

L r __ \ ...,. _ __J I OCokeville r 

\ I 0 Kemnierer 

I I I I 

~~J----~ '"'\~ r I ~ ( L ____ ~,.- __ _ 

S~LT /\!--.J ~A~Y-1 -------~-------- 0 1 0Whiterocks 

~ -l 1 Lake 1 o 

\ \ I Fork I Vernal I \UTAH 

Thistleo 11.._ 0 25 50 MILES 

I•" ,1 I 

FIGURE 3.-Columnar seetions from Big Elk Mountain, Idaho, to Red Grade, Wyo. 

Ul 

8 

~ 

~ 0 

::0 

> 

~ 0 

l'%j 

8 

~ t;j 

~ z 

Q 

::0 

t;j 

t;j 

~ 

t'4 

~ t;j 

~ 0 

~ 

--.:( 
8 

LLJ 

z 

0 

1-

C/) 

LLJ 

::i! 

...J 

~ 

LLJ 

LLJ 

a: 

(.) 

z 

TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

WEST 

16 

Stump Creek 

on Lander Cutoff 

Sees. 27 and 28, T. 6 S., 

R. 45 E., 

Caribou Co., Idaho 

PREUSS SANDSTONE 

GIRAFFE CREEK 

MEMBER 

LEEDS CREEK 

MEMBER 

WATTON CANYON 

MEMBER 

BOUNDARY RIDGE 

MEMBER 

RICH MEMBER 

SLIDEROCK MEMBER 

GYPSUM SPRING 

MEMBER 

NUGGET SANDSTONE 

17 

Cottonwood Creek 

(east of Smoot) 

Sees. 35 and 36, T. 31 N., 

R. 118 W., 

Lincoln Co., Wyo. 

COLOR SYMBOLS 

LIGHT GRAY 

MEDIUM GRAY 

DARK GRAY 

TO BLACK 

GREENISH GRAY 

YELLOW ISH TO 

BROWNISH GRAY 

RED. MAROON, 

PURPLE 

18 

Poker Flat 

on Lander Cutoff 

Sees. 3 and 10, T. 29 N., 

R. 117 W., 

Lincoln Co., Wyo. 

EAST 

19 

South Piney Creek 

(north side) 

Sec. 11, T. 29 N., 

R. 115 W., 

Sublette Co., Wyo. 

0 G. n. 

17319, 20960 

17299 

17298 

LITHOLOGY 

~ 

L:::_i3 

Conglomerate 

L_:j ~ 

Sandstone 

Shaly 

~ 

~ sandstone 

~ 

~ Siltstone 

~ 

~ Shale 

~ limestone 

8 

Shaly limestone 

Sandy 

~ 

~ limestone 

V =Vesicular 

X=Giauconitic 

O=Oolitic 

P=Pebbly 

C=Chert bands 

Brecciated limestone 

22459= Mesozoic fossil locality 

FEET 

0 

200 

400 

600 

800 

~Idaho Falls I 0 Jacks_:j-, 

\_ I r \ 

ID1_Ho jL--] " 

1- ----1 I "-._ 

}, 16 I I \ Soda Springs~ff4m ""' \$--- I 17· I 

\ 1..--118 19 

~~~er0 I ~:'O~IN_G_j r \ I 0 Cokeville r L_..,._---J 

\ J 0 Kemnlerer 

I I I I y , ___ , - 0 Ogde':f\ 1 I 

.__,~ r~ I 

~ c L----1.---- /\_/ , I 

{ 0 ..___) ------l--"---- S~LT ~A~-1 o I0Whiterocks 

~ - l 1 Lake 1 o 

\ t I Fork I Vernal 

I \UTAH 

Thistle 0 1 

.__/\ __ 1.... 0 25 

I I " • I 

50 MILES 

I 

FIGURE 4.-Columnar sections from Stump Creek, Idaho, to South Piney Creek, Wyo. 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 9 

LJ.J 

z 

0 

1-

en 

LJ.J 

:::! 

...J 

~ 

LJ.J 

LJ.J 

0::: 

(.) 

z 

3: 

1-

WEST 20 

Preuss Creek 

(north side) 

Sec. 15, T. 11 S., 

R. 45 E., 

Bear Lake Co., Idaho 

PREUSS SANDSTONE 

GIRAFFE CREEK 

MEMBER 

LEEDS CREEK 

MEMBER 

WATTON CANYON 

MEMBER 

BOUNDARY RIDGE MEMBER 

RICH MEMBER 

SLIDEROCK MEMBER 

GYPSUM SPRING 

MEMBER 

NUGGET SANDSTONE 

21 

Thomas Fork Canyon 

(north side) 

Sees. 19 and 20, T. 28 N., 

R.120 W., 

Lincoln Co., Wyo. 

23 

Sliderock Creek 

(north side) 

Sec. 10, T. 25 N., 

R.118 W., 

Lincoln Co., Wyo. 

COLOR SYMBOLS 

LIGHT GRAY 

MEDIUM GRAY 

DARK GRAY 

TO BLACK 

YELLOWISH TO 

BROWNISH GRAY 

RED, MAROON. 

PURPLE 

24 

Devils Hole Creek 

(North Fork) 

Sec. 15, T. 27 N., 

R. 117 W., 

Lincoln Co., Wyo. 

FEET 

0 

200 

400 

600 

800 

FIGURE 5.-Columnar sections! from Preuss Creek, Idaho, to La Barge Creek, Wyo. 

in figure 4. 

25 

Fontenelle Creek 

(South Fork) 

Sec. 33, T. 26 N., 

R.ll6 W., 

Lincoln Co., Wyo. 

26 EAST 

La Barge Creek 

(north side) 

Sees. 16 and 17, T. 27 N., 

R. 115 W., 

Sublette Co., Wyo. 

20983 

20963, 28639 

X =Glauconitic 

0 =Oolitic 

V =Vesicular 

P =Pebbly 

G. f. = Gryphaea plarwconvexa fraterna Imlay 

209!10 =Mesozoic fossil locality 

~ Idaho Falls I 0 J ac ks:;----1 

L_ I r \ 

ID1_HO jL-1 , 1- ------1 I \.___ 

Sod\ Springs i 0 ~ '""' t--::u Aftot I 

r~~er 20W-: 0 ~IN _<j r \ I 0 23 25 r L __ _,. _ ___J Cokeville 

( j 0 Kemnlerer 

I I I I u ~----r - 0gde~\ 1 I 

-.\~ r..Y-1 I 

\ ( L---~1,---- /)-> ' I { 0 '-..) _ _......---~ __ ....,____ 

s~LT ~A~-1 o Jowhiterocks 

,...._______ - l 1 Lake 1 o 

\ l I Fork I Vernal I \UTAH 

Thistle 0 1 

._/', __ 1.._ 0 25 

I " " I 

50 MILES I 

Explanation of lithologic symbols shown 
10 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

NORTH 

6 

Mosquito Pass 

N1h sec. 34, T. 41 N., 

w GIRAFFE CREEK z MEMBER 

0 

I-

(/) 

w LEEDS CREEK 

~ MEMBER 

....J 

~ 

UJ 

UJ 

a:: 

(.) 

z 

WATTON CANYON 

MEMBER 

R. 118 W., 

Teton Co., Wyo. 

14 15 

Fall Creek Greys River 

(north side) at Sheep Creek 

Sec. 20, T. 39 N., Sec. 4, T. 33 N., 

R. 116 W., R.116 W., 

Teton Co., Wyo. Lincoln Co., Wyo. 

FEET 

0 

200 

400 

600 

800 

COLOR SYMBOLS 

LIGHT GRAY 

MEDIUM GRAY 

DARK GRAY 

TO BLACK 

YELLOWISH TO 

BROWNISH GRAY 

RED. MAROON. 

PURPLE 

18 

Poker Flat 

on Lander Cutoff 

Sees. 3 and 10, 

T. 29 N., R. 117 W., 

Lincoln Co., Wyo. 

23 

Sliderock Creek 

(north side) 

Sec. 10, T. 25 N., 

R.118 W., 

Lincoln Co., Wyo. 

O=Oolitic 

c=Chert bands 

SOUTH 

28 

Leeds Creek 

Sec. 30, T. 23 N., 

R. 118 W., and 

sec. 25, T. 23 N., 

R. 119 W., 

Lincoln Co., Wyo. 

20965, 20987 

---w-h-'.,-.....[..L0966, 20975, 20977, 

-----"~:t-t::=-'=""~ 20981, 28798 G. f. 

G. n.= Gryphaea nebrascensis Meek and Hayden 

G. t.=G. planocO'nvexafraterna Imlay 

20346=Mesozoic fossil locality 

FIGURE 6.-Columnar sections from Mosquito PaBs to Leeds Creek, Wyo. Explanation of lithologic symbols Bhown in figure 4. 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 

SOUTH 

0 =Oolitic 

P =Pebbly 

V =Vesicular 

36 

Monks Hollow 

Sec. 32, T. 4 S., 

R. 5 E., and sec. 5, 

T. 5 S., R. 5 E., 

Utah Co., Utah 

600l 

800J 

G. f.= Gryphaea planoconvexa fraterna Imlay 

28790 =Mesozoic fossil locality 

D . 

0 

Conglomerate 

~ 

~ Sandstone 

~ 

t"~ Shaly sandstone 

~ Siltstone 

LITHOLOGY 

~ 

~ 

Shale 

g 

Limestone 

Shaly ~ limestone 

Sandy 

~ 

~ limestone 

35 

Burr Fork 

NEIA sec. 21, 

SWIA sec. 22'/ 

NWIA sec. 2 , T. 1 N., R. 2 E., 

Salt Lake Co., Utah 

(after Granger, 1953) 

34 

Near Deviis Slide 

Sec. 24, T. 4 N., 

R. 3 E. to sec. 19, 

T. 4 N., R. 4 E., 

Morgan Co., Utah 

(after T. E. Mullens and 

T. H. Cole, unpub. data) 

33 

Birch CreekWatton Canyon Sees. 17-20, T. 9 N., 

R. 6 E., sees. 23 and 24, 

T. 9 N., R. 5 E., 

Rich Co., Utah 

I 

I 

) {Exposed in road cut on east side 

' of Brigham Fork in SWij4 NE\4 

sec. 21, T. 1 N., R. 2 E. 

m "' "' A 

Brecciated limestone 

COLOR SYMBOLS 

LIGHT GRAY 

MEDIUM GRAY 

DARK GRAY 

GREENISH GRAY 

YELLOWISH TO 

BROWNISH GRAY 

RED, MAROON, 

PURPLE 

28 

Leeds Creek 

Sec. 30, T. 23 N., 

R. 118 W., and sec. 25, T. 23 N., 

R. 119 W., 

Lincoln Co., Wyo. 

FIGURE 7.-Columnar sections from Monks Hollow, Utah, to Fontenelle Gap, Wyo. 

11 

30 NORTH 

Fontenelle Gap North-Central sec. 4, 

NEIA sec. 5, 

and NEIA sec. 9, 

T. 24 N., R. 115 W., 

Lincoln Co., Wyo. 
12 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

WEST 

35 

Burr Fork 

NP4 sec. 21, 

SW% sec. 22, 

NW% sec. 27, 

T. 1 N., R. 2 E., 

Salt Lake Co., Utah 

(after Granger, 1953) 

PREUSS SANDSTONE 

GIRAFFE CREEK 

MEMBER 

SLIDEROCK MEMBER 

GYPSUM SPRING 

MEMBER 

NUGGET SANDSTONE 

37 

Northwest of Peoa 

(Weber River) 

Sees. 11 and 14, 

T. 1 S., R. 5 E., 

Summit Co., Utah 

38 

Duchesne River 

(south side) 

Sec. 4, T. 1 S .. 

R. 8 W., 

Duchesne Co., Utah 

39 

Lake Fork 

(west side) 

Sec. 2, T. 1 N., 

R. 5 W., 

Duchesne Co., Utah 

0 =Oolitic 

40 

Whiterocks Canyon 

NW% sec. 19 

and SE% sec. 18, 

T. 2 N., R. 1 E., 

Uintah Co., Utah 

21622 =Mesozoic fossil locality 

I 

_:. :...:.. 11r~~~r :; ;-'I 

I 

I 

,------A--..., 

Measured in NE1,4 

sec.l, T. 1 S., R. 6 E. 

northeast of Oakley 

MEDIUM 

GRAY 

DARK GRAY 

GREENISH 

GRAY 

YELLOWISH TO 

BROWNISH GRAY 

RED, MAROON, 

PURPLE 

LITHOLOGY 

in road cut 

on east side of 

Brigham Fork in 

SW1,4NEV4 sec.21, 

T. 1 N., R. 2 E. 

~ 

~ 

Conglomerate 

r:::::::l 

~ Sandstone 

~ 

~ 

Shaly sandstone 

~ 

~ 

Siltstone 

~ 

~ Shale 

Limestone 

Shaly 

~ limestone 

Sandy 

~ limestone 

Brecciated 

~ limestone 

,. 

Gypsum 

FIGURE 8.-0olumnar sections from Burr Fork to Steinaker Draw, Utah. 

EAST 

41 

Steinaker Draw 

(northeast of Vernal) 

Sec. 7, T. 3 S., 

R. 22 E., 

Uintah Co., Utah 

FEET 

0 

200 

400 

600 

800 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 

TABLE 2.-Stratigraphic thickness in feet of the members of the Twin Creek Limestone in Idaho, Wyoming, and Utah 

[Letters A-G refer to usage of Imlay (1950a)] 

Locality N arne of section Location of section 

(fig. 1) 

-----

Idaho 

!_ __________ Red Mountain __________________ SWU sec. 33 T.4N. T. 44 E _________ 

5_-- -------- West side of Fall Creek _________ SW cor. sec. 18 T.1N. R. 43 E ______ 4 ___________ North side of Willow Creek _____ From NEU sec. 19 to south-central 

part sec. 17 T. 1 N. R. 40 E. 

8_ ---------- North side of Big Elk Mountain_ SWU sec 6 T. 2 S. R. 45 E __________ 

16_-- ------- Stump Creek ___ ---------------- SWU sec. 27 and SEU sec. 28 T. 6 S. 

R.45E. 

20_-- ------- Preuss Creek ____________________ E~ sec. 15, T. 11 S., R. 45 E __________ 27 __________ East of Bear Lake _______________ NE cor. sec. 30, T. 15 S., R. 45 E ______ 

wvomino 

7_- --------- North side of Lower Slide Lake 2 _ Sec. 4, T. 42 N., R. 114 W .. ____________ 

6_- --------- Mosquito Pass __________________ N}1 sec. 34, T. 41 N., R. 118 W -------- 14_--------- North side of Fall Creek ________ NEU sec. 20, T. 39 N., R. 116 W ______ 

9_-- -------- North side of Cabin Creek ______ N}1 sec. 17, T. 38 N., R. 116 w ________ 10_- -------- North side of Mumford Creek ___ SEU sec. 32, T. 38 N., R. 115 W _______ 1L _________ North side of Hoback Canyon ___ EV2 sec. 32 and SEU se('. 31, T. 39 N., 

R.114 W., NEU sec. 6, T. 38 N .• R. 

114 w. 15_- -------- North side of Sheep Creek near Sec. 4, T. 33 N., R. 116 w _____________ Greys River. 17 __________ Cottonwood Creek, 2 miles east W}1 sec. 36 and E.V2 sec. 35, T. 31 N., 

of Smoot. R.118 W 18 __________ Poker Flat_ _____________ -------- Sees. 3 and 10, T. 29 N., R. 117 W _____ 

19_-- ------- North side of South Piney Creek_ Sec. 11, T. 29 N., R. 115 W ------------ 21_-- ------- NorthsideofThOip.asForkCanyon_ Sees. Hl and 20, T. 28 N., R. 119 W ____ 

22 __ -------- North side of Ferney Gulch _____ Sees. 1 and 2, T. 27 N., R. 117~ W., sec. 1, T. 27 N., R. 118 W. 

24_- -------- North Fork of Devils Hole 

Creek. 

Sec.15, T. 27 N .• R. 117 W ____________ 

26_-- ------- La Barge Creek _________________ NWU sec. 16 and NEU sec. 17, T. 27 

N., R.115W. 23 __________ Sliderock Creek~-----------~---- Sec. 10, T. 25 N., R. 118 W ____________ 

25_- -------- South Fork of Fontenelle Creek_ NW}i sec. 33, T. 26 N., R. 116 W _____ 

30_-- ------- Fontenelle Gap ___ -------------- NV2 sec. 4, NEU sec. 5, NEU sec. 9, 

28---------- North side of Leeds Creek _______ T. 24 N., R. 115 W. 

Sec. 30, T. 23 N., R.118 W. and sec. 25, 

T. 23 N., R. 119 W. 

29---------- North side of Twin Creek _______ NEU sec. 1, T. 21 N., R. 119 W _______ 

Utah 

33_- -------- Birch Creek and Watton Canyon_ Sees. 17 to 20, T. 9 N., R. 6 E., sees. 24-26, T. 9 N., R. 5 E. 34 __________ Weber River near Devils Slide __ Sec. 24, T. 4 N., R. 3 E. and sec. 19, T. 

35---------- Near Burr Fork and Brigham 

4 N., R. 4 E. 

NEU sec. 21, SWU sec. 22 and NWU 

Fork. sec. 27, T. 1 N., R. 2 E. 

37---------- Weber River near Peoa and SW}i sec. 11 and NWU sec. 14, T. 1 S., 

Oakley. R. 5 E., NEU sec.1. T. 1 S., R. 6 E. 

38---------- Duchesne River near Hanna ____ swu sec. 4, T. 1 s., R. 8 w ___________ 39_-- ------- Lake Fork_--------------------- NWU sec. 2, T. 1 N .• R. 5 W --------- 40_-- ------- Whiterocks River _______________ NWU sec. 19 and SWU sec. 18, T. 2 

36_-- ------- Monks Hollow __________________ N.,R.1E. 

Sec. 32, T. 4 S., R. 5 E. and sec. 5, T. 

5 S., R. 5 E. 

42_-- ------- Thistle __ --- ____________ --------- W}1 sec. 28, T. 8 S., R. 4 E ____________ 

43_-- ------- Crab Creek_-------------------- SWU sec. 5, T. 10 S., R. 4 E __________ 

1 Not measured. z The beds at Lower Slide Lake formerly referred to members B-F (Imlay, 1956a) are now referred to the lower part of the Sundance Formation. 

Gypsum SlideSpring rock 

Member Member 

A B 

---

12 69 

183 113 

430? 140? 

20 74 

223 281 

129 229 

246 283 

46 21 

80 75 

63 55 

97 97 

107 24 

76 60 

110 45 

102 87 

125 88 82 70 

40+ 188 

140 91 

75 79 

53 75 

150 85 77 68 

75 65 

76 95 

85 97 

36+ 227 

208 100 

140? 150 

22 47 

0 42 0 32 

0 0 

49 91 

9 71 

15 95 

3 Not exposed. 4 Faulted. 

Rich Bound- Watton Leeds Giraffe 

Member ary Ridge Canyon Creek Creek 

Member Member Member Member 

c D E F G 

-------------------

208 36 75 471 26 

374 77 160 628 131 

380 187 151 1,145 295 

228 123 172 520 120 

250 270 400 1,000 90 

271 39 246 1,500+ 71 

(1) (1) (I) (1) (I) 

85 38 57 163 0 

95 45 65 395 25 

151 40 69 477 48 

130 50 89 370 83 

125 71 65 330 41 

104 43 65 290 28 

240 64 155 475 89 

275 66 146 530? 100 

247 83 175 813 102 

85 67 157 262 232? 

315 168 305 1, 625? 111 

252 115 400? 575? 86 

245 69 218 735 102 

208 59 186 402 128 

275 75 154 1,089 186 

184 35 212 487 177 

210 49 57 420 132 

260 108 182 1,118 102 

300 108 62+ (3) (3) 

425 251 399 1, 018? 221? 

540 97 380 1,289 108 

391 102 348 1, 520? 200 

125 107 220 776 82 

91 68 104 280 165 

109 30 109 114 49 

40 21 17 182 85 

123 57 305 275? 288 

183 41 345+ (4) (4) 

140 90 300+ (3) (3) 

13 

Total 

thickness 

---

897 

1,666 

2, 728 

1,257 

2, 514 

2,485 

(1) 

410 

780 

903 

916 

763 

666 

1,178 

1,306? 

1,633 

955 

2, 752 + 1,659 

1,523 

1,111 

2,014 

1,240 

1,008 

1,941 

(3) 

2,577 + 

2, 722 

2,851 

1,379 

750 

443 

345 

1,188 

(4) 

(3) 
Fossils 

Nucula sp. _______ . ___________________ _ 

Grammatodon haguei (Meek) _______________ _ 

Idonearca haguei (Stanton) _________________ _ 

Modiolus subimbricatus (Meek) ______________ _ 

(Musculus) sp. _______________________ _ 

Mytilus cf. M. whitei Whitfield ______________ _ 

Gervillia? montanaensis Meek _______________ _ 

Isognomon cf. I. perplana (Whitfield) ______ .. ___ _ 

Pinna kingi Meek _______________________ _ 

Camptonectes stygius White ________________ _ 

platessijormis White ________ -- -- ___ - ---

cf. C. d·istans Stanton __________________ _ 

Plicatula sp. __________________________ _ 

Lima (Plagiostoma) occidentalis Hall and Whitfield_ 

8~--------------------------------

Ctenostreon cf. C. gikshanensis McLearn ________ _ 

Ostrea (Liostrea) strigilecula White __________ _ 

Lopha sp. ____________ -- -- - - - _________ _ 

Gryphaea planoconvexa Whitfield ____________ _ 

planoconvexa. fraterna Imlay, n. subsp. _______ _ 

nebrascensis Meek and Hayden ____________ _ 

Trigonia americana Meek _________________ _ 

elegantissima Meek ____________________ _ 

Vaugonia conradi (Meek and Hayden) _________ _ 

Myophorella (Promyophorella) montanaensis (Meek)_ 

(Haidaia) yellows~onensis Imlay ___________ _ 

Astarte meeki Stanton ____________________ _ 

(Coelastarte) livingstonensis Imlay _________ _ 

Central and 

Southeastern Idaho, western Wyoming, and north-central Utah! southwestern 

Utah 

-~ ~~ P.< 

oo~ 

s '-< 

::l~ "'s 

~Q) 

c::S 

-

Twin Creek Limestone 

ii:i' ~~ 

.., ~ 

g~ 

~ s ~~ 

s 

~ Q) 

::s 

..<:: 

~ 

~ 

--

--

-

--

- --

r--

-

~~ ~~ 

ii 

1:: s ::l Q) .8::.:: 

1-

~~ .sri! 

u~ 

.§] 

~ s ~~ 

~~ 

f~ u ~ wil 

]~ 

--

r 

~ 

-

~ 

-+-

-

~ f!G 

u~ 

~] 

-~ ~ '-'~ 

Carmel 

Formation 

~ 

;..:::: 

~ $.-4 

~1l 

~ s 

.ss 

1----

f--

--

I--

1----

1---

--

--

1---

-~ ~ ~~ s Q) 

~ s 

~~ 

~~ 

p,_ &~ 

;::>·;;; 

North-central 

Wyoming 

~~ 

~ ~ r;:.;.il 

~~ ·;::: s P,Q) ood 

s.S 

::l gj 

"'s 

~-­ c8 

t-

~ 

j 

-~ 

§ 

00 

J 

Central and 

south-central 

Montana 

b 

§ 

] 

~~ E ~ &.S 

-~1 P..~ 

1---

1---

-+-

---+-

t---

~ 

J 

~ ~ 

Western 

Montana 

~ 

~ 

.s ""' 

i 

00 

--

~ 

~ 

f-----

1---

~ 

~ 

-

t---

~ E 

& 

d 

~ 

~ 

f---- --- --- I--

~ 

t-- 1---+---1~ 

t-- f----- ~ 

-

~ 

! 

$l 

·a 

;::> 

Northwestern 

Wyoming 

Lower Slide Lake sequence below 

"Upper Sundance" 

Formation 

e 

"? 

t-

-~ ;::> 

s 

~ ~ 

$l 

·a 

;::> 

t 

t1-

ffII--

It 

t 

I--

§ 

~ ;::; 

-~ 

:5 

@ 

.... 

Cjl 

:e 

$l 

·a 

;::> 

E 

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-+-

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Opis (Trigonopis) spp. __________ ----------- -

Prorokia fontenellensis Imlay, n. sp. ___________ _ 1- -- 1---

Pronoella cinnabarensis Stanton __________ -- -- - - f-- 1--- I-- tu.intahensis Imlay ____________________ _ -- 1---

cf. P? iddingsi (Stanton) ______________ --- -- -

Qu.enstedtia sublevis (Meek and Hayden) _______ _ 

cf. Q. sublevis (Meek and Hayden) _________ _ 

Protocardia cf. P. schucherti Me Learn _______ _ - 1---

Pleuromya su.bcom.pressa (Meek) _____________ _ - t- -+-

Corbula cf. C. munda McLearn ________ ------- - Pho/adomya kingi Meek ___________________ _ 

inaequiplicata Stanton __________________ _ 

Homomya cf. H. gallatinensis Stanton _________ _ 

~I 1---

t-

- I1-

Goniomya montanaensis Meek _______________ _ 

Myophola.s hardyi Imlay __________________ _ 

Cercomya punctata Stanton ________________ _ 

1 -1 I - -

I-- 1---

1-

~ 

Platymya rockymontana Imlay, n. sp. _________ _ 1---

Thracia -weedi Stanton ____________________ _ 1--- 1--- 1--- fFIGuRE 9.-Stratigraphic distri!bUJtion of Bajocian and Callovian pelecypods in the Twin Creek Limestone and contemporary formations in the western interior of 

the United States. A question mark(?) indicates that the identification is uncertain; black bars show known stratigraphic ranges of species within the 

formations and members. The letters A-G show informal member names as previously pUJblished by Imlay (1950a, 1953'a, 1956a). 

lfJ. 

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Ol 
16 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Twin Ot·eek Limestone (incomplete type section) measured about 

three-fourths of a mile 110rth of Twin Greek in a southward 

draining gulch in NE% sec. 1, T. 21 N., R. 119 W., Lincoln 

County, Wyo. 

[Fig. 1, loc. 219] 

Twin Creek Limestone (incomplete) : 

Watton Canyon Member (incomplete) : Feet 

17. Limestone, mostly thin bedded, dense, mediumbrownish-gray; some medium-bedded limestone -------------------------------------- 43 

16. Limestone, medium-bedded, dense, medium-brownish-gray; forms top of ridge__________________ 16 

15. Limestone, medium-bedded, oolitic, mediumgrayish-brown ------------------------------ 3 

Boundary Ridge Member : 

14. Siltstone, very soft; brownish red at base, becoming purplish and yellowish brown at top; overlain sharply by oolitic limestone______________ 42 

13. Limestone, medium- to thin-bedded, dense, silty 

to finely sandy, medium-gray----------------- 13 

12. Limestone, medium- to thin-bedded, oolitic, hard, 

medium-gray ; weathers brownish gray________ 5 

11. Limestone, hard, medium- to thin-bedded, slightly 

sandy, dense, medium-gray; weathers brownish 

gray --------------------------------------- 5 

10. Siltstone, very soft, yellow to greenish-gray_____ 10 

9. Limestone, thin- to medium-bedded, silty, mediumto light-yellowish-gray, ripple-marked; some 

cross-bedding; weathers brownish gray_______ 17 

8. Siltstone, shaly, soft, medium yellowish-gray____ 2 

7. Limestone, thin-bedded to shaly, silty, lightyellowish-gray, ripple-marked________________ 5 

6. Siltstone, shaly, very soft, medium-yellowishbrown; poorly exposed_______________________ 9 

Rich Member : 

5. Limestone, shaly to very thin bedded, light-graybrown; upper part forms cliff; weathers into 

plates rather than splinters__________________ 182 

4. Limestone, shaly, soft, light-grayish-brown; 

chunky; weathers light gray; some thin hard 

beds contain crinoid columnals, echinoid spines, 

and Oamptonectes fragments; partly covered in 

bottom of gulch______________________________ 118 

Sliderock Member : 

3. Limestone, medium- to thin-bedded, uneven bedded, 

medium-grayish-brown, rubbly ---------------- 82 

2. Limestone, thick-bedded, oolitic, grayish-brown ; 

forms ledge__________________________________ 15 

Gypsum Spring Member : 

1. Siltstone; soft, red, poorly exposed ; some rubbly 

to vuggy limestone from 10 to 20ft above base__ 85 

Total thickness_________________________________ 652 

Nugget Sandstone. 

Twin Greek Limestone Oth north side of Leeds Greek Canyon 

in SW% sec. 30, T. 23 N., R. 118 W., and SE1j4- sec. 25, T. 23 N., 

R. 119 W., Lincoln County, Wyo. 

Preuss Sandstone. 

Twin Creek Limestone : 

[Fig. 1, loc. 2'8] 

Giraffe Creek Member: Feet 

17. Limestone, thin-bedded to shaly, silty, light-yellowish-gray, ripple-marked; weathers slightly 

lighter ; some hard oolitic beds near base and 

top contain rounded to angular limestone 

nodules. Contact with Preuss Sandstone is 

transitional within several feet______________ 102 

Leeds Creek Member : 

16. Limestone, shaly, soft, light-gray ; contains a few 

hard beds, some of which consist mostly of 

crinoid columnals; others have Oamptonectes 

shells in addition___________________________ 164 

15. Lim~stone, shaly, soft, medium- to light-gray____ 814 

14. Limestone, shaly, medium-gray; a few beds of 

yellow silty ripple-marked limestone from 3 to 

6 in. thick__________________________________ 37 

13. Limestone, silty, light-yellow, ripple-marked; 

contains Oamptonectes and Vattgonia (Mesozoic 

loc. 20991)---------------------------------- 6 

12. Limestone, shaly, soft, light-gray--------------- 22 

11. Limestone, shaly to thin-bedded, medium- to 

light-gray ---------------------------------- 75 

Watton Canyon Member: 

10. Limestone, medium- to thin-bedded, mostly dense, 

some oolitic, hard, medium-gray, cliff-forming; 

becomes thinner bedded upwatd-------------- 182 

Boundary Ridge Member : 

9. Covered--------------------------------------- 45 

8. 'Siltstone, soft, red, partly covered_______________ 45 

7. Limestone, thin-bedded, silty, yellow------------ 18 

Rich Member : 

6. Limestone, shaly, medium- to light-gray; soft at 

base; forms low ridge at top wh~re some beds 

are an inch thick ; some beds pinkish to yellowish 

gray; weathers light yellowish gray; contacts 

sharp (Mesozoic loc. 20965 from 38 ·to 40 ft 

above base)---------------------------------- 260 

Sliderock Member : 

5. Limestone, thin- to medium-bedded, 2-18 in. thick; 

becomes thicker bedded upward ; dense, hard, 

dark gray to black (Mesozoic lo.cs. 20966 and 

20981 from upper 20 ft) ---------------------- 85 

4. Limestone, oolitic, hard, dark-gray to black______ 10 

Gypsum Spring Member: 

3. Siltstone, soft, red ; 2 ft of yellow siltstone at top; 

some thin beds of sugary-textured limestone near 

middle -------------------------------------- 27 

2. Limestone, brecciated, medium-yellowish-gray; 

some red spots______________________________ 6 

1. Siltstone, very soft, red, poorly exposed ; several 

feet of gray limestone from 6 to 9 ft above base; 

contact with Nugget covered__________________ 43 

Total thickness _________________________________ 1, 941 

Nugget Sandstone. 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 17 

A smnm.ary of the lithologic, stratigraphic, faunal, 

and ecological features of the Twin Creek Limestone is 

presented in table 1. 

GYPSUM SPRING MEMBER 

DEFIN:ITION 

The term Gypsum Spring Member of the Twin Creek 

Limestone was suggested by Imlay (1950a, p. 39) and 

defined by Oriel (1963) for a basal unit consisting of 

red beds and associated brecciated limestone and chertbearing limestone that rest directly on the Nugget Sandstone. These beds were once called member A by Imlay 

(1950a, p. 37-39; 1953a, p. 54). They were not noted 

by Veatch (1907, p. 56, 57) in his definitions of the Nugget Sandstone and Twin Creek Limestone, but were ineluded by Mansfield (1927, p. 96, 97) partly in the Nugget Sandstone and partly in the Twin Creek Limestone. 

The ?eds were recognized by W. W. Rubey (1958) as 

forming the basal part of the Twin Creek Limestone. 

DISTRIBUTION AND THICKNESS 

The Gypsum Spring Member crops out in the same 

areas as the overlying members of the Twin Creek 

Limestone except in the Uinta Mountains Utah where ' ' the easternmost known exposure is northeast of Oakley 

on the west side of Sta:te Highway 213 in the NE%, sec. 

1, T.1 S.,R. 6E. 

Th~ Gypsum Spring Member characteristically thins 

or thickens markedly within short distances. Regionally it thickens westward in an irregular manner from 

an average of about 75 feet in western Wyoming to 

about 400 feet in the Blackfoot Mountains in Idaho 

(table 2; figs. 2-5). It also thickens considerably 

southwestward from a minimum of 12 feet at Red 

Mountain, Teton County, Idaho, toward the Blackfoot 

Mountains. In northern Utah it thickens westward 

from a few feet near the west end of the Uinta Mountains to about 200 feet near Devils Slide in the Wasatch 

Mountains (fig. 7). Southward in the ·vvasatch Mountains ·the member thins to 49 feet at Monks Hollow (fig. 

7) and 9 feet at Thistle. 

LITHOLOGIC AND STRATIGRAPHIC FEATURES! 

The Gypsum Spring Member is characterized by 

soft brownish-red to yellow siltstone and silty claystor{e 

that contains interbeds, or units, of brecciated or honeycombed limestone and chert-bearing limestone. In 

western Wyoming and loc.:'tlly in easternmost Idaho the 

lower part of the member contains a conspicuous unit 

of brecciated gray to yellowish-gray limestone that 

ranges from 10 to 50 feet in thickness (figs. 2-7). This 

unit is a jumble of sharply angular blocks, generally includes some red siltstone, and shows faint stratification. 

The position of this brecciated limestone unit is occupied 

by thick masses of gypsum in the southeast corner of 

the Jackson quadrangle, Wyoming, in the Elh sec. 36, 

T. 36 N., R. 115 W., about 32 miles SSE of Jackson, 

Wyo. Locally, gypsum has been found in the lower 

part of the member near the head of Crow Creek, Caribou County, Idoaho (Mansfield, 1927, p. 96) about 14 

miles northeast of Montpelier. In addition, about 60 

feet of gypsum is reported from the base of the member 

in the subsurface of north-central Utah (Peterson, 

1957a, fig. 9 on p. 418). 

In many places the middle and upper parts of the 

member contain one or more thin units of yellow honeycombed or brecciated limestone that are generally 

inconspicuous. Pieces of such limestone lying on soilcovered areas are of considerable aid in recognizing and 

tracing the member. 

In southeastern Idaho the 1niddle and upper parts of 

the member contain beds or· units of dense gray, 

yellowish-gray, or white limestone that is siliceous and 

bears nodules and lenses of brownish-gray chert. Such 

beds are about 70 feet thick on Stump Creek in the SW 

~sees. 27 and 28, T. 6 S., R. 45 E., Caribou County, and 

at least 140 feet thick on Williams Creek in the SElJ:a, 

sec. 12, T. 2 S., R. 39 E., Bingham County. Similar 

chert-bearing limestone beds from 1 to 2 feet thick occur near the middle of the rnemher in several sections 

near the Wyoming-Idaho border. They have been 

observed on Cabin Creek in the Slh sec. 17, T. 38 N., R. 

116 W., Teton County, Wyo., on the South Fork of 

Fontenelle Creek in the NW:L4 sec. 33, T. 26 N., R. 116 

W., Lincoln County, Wyo., and at the mouth o£ 

Dunns Canyon near center o£ sec. 10, T. 11 S., R. 44 

E., Bear Lake County, Idaho. The chert-bearing limestone beds in Idaho are interbedded with and overlie 

limestone beds that are silty to finely sandy, a feature 

that appears to become more common westward. 

Sandstone forms a minor part of the Gypsum Spring 

Member. Most sections, however, contain a little 

brownish-red fine-grained sandstone interbedded with 

red siltstone. Fairly thick units o£ brownish-red sandstone occur near the middle of the member on the South 

Fork of Fontenelle Creek, Wyo., and on Preuss Creek, 

Idaho. Yellowish-white sandstone has been noted 

below the brecciated lim.estone at or near the base of the 

member at Sliderock Creek northeast of Cokeville, 

Wyo., at Cottonwood Creek east of Smoot, Wyo., and 

on Sheep Creek near ·Greys River, Wyo. Northeast of 

Oakley, Utah, the entire 22 feet of the member consists 

o£ red, pink, and yellow silty sandstone and sandy siltstone except for a few inches of green tuff at and near 

the top. 

The claystone and siltstone beds and units are mostly 

brownishred to brick red, but some are gray, yellowish 
18 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

gray, greenish yellow, or green. Green streaks occur 

rarely in the units that are dominantly red. 

The basal beds of the outcropping Gypsum Spring 

Member may consist of red siltstone, of soft yellow sandstone, or of brecciated limestone that invariably rests 

sharply on the hard quartzitic Nugget Sandstone. The 

upper contact is generally 1narked by an equally sharp 

change from soft red, pink, or gray siltstone to oolitic, 

or sandy, or dense limestone at the base of the Sliderock 

Member. Locally the top of the Gypsmn Spring Member consists of green to white volcanic tuff as at Thmnas 

Fork Canyon and Cottonwood Canyon in western 

Wyoming and northeast of Oakley, Utah. At Red 

Mountain, Teton County, Idaho, the entire Gypsum 

Spring Member consists of only 12 feet of limestone 

breccia th3!t is overlain directly by thin- to mediumbedded granular limestone at the base of the Sliderock 

Member. Similar limestone breccia elsewhere in Idaho 

and Wyoming are separated from the Sliderock Member by a unit of soft red siltstone. 

In most places the Gypsum Spring Member is poorly 

exposed because it consists mainly of soft siltstone. Its 

position is generally marked, however, by a soil-covered 

ravine lying between the much harder Nugget Sandstone and the limestone of the Sliderock Member of the 

Twin Creek Limestone. In western Wyoming the unit 

of brecciated limestone at or near the base of the memher forms low ridges or cliffs, and the position of the 

red siLtstone is indicated by red soil, by fragments of 

yellow honeycombed limestone, and by some outcrops. 

Westward in Idaho the brecciated unit at the base becomes inconspicuous, but the chert-bearing siliceous 

limestone beds in the middle and upper parts of the 

member become locally conspicuous. Well-exposed sections of the member occur in Wyoming at La Barge 

Creek, Slide rock Creek, and at Cottonwood Creek (see 

locations in fig. 1 and table 2). In Idaho, fairly complete sections are exposed at Willow Creek, Stump 

Creek, and on the road from Crow Creek to Sage Valley 

in the SEi4 sec. 9, T. 9 S., R. 46 E. In Utah the best 

exposed sections are on the Weber River near Oakley 

in the NE1;4 sec. 1, T. 1 S., R. 6 E., and near Devils 

Slide. Many of the exposures in Utah and Idaho have 

been made during road building or quarrying 

operations. 

Many sections of the Gypsum Spring Member have 

been described previously as member A (Imlay, 1950a, 

p. 42-45 ; 1953a, p. 60-62) . The following sections of 

the Gypsum Spring Member illustrate the major lithologic and stratigraphic changes from east to west. 

Gypsum Spring Member on north side of South Piney Creek, 

sec. 11, T. 129 N., R. 115 W., Sublette County, Wyo. 

[Fig. 1, loc. 19] 

3. Siltstone, soft, olive-green; overlain sharply by dark 

gray to black medium-bedded, oolitic limestone at 

base of Sliderock Member _______________________ _ 

2. Siltstone, soft, brownish-red ________________________ _ 

1. Breccia of gray to yellowish-gray limestone containing fragments of many sizes; some pieces much 

honeycombed; weathers medium gray. Rests 

sharply on Nugget Sandstone ___________________ _ 

·,Total thickness _______________________________ _ 

Feet 

5 

42 

35 

82 

Gypsum Spring M em bet· on Cottonwood Creek in Wlh sec. 36, 

T. 31 N., R. 118 W., Lincoln County, Wyo. 

[Fig. 1, loc. 17;] 

13. Tuff, dense, hard; white at top, light green at base; 

overlain sharply by sandy, thick-bedded limestone 

Feet 

at base of Sliderock Member____________________ 6 

12. Claystone, soft, yellowish-gray_____________________ 2 

11. Limestone, brecciated, light-gray, honeycombed; contains fragments of red and green siltstone________ 8 

10. Siltstone, soft, mostly brownish red, partly yellowish 

green to gray___________________________________ 22 

9. Siltstone, soft, light-yellow------------------------ 3 

8. Limestone, brecciated, massive, gray---------------- 3 

7. Breccia of yellow limestone and red siltstone________ 6 

6. Limestone, thick- to medium-bedded, brecciated; contains fragments of red and yeUow siltstone_______ 35 

5. Limestone, yellow to brown, honeycombed__________ 5 

4. Siltstone, soft, yellowish-gray---------------------- 2 

3. Sandstone, massive, :fine-grained, light-yellow; .some 

green specks on fresh surfaces___________________ 2 

2. Sandstone, shaly, :fine-grained, rather soft, yellowishwhite ------------------------------------------ 7 

1. Siltstone, dark-red; rests sharply on Nugget Sandstone ------------------------------------------ 1 

Total thickness -------------------------------- 102 

Gypsum Spring Member near Old Lander Trail south of Stump 

Creek in SElft sec. 28 T. 6 S., R. 45 E., Caribou County, 

Idaho 

[Fig. 1, loc. 16] 

Siltstone, light-gray to pink; interbedded with yellowish-gray thin-bedded limestone that locally is 

brecciated and honeycombed. Overlain sharply by 

sandy fossiliferous limestone at base of Sliderock 

Member ----------------------------------------- 3. Siltstone, soft, brownish-red, poorly exposed ________ _ 

2. Limestone, medium- to thin-bedded, medium-gray; 

weathers light gray; contains much brownish- to 

reddish-gray chert as granules, nodules, and short 

lenses ; contains abundant crinoid fragments _____ _ 

1. Sandstone, fine-grained, and siltstone ; both brownish 

red, poorly exposed. Contact with Nugget Sandstone 

not exposed but mapped as a fault _______________ _ 

Total thickness---------------------------------

Feet 

41 

65 

70 

47 

223 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 19 

Gypsum Spring Member along north side of Willow Oreek in 

NE1f4 sec. 19, T. 1 N., R. 40 E., Bonneville Oounty, Idaho 

[Fig. l, loc. 4] 

Feet 

22. Covered in ravine. Overlain by fossiliferous shaly 

to thick-bedded limestone of the Sliderock Member. 

Assignment to Gypsum Spring Member uncertain__ 54 

21. Limestone, medium-bedded, medium-gray ; contains 

some dark chert near base ; becomes sandy and 

thicker near base ; becomes sandy and thicker bedded upward; forms top of ridge__________________ 38 

20. Limestone, thin-bedded, laminated, finely sandy, light 

gray; contains several chert bands ______________ _ 

19. Siltstone, light-yellowish-gray----------------------

18. Limestone, thick-bedded, dense, silty, dark- to mediumgray, cliff-forming; upper 45 ft contains lenseH of 

brownish-black chert and traces of silicified 

9 

2 

fossils ----------------------------------------- 60 

17. Limestone, medium-bedded, medium-gray; vuggy, 

much fractured ; weathers yellow---------------- 4% 

16. Limestone, medium-bedded, mostly dense, mediumgray, brittle, much fractured____________________ 34 

15. 'Siltstone, calcareous, yellowish-gray, vuggy _________ 2 

14. Limestone, thin-bedded, finely laminated, mediumgray, brittle ; some lamellae curved ; weathers light 

gray --------------------------------------~--- 4 

13. Claystone, silty, calcareous, thinly laminated, 

medium-gray ----------------------------------- 5 

12. Siltstone, sandy, soft, yellowish-gray; some thinly 

laminated ; contains some vuggy calcareous beds___ 9 

11. Limestone, massive, silty, light-grayish-white_______ 4 

10. Limestone, medium- to thin-bedded, silty, mediumyellowish-gray, brittle___________________________ 17 

9. Limestone, medium-bedded, sandy, light-yellowishgray; grades downward into yellow siltstone______ 2 

8. Siltstone, light-yellowish-gray; interbedded with finegrained thin-bedded sandstone ; grades downward 

into red siltstone ___________ ...:.____________________ 5 

7. Siltstone, soft, red; a few white streaks near base___ 6 

6. Limestone, silty, light-yellowish-gray--------------- 1 

5. Limestone, thin~bedded, thinly laminated, mediumgray ------------------------------------------ llh 

4. Limestone, thinly laminated, massive, granular, 

dark-gray, chert-bearing_________________________ 4 

3. Siltstone, medium-bedded, thinly laminated, greenishyellow ----------------------------------------- llh 

2. 'Siltstone, thin-bedded, thinly laminated, greenishyellow; grades at base into red siltstone__________ 6 

1. Siltstone, soft, and fine-grained brownish-red sandstone. Unit rests on quartzitic Nugget 'Sandstone_ 160 

Total thickness ________________________________ 430? 

FOSSILS, AGE, ANn CORRELATIONS 

The most abundant fossils found in the Gypsum 

Spring Member of the Twin Creek Limestone are 

crinoid columnals and echinoid spines. In addition some 

fragments of Oamptonectes similar to 0. distarus Stanton have been obtained from the top of the Gypsum 

Spring Member in the NE:l4 sec 5, T. 24 N., R .. 116 W., 

Lincoln County, Wyo. (USGS Mesozoic loc. 25261). 

The age of the Gypsum Spring Member is :probably 

middle Bajocian inasmuch as it is overlain concordantly 

by the Sliderock Member which contains ammonites of 

earliest late Bajocian age, as described herein. Furthermore, in northwestern Wyoming the Gypsum 

Spring Member passes eastward from Jackson Hole 

into the lower part of gypsiferous red beds (fig. 3, locs. 

12 and 13) that grade upward into a limestone member 

of probable late Bajocian age. This age is based on 

the presence of the ammonite S ohlites, identical with 

ammonites in the Rich Member of the Twin Creek 

Limestone, found near the middle of the same limestone 

member (Imlay, 1956b, p. 583) a few miles south of 

Cody, Wyo. 

Correlation of the Gypsum Spring Member of the 

Twin Creek Limestone with the lower part of the Gypsum Spring Formation of north-central Wyoming is 

supported not only by apparent lateral continuity but 

by a similar stratigraphic position between the Nugget 

Sandstone below and limestone beds above (fig. 10) and 

by striking lithologic resemblances. The fact that the 

brecciated limestone unit in the lower part of the Gypsum Spring Member is replaced by thick gypsum masses 

in the southeastern part of the Jackson quadrangle 

implies that the brecciated limestone unit represents 

beds that collapsed because of the removal by solution 

of interbedded gypsum. It implies, also, that the 

honeycombed or vesicular limestone beds in the Gypsum 

Spring Member represent the positions of former thin 

beds of gypsum. If these conclusions are correct, the 

Gypsum Spring Member along the Wyoming-Idaho 

border must have once been lithologically identical with 

the lower part of the Gypsum Spring Formation to the 

east in the Wind River Mountains (Richmond, 1945) 

and Wind River Basin (Love and others, 1945). 

Similarly, an approximate correlation may be made 

between the Gypsum Spring Member of the Twin Creek 

Limestone and the lower red-bed member of the Piper 

Formation in southern Montana by means of close lithologic and stratigraphic resemblances and by ·tracing 

beds laterally northward through the Bighorn Basin, 

as discussed in detail elsewhere (Imlay, 1956b, P• 579, 

584, 585, figs. 3-7). This correlation is supported by 

the fact that the lower red-bed member of the Piper 

Formation grades upward (Imlay, 1954, p. 54) into a 

middle limestone member that contains the same ammonites as occur in the Rich member of the Twin Creek 

Limestone. Inasmuch as these ammonites, described 

herein, have been found in all parts of the middle limestone member of the Piper Formation, the underlying 

lower red-bed member probably is equivalent to both 

the Sliderock and Gypsum Spring Members of the Twin 

Creek Limestone. 

Correlation of the Gypsum Spring Member of the 

Twin Creek Limestone with beds elsewhere in the west-
European Standard zones in Nort hwestern and Southwestern Central and Northern and eastern Southeastern Idaho, Central Utah Central Utah 

northwest Europe north-central south-central Wyoming and western western Wyoming, and west of the west side of South western Utah stages (Arkell, 1956; Callomon, 1955) Montana Montana Montana South Dakota north-central Utah Wasatch Plateau San Rafael Swell 

Quenstedtoceras 

Iamberti 

" 

"' Peltoceras ., "' athleta ... 

;:I ? ? ? ? ...., Ery mnoceras Twist Gulch corona tum. ... Member Entrada ~ Callovian -., Lak Member Preuss Sandstone Ent rada Sandstone Kosm.oce·ras u g -.-?- v p, Sandstone jason ., ,::: Salt and ;:J ]:3 red beds ? ;:I., 

... Oolitic limestone 00 S Hulett Sandstone '------?- Giraffe Creey Q) Sigaloceras ? ? and sandstone ~ S Member Member Gypsiferous red Gyp siferous red ~ ca llov'ien se Rierdon ~ ... r---- - to gray silt- san dstone and 0 

F~~~~n ~ Sfllia~:'}d,B,;:i:;~r Leeds Creek stone and ...< Rierdon Formatior. Member sandstone siltstone 

Mac·rocephalites Formation 

/ Oolitic limestone / Oolitic limestone Canyon Springs Watton Canyon t--o;;ITti~ sandy ~?~ L tone and macrocephalus and sandstone Sandstone Member Member Gray limestone psum 

?~ ?~ ? ? r--?- ? Clydoniceras 

~ ~ ~ limestone 

discus and Gypsiferous G;ili~ ferous red ... tone and "' claystone Oppelia .,9:1 ,0 claystone sandstone ., s ,::: and aspido·ides ,.<::: 

~ Some thin ·.§ Q) 

w. ., Yellowish- siltstone Siltstone, Red beds ,::: @ ,::: beds of E Tulites Red \ ~ Boundary ·a 0 

Bathonian sandstone & gray sandstone, grading » subcon tra.ctu.s Ridge ~ ,::: 

siltstone beds \ s ., and sandy westward and § ...:: u 'i ;::! Member s Thousand Graciz.isphinctes ,::: and silty ,::: limestone northward -~ ~ ... Pockets ·.§ 0 ,::: _,. ·s ., Tongue progracil1:s to sandy ., -~ E ., ., Q) u into sandy ~ ~ E ~ 0 limestone E limestone ... u Zigzag·iceras ... 

0 

... bll .s ~ z·igzag 0 

-~ ~ E-< ,.<::: 

~ ... "'0 ... -?-~ E-< r---?- r---?- "' -? ------' ~?--? 

Parkinsonia B ~ p, Limestone and S Limestone and Limestone, sandy, Judd H ollow Tongue 

" ~ Gray shale i5:; oolitic, o f Carmel parkinsoni ., ., claystone, ;:I claystone, locally pebbly F ormation 

"' w. w. and ~ Rich 1----n---;- ? - til Dark shale locally » ., Garantiana locally Member Red to gray Gray li mestone 

limestone C,!) sandstone, and shale ... garantia.na and pebb_!r..-- pebb~ siltstone, and ;:I 

---? 1----- ? ...., limestone limestone -?- ? Q) Strenoceras Sandstone and Sliderock subfurcatum. siltstone Massive Massive 

"" Member 

"" ?/ gypsum and gypsum and 

):;1 Stephanocera.s --?- red beds red beds Gypsum Spring I 

II II? 111111 Red Temple Cap 

humphriesianum Sandstone Member , beds Member 

? ? ? ? Otuites Bajocian sauzei Possibly Possibly 

Sonninia. represented represented Navajo 

sowerbyi by part of the by part of the Sandstone 

Nugget or Navajo Navajo (upper part) Ludwigia Sandstone Sandstone m urch1:nsoni 

Tmetoceras 

sc1."ssu,m 

Leioceras 

opalinum 

FIGURE 10.-0orrelation of Midd,le and lower Upper Jurassic formations in the western interior of the United States. Vertica,l ruled lines indieate a hiatus. 

t..,J 

0 

~ ~ 

z 

0 

~ 

trJ 

trJ 

~ 

~ 

U1 

~ 1-3 

0 

z 

trJ 

~ 

z 

1-3 

111 

trJ 

:a 

trJ 

~ trJ 

~ 

@ 

8 

trJ 

t:! 

U1 

~ U1 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 21 

ern interior region is much less certain. The member 

may be in part equivalent to the lower membHr of the 

Piper Formation in the subsurface of eastern ]\fontana 

(Sandberg, 1959), but it shows more resemblance lithologically to the underlying Nesson For1nation of Nordquist ( 1955), and it could be equivalent to both of those 

units .. It 1nust be older than the middle member of 

the Piper Formation of the subsurface, because tha~t 

member near its 1niddle has furnished Gryphaea planoconvexa Whitfield, a fossil typical of the Rich Member 

or the Twin Creek Limestone, obtained at a depth of 

2,896-2,906 feet in The Texas Co. Bowdoin Unit 1 

( 822) , Phillips County, Mont. 

The Gypsum Spring Member of the Twin Creek 

Limestone should be equivalent to part of the Sawtooth 

Formation in northwestern Montana (Cobban, 1945, p. 

1272-75; Imlay and others, 1948) beneath the upper 

siltstone member, which contains ammonites of Bathonian or earliest Callovian age (Imlay, 1962a, p. C-12). 

Inasmuch as the middle shale member of the Sawtooth 

Formation grades downward from the upper siltstone 

member and near its base contains the middle Bajocian 

ammonites Ohondroceras (Imlay, 1948, p. 19) and 

Stemma.toceras (see S. arcicostunz, Imlay, n. sp.), the 

upper part of the shale member should be of late 

Bajocian age, equivalent to the Sliderock and Rich 

Members of the Twin Creek Limestone. Ammonites of 

definite late Bajocian age, however, have not been found 

in the middle shale member, or anywhere to the north 

in the Fernie Group of Canada ( Frebold, 195~~, p. 1). 

The Arapien Shale of central Utah at one place is 

marked basally by 1-10 feet of soft red siltstone that 

is comparable lithologically and stratigraphically to 

the Gypsum Spring Member of the Twin Cree:k Limestone. This place is in the bot.tom of Red Canyon about 

2 miles northeast of Nephi, Utah. Elsewhere in Red 

Canyon (Johnson, 1959, p. 20-25; Imlay, 1964a, p. C6), 

the Nugget Sandstone is overlain directly by sandy 

oolitic limestone similar to the Sliderock Member of 

the Twin Creek Limestone, but the contaot is marked 

by slickensides and could be a fault. 

Elsewhere in central Utah the charaoteristies of the 

basal beds of the Arapien Shale are unknown. In the 

area southwest of Marysvale, the Navajo Sandstone is 

reported to be overlain by 1,000 feet of 1nedium- to 

thin-bedded limestone (Willard and Callaghan, 1962), 

but the nature of the contact is not described. A thin 

unit of soft red siltstone at the base of the limestone 

could easily be overlooked, or perhaps considered pal"t 

of theN avajo Sandstone. 

The Carmel Formation in southwestern Utah between 

Gunlock and Cedar City has a basal red-bed unit 

(Wright and Dickey, 1963a) that is simih,r lithologically to the Gypsum Spring· Member of the Twin 

Creek Limestone, is overlain by limestone containing 

pelecypods that are characteristic of the Sliderock and 

Rich Members (Imlay, 1964a, p. C3), and contains bentonite beds (Schultz and Wright, 1963, p. E67; Wright 

and Dickey, 1963a, p. E66) that could be related in 

origin to the green tuff beds at the top of the Gypsum 

Spring Member. This basal red-bed unit is replaced 

eastward in Zion National Park by the Temple Cap 

Member of the Navajo Sandstone (Cook, 1957, p. 34; 

Wright and Dickey, 1963a, p. E64-65). 

ORIGIN 

The lithologic and stratigraphic characterishcs of the 

Gypsum Spring Member of the Twin Creek Limestone 

indicate that it was the initial deposit in a very shallow 

sea that advanced eastward frmn a basin whose center 

lay west of the present western limit of Jurassic o~tcrops. In westernmost Wyoming the oldest depos~ts 

at most places consisted of a few feet of red to white 

sand or red silt. These were overlain by a thick mass 

of gypsmn that was interbedded with some calcareous 

mud and red silt. Then followed deposition of red silt 

and clay that included minor amounts of gypsum and 

calcareous mud. Some of the mud contained siliceous 

material that now forms nodules and lenses of chert. 

This succession was terminated abruptly by deposition 

of the fossiliferous oolitic limestone of the Sliderock 

Member that represents a sudden deepening of the se.a. 

West ward in southeastern Idaho the oldest deposits 

consisted mostly of red, yellow, or gray silt and sand 

but included some gypsum and calcareous mud. Then 

followed deposition of considerable calcareous mud that 

included some siliceous material, sand, and fossil remains. Deposition of this mud was interrupted at times 

by an influx of yellow to gray silt and clay and some 

gypsum. The latest beds deposited generally consisted 

of red silt and clay tha;t included some calcerous mud 

and a little gypsum. This succession was terminated 

abruptly by deposition of sandy dense to oolitic fossiliferous calcareous mud that is included in the Sliderock Member. Overall, the deposition of gypsum 

increased eastward and the deposition of calcerous mud 

and siliceous ma;terial increased westward. Fossil remains are scarce throughout but become more common 

westward. These features plus an irregular westward 

thickening of the member indicate that the sea deepened 

somewhat westward. 

The position of the Gypsum Spring Member of the 

Twin Creek Limestone at the western edge of a sheet 

of red beds and gypsum that extends eastward to the 

Black Hills of western South Dakota (Imlay, 1950b, 

p. 85) and northward through eastern Montana (Imlay 

and others, 1948; Rayl, 1956, p. 36; Peterson, 1957a, 
22 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

p. 406, 422) into Manitoba and Saskatchewan (Francis, 

1956, p. 23-25, 46) indicates that the sources of clastic 

sediment lay to the south and west, or possibly northwest. Probably some sand or silt was derived also 

from the formations over which the sea transgressed. 

The presence of extensive masses of gypsum over such 

an enorn1ous area suggests that the land surrounding 

the sea were so hot and arid that few streams entered 

the sea. Highly oxidizing conditions during the deposition of the red beds and gypsmn is indicated by a 

complete lack of fossils in such beds. 

SLIDEROCK MEMBER 

DEFINITION 

The Sliderock Member of the Twin Creek Limestone 

consists of dark fossiliferous medium- to thin-bedded 

limestone, from 20 to nearly 300 feet thick, that rests: 

sharply on the red beds or brecciated limestone of the 

Gypsum Spring Member and is overlain gradUJtionally 

within a few feet by soft shaly limestone at the base of 

the Rich Member. The type section (fig. 1, loc. 23) is 

designated as on the west side of Grade Creek at the 

junction with Sliderock Creek in NEll.! sec. 10, T. 25 N., 

R. 118 W., Lincoln County, Wyo. At this locality the 

lower 12 feet of the member consists of hard black 

oolitic limestone that overlies soft red siltstone. The 

upper 73 feet consists of black to medium-gray very 

fossiliferous cmnpact thin- to medium-bedded limestone 

that is 1nore resistant than the overlying shaly limestone. The Sliderock Member has previously been 

called member B by Imlay (1950a, p. 39; 1953a, 

p. 54, 55). 

DISTRIBUTION AND THICKNESS 

The Sliderock Member crops out in the same areas 

as the overlying members of the Twin Creek Limestone. 

In the Uinta Mountains its easternmost exposures are 

on Lake Fork (Imlay, 1953a, fig. 3). In no~thwestern 

Wyoming east of the area of Twin Creek Limestone it 

is represented at Lower Slide Lake, Teton County, by 

units 7-9 of the published section (Imlay, 1956a, p. 70). 

The Sliderock Member thickens westward from a 

minimum of 21 feet in the Jackson Hole area, Wyoming, 

to about 285 feet in southeastern Idaho. In northern 

Utah it thickens westward from 32 feet near Lake Fork 

in the Uinta Mountains to 150 feet near Burr Fork in 

the Wasatch Range. Southward in that range it thins 

to less than 100 feet near Thistle (table 2). 

LITHOLOGIC AND STRATIGRAPHIC FEATURES 

The Sliderock Member consists mainly of medium- to 

thin-bedded grayish-black to brownish-gray dense to 

granular limestone (figs. 2-8). In Wyoming the basal 

5-15 feet is generally a dark hard thick-bedded oolite 

that contains some sand grains and some pyrite. In 

southeastern Idaho the basal 20-60 feet generally con· 

sists of brownish glauconitic sandy crossbedded limestone that locally contains tiny pebbles of red, g~n, or 

gray chert. Pebbles have been noted in the member 

at Willow Creek, Stump Creek, Preuss Creek, and 1lh 

miles east of Bear Lake on the road to Pegram. 

Locally in Idaho, similar sandy beds occur at higher 

levels. On Willow Creek, east of Idaho Falls, a 4·foot 

bed of quartzitic sandstone occurs 34 feet below the top 

of the member beneath a bed of bentonite. Oolites may 

occur in the basal beds, or higher, but are somewhat less 

common than in Wyoming. Units of so:£t claystone 

occur in the member on Willow Creek, Idaho, and on 

Birch Creek, Utah. Overall toward the west the Sliderock Member becomes increasingly sandier, more clayey, 

and less oolitic. 

The Sliderock Member in southeastern Idaho also 

contains some light-green to white volcanic ~tuff in beds 

that range from 4 to 10 feet in thickness (Mansfield, 

1927, p. 97; Gulbrandsen and Cressman, 1960, p. 458-

464). On Preuss Creek, tuff occurs 153 feet above the 

base of the member and 71 feet below its top. About 

2 miles northwest of Pegram in the noDth-central part 

of sec. 36, T. 14 S., R. 45 E., tuff occurs in the upper 

part of the member in two beds separated by 12 feet 

of limestone. Other occurrences are reported to be 150-

200 feet above the base of the member (Gulbrandsen 

and Cressman, 1960, p. 459). 

In northern Utah the Sliderock Member is similar 

lithologically to the member in southeastern Idaho. 

rhe basal units are generally oolitic, or sandy, or both. 

The upper beds are generally dense to granular. Near 

Thistle a bed of sandstone 6 feet thick occurs 23 feet 

above the base of the member. On Birch Creek west of 

Woodruff the lower 43 feet of the member contains two 

beds of ripple-marked sandstone as well as a basal unit 

of highly. sandy oolitic limestone. 

Many sections of the Sliderock Member in Wyo· 

ming and Idaho have been described previously as 

member B (Imlay, 1950a, p. 42-45; 1953a, p. 60-62). 

The following sections illustrate the major changes that 

occur in the member in northern Utah. 

Sliderock Member on west side of Lake Fork in NW~ sec. 2, T. 

1 N., R. 5 W., Duchesne County, Utah 

[Fig. ,1. Ioc. 39] 

1. Limestone, medium-bedded, finely sandy; light-yellowish-gray; some crossbedding present; contains fragments of crinoids and Oamptonectes. Rests sharply 

on Nugget Sandstone. Contact with the Rich Member is covered in a gully--------------------------

Feet 

32 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 23 

Slide1·ock Member on southwest side of Duchesne River ~in SW* 

sec. 4, T. 1 S., R. 8 W., Duchesne County, Near Hanna, Utah 

[Fig. 1, loc. 381 

2. Limestone, massive, oolitic, medium-yellowish-gray, 

cliff forming; contains small oysters on upper surface. Overlain by soft shaly limestone at base of 

Rich Member -------------------------------------

1. Limestone, medium- to thin-bedded, finely sandy, lightyellowish-gray to pinkish-gray; some beds oolitie ; 

passes laterally into massive cross:bedded sandstom~; 

rests sharply on salmon-colored Nugget Sandstone __ _ 

Feet 

12 

30 

Total thickness__________________________________ 42 

Sliderock Member on west side of Sta-te Highway 213 northeast 

of Oakley in NE1f4 sec.1, T.1 S., R. 6 E., Summit County, Utah 

[About 7 miles east-northeast of loc. 37 as shown in fig. 1] 

5. Limestone, medium-bedded, medium- to dark-gray, cliffforming; some pink beds near base; weathers 

medium gray; overlain by soft shaly limestone at 

Feet 

base of Rich Member______________________________ 30 

4. Limestone, dense, yellowish-gray, brittle; weathers 

sa1ne ------------------------------------------·-

3. Limestone, medium- to thin-bedded, brownish-gray; 

weathers light gray; fossiliferous (USGS Mesozo:lc 

loc. 21619 from base) ____________________________ _ 

2. Limestone, slightly oolitic, finely :sandy, medium- to 

pinkish-gray -----------------------------------·-

1. Limestone, sandy, mottled pink and gray; rests on 3--6 

in. of dark-green volcanic tuff at the top of the Gyp- sum Spring Member ______________________________ _ 

1 

6 

9 

1 

Total thickness_________________________________ 47 

Sliderock M embe1· on north side of road north of Thistle in 

1Vlf2 sec. 28, T. 8 S., R. 4 E., Utah County, Utah 

[Fig. 1, loc. 42] 

7. Limestone, medium-bedded, moderately soft, dark-yellowish-gray; weathers light gray ; contains lenses of 

crinoidal debris in upper 6 ft and a few fragments 

of Gryphaea; overlain by soft shaly limestone at 

Feet 

base of Rich Member_____________________________ 31 

6. Limestone, medium-bedded, hard, pink; upper surface 

greenish pink; weathers light gray---------------- 7 

u. Limestone coquina of crinoid and echinoid fragments__ 1 

4. Limestone, thin-bedded to shaly, yellowish- to darkgray -------------------------------------------- 3 

3. Sandstone, medium-bedded, hard, pinkish-gray_______ 6 

2. Limestone, sandy, soft, pink to gray----------------- 8 

1. Limestone, oolitic, pink to gray; :contains crinoid 

columnals, echinoid spines, and fragments of 

Gryphaca. Rests sharply on 9ft of soft red siltstone 

comprising the Gypsum Spring Member___________ 15 

Total thickness_________________________________ 71 

Sliderock Member a-long north side of Birch Creek in NW1f.t 

soo. 20, T. 9 N., 6 E., Rich County, Utah 

[Fig; 1, Ioc. 331 

Feet 

28. Limestone, medium- to thin-bedded, uneven-bedded, 

medium-gray, ridge-forming, nodular; weathers 

light gray ; overlain by soft shaly limestone at base 

of Rich Member; contains ammonites and pelecypods (Mesozoic locs. 28644-46, 28648) ------------

27. Claystone, soft, medium-gray, fissile to chunky _____ _ 

26. Siltstone, soft, reddish-purple _____________________ _ 

25. Limestone, pink to yellow, nodular; contains traces of 

fossils ----------------------------------------- 24. Siltstone, reddish-brown to yellowish-gray----------

23. Limestone, shaly, gray, nodular __________________ _ 

22. Siltstone, soft, brownish-red to gray----------------

21. Claystone, soft, medium-gray, chunky to fissile _____ _ 

20. Claystone, calcareous, yellowish-gray, nodular ______ _ 

19. Limestone, yellowish-gray, rubbly ------------------

18. Limestone, shaly, soft, medium-yellowish-gray ______ _ 

17. Claystone, soft, purplish-red _______________________ _ 

16. Limestone, yellowish-gray, rubbly ------------------. 

15. Claystone, fairly soft, yellowish-gray, chunky------

14. Limestone, yellowish-gray, rubbly; contains Camp62 

4 

lh 

1 

14 

2 

3 

9 

3 

1 

3 

1 

2 

5 

toneotes --------------------------------------- 1% 

13. Claystone, soft, medium-gray----------------------

12. Limestone, shaly, medium-gray--------------------

11. Limestone, gray, rubbly; contains Gryphaea and 

Camptonectes (Mesozoic loc. 28647) --------------

10. Sandstone, medium-bedded, very fine grained, pinkishgray, ledge-forming; weathers brownish gray; 

passes at top into brownish-gray sandy limestone_ 

9. Siltstone, sandy, yellowish- to pinkish-gray; bedding 

18 

5 

5 

16 

irregular -------------------------------------- 5 

8. Covered ----------------------------------------- 10 

7. Sandstone, medium-bedded, very fine grained, pinkishto yellowish-gray-------------------------------- 3 

6. Covered ------------------------------------------ 10 

5. Sandstone, thin-bedded, very fine grained, pinkishgray, ripple-marked----------------------------- 4 

4. Mostly covered ; some red to light-green soft siltstone 

exposed at top__________________________________ 12 

3. Sandstone, medium- to thin-bedded, fine-grained, 

pinkish- to greenish-gray, ripple-marked__________ 5 

2. Siltstone, soft, red, poorly exposed__________________ 2 

1. Limestone, thick- to medium-bedded, oolitic, very 

sandy, brownish- to yellowish-gray, cliff-forming; 

some crossbedding; contains fragments of fossils 

throughout ; underlain by a covered interval of 

16 ft and then by 20 ft of reddish-brown purple, 

and lavender siltstone of the Gypsum Spring Member. Contact with Nugget Sandstone is not exposed 20 

Total thickness ________________________________ 227 

In the above sequence only the upper 62 feet of ridgeforming limestone and the lower 20 feet of cliff-forming 

oolitic limestone resemble the Sliderock Member as exposed in western Wyoming. In between are seven units 
24 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

TABLE 3.-Geographic distribution of marine Jurassic megafossils in the Sliderock Member of the Twin Creek Limestone 

[Gastropod identifications are by N. F. Sohl. All one- and two-digit numbers refer to localities in figs. 16 and 17. Higher numbers are USGS Mesozoic locality numbers 

identifying individual collections] 

Idaho Wyoming 

2 3 4 10 12 13 16 18 19 20 21 23 31 34 36 38 40 45 46 47 48 49 

- - - - - ---- - - - - ----- <Zl ...... 0 ...... ~ 10 <Zl 10 <Zl 0 ...... ;:: C'l <':> 10 <Zl t-- <Zl 00 0> 0> ...... 00 0> <':> 00 ...... ~ C'l t-- 00 0> 0 0 S5 S5 <':> <':> 0 ~ <':> <':> ~ 0 ...... C'l 0 ...... <':> 00 0> <':> C'l ...... ...... ...... t-- 00 C'l 00 $ C'l <Zl t-- ...... 10 ~ 10 10 10 <Zl <Zl ~ 0 ;:: ;:: 0> ...... <':> t-- 00 <Zl <':> <':> 0 0 0 ...... 0 8 0> t-- ~ <Zl 0> 0> 8 0 00 ~ 00 00 00 00 00 00 00 ~ ;:: 00 0 C'l ~ <':> 0 ~ <Zl <Zl <Zl ...... C'l C'l C'l 0 t-- <':> ...... 0 0 C'l C'l C'l C'l C'l C'l C'l C'l C'l C'l C'l C'l C'l 0> ...... t-- <Zl ~ <Zl <Zl ...... 0> ...... ...... .... C'l C'l C'l C'l <Zl C'l C'l C'l C'l C'l 

-------------1----------------------------------------

Worm tubes ______________________ X ________________________________________________________________________ X ___ --- --- --- --- --- --- --- --- --- --- X 

Echinoid spines _________________ X ____________________________________________________________________________________ --- ___ --- --- --- --- --- --- ---

g~~~a~d~~~a~~ei-(M:eekf_-=~~~ ~== -~- === === === === === === === === === ?:_ ·x.- === === === === === === === === === === -~- === === -~- === === === === -x.- === === === === -~- === ldonearca haguei (Stanton) ______________________________________________________________________________________________ --- --- --- --- --- --- --- --- ---

. Modiolus subimbricatus (Meek) __________________________________________________________________________________________ --- ______ --- --- ___ --- --- --- Mytilus cf. M. whitei Whitfield _________________________________________________________________________________________________ --- --- --- --- --- --- --- Pinna kingi Meek _______________________________________________________________________________________________________ --- --- X X --- --- --- --- --- Camptonectes stygius White __________________________________________________________________________________________________________ --- ___ --- --- --- platessijormis White ____________________________ X ____________ X ______ X __________________________________________ --- --- --- --- X --- --- X ---

Plicatula Chl:~ys?-si)--~=================== sp _______________________________________________________________________________________________________________________________ -~- === === -x.- === === === === === === -~- -~- === === === === === === -~- === === === === === === -~- === -~- === === === === === === === ---

=== === 

--- === --- Lima (Plagiostoma) sp _______________________________________________________________________ ·-- __________________________________________ --- --- X 

Ctenostreon cf. C. gikshanensis McLeam _____________________________________________________________________________________________________ X ____________ --- --- --- --- --- X X 

Ostrea (Liostrea) strigilecula 

Lo~~!t~p========================= Gryphaea -~- === === === === === === === === === === x: === -x.- === === === === === -~- === -~- === === === === === -~- === === === === === === === === === === planoconvexa fraterna 

Jr;;~ld ~m~~~~~a-:Meek:========= === -~- === === -~- -~- =~= -~- ?:_ -~- -~- -~- === -~- -~- === ?:_ -~- === === ?:_ === === -x ~- === === === ?:_ ~- ?:_ -~- -~- === -~- -~- ~- === Myophorella (Promyophorella) montanaensis (Meek) ________________________________________________________________________________________________________ X ___ --- --- --- --- ---

Astarte meeki Stanton _______________ X ________________________ X __________________________________________________________________ --- --- --- --- ---

( Coelastarte) livingstonensis 

Imlay __________________________________ ------------------------------ --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- Opis (Trigonopis) sp _______________________________________________________ ---· ______________________________________________________ --- --- --- --- ---

Prorokia jontenellensis Imlay, n. sp _____________________________ --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- Pronoella cinnabarensis (Stanton) __________________________________________________________________________________________________________ X ______ --- ___ --- --- --- --- ---

uintahensis Imlay _______________________________________________________________________________________________________________ --- --- --- --- --- cf. P.? iddingsi (Stanton) ___________________________________________________________________________________________________ --- ___ --- --- --- --- ---

Mac~fomiia?-sp=================== Tancredia? sp ______________________________________________________________________________________ === === === === === === === === === === === === === === === === === === === === === === === X -~- ______________________________ === === === === -~- === === -~- === === 

--- --- --- === === ·---

=== === 

---

~~J~~~r:~~~7~ff;~-~~~~~!~~ === === ==~ === === === === ~~~ === === === === ~~ === === === === === === === === === =~= === === === === === === -~- ~~ ~~~ ~~~ === === ~~~ =~= === inaequiplicata Stanton _____________________________ X ___________________________________________________ --- --- --- --- --- --- --- --- --- --- --- --- ---

Homomya cf. H. gallatinensis Stanton _____________________________________________________________________________ ·-- _____________________ ---· __________________ --- --- --- --- ---

~~~~~7~Y~e":Jr~~~~~~~i_s_~~~~==== Teinostomopsis? sp __________________________________________________________________________________________________________________ === === === === === === === === === === === === === === === === === === === === === === === === === === === === ?:_ === === === === --- ---=== === ---=== --- ---=== === 

~X~t~~c~~o:;e~g-~~!~~==~==~~~~== === === === === ::: === === =~~ ::: === ::: ~== === === === === === === === === === === === ~- === === === === ::: === === === === === === =~= === === 

~i~~=~~-~- = =~~ -X :_ ~- :__ ~~: :~- __ : :- := _: ~- -=- m m :__ ~~- _:~ ::_ :__ ::: ::: =:: ::: ~ :~: ~:: =:: :~: m ~~: =~: m === ::=::: ~=~ ~== ::: 

Gastropods undet ______________________ X _________ X ___________________________ --·· _________________________________________________ --- --- --- --- ---

Spiroceras cf. S. orbignyi Baugier and Sauze ____________________________________________________________________________________ -~- _________________________________ --- --- --- --- ---

Stephanoceras cf. S. skidegatensis 

St:~!~r~::~~~;;:Yci~-o~~!nste<:ft)~= :== =~= === === === === === === === === === === -~- === === === === === === === === === === === === === === === === === === === === === === === === === Imlay, n. sp ______________________________________________________________________________________________________________________ --- --- --- ---- ---

n. sp. aff. S. albertense 

McLeam __ ---------------- _________________________________ X ______ X __________________________________________ X ___ X --- --- --- --- --- ---

N<ii~~~~~~~f~~~·~~~i~~~~~~~~~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ -~- ~~~ ~~~ ~~~ -~- ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ ~~~ Megaspaheroceras cf. !.1. rotundum 

I~#_Y.M~-io'tiintiurii-iilliay~====: === === === === === === === -x.- === === === -x.- === === =-: === === === === === === === === === === === === -~- === === === ~ === === === === ==: === 

Eocephalites ~~p~~~~~===============~==~= primus Imlay, n. sp ________________________________________________________________________________________ === === === === === === === === === :== === === === === === === === === === === === === === === === === === === === === X X X === === ---

=== === X X X === === === --- === 

--- sp _____________________________________________________________________________________________________________________ X --- --- --- --- --- --- --- ?sp. undet. A _____________________________________________________________________________________________________________ --- --- --- --- --- --- ---

~i~~~-~~;~~~================ === === === === === === === === === === ~== === === === === === === === === === === === === === === === === === ===; === === === === =~= === === === 

Eutrephoceras sp. undet. _______________________________________________________________________________________ --- --- --- --- --- --- --- --- --- --- --- __ .. 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 25 

TABLE 3.-Geographic distribution of marine Jurassic megafossils in the Sliderock Member of the Twin Creek Limestone-Continued 

Wyoming Utah 

54 55 56 57 58 63 64 65 66 68 69 71 72 73 75 77 81 82 83 84 93 95 96 

- - - - -;---;--~-,--11--- ---------------------- 0> 10 '<!' 0 ~ '<!' '<!' ao 10 0> '<!' <0 10 t- ao '<!' t- t- ao 0> t- '<!' <0 ~ <0 t8 8 ~ 8 8 '<!' t- 8 ~ 0> 0 ao ao 0 0 8 8 C'l C'l C'l C'l C'l C'l C'l C'l C'l C'l C'l C'l C'l 

---------------1---------------------------------------

J!Eg~il~@.i~i=-=~~~~ ~~~ =~~ ~J ~~~ ~~~ ~~~ ~~~ ~~~ =l~ ~=~ ~~~ :~= ==~ m ~~~ m ~~~ m ~~= ~~~ =~= ~~~ m ~== ~~~ ~=~ ~~~ ~~= =~= :~ ~~~ ~~~ t: ~~ ~~- ~~~ ::~ ~: M~tfz~~~ril~h~~~fw~ft~:tl_-_-_~ ==~ === === === === === -~- === -~- === === === === === === === === === === === -~- === === === === === === === === === === === === === === === === === 

b~r:nn;t:!~gfe~~~~iU8wl1£ie======= platessiformis White __________________________________ === === === === === === === === X ~ ::: _____________________________________________________________________ -~- === === ~- === === -~- === === ::: -~- === === ~- === === === === === -~- -~- === === X X === === _________ ::= === ~-X 

sp ___________________________ --- --- X --- X --- --- --- --- --- --- --- X --- --- ___ X X --- ____________ --- ___ X ___ --- X --- --- --- ___ --- X --- --- --- Chlamys? sp ______________________________________________________________________________________________________________________________________ _ Plicatula sp ______________________________________________ X _____________________________________________________________________ X X ___________ _ Lima (Plagiostoma) sp ___________________________________ X ________________________ : ___________________________________ --- --- --- --- --- --- --- --- ---

Ctenostreon cf. C. gikshanensis McLearn _____________________________________ X ______ X ___ X ____________________________________ X __________ · _____________________________ X 

Ostrea (Liostrea) strigilecula White ____________________________________________________________________________________________________________________ X X X ____________ X 

Lopha sp _______________________________ --- ______ --- --- --- ___ --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- Gryphaea planoconvexa fraterna 

Myophorella T~~:r,; ~~~~~~Rii-Meek:========= === === -~- -~- -~- _>:-_ === === -~- === === -~- -~- -~- === --x- ::: -~- -~- -~- -~- === ~- -~- -~- === === === === -~- === === === --x- === === === === (Promyophorella) montanaensis (Meek) _________________ X ___ X ______________________________ X _________ X X ______ X _________________________________________ _ 

Astarte meeki Stanton __________________________________________ X _______________ X X _______________ X ____________ X X _______________________ _ 

( Coelastarte) livingstonensis Imlay ____________________________ X ___________________________________________________ X __________________ X _____________________________ _ 

Opis (Trigonopis) sp _____________ --- --- X --- ___ --- --- --- --- --- --- --- --- --- --- --- --- --- --- _________ --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- Prorokia fontenellensis Imlay, n. sp _____________________________ --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- _________ --- ________________________ X ______ --- ______ --- ______ --- ---

Pronoella cinnabarensis (Stanton) _______________________________________________________________________________________________________________________________________ _ 

uintahensis Imlay __________________ --- ____________________________________________________________ X _____________________ X _________________ _ cf. P.? iddingsi (Stanton) ____ --- _______________ --- __________________ X _______________________________________________________________________ _ 

sp ___________________________ --- --- X X X --- --- --- X --- X --- --- X --- --- X --- --- X --- --- --- X --- --- --- --- --- --- X X --- --- X --- --- X 111actromya? sp ___________________ --- _______________ --- ____________ X ___ X _____________________________________________ X _______________________ _ 

Tancredia? sp ____________________ --- --- --- X --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ~-- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---

Quenstedtia sp ____________________ --- ______________________________________________________________________________________________________________ _ 

Pleuromya subcompressa (Meek) __ --- --- X X ___ X --- ___ X X X X X X --- X X X X X X ______ X ___ X ___ X X --- --- --- --- --- --- --- --- X Pholadomya kingi Meek __________ --- ___ X ___________________________________________________ X __________________ X _____________________________ _ inaequiplicata Stanton ________________________________________________________________________________________________________ X _________ --- ---

Homomya cf. H. gallatinensis Stanton __ --------------------- --- _____________________ X _______________ --· ____________________________________________________________________ _ 

Goniomya montanaensis Meek ____ --- ____________________________________ X ___ X _________________________________________________________________ _ Thracia weedi Stanton ____________ --- --- --- --- ___ --- --- __________________ X _________________________________________________________ --- ___ --- --- --- Teinostomopsis? sp _______________ --- --- --- ______ --- --- _______________________________________________________________________________________ X __ _ Lyosoma powelli White _______________________________________________________________________________________________________________________ X --- Rhabdocolpus sp __________________ --- --- ---· --- ___ --· --- --· ____________ ---· ___ --· ______________________________________________________ --- ___ X --- X 

Pseudomelania? sp _______________ --- _________________________________________________________________________________________________________ X ---

Naticiform gastropods ___________ --- --- --- --- ___ --- --- --- X --- --- --- --- --- --- --- --- --- --- _________ --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---

g~:ft:Zr:::~~ ~~~================== === === === === === === === === === === === === === === === === === === === === === === === === === === === === === === === === === === === === === === Cylindrobullina? sp _______________ --- _______________ --- ___ ·-- ______________________________________________________________________________ X X X 

Gastropods undet ________________ --- --- --- ______ --- --- ___ X ____________ --· ______ X _________________________________ X ____________ X ___ --- --- ---

Spiroceras cf. S. orbignyi Baugier and Sauze _____________________ --- X --- --- ___ --- --- ______ X ___________________________________________________________________________ --- --- ---

Stephanoceras cf. S. skidegatensis (Whiteaves) _ ---~-------------- --- ______ ·-- _____________________ ·-- ________________________ ·-- _____________________________________________ --- --- aff. S. nodosum (Quenstedt) __ --- --- --- ______ --- --- ______________________________ X ______________________________________________________ --- ---

Stemmatoceras arcicostum Imlay, n. sp _______________________________________________________________________________ X __________________________________________________ _ 

n. sp. aff. S. albertense 

McLearn. ----------------- --- --- X --- --- --- X --- X ______ --- _______________ X --- X X X ______ X --- ______ --- ___ --- --- --- --- --- --- --- --- sp ___________________________ --- --- --- --- --- --- --- --- --- X --- --- --- --- --- --· --- --- --- --- --- --- --- --- X --- X --- --- --- --- --- --- --- --- --- --- --- Normannites? cf. N. crickmayi (McLeam) _____________________ --- --- --- --- --- --- X X _________ ---· ________________________ X ___________________________ --- ___ --- --- --- --- --- ---

Megasphaeroceras cf. M. rotundum 

Imlay __________________________ ------ X --- --- --- X ___ --- ___ --- ___ --- ____________ X --- X X ______ X ___ --- _______________ --- --- --- --- --- --- --- aff. M. rotundum Imlay ______ --------- ______ ------ X _______________ --- ____________ --- ____________ --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---

sp. undet_ ___________________ --- --- --- ___________________________ X __________________________________________________________________ --- --- --- ?sp __________________________ --- --- --- --- ___ --- --- ______ X _______________________________________ -~- ______________________________ --- --- --- ---

Eoce:,~~l~t-e~-~~~~-~~~~~:':_~:_s~~ ::: ::: -~- -~- -~- -~- ::: ::: -~- ::: ::: -~- ::: ::: ::: === === -~- === -~- -~- ::: ::: -~- ~~- === === === === === === === === === === === === -X- '!sp. undet. A ________________ --- __________________________________________________________________ X ____________ --~ _____________________ --- ---

'!sp. undet. B ________________ X --- --- --- ___ --- --· _________ --- ___ --- --- _________ X ___ X _________ X ________________________ --- --- --- --- --- --- ?sp. undet. c ________________ -~- --- ______ --- ________________________ --- _______________ X _________________________________ --- ______ --- --- --- ---

?sp __________________________ --- --- --- --- --- --- X --- --- --- --- --- --- --- --- X ___ --- X ___ --- X --- --- X --- --- --- --- --- --- --- --- --- --- --- --- --- Eutrephoceras sp. undet __________ --- ___ --- ___ 1 

___ --- ____________ --- ______ --- X ______ --- --- _________ --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---

221---691 0-6&--3 
26 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

of soft red siltstone and claystone that resemble red beds 

in the Gypsum Spring Member and have not been found 

elsewhere in the Sliderock Member. Interbedded with 

these red units, however, are many units of gray claystone, shaly to nodular limestone, and medimn- to thinbedded sandstone that can be matched readily in the 

lower part of the Sliderock Mmnber in southeastern 

Idaho and in north-central Utah and are unknown in 

the Gypsum Spring Member. The lithologic evidence, 

therefore, favors assigning the entire sequence, as described, to the Sliderock Member. It is possible, of 

course, that the base of that member becomes older toward the west and that most of the sequence below the 

highest ridge-forming limestone on Birch Creek was 

never deposited farther east in western Wyoming, or in 

the western part of the Uinta Mountains. 

The following section is of interest because it is the 

most western in Idaho, is exceptionally well exposed, 

and contains units of soft fissile claystone as does the 

section on Birch Creek, Utah. 

Sliderook Member on north side of Willow Creek -in NE¥4 sec. 19, 

T.1 N., R. 40 E., Bonneville County, Idaho 

[Fig. 1, loc. 4] 

8. Limestone, thin- to medium-bedded, nodular, mediumgray ; interbedded with calcareous silty claystone; 

contains Gryphaea planocovea:a tratet·na Imlay and 

the ammonites Stemmatoceras and Megasphaeroceras (Mesozoic locs. 28501 and 28635). Contact 

with overlying Rich Member covered by soil in 

Feet 

gully ------------------------------------------- 30~ 7. Claystone, papery, soft, gray to yellowish-gray (Mesozoic loc. 28502) ; bentonitic at base_________________ 4 

6. Sandstone, thin-bedded, quartzitic, yellowish-gray, 

crossbedded, ripple-marked; weathers yellowish 

gray -------------------------------------------- 14 

5. Claystone, silty, soft, gray, fissile; interbedded with 

thin beds of nodular limestone ( lh-3 in. thick) ; 

grades upward into quartzitic sandstone; Gryphaeas 

occur near base; one thin bed of limestone 13 ft. 

above base contains small gastropods and pebbles 

(Mesozoic loc. 28500) ----------------------------- 28 

4. Limestone, shaly, soft, medium-gray, nodular; contains 

Gt·yphaea (Mesozoic locs. 28500, 28636) ____________ 7 

3. Claystone, soft, medium-gray, fissile (Mesozoic loc. 

28505) ------------------------------------------ 6lh 

2. Limestone, medium-brownish-gray; soft and shaly at 

base; becomes moderately hard and massive toward 

top ; weathers yellow ; contains many :fossil fragInents ------------------------------------------ 50 

1. Limestone, medium-gray.; forms ledge; weathers yellow; contains Camptonectes, Ostrea, and Pleurotnya. 

Overlies a covered interval 40 ft thick marked by a 

ravine and interpreted as the top of the Gypsum 

Spring Member__________________________________ 1 

Total thickness _________________________________ 140? 

FOSSILS, AGE, AND CORRJl2LATIONS 

The Sliderock Member is fairly fossiliferous and becomes more fossiliferous upward. Cepahalopods have 

been found only in the upper half. Pelecypods and 

cephalopods are much more common than gastropods. 

Worm tubes, echinoid spines, and crinoid columnals are 

fairly common but generally have not been collected. 

All the megafossils are given by localities in table 3. In 

addition some megafossils that are probably in part 

from the Slide rock Member :are given in table 4. 

The known stratigraphic ranges of the pelecypods 

within the Twin Creek Limestone and contemporary 

formations in the western interior are shown in figure 

9 · the stratigraphic ranges of the ammonites are shown 

i1~ figure 11. These figures show that ammonites in the 

Sliderock Member belong to different genera than those 

in the overlying Rich Member except possibly for some 

fragments assigned questionably to Stephanoceras. In 

contrast all the pelecypod species that occur in the Sliderock Member range higher. In practice, the pelecypod 

Gryhaea planoconvewa fraterna Imlay, n. subsp., is a 

useful marker for the Sliderock Member, although it occurs rarely in the lower part of the Rich Member. It 

has not been found outside of the area of distribution of 

the Twin Creek Limestone. 

Other pelecypods of stratigraphic value include 

Astarte ( Ooelastarte) livingstonensis Imlay, Prorokia 

fontenellensi.s Imlay, n. sp., Goniomya montanaensis 

Meek and T hraci.a weedi Stanton. Of these, the first 

species ' ranges from 'd the Slide rock to the Boundary R1 ge 

Member, inclusive. The other three species have been 

found only in the Sliclerock and Rich Members. Elsewhere in the western interior these four species have been 

found only in formations or members of Bajocian, or 

probable Bajocian, age, including the lower limestone 

member of the Carmel Formation in southern Utah, the 

limestone 1nember of the Gypsum Spring Formation in 

Wyoming, the middle limestone member of the Piper 

Formation in Montana, and the lower part of the Sawtooth Formation in southwestern Montana (fig. 9). 

The age of the middle and upper parts of the Sliderock Member of the Twin Creek Limestone is late Bajocian as shown by the presence of the ammonites llf ega.rsphaeroceras and Spiroce1•as in association with Stephanoceras. Stemnwtoceras, and questionable N orrrwnnites. 

Of thes~ 111 egasphaeroceras has been recorded previous- ' . ly only in southern Alaska (fig.12) in beds of late BaJocian age (Imlay, 1962b, p. A1-A3, A9-A11). In Eurasia, Spirocera:s ranges from upper Bajocian to lower 

Bathonian (Arkell and others, 1957, p. L 206); S'tem.-

1natoceras occurs only in the middle Bajocian zones of 

Otoi.tes sauzei. and Stephanoceras hutnphriesiarnunu (Arkell and others, 1957, p. L 289); Stephanoceras ranges 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 27 

TABLE 4.-Geographic distribution of marine Jurassic megafossils in the lower part of the Twin Creek Limestone and probably mostly 

from the Sliderock and Rich Members 

[Gastropod identifications are by N. F. Sohl. All one- and two-digit numbers refer to localities shown in figs. 16 and 17. Higher numbers are USGS Mesozoic locality members 

identifying individual collections] 

Idaho Wyoming Utah 

8 15 18 86 87 95 

----------------.----1--,----;---l---,----

------------------------1-----------------------------------

Coral undet_ ______________________________________________________________________________________________ X _______________________________________________ _ Worm tubes ___________________________________________________________________________ X ________________ X ____________________________ X _______________ _ Crinoid columnals ____________________________________________________________________________________________ X ________ X ____________ X ____ X ____ X 

Starfish arm ___________________________________________________________________________________________________________________________________ X ___________ _ Grammatodon haguei (Meek) ___________________________________________________________________________ X ____________________________________ X ____ X ___ _ 

Gervillia? montanaensis Meek __________________________________________________________________________________________________________________ X ___________ _ Mytilus cf. M. whitei Whitfield _____________________________________________________________________________________ X _______________________________________ _ Pinna kingi Meek _____________________________________________________________________________________________________________________________ X ___________ _ Camptonectes platessiformis White __________________________________________ --~- ____ ____ ____ ____ ____ ____ ____ ____ ____ ____ X ____ ____ X X ____ X ___________ _ 

SP--------------------------------------------------------------------- ---- X ---- X X X ---- ---- ---- ---- X ---- ---- ---- ---- ---- ---- ---- ---- ---- ----

Lima (Plagiostoma) occidentalis Hall and Whitfield _________________________________________ X ____________ X ________________ X ___________________________ _ 

Ctenostreon cf. C. gikshanensis McLearn ________________________________________________ X ________________________ ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- Ostrea (Liostrea) strigilecula White _____________________________________________ X X ________________________________ X ________________ X X ____ X X 

Gryphaea sp_______________________________________________________________ X ____________________________ ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- Irigonia americana Meek___________________________________________________ ____ ____ ____ ____ ____ ____ ____ ____ ____ ____ X _______________________________________ _ 

elegantissima Meek ________________________________________________________________________________________ X ___________________________________________ _ 

:J~;J;J~~~~~~~~~~~~=··-~,,.i: -_ ---~-- :_ --~:: :-:: ~=~~ :-:: :::: ::~~ ~==~ ~~~= ::~~ ~~~= -~- :~: :::: ~~~~ :=~~ ~~~: :::: =~== :~: :~:: ~~~= :~:: Pronoella uintahensis Imlay _______________________________________________________________________________________ X ____ X _______________________________ _ 

Mac;io%v~i~~~:~~~~-t~~~~~~====~======~==~=====~===~=========~=====~==~== ~~=~ ==~= ==~= ==~~ ~=== ~=== -~- =~== ~=~~ ~==~ ~==~ ~=== ~=== ===~ ~=~~ ==== ~=== -x- ~=== ~~== ~~~~ Tancredia? sp _____________________________________________________________ ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- Corbicella? sp______________________________________________________________ ____ ____ ____ ____ ____ ____ ____ ____ ____ X ___________________________________________ _ 

!~~r£e~Y~1f~:t·~i --===:-~----:::::::::: ::::=~~~~~ ==~~ ~~=~ ::== ~~~= ~~~~ :~; ~=~: ::~~ ~:~~ }_ ~:~: ~:=: ~:~~ ~=~~ ~:~~ ~~~: ~~~~ -~- :~~~ ~~~~ ::_: Lyosoma powelli white _________________________________________________________________________________________________________________ X ________ ---- _______ _ 

8~X;g;:tl~~~~~~;===================================================== ==== ==== ==== ==== ==== ==== ==== ==== ==== ==== -~- ==== ==== ==== ==== ==== ==== =~= ==== ==== =~= from the middle Bajocian (upper part of Sonninia 

sowerb-yi zone) into the lower part of the upper Bajocian 

(Strenocera.s subfurcatum zone) (Arkell, 1956, p. 232, 

264, 300; 1952, English Bathonian Ammonites, p. 7 4) ; 

and N ormannites ranges frmn the middle Bajocian 

(upper part of Otoites sauzei zone) into the lower part 

of the upper Bajocian (Westermann, 1954, p. 124; 

Arkell, 1956, p. 176, 232, 300). 

These ranges indicate that the Sliderock Member correlates with the low.er part of the upper Bajocian Stage 

and probably with the zone of Strenoceras sub furcatum .. 

In fact, the presence of Stemmatocera.s indicates a 

position near the boundary of the 1niddle and upper 

Bajocian. As no ammonites were obtained from the 

lower part of the Sliderock Member, that part could be 

as old as middle Bajocian. 

A late Bajocian age for most of the Sliderock Member is indicated also by the presence of M egasphaeroceras, which in Alaska is associated with the typical 

late Bajocian genus Leptosphinctes. It is indicated by 

the absence of Ohondrocera.s which is common in beds 

of middle Bajocian age but has not been reported in 

beds of late Bajocian age. A late Bajocian age is favored by the presence of E ocephalites, n. gen., that 

greatly resembles Paracephalite8 from beds of probable 

Bathonian age in Canada (Frebold, 1963, p. 8-13, pis. 

1, 2, 4) and northwestern Montana (Imlay, 1962a, pis. 

3-5) and likewise resembles Oranocephalites from beds 

of Bathonian to possible late Bajocian age in Greenland 

(Spath, 1932, p. 14--31; Oallomon, 1959, p. 507-511) 

and in the Canadian Arctic (Frebold, 1958, p. 8, 9; 19611 

p. 12-15). These resemblances plus the a!bsence of any 

closely si-milar genera in beds of middle Bajocian age 

in Alaska, or elsewhere, suggest an age at least as young 

as late Bajocian for the Sliderock Member. 

The Sliderock Member is traeeahle as a lithologic 

unit into the "Lower Sundance" Formation exposed 

along the north side of. Lower Slide Lake, Teton 

County, Wyo. At this place it is represented by 5 

feet of so£t shaly limestone and claystone and by the 

overlying 16 feet of oolitic medium- to thin-bedded 

limestone (units 7-9 of Imlay, 1956a, p. 70). The member is overlain by 85 feet of soft shaly limestone typical 

lithologically ·and faunally of the Rich Member, and is 

underlain by about 45 feet of red siltstone, yellow to 

pink limestone, and brecciated limestone typical of the 

Gypsum Spring Member of the Twin Creek Limestone. 
Spiroceras cf. S. orbignyi Baugier and Sauze _______ _ 

St~f/i:J;ao~::::i~fi -$·hi tea ves) __ 

aff. S. nodosum (Quenstedt) _______ _ 

? sp. - - - - - - - - - - - - - -

Stemmatoceras arcicostum Imlay, n. sp. ________ ·- __ _ 

cf. S. arcicostum Imlay, n. sp ____ ----

n. ~lbe~~;;,~~ McLearn __ _ 

cf. S. palliseri McLearn _________ _ 

sp. - - - - - - - - - - - - - - -

Sohlites spinosus Imlay, n. sp. ________ _ --

Normannites? cf. N. crickmayi (Me Learn) _____ _ 

Chondroceras cf. C. allani (McLearn) ____________ _ 

Parachondroceras andrewsi Imlay, n. sp. ___________ _ 

cf. P. andrewsi Imlay, n. sp. _______ _ 

jilicostatum Imlay, n. sp. ___________ _ 

cf. P. jilicostatum Imlay, n. sp. _______ _ 

Megasphaeroceras cf. M. rotundum J mlay ________ _ 

aff. M. rotundum Imlay __ 

sp. undet. _____ - - - __ _ 

? sp.-- -- -- -- -- -- --

Eocephalites primus Imlay, n. sp. _______________ _ 

sp. - - - - - - - - - - - - - - -

? sp. undet. A ____ - __ _ 

Southeastern Idaho, 

north-central Utah, 

and western Wyoming 

Twin Creek Limestone 

(Lower part) 

Sliderock 

Member 

--

-

-

--

--

--

--

-

-

-

Rich 

Member 

-

--

? sp. undet. B-- -- -- --I -

? sp. undet. C-- -- -- -- I -

? sp.----- --1 -

Central Utah 

West side of 

San Rafael Swell 

Carmel Formation 

Lower 

limestone 

member 

Upper gypstferous 

shaly silt member 

1--

North-central Wyoming 

Bighorn Basin 

Gypsum Spring Formation 

Lower 

red-bed 

member 

Middle 

limestone 

member 

Upper 

red-bed 

member 

South-central Montana 

Gallatin Range and north side 

of Pryor and Beartooth Mountains 

Lower 

red-bed 

member 

Piper Formation 

Middle 

limestone 

member 

--

-

Upper 

red-bed 

member 

Southwestern Montana 

Madison, Tobacco Root, Gravelly 

and Centennial Ranges and 

Tendoy Mountains 

Sawtooth Formation 

Sandstone 

member 

(generally 

absent) 

Middle 

limestone 

and shale member 

1--

Upper 

siltstone 

member 

Northwestern and 

west-central Montana 

Drummond area and 

Swift Reservoir 

Sawtooth Formation 

Upper 

silty to sandy 

Sandstone 

member 

(absent at Drummond) 

Middle 

shale 

member member 

t 

if.-

1--

FIGURE H.-Stratigraphic distribution of Bajocian ammonites in the western interior of the United States. A question mark(?) indicates that the identification 

· is uncertain. Black bars show stratigraphic ranges of species within the formations and members·. 

tv 

00 

~ 

z 

0 

~ 1::".:1 

~ 

t"4 

~ ~ 0 

~ 

z 

~ 

~ 

~ 1::".:1 

~ 

~ 1::".:1 

1::1 

~ 

U2 

~ 
European 

stages 

<.> 

00 

00 

ol 

;:$ '"'

...., 

Q) 

.... Callovian ol 

..:I 

:: 

'"' ol 

""' 

STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 

Standard zones in 

northwest Europe 

(Arkell, 1956; Callomon, 1955) 

Quenstedwceras Iamberti 

Pelwceras athleta 

Erymnoceras coronatum 

Kosrnoceras jason 

Sigaloceras ca.lloviense 

Macrocephalites macrocephalus 

Suggested guide fossils 

in the western interior 

of the United States 

(Imlay, 1953b, 1962a) 

Kepplerites mclearni ., 

f--;,~,.-ep-p...,.le-rt..,.,.te-s-a"'ff.- . ..,.,K~. t,....y.,..,cfw--n.,-i~s -----i ] 

G<Jwericeras sum:tum !;; ~-----------------~~ G<Jwericeras cost-idensum 

Characteristic fossils 

in the western interior 

of Canada 

(Frebold, 1963, 1964) 

/'fmlayoceras miettense 

Kepplerites mclearni 

Kepplerites aff. K. tycfwnis 

Not identified 

Warrenoceras henryi 

Suggested guide fossils 

in southern Alaska 

(Imlay, 1964b) 

Pseudocadoceras grewingki 

Xenocephalites vicarius 

Warrenoceras codyense ~ 1-------------------------+-------------------------i r----+----------~------------------------~--------7--------~<:j·~ 

<.> 

"' "' ol 

;:$ '"' 

...., 

~ 

"tl 

"tl 

·~ 

~ 

Bathonian 

Clydoniceras discus 

Oppelia aspidoides 

~ ~ "" <:j ~-~ 

" ~ 

-~ 

Paracephalites sawtoothensis Paracephalites glabrescens 

1----------- ? ----------~ 

~----------------------~---------?----------~~------------?----------~ 

Tulites subcontractus 

Gracilisphinctes progracilis 

Zigzagiceras zigzag 

Not 

identified 

Cranocephalites pompeckji 

~---------?----------~ 

r---------~------------------------4------------?--------r--

Not 

identified 

Bajocian 

Parkinsonia parkinsoni 

Garantiana garantiana 

Strenoceras subfurcatum 

Stephanoceras humphriesianum 

Otoites sauzei 

Sonninia sowerbyi 

Ludwigia murchisoni 

Trnewceras scissum 

Leioceras opalinum 

Sohlites spinosus and 

Parachondroceras andrewsi 

Megasphaeroceras cf. M. rotundum 

and Gryphaea planoconvexa.fraterna 

Chondroceras . .and 

Stemmatoceras 

Not 

identified 

Teloceras, Stemmawceras, 

Stephanoceras and Chondroceras 

Not 

identified 

Megasphaeroceras rotundum 

Teloceras iiinsae 

Skirroceras kirschneri 

Parabigotites crassicostatus 

Sonninia sp. 

1-----------?----------~ 

Tmewceras regleyi 

1-----------?----------~ 

FIGURE 12.--Correlation of some Middle and early Late Jurassic faunas in North America. 

29 

Near Green River Lakes in the Wind River Moun~ 

tains, Wyo., the Sliderock Member is possibly represented (Imlay, 1953a, fig.l) by 14 feet of dense mediumbedded limestone overlying 75 feet of red beds and 

gypsum that Richmond (1945) assigned to the Gypsum 

Spring Formation. Above the limestone are 33 feet of 

gray calcareous shale that resembles the Rich Member 

and above follows a red unit that probably correlates 

with the Boundary Ridge Member of the Twin Creek 

Limestone. 

Farther east in the Wind River Basin neither the 

Sliderock Member or the Rich Member are recognizruble 

as lithologic units. Limestone occurring in the middle 

or upper parts of the Gypsum Spring Formation (Love 

and others, 1945) of the Wind River Basin could be 

equivalent to either the Sliderock Member, or the Rich 

Member, or both. Likewise neither member is recogniz-

·able lithologically in the Gypsum Spring Formation in 

the Bighorn Basin of northern Wyoming or in the 

Piper Formation in southern Montana. In both areas, 

however, limestone members within these formations 

contain mollusks that in the Twin Creek Limestone are 

either restricted to the Rich Member or have never been 

found below the Rich Member. These facts, which are 
30 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

discussed in detail in the description of the Rich Memher (p. 33, 34), suggest that limestone deposition during 

the time represented by the Sliderock Member did not 

extend very far east or north of the area in which the 

Twin Creek Limestone occurs. 

Beds equivalent to the Sliderock Member of the Twin 

Creek Limestone probably exist in both central and 

southwestern lTtah. In central Utah the the lowermost 

lin1estone unit of the Arapien Shale exposed on the south 

side of Red Canyon about 2 1niles northeast of Nephi 

consists of 50-80 feet of oolitic sandy medium- to thickbedded limestone that greatly resembles the Sliderock Member. This limestone is overlain by at least 

100 feet of light-olive-gray shaly limestone that resembles the Rich Member (I1nlay, 1964a, p. C6). It is 

underlain locally by 1-10 feet of sof>t red siltstone, but 

in 1nost places is under lain directly by the Nugget 

Sandstone. 

In southwestern Utah near Gunlock and along the 

south side of the Pine Valley Mountains (Cook, 1957, 

p. 34, 41-!4; Wright and Dickey, 1963a, p. E63-E67, 

fig. 197; 1963b), the Carmel Formation consists of a 

basal red-bed unit overlain by 300 feet or 1nore of limestone and shale. From the li1nestone beds near the top 

(Mesozoic loc. 27 466) and near the bottom (Mesozoic 

loc. 28493) has been obtained the pelecypod Thracia 

weedi Stanton which is characteristic of the Sliderock 

and Rich Members of the Twin Creek Limestone and 

has not been found higher. The limestone and shale 

beds are not divisible lithologically into two members 

comparable to the Sliderock :and Rich Members and 

have not furnished any fossils that are restricted to 

just one of those 1nembers. 

ORIGIN 

The Sliderock Member of the Twin Creek Limestone 

was deposited in shallow marine ·waters of nonnal sa.-

linity as shown by the presence of oolites and Ostrea in 

the basal beds, by the presence of Ostrea and Gryplwea 

throughout the renrainder of the formation, by the darkgray to black color of the limestone, and by a fair 

abundance of bottom-dwelling as well as free-swimming and free-floating organisms. Deepening of the 

sea bottom during deposition is shown by an upward 

increase in the number and variety of organis1ns and 

particularly by the appearance of cephalopods. The 

presence of nerineid gastropods and a coral (Mesozoic 

loc. 3800) suggests that the waters were wa.im. A 

western source for the clastic sediments is shown by a 

westward increase in the amount of sandy, silty, and 

clayey material and by the occurrence of small pebbles 

in the 1nember in Idaho but not in Wyoming. 

RICH MEMBER 

DEFINITION 

The Rich Member of the Twin Creek Limestone consists mostly of medium-gray shaly limestone that is very 

soft basally but becomes harder upward, contains some 

thin beds of limestone near its top, ranges in thickness 

from a few feet to about 500 feet, and grades fairly 

abruptly into the adjoining members. Its base is generally 1narked topographically by a shallow depression 

where it overlies the harder thicker bedded limestone of 

the Sliderock Member. The top of the Rich Member is 

generally marked by a change upward from silty yellowish-gray shaly limestone to red siltstone, or to finely 

sandy thin- to thick-bedded yellowish-gray limestone, 

or to yellowish-gray sandy siltstone. It weathers characteristically into long slender pencillike splinters. The 

Rich Member has previously been called member C by 

Imlay ( 1950a, p. 39, 1953a, p. 55). 

The type section (fig. 1, loc. 33), is designated as the 

north side of Birch Creek about 8 miles west of Woodruff, Rich County, Utah, near the line of sees. 18 and 

19, T. 9 N., R. 6 E. The base of the section starts about 

half a mile west of the confluence of Birch Creek and 

Watton Canyon on the west side of the westernmost 

of the three small anticlines that are developed in the 

Twin Creek Limestone west of the middle of sec. 20. 

At this locality the Rich Member consists of about 425 

feet of light-yellowish-gray shaly limestone that weathers a little darker and is fairly soft in its lower 75 feet 

but becomes harder and ledge-forming upward. The 

lower beds weather into chunky fragments. The upper 

beds weather into long, slender fragments. The basal 

shaly limestone beds pass downward rather abruptly 

into much harder limestone beds at the top of the Sliderock Member. The uppermost shaly limestone beds of 

the Rich Member are overlain gradationally by 5 feet 

of sandy thin-bedded to shaly limestone at the base of 

the Boundary Ridge Member. 

DISTRIBUTION AND THICKNESS 

The Rich Member crops out in the sa1ne areas as the 

Sliderock Member except in the U nita Mountains where 

it is recognizable lithologically as far east as Whiterocks Canyon (Imlay, 1953a, p. 55, fig. 3). 

The Rich Member thickens westward from less than 

100 feet in its easternmost exposures in western Wyoming to about 380 feet at Willow Creek near Idaho 

Falls, Idaho (table 2). In northern Utah the member 

thickens westward from about 40 feet on the Whiterocks 

River, where it is included in the basal part of the 

Carmel Formation, to about 390 feet near Burr Fork 

and to more than. 500 feet near Devils Slide in the 

W asateh Range. Southward in that range it thins to 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 31 

about 180 feet near Thistle. This westward thickening 

is not uniform. Locally there is considerable variation 

in thickness which could be depositional, or could be a 

consequence of strong deformation. This last possibility is suggested because of the softness of the member, 

its position between more competent limestone members, 

and the pronounced folding and thrusting that exists 

throughout most of its area of outcrop. 

LITHOLOGIC AND STRATIGRAPHIC FEATURES 

The Rich Member changes very little lithologically 

throughout its area of outcrop (figs. 2-8). Most sections show a marked resemblance .to the type section as 

previously described. Toward the east as the member 

thins it becomes slightly more clayey and fairly fossiliferous. Toward the west as the member thickens it becomes more calcareous and poorly fossiliferous. Near 

the middle of the member in Idaho at Stump Creek, 

Willow Creek, and Red Mountain is a unit of limestone 

that contains the pelecypod Gryphaea planoconvexa 

Whitfield. At Fall Creek, Idaho, a unit of mediumbedded limestone 32 feet thick occurs above the middle 

of the member. Near the top of the member in most sections are thin beds of limestone, from 1 to 6 inches thick, 

that may be dense, granular, oolitic, or silty, or composed of a mass of crinoid columnals. These thin beds 

in Wyoming locally weather as low ledges. 

The contact between the Rich and Sliderock Members 

is easy to select because of marked differences in the 

hardness and competency of the members. The contact 

between the Rich Member and the overlying Boundary 

Ridge Member is 1nore difficult to select and generally 

has been chosen at the base of the lowest medium-bedded 

silty to sandy or oolitic yellowish- to brownish-gray 

limestone. In some places in the northern part of Jackson Hole, Wyo., the shaly limestone typical of the Rich 

Member is overlain directly by red siltstone or greenishgray claystone. 

FOSSILS, AGE., AND CORRE(LATIONS 

The Rich Mmnber is fairly fossiliferous in Wyoming 

and in the Uinta Mountains, Utah (figs. 9, 11, and table 

5), where the Inember is thinnest and most clayey. Elsewhere the Inember has furnished few fossils. Compared 

with the Sliderock l\fmnber, it is much less fossiliferous 

but has furi1ished a greater variety of pelecypods and 

a much smaller variety of gastropods and cephalopods. 

The gastropods are sparse, poorly preserved, and undeterminable. The cephalopods are unco1nmon and belong to different genera than occur in the Sliderock 

Member except possibly for one specimen assigned questionably to Stephanoceras. The pelecypods include 

the same species as occur in the Sliderock Member and, 

in addition, include some species that have been found 

only in the Rich Member or that begin in the Rich 

Member ·and range higher. 

Pelecypods ~that have been found only in the Rich 

Member include Gervillia.~ montanaensis Meek, Gryphea planoconvexa Whitfield, Myophorella (Haidaia) 

yellmostonensis Imlay (fig. 9). Pelecypods that have 

been found only in the Sliderock and Rich Members include Gryphaea planoconvexa fraterna Imlay, n. subsp., 

Prorokia fontenellensis Imlay, n. sp., Goniomya mO'ntanaensis Meek, and Thracia weedi Stanton. P~lecypods that begin in the Rich Member and range higher 

include Isognomon cf. I. perplana (Whitfield), Trigonia elegantissima Meek, V augonia conradi (Meek and 

Hayden), Myophorella (Promyophorella) rnontanaensis (Meek), Quenstedtia sublevis (Meek and Hayden), 

and 0 erc01nya punctata (Stanton) . There are 13 species listed above that Inay be useful in correlating_ the 

Rich Member with equivalent beds elsewhere in the 

western interior region. 

The ~ge of the Rich Member is not as well established 

as .that of the Sliderock Member. J.t is probably late 

Bajocian inasmuch as it grades downward into the 

Sliderock Member which has furnished substantial 

faunal evidence of an early late Bajocian age. FurtherInore, it grades upward into the Boundary Ridge_Member which is probably of Bathonian age, as indicated by 

regional correlations with the upper siltstone member 

of the Sawtooth Formation in northwestern Montana 

(Cobban, 1945, p. 1270-77; Imlay and others, 1948; Inllay, 1962a, p. C5-C14; Frebold, 1963, p. 28, 29). 

The ammonites in the Rich Member (fig. 11), although belonging mostly to new genera, also favor a late 

Bajocian age assignment. Thus Sohlites, n. gen., resembles Zemistephanus McLearn from beds of late middle Bajocian age in British Columbia (McLearn, 1927, 

p. 72; 1929a, p. 18) and Alaska (Imlay, 1964b, p. B51-

B53). It shows some resemblance, also, to Ermoceras 

Arkell (1952, p. 272) from beds of late Bajocian age. in 

Arabia. Parachondroceras, n. gen., resembles the middle Bajocian genera ChondroceraJJ Mascke (Westermann, 1956, pis. 1-12) and Labyrinthoceras Buckman 

( 1919, pl. 134) more than any described ammonites of 

Bathonian age. These ammonites, therefore, along with 

the fragments assigned questionably to Stephanoceras 

(USGS Mesozoic loc. 22459) suggest an age older than 

Bathonian. 

The Rich Member is traceable as a lithologic unit into 

the lower part of the "Lower Sundance" Forma;tion at 

Lower Slide Lake, Teton County, Wyo. At this place 

it is represented by 85 feet of shaly limestone (units 10-

12 of the section published by Imlay, 1956a, p. 70) that 

contains ammonites and pelecypods (fig. 9) character-
32 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

TABLE 5.-Geographic distribution of marine Jurassic megafossils in the Rich Member of the Twin Creek Limestone 

[Gastropod identifications are by N. F. Sohl. All one- and two-digit numbers refer to localities shown in figs.16 and 17. Higher numbers are USGS Mesozoic locality numbers 

identifying individual collections] 

Idaho Wyoming Utah 

2 3 12 13 42 50 51 52 53 61 62 65 66 68 70 75 79 89 90 91 92 

- - ----- ----- - - --- - --- - ., ~ 0> 0 <:0 t- t- 0 CQ 0> <:0 0> 0> 0 :s ., <:0 t-

~ ., t- 0 N ~ 0 * ~ CQ 00 S! ~ ., ~ 0> 0> 0> 00 <:0 12 0> ., N 00 0> ...... 00 00 00 <:0 00 ., 12 ;X\ ~ ., ., 0> 

""' t- t- N 0> 8 t- ...... <:0 t- t- N 00 t- t- t- 8 8 CQ <:0 ., CQ t- t- 00 00 N N 00 00 t- 0 00 N 00 ...... 00 00 00 ~ 00 00 00 00 0 12 ;X\ 12 00 00 ~ 0 00 00 ...... N N N N N N ...... N N N N CQ N N N CQ N N N N N N N N 0> N N N N N 

------------------------------------------------------------

§~~~:~ ~::~~~:~:=~::::: :~: ~:: :~: ~:: ~=: ~~= ;~ :~: ~~ i~i :~~ ;~ ;~; m ~~: ;~ =~= ~~ ;~ ~:: ;~ ~~~ ;~ ::~ ::: ~:: ~:: :~~ ;~ ~~ :~: ::: :~~ ~1= ~~~ 

~~iE~l~~~~~~~~)=j~~~~~~ ~~~ ~=~ ~~~ ~~~ ~~=~~==I=~~ =1~ ~~~ ~~~ j=~ :~: j~i :~ j~i j~: ~~ ~=i j:i ;~; ~~ jj: ~~: ~~: ~:: j:: ~~: =~~ ~=: j:~ ~:: ~~~ j:~ ~~~ 

can;r;gr;:;t;clt;~~~i~~~~~8-~-h_!:~~~::======= === === === === === === -~- === === === === === -~- === === ·x.- ~== === === === ~== ~== ~== === ~== ~== === ~== === === === === === === -~- jJ~~f~~~!~i~~~~~~i~~~===================== ~= === === === === === === =~= === === === === === === === === =~= ~~ ~~ === -~ === === =~= === === === === === -~- === === =~= =~= === 

Gryphaea g:~i~a~iil~~~~~~~~~fe;tri~tli~~-========== planoconvexa ~~~ ~~~ === === ~~~ === =~= =~~ === === === === =~= === === :~: === =~= === === === === === === :~: === :~: === === =~= === === === === === fraterna Imlay, n. 

r:~~~a-am-erican~iM:ee-k::=============·=-- ::: ::: =:: -~- ::: -~ ::: ::: ::: :=: ::~ ::~ ::: ::: ::: ::~ ::: ·x.· ::: ::: ·x.· ::: ::: ::: ::~ ::: ::: ::~ ::: -x_- ::: ::: ·x ::: ::: 

Mvophoreua vau~~~~:~~~:;d/'tr-1e:ek"an.d.-naycfeii)~===== (Promyophorella) montanaensis === === === === === === ·x.- === -~- ~- ·x === === ::: === ·x.- === ~- === === ~- -~- === === === === === === === ::: === ·x.- ·x.- === ~-

A~~Fe~:J:l~i~~:~~~~i~~~~~~========= === =~= === === === === === === ~ === === =~= ~= === === === -~- =~= === === ::: =~ ::: =~ === === === === === =~= === === =~= ~= === 

~~~~~~1/{~~~~Ii~!~Yi~~:~~ft=~=====:: === ==: ::: :=: ::: ==: E_ ::: =~= ::: =:: :=: :=: ::: -~ :=: ::: ::: =:: ::: ~~~ ::: =~= ::: ::: ::: ::: :=: =:: ::: ==: ::: ::: ==: ::: 

Luc~~a?-si>================================== === ~- === === === === -~- ~- -~- === -~- === === === -~- === === ~ === -~- ~ === === === === ::: === === === === === === === === ::: 

!i:~~~;:~ii.~J~~~~~~ji~~i=~== ~~~ ~~~ :~~ =~~ ::: :~~ :~ ~~~ ~~~ ~=~ ~~~ :~ ~~= :~: ~~~ ~== ~~: ~== ~~~ ~~: ~~: ~~~ ~:~ ~~~ ~== ~=~ ~~= ~~ ~== ~:: ~~~ ~~= ~= ~~= ;~: SP-------------------------------------- --- --- --- --- --- --- --- --- X --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- X --- --- --- --- --- ---

i~~~~f~i:~~t{Jm\\m~ ~~= ~r~ ~=~ ~== ~j~ ~~~ ;~ ~:~ =~= ;~ ==~ ~~~ ;~ :~~ ~~j :=~ ~~~ :~ ;~ ~~ ;~ ;~; ~jj :~~ ;~ !~j ;~; :jj ;~; ;r: ~~~ ~~~ i~ ;~ i~i 

~~~~~0~m=:=mm~~ ~1= m m m m ~~~ ~~ m =~ j~j m ;~ ~ ~=j ~!j ~~! ~=j ;~; ~j! ~~j ;: ~~~ ~~: ~=j ~:j ~jj ~=j j:! j~= m ~j: ~~j ~~ ~~~ ~~ 

~E:i~~~r~~=~~H.~=;~~=~m =~ m ~f: j~: m m ~~! m m m m m ~~ jjj j~: ==j m ~~~ =~~ m =:j =~j jjj =!j =j! ~~ m ~~ m ;~ :~: m m =~! ~j: 

~.icp~tfi~icr::s~rgJ:;n~:~;Ii:s"P---_-_-======= === === === === === === === === -~- === === === === === === === === === === === === === === === === :== === === === === === === === === ·x.· 

istic of the member. The Rich Member also may extend 

as far east as Green River Lakes (Imlay, 1953a, fig. 1) 

and as far south as Nephi, Utah, (Imlay, 1964a, p. C6) 

as discussed with correlations of the Sliderock Member. 

(p. 29,30). 

The Rich Member correlates closely with the lower 

40-65 feet of the limy lower unit of the Carmel Formation exposed along the west side of the San Rafael Swell 

in central Utah (fig. 10). This correlation is shown 

by the presence in the Carmel Formation of species that 

in the Twin Creek Limestone are restricted to the Rich 

Member or that have not been found below that memher. These species include in particular Sohlites spinosus Imlay, n. sp., (USGS Mesozoic loc. 12555) and Trigonia elegantl_ssima Meek (USGS Mesozoic loc. 12555) 

from 6 to 12 feet above the base of the Carmel Formation, MyophoreUa (Haidaia) yellow~stonensis Imlay 

(USGS Mesozoic loc. 13531) from 17 feet above the 

base, and Goniornya montanaensis Meek (USGS Mesozoic loc. 20351) from 25 feet above the base. With 

these are associated such species as Gerv-illia cf. G. montanaensis Meek, Vaugonia conradi (Meek and Hayden), 

and 11/ y o p h o r e ll a ( Prmnyophorella) montanaenfJis 

(Meek) which range throughout most or all of the limy 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 33 

lower unit of the Carmel Formation. Clearly the 

Carmel Formation along the west side of the San Rafael 

Swell does not include any beds as old as the Sliderock 

Member of the Twin Creek. On the basis of these fossils the Rich Member may be correlated approximately 

with units 50-66 of the section at the south end of Cedar 

Mountain (Gilluly and Reeside, 1928, p. 98, 99) and 

units 100-108 on Muddy River ( Gilluly and Reeside, 

1928, p. 85-87) . 

Similarly the Rich Member is correlated with the 

basal part of the Carmel Formation in some areas in 

southern Utah. At Teasdale, V augon:ia conradi (Meek 

and Hayden) (USGS Mesozoic loc. 25669), whieh has 

not been found below the Rieh Member, was eolleeted 

about 44 feet above the base of the Carmel Forn1ation in 

an interbedded limestone-elaystone 1nember about 85 

feet thick. In Antimony Canyon, Gervillia rrwntanaensi8 Meek (USGS Mesozoic loc. 26308), which is characteristic of the Rich Member, was eollected 20-30 feet 

above the base of the Carmel Formation in a limestone 

member about 110 feet thick. Near Hells Baekbone 

bridge, northwest of Boulder, the lower 15-20 feet of 

the Carmel Formation has furnished V augonia conradi 

(Meek and Hayden) and Oossmannea n. sp. (USGS 

Mesozoic loc. 28492; identified by N. F. Sohl). At Deep 

Creek north of Escalante the lower 12-20 feet of the 

Carmel Formation has furnished many fossils (USGS 

Mesozoic locs, 25671 and 26307) including the coral 

Actinastrea hyatti (Wells), the nerineid gastropod 

Oossmannea n. sp., and the pelecypods Thracia weedi 

Stanton, Oeroomya punctata Stanton, and V augonia 

conradi (Meek and Hayden). Of these fossils, the 

coral is characteristic of the Middle Jurassic, although 

ranging higher, the gastropod has been found elsewhere only in the Sliderock Member of the Twin Creek 

Limestone, and the pelecypods in association are characteristic of the Rich Member, as discussed on page 31. 

In southwest Utah between Mount Carmel in Kane 

County, and Shurtz Creek in Iron County, the middle 

and upper .Parts of the limy lower unit of the Carmel 

Formation may likewise be correlated with the Rich 

Member by means of the association of such species as 

Thracia 1JJeedi Stanton, Trigonia elegantissima Meek, 

and Va1tgonia conrad,i (Meek Hayden) (Imlay, 1964a, 

p. C 3', 4). The lowest 50-100 feet of the limy part of 

the Cannel Formation, however, is poorly fossiliferous 

and could be equivalent to the Sliderock Member. Still 

farther west along the south side of the Pine Valley 

Mountains the limy lower unit of the Carmel Formation 

could inelude beds older than the Rich Member as discussed previously under correlations of the Sliderock 

Member. 

The Rich Member correlates faunally with the limestone and shale in the lower part of the Sawtooth 

Formation in southwestern Montana, with the limestone 

member of the Piper Formation in southern Montana 

(Imlay, 1954, p. 58; 1956b, p. 576, 577), and with at least 

the middle and upper parts of the Middle Jurassic 

limestone exposed near Cody on the west side of the Bighorn Basin (Imlay, 1956b, p. 581-585). The Gypsum 

Spring Formation along the east side of the Bighorn 

Basin has furnished only a few long-ranging pelecypod 

species that do not permit close age determinations 

(Imlay, 1956b, p. 580, 581). The Gypsum Spring 

Formation in the Wind River Basin has furnished few 

fossils except near the top of the formation at one locality on Mill Creek (Love and others, 1945) west of 

Lander. The species present at this place (USGS 

Mesozoic loe. 19357) are mostly undescribed, however, 

and at present are not useful :for correlation with any 

member of the Twin Creek Limestone. 

Most of the Middle Jurassic limestone exposed near 

Cody, Wyo., is correlated with the Rich Member of the 

Twin Creek Limestone on the basis of the ammonite 

Sohlites (table 6), formerly called Teloceras (Zemistephanus) (Imlay, 1956b, p. 583), such pelecypods as 

Triqonia elegantissima Meek, V augonia conradi (Meek 

and Hayden) and Isognomon perplana (Whitfield), 

and the coral Actinastrea hyatti (Wells) (1942). Of 

these, Sohlites and the coral were found about 6 miles 

south of Cody (USGS Mesozoic loc. 17105) near the 

top of the lower third of the limestone (Imlay, 1956b, 

p. 567-583). Vaugonia conradi (Meek and Hayden) 

was found in the gorge 2 miles west of Cody in the lower 

5 feet (USGS Mesozoic loc. 17093) as well as in the 

uppermost 21f2 feet (USGS Mesozoic loc. 20361) of the 

limestone. Trigonia elegantissima Meek, I sognomon 

perplana (Whitfield) , and the coral were found in the 

gorge near the middle of the limestone sequence (USGS 

Mesozoic locs. 20336t 20375). Of these fossils, S ohlites 

is characteristic of the Rich Member (fig. 11) , the pelecypods have not been found below the Rich Member 

(fig. 9), and the coral is widespread in the western interior in beds of Bajocian age (Imlay, 1956b, p. 577, 

578; 1964a, p. C4), although occurring also in the 

Wolverine Canyon Limestone Member of the Preuss 

Sandstone in Idaho (Imlay, 1952a, p. 1740-1742). 

The middle limestone member of the Piper Formation in southern Montana (Imlay, 1956b, p. 576, 578) 

and the Sawtooth Formation in southwestern Montana 

contain many fossils that permit close correlation with 

the Rich Member and do not contain any fossils that 

indicate a correlation with the Sliderock Member. 

Significant fossils include the ammonites Sohlites and 

Parachondroceras (table 6 and fig. 11), described herein 
34 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

TABLE 6.-Geographic distribution of marine Bajocian ammonites in Montana and northwestern Wyoming north of area of Twin Creek 

Limestone 

[All two-digit numbers refer to localities shown on fig. 18. Higher numbers are USGS Mesozoic locality numbers identifying individual collections] 

Gypsum "Lower 

Sawtooth Formation Piper Formation Spring Sundance'' 

Forma- Formation 

tion 

Montana Wyoming 

98 99 100 101 102 103 104 105 106 107 108 109 110 111 

1-~--1- ----------f---~--1------,-------------

------------1----------------------------------------------------------

Stemmatoceras arcicostum Imlay, n. 

<~1~;,!~~~;~~"-:::: ~~-= r ~~; :~: ~~~ ~~~: ~~= =~:~ :~:~ ~~~= m= ==~~ ~~~~ =~= =~~ ~m =~~= =~= ~~~ ~~: ~~~ ~~~~ ~~:~ ~~~ ~~~ ==~~ m~ ~~== ~=~~ Normannztes? cf. N. crickmayi (McParachondroceras c~';~~~eras-cCC~allani(McLeaiii5== -x- ==== ==== -~- ==== ==== ==== ==== ==== ==== ==== ==== ==== ==== ==== ==== ==== ==== ==== ==== ==== ==== :::= ==== :::: :::: :::: :::: :::: andrewsi Imlay, n. 

'P~~~~iii~~f~"~~~~~ ~=:: ~~:: --:: ~::: :-== -~- ~=:: ;~: :~~ ~::: t ~~: ~~: :~: :~: ): :~: ~~: :~: ==~~ ~~~~ :~: :~: =~=~ ~~~~ ~~~ :~: ~~~ :~: 

and the pelecypods Gryphaea planoconvewa Whitfield, 

Thracia 1oeedi Stanton, Prorokia fontenellen.sis Imlay, 

n. sp., and Goni.mnya montanaensis Meek. Associated 

with these fossils are certain pelecypods that occur also 

in the Rich Member (fig. 9) but range higher, including 

Trigon1~a elegantis8irJna Meek, Vaugonia conra.di (Meek 

and Hayden), j}fyophorella (Prmnyophorella) nwntanaen.sis Meek, Isognomon perplana Whitfield, and 

0 oelastarte livingstonen.sis Imlay. Inasmuch as these 

fossils occur in all parts of the limestone member of 

the Piper Formation as well as farther west in the 

lower to middle parts of the Sawtooth Formation, there 

appears to be no normal marine equivalent of the 

Sliderock Member in southern and southwestern Montana. Equivalent beds, if present, 1nust be represented 

by red beds in the lower part of the Piper Formation 

or possibly by ·unfossiliferous sandstone beds locally in 

the basal part of the Sawtooth Formation. 

Marine equivalents of the Rich Member of the Twin 

Creek Limestone are present also in north-central 

Montana in the Bearpa w Mountains and in the subsurface. The evidence from the Bearpa w Mountains 

consists partly of a fossil collection (USGS Mesozoic 

loc. 8096) made from several Jurassic formations in 

the SWl;iSW%, sec. 1, T. 28 N., R. 20 E:, about 5 miles 

southeast of Cleveland. The fossils present include 

belemnites and the pelecypod Owytmna from the Swift 

Formation and Gryplwea nebrasce,nsis Meek and Hayden from the Rierdon Formation. They also include 

two external molds of Sohlites spinosus Imlay, n. sp., 

and two specimens of Gryphaea planoconv·ewa Whitfield. These and some other pelecypods are preserved 

in a dense grayish-black limestone identical with that 

constitutingthe lower 70 feet of the Sawtooth (Piper) 

Formation (Reeves, 1924, p. 93). 

Confirmation of the presence of Gryphaea planocon-

,vewa Whitfield in the basal limestone of the Jurassic 

sequence is furnished by a fossil collection made at the 

same locality by Frank Reeves ( 1924, p. 93, 94). His 

collection (USGS Mesozoic loc. 18757) contains one 

well-preserved specimen of G. planoconvewa Whitfield 

plus Trigonia americana Meek, Pinna kingi Meek, 

Omnptonectes platessiformis White, Idonearca cf. I. 

haguei (Stanton), bwcyprina cf. I. cinnabaren.sis 

(Stanton), I sognomon cf. I. perplana (Whitfield), and 

Astarte ( 0 oelastarte) living stonensis Imlay. 

The presence of the last two species shows that the 

basal limestone is not older than the Rich Member of 

the Twin Creek Limestone. The presence of Sohlites 

spinosus Imlay, n. sp., and Gryphaea planoconv·ewa 

Whitfield provides definite correlation with the outJ 

cropping middle limestone member of the Piper Formation in southern Montana, the lower to middle parts of 

the Sawtooth Formation in southwestern Montana, and 

the Rich Member of the Twin Creek Limestone. 

The evidence .for marine equivalents of the Rich 

Member in the subsurface of north -central Montana 

consists mainly of some fossils obtained in cores at the 

depth of 2,896-2,906 feet in The Texas Co.'s Bowdoin 

Unit 1 (822) test well in the center of the SW14SW% 

sec. 8, T. 32 N., R. 32 E., Phillips County, Mont. In 

this well the top of the middle limestone member of 

the Sawtooth (Piper) Formation was picked at the 

depth of 2,845 feet and the bottom at 2,945 feet. The 

fossils include Gryphaea planoconvewa Whitfield, 

Gervillia 1nontanaensis Meek, Isocyprina cf. I. cin-
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 35 

nabarensz/s (Stanton), /.? iddingsi ('Stanton), and 

fragmentary specimens of A8tarte, Pinna, and OamzJtonectes. Of these species, the first two are diagnostic 

of the Rich Member as well as of the outcropping 

limestone of the Piper Formation. 

Another bit of evidence concerning the age and cor~ 

relation of the subsurface Piper Formation is the 

presence of the coral Aotinastrea hyatti (Wells) (1942, 

p. 1-3) in cores from depth of 5,243-5,248 feet in the 

Socony Vacuum Rhodes well F-11-6 in sec. 6, T. 33 N., 

R. 47 E., Daniels County, Mont. This species, originally obtained from an oolitic limestone in the gorge 

2 miles west of Cody, Wyo., has been found at several 

localities in the Middle Jurassic limestone near Cody 

(Imlay, 1956b, p. 583, 584); in the Gypsum Spring 

Formation at Sykes Mountain northeast of Lovell in 

the SW% sec. 2, T. 47 N., R. 95 W., Bighorn County, 

Wyo.; in the middle limestone member of the Piper 

Formation in the Pryor Mountains (Imlay, 1956b, 

p. 577, 578), Carbon County, Mont.; in the limy lower 

unit of the Carmel Formation in southern Utah (Imlay, 

1964a, p. C4); in the Wolverine ·canyon Limestone' 

Member of the Preuss Sandstone in southeastern Idaho 

(Imlay, 1952a, p. 1740-42); and as pebbles at the base 

of the Swift Formation in the Centennial Range, 

Beaverhead County, Mont. (Imlay, 1952a, p. 1744). 

Except for the last two occurrences, the only corals 

found in the Jurassic of the western interior region 

have been in beds of l\Eddle Jurassic (Bajocian) age. 

The presence of corals, therefore, is of some value for 

correlation purposes. For this reason, all coral occurrences (identifications by J. W. Wells, Feb. 8, 1949, and 

Sept. 10, 1962) are given below by formation and 

localities. 

Swift Formation (pebbles only) : 

A.ctinastrea hyatti (Wells) (USGS Mesozoic loc. 20999) 

Thamnasteria.sp. (USGS Mesozoic loc. 20999) 

Preuss Sandstone 0Volverine Canyon Limestone Member): 

Actinast1·ea hyatti OVells) (USGS Mesozoic locs. 9748, 

16192) 

Aotinastrea n. sp. (USGS Mesozoic locs. 9748, 13897) 

Periserisn. sp. (USGS Mesozoic loc. 9749) 

Twin Creek Limestone (probably Sliderock Member) Coral 

undet. (USGS Mesozoic loc. 3800) 

Carmel Formation (limy lower unit) : 

Actinastrea hyatti (Wells) (USGS Mesozoic locs. 24258, 

26307,26309,28468,28473) 

Tharnnasteria sp. (USGS Mesozoic locs. 28461, 28470) 

Oppelismilia sp. (USGS Mesozoic locs. 28461, 28462) 

Gypsum Spring Formation or Middle Jurassic limestone: 

Actinastrea hyatti ('Veils) (USGS Mesozoic locs. 17097, 

17103, 17105, 17107, 17096, 17659, 19403, 24669, 27721) 

Piper Formation (middle limestone member) : 

Actinastrea hyatti (Wells) (USGS Mesozoic locs. 13794, 

17659, 17662, 17665, 17666, 17667, 24644, 24673, 24675) 

Coral undet. (USGS Mesozoic loc. 25866) 

Sawtooth Formation (limestone member) : 

Actinastrea hyatti (Wells) (USGS Mesozoic loc. 29273). 

The Rich Member of the Twin Creek Limestone 

should be equivalent to part of the middle shale member of the Sawtooth Formation in northwestern Montana (Cobban, 1945, p. 1270-1277; Imlay and others, 

1948). The stratigraphic evidence is based partly on 

correlation of the Rich Mmnber with the middle limestone 1nember of the Piper Formation, as just described, 

and on the fact that this limestone member in northcentral Montana is overlain (Reeves, 1924, p. 93; Imlay 

and others, 1948; Schmidt and others, 1961, p. 165) by 

sandstone, siltstone, and silty to sandy limestone that is 

identical lithologically and stratigraphically with the 

upper member of the Sawtooth Formation of northwestern Montana. Furthermore, in the Sweetgrass 

Hills, the middle member of the Sawtooth Formation 

consists of interbedded black limestone and shale (Cobban, 1945, p. 1274-1275) which is intermediate in character between the limestone member of the Piper Formation and the middle shaly member of the Sawtooth 

Formation. 

The faunal evidence for correlating the Rich Member with part of the medium shale member of the Sawtooth Formation is convincing only in the Sweetgrass 

Hills. From 27 feet of interbedded dark shale and 

limestone exposed on East Butte of the Sweetgrass 

Hills (Cobban, 1945, p. 127 4; Imlay and others, 1948) 

have been obtained the pelecypods (USGS Mesozoic loc. 

19190) Ostrea sp., Modiolus subimbrioatus (Meek), 

Trigonia americana Meek, J/ yophorella montanaensis 

(Meek), Omnptoneotes platessiformis White, Astarte 

( Ooelastarte) Zivingstonensis Imlay, Pseudolimea cf. 

P. dnnabarensis (Stanton) , Gervillia? montanaensis 

Meek, G. dolobrata Crickmay, ldonearoa haguei Stanton and lsooyprina cf. I. iddingsi (Stanton). Of these, 

Gervillia? 1nontanaensis in association with M. M ontanaensis and Astarte livingstonen8is is good evidence 

for correlation with the Rich l\fember. Furthermore, 

Pseudolimea oinnabarensis (Stanton) and Gervillia. 

dolobrata Crickmay are characteristic of the Piper ForInation near Livingston and on Cinnabar Mountain in 

southern Montana and have not been found in higher 

formations. 

The middle shale member of the Sawtooth Fonnation 

west of the Sweetgrass Arch has furnished very few 

fossils (Imlay, 1962a, p. C21). Near the base was obtained the ammonite Ohondrooeras (Imlay, 1948, p. 19, 

pl. 5, figs. 1-5) which shows that the basal part of the 

shale 1nember is older than either the Rich or Sliderock 

Members of the Twin Creek Limestone. Higher beds 

have furnished belemnite fragments, undetermined 

gastropods, and a few pelecypods. Of these, Astarte 
36 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

111;orion Crickmay (1936, p. 549, 559, pl. 3, fig. 1, 2) may 

be of some value stratigraphically because the type specimens are from the 1niddle limestone member of the 

Piper Formation on Cinnabar :Mountain, Mont. Myophorella 1nontanaensis (Meek), obtained from the lower 

part of the shale member (USGS Mesozoic loc. 18711), 

has not been found below the Rich Member of the Twin 

Creek Liinestone. 

ORIGIN 

The Rich Member was deposited in a shallow sea that 

deepened westward or southwestward. This deepening 

is shown by the fact that the pelecypod fauna is fairly 

abundant and varied in easternmost Idaho and western 

Wyoming where the member is thinnest and most clayey 

but raltl1er sparse el~where. It is shown, also, by the 

presence of Ostrea and Gryphaea mostly in the eastern 

exposures in Utah and Wyoming and in the northern 

exposures in Idaho. ·The abrupt change !rom fairly 

fossiliferous medium-bedded limestone aJt the top of the 

Sliderock Member to very soft poorly fossiliferous shaly 

limestone at the base of the Rich Member suggests a sudden deepening of the sea accompanied by rapid deposition of calcareous mud. Shallowing of the sea during 

deposition is indicated by the presence of some oolitic or 

coquinoid beds, or units of thin-bedded to Inedium-bedded limestone in the middle and upper parts of the 

member. The sea bottom in n1ost places was probably 

much softer and Inuddier than during deposition of the 

Sliderock Member as indicated by an increase in the 

variety of benthonic pelecypods, by a marked decrease 

in the nu1nbers of gastropods and of H1e pelecypod GryJJhaea, and perhaps by a paucity of ammonites. 

BOUNDARY RipGE MEMBER 

DEF'INITION 

The Boundary Ridge Member of the Twin Creek 

Limestone consists of interbedded soft red, green, or 

yellow siltstone, silty to finely sandy limestone, oolitic 

limestone, and greenish-gray silty claystone. It ranges 

in thickness frmn about 30 to 285 feet. It grades downward within 1 to 2 feet into the shaly limestone of the 

Rich Member and at 1nost places is overlain sharply by 

the cliff-fonning limestone of the \Vat.ton Canyon Member. The Boundary Ridge Mmnber has previously been 

called member D (Imlay, 1950a, p. 39; 1953a, p. 55). It 

is named after Boundary Ridge \Yhich lies east and 

southeast of the type locality. The type se0tion is designated as the exposures at and immediatelv north of the 

railroad cut about 1 1nile southwest of Pegram in the 

NW:lh sec. 12, T. 15 S., R.. 45 E., Bear Lake County, 

Idaho. This section from top to bottom 1nay })e 

described as follows: 

Bottndary Ridge .Member (type 8ection) of Twin Creek Limestone empo8ed at and immediately north of railroad cut, 1 mile 

southwest of Pegratn, in NWlA, sec. 12, T. 15 S., R. 45 E., Bear 

Lake County, Idaho 

Twin Creek Limestone (incomplete) : 

Watton Canyon Member (incomplete) : Feel 

Limestone, mediu~ to thin-bedded, even-bedded, 

dense, medium- to brownish-gray; about 150 ft. 

exposed. 

Boundary Ridge Member : 

16. Limestone, oolitic, slightly sandy, yellowish-gray; 

weathers pinkish gray; forms ledge_____________ 2 

15. Siltstone, sandy, soft, yellowish-gray------------ 6 

14. Claystone, soft, medium-gray------------------- 18 

13. Covered -------------------------------------- 24 

12. Limestone, dense to granular, yellowish-gray, 

vuggy, partly brecciated ; forms cliff_____________ 11 

.;.1. Siltstone, soft, reddish brown__________________ 25 

10. Limestone, thin-bedded to shaly, slightly sandy, 

granular light-yellowish-gray, poorly exposed___ 36 

9. Limestone, thin- to medium-bedded, brownishgray; weathers light yellowish gray; forms low 

ridge; makes sharp contact with underlying uniL 52 

8. Limestone, thick-bedded (beds %-6 ft thick), 

oolitic, medium-brownish-gray; weathers yellowish gray ; forms prominent ridge; Camptonecte8 

common on bedding surfaces___________________ 72 

7. Siltstone, soft, sandy; yellowish gray at top, becoming greenish gray at bottom; overlain abruptly 

by massive oolitic limestone along an undulating 

bedding plane; grades into underlying unit______ 5 

6. Siltstone, soft, sandy, purplish- to reddish-brown ; 

contains some green streaks near base___________ 6 

5. Siltstone, thin-bedded to shaly, calcareous, slightly 

sandy ; exhibits some crossbedding; yellowishgray at top, becoming greenish gray at base; surfaces wavy and crenulated; makes sharp· contacts 

with adjoining units___________________________ 9 

4. Siltstone, soft, yellowish-gray------------------ 1 

3. Siltstone, soft, brownish-red____________________ 5 

2. Siltstone, soft, yellowish-gray------------------- 3 

1. Claystone, thinly laminated, silty, soft, mediumgray, crinkled -------------------------------- 10 

Total thickness of Boundary Ridge Member____ 285 

Rich Member : 

Limestone, shaly, medium-brownish-gray, chunky; 

only 7 ft. exposed. 

DISTRIBUTION AND THICKNESS 

The Boundary Ridge Member crops out in the same 

areas as the Twin Creek Limestone as a whole but extends eastward in the Uinta Mountains, Utah, as far 

as the Whiterocks River (Imlay, 1953a, fig. 3) and in 

northwestern Wyoming passes gradually eastward into 

reel beds (Imlay, 1950a, p. 40; 1953a, fig. 1) that have 

been included in the "Lower Sundance" Formation. 

The Boundary Ridge Member thickens westward in a 

highly irregular manner (table 2). On the north it 

thickens westward from about 40 feet in the Jackson 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 37 

Hole area, Wyoming, to 187 feet at Willow Creek, 

Idaho (fig 2). Fa~ther south it thickens westward 

from about 50 feet at Fontenelle Gap (fig. 7) Wyo., to 

285 feet near Pegram, Idaho. In northern Utah the 

Boundary Ridge Member thickens westward from 30 

feet at Lake Fork to about 100 feet near Burr Fork 

and Devils Slide (fig. 8). Southward in the Wasatch 

Range from Devils Slide, it thins to about 40 feet near 

Thistle. 

LITHOLOGIC AND STRATIGRAPHIC FEATURES 

Overall, the Boundary Ridge Member is characterized by yellow silty sandy and oolitic limestone interbedded with soft red siltstone. As the mmnber thickens 

westward, the proportion of limestone increases, and 

that of the red beds decreases as does the amount of 

silty and sandy material. Gypsum was noted in red 

siltstone at one place and is indicated locally by brecciated limestone, but probably was never common. A 

limestone unit commonly occurs at the base of the memher. In Idaho ·and north-central Utah the sequence of 

beds appears to vary within short distances. 

The easternmost exposures of the Boundary Ridge 

Member in Wyoming consist mostly of soft brownishred siltstone that is underlain by a thinner unit of silty 

to sandy or oolitic yellowish-gray thin- to mediumbedded li1nestone. The upper part of the siltstone in 

most sections changes upward within the top 1-15 feet 

from red to pinkish- greenish- or yellowish-gray. 

Vuggy limestone occurs near the top at Fontenelle Gap. 

West ward in Wyoming, as the member th~ckens, interbeds of similar limestone appear within or at the top 

of the 1nember (figs. 2-9) . 

Near the boundary of Wyoming with Idaho and Utah 

and westward for about 15 miles the ·member is represented by the same lithologic types as in the type section near Pegram. In this area the member is charaeterized by silty, sandy, or oolitic yellowish-gray thinto thick-bedded lilnestone of which some units are cliff 

for1ning. Some of the limestone is vuggy and breeciated, which features suggest the former presence of 

gypsum such as reported from the subsurface (Peterson, 1957a, fig. 9 on p. 418). Between the limestone 

units are subordinate units of soft claystone, siltstone, 

and sandy siltstone that are 1nostly reddish brown but 

loeally are gray, greenish gray, or yellowish gray. The 

softer beds are generally poorly exposed and lenticular. 

Still farther west in Idaho near Willow Creek the member consists dominantly of dense, or oolitic, or silty 

thin- to thick-bedded limestone but includes 'a few feet 

of red beds in its upper part. 

In the Uinta Mountains, Utah, similar westward 

changes occur in the Boundary Ridge Member (fig. 8.). 

Between Lake Fork and Peoa the basal unit consists of 

shaly to thin -bedded grayish-yellow limestone that is 

sandy and ripple marked. This unit is overlain by a 

somewhat thicker unit of soft brownish-red siltstone 

that near the Duchesne River contains a little gypsum. 

At Lake Fork the red m1it is overlain by several feet of 

greenish-gray claystone and on the Duchesne River 

near Tabiona by 10 feet of brownish-gray soft limestone. Still farther west at Devils Slide the member 

consists mostly of massive brownish-gray oolitic limestone that rises vertically as high cliffs but basally ineludes some fine grained sandstone and silty limestone. 

Red beds were not observed but could be present beneath 

soil-covered intervals tot'aling 26 feet. A similar section near Burr Fork includes 10 feet of red soft siltstone a little below the 1niddle of the member. These 

westernmost sections of the member at Devils Slide and 

Burr Fork resemble the sections to the north at Birch 

Creek, Utah, and near Pegram, Idaho, except for a 

scarcity of red siltstone. South of Burr Fork near 

Monks Hollow and Thistle the Boundary Ridge Memher consists of a fairly thick unit of silty to sandy 

ripple-marked shaly to thin-bedded lilnestone overlain 

by a thin unit of soft red to green siltstone. 

The eontact of the Boundary Ridge Member with the 

underlying Rich Member is transitional, as described 

previously. The contact with the overlying Watton 

Canyon Member is generally marked by a sharp change 

from soft red, green, or gray siltstone or claystone to 

massive cliff-forming gray limestone. Locally in western Wyoming, as at Thomas Fork Canyon and Cottonwood Creek, the contact is marked by an abrupt change 

from silty yellowish- or brownish-gray shaly limestone 

to medium-gray medium-bedded compact dense to 

oolitic cliff-forming limestone. Locally in north -cent~al Utah, as at Devils Slide and Burr Fork, the highest 

beds of the Boundary Ridge Member consist of thickbedded brownish-gray oolitic limestone. This limestone is overlain by medium- to olive-gray medium- to 

thin-bedded limestone that is generally dense and is 

characterized by remarkably even bedding and by conspicuous rectangular jointing. These bedding and 

jointing features characterize the limestone of the W atton Canyon Member and generally differentiate it 

readily from the limestone of the adjoining members. 

Many sections of the Boundary Ridge. Member in 

Wyoming and Ida;ho have been described previously as 

member D (Imlay, 1950a, p. 42-45; 1953a, p. 60-62). 

The following sections illustrate the major changes that 

occur in the member in northern Utah. 
38 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Eastward extension of equivalents of Bottndary Ridge Member 

and adjoinin.g 'marine members of Twin Oreek Limestone in 

lower part of Oarmel Formation on ca.st side of Whiterocks 

River in SE1f4SE1j4 sec. 18, T. 2 N., R. 1, E., Uintah Oottnty, 

Utah. 

[Fig. 1, loc. 410] 

Carmel Formation (incomplete) : 

Watton Canyon equivalent: Feet 

12. Limestone, lithographic, grayish-white; overlain 

by soft green claystone and massive gypsum___ 11 

11. Limestone, shaly, light-gray------------------- 6 

Boundary Ridge equivalent: 

10. Limestone, thin-bedded, slightly sandy, oolitic, 

light-yellowish-gray, ripple-marked___________ 4 

9. Siltstone, soft, red____________________________ 17 

Rich equivalent: 

8. Claystone, bentonitic, soft, greenish-gray________ 2 

7. Sandstone, ·fine-grained, soft, white____________ 2 

6. Claystone, very soft, greenish-gray------------- 6 

5. Limestone, yellowish-gray ; thin bedded in middle; oolitic at top and bottom; contains pleuromya and Oamptonectes____________________ 4 

4. Claystone, soft, yelldwish-gray; contains thin 

beds of yellow nodular limestone that bear 

oysters ------------------------------------ 12 

3. Claystone and sandy siltstone, soft, light-gray___ 872 

2. Limestone, sandy, oolitic, brown; contains Oamptonectes ----------------------------------- lh 

1. Claystone, soft, yellowish-green ; contains many 

thin beds ( lf4 -~ in. thick) of light-yellowishgray shaly sandstone ; rests sharply on asphaltic Navajo Sandstone___________________ 5 

Navajo Sandstone (not measured). 

Bonndary Ridge Membet· on west side of Lake Fork in NW1f4 

sec. 2, T. 1 N., R. 5 1V., Duchesne Oounty, Utah 

[Fig. 1, loc. 39] 

3. Claystone, calcareous, greenish-gray; overlain by 2 ft 

of medium-gray oolitic limestone at base of 'Vatton 

Canyon ~ember _________________________________ _ 

2. Siltstone, soft, brownish-red _______________________ _ 

1. Limestone, mostly thin bedded, some shaly, silty to 

sandy, light-yellow, ripple-marked; forms low cliffs; 

underlain by light-gray shaly soft limestone of Rich 

~en1ber -----------------------------------------

Total thickness ________________________________ _ 

Feet 

3 

14 

13 

30 

Boundary Ridge lllembe'r on south side of Duchesne Rive1· in 

SW1f4 sec. 4, T. 1 S., R. 8 W., Duchesne Oounty, near Hanna, 

Utah 

[Fig. 1, loc. 38] 

3. Limestone, medium-bedded, silty, soft, brownish-gray; 

overlain by 8 ft of massive oolitic limestone at base 

of 'Vatton Canyon ]dember ______________________ _ 

2. Siltstone, soft, brownish-red ; pieces of white gypsum 

present about 6 ft above base ___________________ _ 

1. Limestone, shaly to thin-bedded, finely sandy, lightgrayish-yellow, ripple-marked; becomes harder and 

thicker bedded upward, some beds as much as 10 in. 

thick; forms low cliff, underlain by shaly limestone 

of Rich ~ember ________________________________ _ 

Total thickness __________________________ _ 

Feet 

10 

32 

26 

68 

Bowndm·y R·idge Member 1 mile nCJrthwest of Peoa in NW% 

sec. 14, T. 1 S., R. 5 E., Smum.it Oounty, Utah 

1[Fig. 1, Ioc. 37] 

5. Siltstone, soft, red to brown; overlain by mediumbedded cliff-forming limestone of Watton Canyon 

Feet 

~ember ---------------------------------------- 10 

4. Covered ------------------------------------------- 60 

3. Limestone, thin-bedded to shaly, partly sandy to silty, 

yellowish-gray ---------------------------------- 21 

2. Limestone, shaly, pinkish- to greenish-gray----------- 8 

1. Limestone, shaly to thin-bedded, slightly sandy;. yellowish-gray; underlain by light-gray shaly soft 

limestone of Rich ~ember________________________ 8 

Total thickness--------------------------------- 107 

Boundary Ridge JJiembm· on no1·th side of Emigrant Oanyon 

along east side of Br-igham Fork in SWlf4NElf4 sec. 21, T. 1 N., 

R. 2 E., Salt Lake Oounty, Utah 

[Half a mile west of Burr Fork, fig. 1, loc. 36] 

6. Limestone, massive, oolitic, yellowish-gray; some beds 

contain fragments of Oamptonectes, Modiolus, echinoid spines, and crinoid columnals. Overlain by 

medium-bedded, even-bedded limestone at base of 

Feet 

Watton Canyon ~ember__________________________ 15 

5. Limestone, medium-bedded, silty, yellowish-gray------ 30 

4. Limestone, massive, oolitic, yellowish-gray----------- 5 

3. Limestone, thin-bedded to shaly, silty, yellowish-gray_ 5 

2. Siltstone, soft, brownish-red________________________ 10 

1. Limestone, shaly to thin-bedded, silty to finely sandy, 

medium-yellowish-gray, ripple-marked; transitional 

within several feet into underlying light-gray shaly 

soft limestone of Rich ~ember____________________ 37 

Total thickness--------------------------------- 102 

Bounda1·y Ridge Member along spillway of reservoir on north 

side of Birch Oreek in NW1,4 sec. 19, T. 9 N., R. 6 E., Rich 

Oounty, Utah 

[Fig. 1, loc. 33] 

26. Claystone, gray, chunky; grades within several inches 

into medium- to thin-bedded, even-bedded limestone 

Feet 

at base of ·watton Canyon ~ember_______________ 3 

25. Limestone, dense, yellowish-gray___________________ 1 

24. Sandstone, thin-bedded to shaly, fine-grained, gray 

to pinkish-gray, crossbedded______________________ 5 

23. Siltstone, thin-bedded to shaly, soft, yellow, greenish22. 

21. 

20. 

19. 

18. 

17. 

16. 

15. 

gray, red; makes sharp contact with overlying 

sandstone. Well exposed in spillway below upper 

da111 ------------------------------------------- 34 

Limestone, oolitic, sandy, gray, yellowish-gray, partly 

brecciated --------------------------------------

Siltstone, soft, red---------------------------------

Lilllestone, thin- to thick-bedded, dense, light-yellowish-gray; forms low cliff; upper 2 ft brecciated __ _ 

Siltstone, soft, light-red, 111uch crumpled ___________ _ 

Sandstone, thin-bedded to shaly, light-red __________ _ 

Siltstone, soft, mediu111-red------------------------

Li111estone, thin-bedded, sandy, pinkish-gray, crossbedded, ripple-marked; weathers pink ___________ _ 

Sandstone, thin-bedded, silty, soft; pink at base; be- comes yellowish gray above _____________________ _ 

5 

5 

10 

4 

3 

5 

12 

28 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 39 

Bottndary Ridge Member along spillw·ay of reservoir oo north 

side of Birch Ot·eek in Rich County, Utah-Continued 

Feet 

14. Limestone, shaly to thin-bedded, dense, light-yellowish-gray --------------------------------------- 8 

13. Limestone, thin- to medium-bedded, dense light-yellowish-gray, even-bedded __________________________ _ 10 

12. Limestone, medium-bedded, medium-gray; contains 

oolitic streaks __________________________________ _ 4 

11. Limestone, massive, oolitic, medium-gray____________ 17 

10. Limestone, thin-bedded, partly oolitic, yellowish-gray; 

forms recess in cliff_____________________________ 4 

9. Limestone, massive, oolitic, medium-gray; fragmented 

fossils present; shows traces of bedding at an angle 

to the top and bottom of the unit ; slightly sandy 

near base______________________________________ 40 

8. Covered ------------------------------------------ 10 

7. Siltstone, very soft, brownish-red__________________ 10 

6. Siltstone, very soft, greenish-gray------------------ 3 

5. Sandstone, very finely bedded to shaly, yellowish- to 

pinkish-gray, ripple-marked______________________ 2 

4. Siltstone and silty claystone, very soft, yellowish- to 

pinkish-gray ----------------------------------- 6 

3. Sandstone, calcareous, fine-grained, grayish-red______ 1 

2. Siltstone, very soft, greenish- to yellowish-gray------ 16 

1. Limestone, thin-bedded to shaly, sandy to silty, lightyellowish-gray, wavy-bedded; some crossbedding; 

gradational into underlying light-gray shaly limestone of the Rich Member_______________________ 5 

Total thickness _________________ ,_______________ 251 

FOSSILS, AGE., AND CORREILATIONS 

The Boundary Ridge Member has furnished very few 

identifiable species (table 7), although fragments of 

Oarnptoneotes, crinoid columnals, and echinoid spines 

are n1oderately common. The only species present that 

appears to have much stratigraphic significance is 

Astarte ( Ooelastarte) livingstonensis Imlay, which was 

collected near the top of the mmnber. Elsewhere it has 

been found (fig. 9) in the Sliderock and Rich Members 

of the Twin Creek Limestone, in the limy lower part 

of the Carmel Formation in central and soutlnvestern 

Utah, in the Arapien Shale in central Utah, in limestone 

of Iniddle Jurassic age in the Bighorn Basin, and in the 

Piper and Sawtooth Formations in Montana. All occurrences that can be dated by ammonites are of Bajocian age. The occurrence in the Boundary Ridge 

Member, however, is probably of Bathonian age inasmueh as the underlying members are of late Bajocian 

age and the overlying Watton Canyon Member within 

the Jackson Hole area, Wyoming, passes eastward at 

Lower Slide Lake into beds eontaining the Callovian 

amn1onite Oadooeras (Imlay, 1956a, p. 70). 

In north-eentral Wyoming the Boundary Ridge Member continues far to the east (figs. 3, 10) as a soft red 

claystone and siltstone that locally contains nodules 

and lenses of gypsum and that in most places contains 

a thin unit of dolomitic or laminated limestone. This 

TABLE 7.-Geographic distribution of marin~ Jurassic r_negafossils 

in the Boundary Ridge Member of the Tw~n Creek L~mestone 

[Gastropod identifications by N. F. Sohl. Two-digit number~ refer. to localities 

shown in figs. 16 and 17. Higher numbers are USGS Mesozmc locahty numbers 

identifying individual collections] 

Wyo- Utah 

ming 

45 80 89 

__________________________ 20990 28791 21621 , __________ __ Camptonectes platessiformis White________ X --------X--

Trigonia elegantissima Meek_----------- ------ ------ X 

Astarte meeki Stanton __________________ ------------

(Coelastarte) livingstonensis Imlay---- ------ ------ X 

Pronoella uintahensis Imlay------------- ------ X 

Neri!~~?-~p~~~======================== Q --~-- --~--

red-bed member in the Wind River Mountains (Richmond, 1945) and in the Wind River Basin has generally 

been included in the lower part of the "Lower Sundance" Formation (Love and others, 1945) . In the 

Bighorn Basin it has been included in the upper part 

of the Gypsum Spring Formation (Imlay, 1956b, p. 

578, units 6-8) and in the upper part of a sequence of 

Middle Jurassic age (Imlay, 1956b, p. 582). 

In south-central, central, and eastern Montana the 

equivalents of the Boundary Rid~e Member a~ called 

the upper red-bed member of the Piper FormatiOn (Imlay and others, 1948; Imlay, 1954; 1956b, p. 575; Sand· 

berg, 1959) and the Bowes Member of Nordquist ( 1955, 

p. 102) of the Piper Formation (Rayl, 1956, p. 43). In 

southwestern Montana this red-bed member passes west· 

ward in the Madison Range into brownish-gray siltstone that is ealled the upper member of the Sawtooth 

Formation (Imlay and others, 1948). Similarly in 

north-central Montana the upper red-bed member of 

the Piper Formation is replaced westward in both sur· 

face and subsurface by silty to sandy beds (Imlay, and 

others 1948 · Peterson, 1947, p. 407; Rayl, 1956, p. 42, 

43; N ~rdqui~t, 1955, p. 102) that are lithologically identical with the upper siltstone member of the Sawtooth 

Formation in northwestern Montana (Cobban, 1945, p. 

1270-1277 · Selunidt and others, 1961, p. 165). Thus 

the red-bed member of the Piper Formation shows the 

same facies changes westward and northward as occur 

in the Boundary Ridge Member of the Twin Creek 

Limestone from western Wyoming westward into eastern Idaho. 

The validity of these facies changes has been confirmed by many surface and subsurface sections as well 

as by superposition, of the beds in question. between 

marine beds of late Bajocian and early Callovian ages. 

It appears reasonable, therefore, to correl~te the Bou~dary Ridge Member of the Twin Creek Limestone w.Ith 

the upper siltstone member of the Sawtooth Formatwn 
40 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

in northwestern Montana which contains ammonites of 

probable Bathonian age (Imlay, 1962a, p. C12-C13; 

Frebold, 1963, p. 28, 29) . 

The Boundary Ridge Member in the Uinta Mountains of northwestern Utah pinches out eastward between the Whiterocks River and Steinaker Draw near 

Vernal (Imlay, 1953a, p. 58). In the Wasatch Range 

its southernmost known exposure is on Crab Creek, a 

few miles south of Thistle. It has not been identified 

definitely in the Arapien Shale in central Utah but is 

possibly represented by about a hundred feet of yellowsandy oolitic limestone and some interbedded red siltstone cropping out on the south side of Red Canyon 

northeast of Nephi (Imlay, 1964a, p. C6). On the west 

side of the San Rafael Swell in central Utah smne soft 

gypsiferous gray green to red claystone, siltstone, and 

sandstone may be correlated with the Boundary Ridge 

Member on the basis of the fact that they overlie basa] 

limy beds that contain the same fossils as the Rich 

Member and underlie some ledge-forming sandstone and 

sandy to oolitic limestone that stratigraphically occupy 

the same position as the Watton Canyon Mmnber of the 

Twin Creek Limestone. The possible equivalents of the 

Boundary Ridge Member on Muddy Creek include units 

89-99 of the section described by Gilluly and Reeside 

(1928, p. 87) and at the south end of Cedar Mountain 

include units 46-49 ( Gilluly and Reeside, 1928, p. 98). 

Southwest of the San Rafael Swell near Teasdale and 

Antimony Canyon the Boundary Ridge Member is probably represented by gypsiferous siltstone and claystone 

that lie between limestone Inembers. The correlation is 

based again on the presence low in the lower limestone 

member of such pelecypod species as Ger'l'illa.'l rnwntanensis Meek and Vaugonia conrad-i (Meek and Hayden) 

that characterize or are not known below the Rich Mmnher of the Twin Creek Limestone. The correlation is 

based also on the assumption that the highest limestone 

member is equivalent to the Watton Canyon. Member. 

In southern Utah the approximate equivalent of the 

Boundary Ridge Member appears to be the Thousand 

Pockets Tongue of the Navajo Sandstone (Wright and 

Dickey, 1963a, p. E64-E65) . This tongue overlies the 

thin Judd Hollow Tongue of the Carmel Formation 

which has furnished fossils of Middle Jurassic age. At 

bvo localities (see p. 33) fossil collections made from 

12 to 20 feet above the base of the Carmel Formation 

contain an association of pelecypods that is characteristic of the Rich Member. It seems reasonable, therefore, that· the overlying sandstone beds forming the 

Thousand Pockets Tongue are equivalent to at least part 

of the Boundary Ridge Member of the Twin Creek 

Limestone. If so, the uplift and erosion of the Navajo 

Sandstone that furnished the sand of the tongue coincide in time with the marine regression represented by 

the Boundary Ridge Member and with an uplift in 

Canada that furnished the sand and silt present in the 

upper member of the Sawtooth Formation in northcentral and northwestern Montana. 

In southwestern Utah between Mount Carmel and 

Gunlock the equivalents of the Boundary Ridge Member should include some part or all of the red siltstone 

and sandstone directly overlying the limy lower part of 

the Carmel Formation. This correlation is indicated by 

the presence of fossils of Middle Jurassic age not younger than the Rich Member of the Twin Creek Limestone 

near :the top of tJ1e limy part of the Carmel Formation 

at several places between Mount Cannel and Pintura 

(USGS Mesozoic locs. 28494,28495,27465, 27466). The 

Boundary Ridge Member is therefore apparently equivalent at least in part to the Entrada Sandstone as used 

by Gregory (1950a, p. 94-96, 125-127; 1950b, p. 40, 41, 

80, 83, 89) in the area from Mount Carmel westward. 

ORIGIN 

The .. Boundary Ridge Member was deposited in a shallowing retreating sea under climatic conditions similar 

to those that existed during deposition of the Gypsum 

Spring Member. Such an origin is indicated by its 

strtt.tigraphic position between marine limestone members that represent transgressive deposits, by the red 

color of much of the claystone and siltstone, by th.e interbedding of irregular lenses of oolitic or silty to sandy 

crossbedded limestone with the red beds, and by a general scarcity of fossils except for fragments of echinoids, crinoids, and mollusks in some limestone beds. The 

scarcity of gypsum and the scanty evidence for the 

former presence of gypsum suggests that there were 

fairly broad marine connections with the main seaways 

to the west, or that the basins in which gypsum might 

form were very shallow and transient, or even that the 

deposition was partly inte~tidal or subaerial. 

The fairly thick lenticular units of oolitic limestone 

within the Boundary Ridge Member along and a little 

west of the western boundary of Wyoming contrast with 

the che~t-bearing limestone units in the ·Gypsum Spring 

Member in the same area and suggest deposition in shallower waters. The fact that the oolitic limestone units 

attain their greatest development at the western edge of 

a facies that consists mostly of red siltstone and claystone and that they pass westward at Willow Creek, 

Devils Slide, and Burrs F'ork into a facies that is mostly 

limestone shows that they were formed a:t the margin 

between two different environments of deposition. To 

the west, deposition evidently occurred in a very shallow 

agitated sea containing abundant calcium carbonate. 

To the east, deposition occurred in an even shallower 

sea or in intertidal areas, or subaerially. Possibly some 

of the oolitic and sandy crossbedded limestone was 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 41 

formed as offshore bars or as submarine swells behind 

which the red and green siltstone accumulated. 

WATTON CANYON lVIE.lVIBER 

DEFINITION 

The 'V atton Canyon Member of ~the T·win Creek Lime· 

stone consists mostly of medium- to brownish-gray compact brittle medium- to thin-bedded limestone and 

ranges in thickness from about 60 to 400 fe~t. It becomes thinner bedded upward and grades into the overlying Leeds Creek Member. Most of the beds are dense 

but oolitic beds occur· throughout. Generally the basal 

bed is massive and oolitic. The member is characterized 

by having unusually even bedding and conspicuous rectangular jointing that permit recognition at considerable distance and ready differentiation from adjoining 

members. In Wyoming and in the Uinta Mountains, 

Utah, the member is the main ridge former in the Twin 

Creek Limestone. In Idaho and adjoining parts of 

north-central Utah, both the Watton Canyon and 

Boundary Ridge Members form prominent ridges. The 

Watton Canyon Member has previously been called 

me~be~ E (I~lay, 1950a, p. 40; 1953a, p. 55). The type 

sect10n IS designated as the continuous exposures on the 

north side of Birch Greek about 8 miles west of Woodruff, Utah. The base of the. member is exposed just below the upper dam across Birch Creek. The top of the 

member is exposed in a small southward-draining gully 

at the extreme northwest corner of sec. 19. The member is named after WaJtton Canyon, just south of Birch 

Creek, where the sequence of beds appears to be the 

same as on Birch Creek. The type se~tion is described 

below. 

Watton Canyon JJfembet· (type section) o·n north side of Birch 

Creek in NW% sec. 19, T. 9 N., R. 6 E., Rich County, Utah 

[Fig. 1, loc. 33] 

5. Limestone, shaly to thin-bedded, yellowish-gray, splintery ; grades upward into soft silty shaly limestone 

at base of Leeds Creek Member; contact selected at a 

small gully ---------------------

4. Limestone, thin-bedded, medium-yell~~f~h~;;~;~~l;t;~ weathers light gray _________________ _ 

3. Limestone, oolitic, slightly sandy, yellowi;h-~r~;-~~h~l; 

at base; becoming medium-bedded and ledge forming at top __________________________________________ _ 

2. Sandstone, very thin bedded to shaly, calcareous finegrained, ripple-marked; grades upward into ~andy 

limestone -------------------

1. Limestone, medium- to thin-bedded~-d;~;;,-~e-di~~~~;;l~ 

lowish-gray, cliff-forming, even- bedded, brittle· becomes thicker bedded upward; weathers to a iight 

yellowish gray ; bears a conspicuous rectangular 

fracture pattern both on bedding surfaces and on 

cross sections ; overlies gray chunky claystone at top 

of Boundary Ridge Member ______________________ _ 

Total thickness 

Feet 

75 

80 

17 

22 

205 

399 

DISTRIBUTION AND THICKNESS 

The Watton Canyon Member crops out in the same 

areas as the Twin Creek Limestone as a whole. In 

Utah it extends as far south as Crab Creek south of 

Thistle and at least as far east as the Whiterocks River. 

The lowest few feet of limestone about 15 feet above 

the base of the Carmel Formation north of Vernal in 

sec. 26, T. 3 8., R. 21 E., is probably the easternmost 

limit of the member. In northwestern Wyoming, east 

of the area of the Twin Creek Limestone, the member 

is probably represented at Lower Slide Lake, Teton 

County, by units 16-21 of the published section (Imlay, 

1956a, p. 70). 

The Watton Canyon Member thickens westward from 

about 60 feet in Wyoming to about 400 feet in southeastern ~daho (table 2). In Utah it thickens westward 

from 17 feet on the Whiterocks River to 380 feet at 

Devils Slide. In the Wasatch Range its thickness 

ranges from 300 to nearly 400 feet. 

LITHOLOGIC AND STRATIGRAPHIC FEATURES! 

The Watton Canyon Member is characterized by 

dense medium- to thin-bedded medium- to brownishgray hard brittle limestone that is cliff forming, is 

remarkably even bedded, weathers into a noticeable 

rectangular fracture pattern, and persists with little 

change over lar.ge areas (figs. 2-8) . Most beds are 

dense to sublithographic, but some beds are granual or 

oolitic. Silty to finely sandy limestone beds showing 

some crossbedding are uncommon but have been noted 

in Wyoming at La Barge Creek, Fontenelle Gap, and 

the South Fork of Fontenelle Creek, in Utah at Lake 

Fork, Burr Fork, and Birch Creek, and in Idaho at 

Stump Creek and Big Elk Mountain. Sandstone beds 

occur in Utah at Lake Fork and Birch Creek. Generally the basal limestone bed is massive and oolitic 

and rises as a cliff above soft beds at the top of the 

underlying member. In the Wasatch Range near 

Devils Slide the Watton Canyon Member is quarried 

:for cement and is reported to be a natural cmnent rock 

that needs the addition of only a little silica. 

The upper part of the Watton Canyon Member in 

most sections contains units of thin-bedded to shaly 

ripple-marked limestone that are transitional into the 

soft shaly limestone of the Leeds Creek Member. Gen· 

erally the boundary can be selected at the top of a 

hard bed of oolitic or crinoidal limestone underlying 

much softer beds, but in some places the boundary 

must be chosen -arbitrarily within an interval of 30-50 

feet. The most difficult area in which to select the 

boundary is in Wyoming between South Piney Creek 

and Slate Creek where the Leeds Creek Member con· 
42 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

tains 1nany beds of oolitic limestone. In that area the 

boundary has been selected arbitrarily at the base of 

a unit of shaly limestone overlying a series of massive 

cliff-forming limestone. Overall the two members ·~re 

readily distinguished by marked differences in bedding, 

in hardness as expressed by topographic relief, and in 

fracture pattern. 

Many sections of the Watton Canyon Member in 

Wyoming and Idaho have been described previously 

as member E (Imlay, 1950a, p. 42-44; 1953a, p. 60-62). 

Of these the described section near La Barge Creek 

should be 1nodified by placing the top of the Watton 

Canyon Member between units 15 and 16 (Imlay, 

1950a, p. 42) . This position is in harmony with the 

tops selected in sections to the north and south. The 

following sections illustrate the lithologic features of 

the n1ember in northern Utah. 

Watton Canyon Membet· on west side of Lalce Forlc in NW%, sec. 

2, T. 1 N., R. 5 W., Duchesne County, Utah 

·[Fig. 1, loc. 39] 

Feet 

9. Limestone, thin -bedded, oolitic1 sandy, yellowish-gray ; 

underlies soft greenish-gray calcareous claystone___ 4 

8. Mostly covered; some sandy limestone in upper 17ft___ 24 

1: Limestone, thin-bedded, sublithographic, very light 

gray; weathers white____________________________ 37 

6. Covered ------------------------------------------ 13 

5. Limestone, sandy, light-yellowish-gray--------------- 4 

4. Covered ------------------------------------------ 12 

3. Sandstone, light-yellow----------------------------- 2 

2. Limestone, thin- to medium-bedded, dense, hard, lightyellowish-gray ---------------------------------- 11 

1. Limestone, oolitic, medium-gray; overlies greenishgray calcareous soft claystone at top of Boundary 

Ridge Member___________________________________ 2 

Total thickness ______________________________ 109 

Watton Canyon JJfember on south side of Duchesne River in 

SlV1,4 sec. 4, T. 1 S"., R. 8 lV., Dttehesne County, near Hanna, 

Utah 

[Fig. 1, loc. 38] 

4. Limestone, very thin bedded, sublithographic, very 

light gray, platy, cliff-forming; weathers nearly 

Feet 

white ------------------------------------------ 44 

3. Limestone, massive, oolitic, brownish-gray; fossiliferous at top (USGS Mesozoic loc. 21623) _____________ 4 

2. Limestone, shaly to medium-bedded, mostly thin 

bedded, light-yellowish-gray, cliff-forming, fossiliferous (USGS Mesozoic loc. 21622) ------------------ 48 

1. Limestone, massive, oolitic, fossiliferous (USGS 

Mesozoic loc. 21624) ----------------------------- 8 

Total thickness ______________________________ 104. 

Watton Canyon Me.mber 1 mile northwest of Peo'a in NWlA, .'lee. 

14, T.1 S., R. 5 E., Summit County, Utah 

[Fig. 1, loc. 37] Feet 

3. Limestone, medium- to thin-bedded, mostly dense, oolitic, medium-gray, cliff-forming; weathers light gray; 

overlain by soft shaly limestone___________________ 105 

2. Limestone, shaly, gray to greenish-gray-------------- 8 

1. Limestone, medium-bedded, medium- to light-gray, 

cliff-forming; oolitic in part, underlain by soft red 

to brown siltstone________________________________ 107 

Total thickness--------------------------------- 220 

Watton Canyon Membm· on. north side of Emigrant Canyon on 

e.ast side of Brigham Fork in NElA, sec .. 21, T. 1 N., R. 2 E .• 

Salt Lake County, Utah 

[Half a mile west of Burr Fork, fig. 1, Ioc. 3.5] 

3. Limestone, medium-bedded, even-bedded, dense, light·· 

yellowish-brown; weathers yellowish gray; fractures 

in a rectangular pattern; contains one bed of massive oolite about 100 ft above base. Overlain by 4 

ft of silty brecciated limestone and then by 7 ft of 

Feet 

soft red siltstone_________________________________ 150 

2. Limestone, massive, sandy, yellowish-gray------------ 10 

1. Limestone, medium-bedded, mostly dense, light-yellowish-gray; weathers a little darker ; basal part oolitic; 

fractures in a rectangular pattern; rests on massive oolitic limestone at top of Boundary Ridge 

Member ----------------------------------------- 188 

Total thickness--------------------------------- 348 

Watton. Canyon Member Ot~ north side of ro.ad near Thistle in 

W¥2 sec. 28, T. 8 S"., R. 4 E., Utah County, Utah 

,[Fig. 1, loc. 42]1 Feel 

7. Limestone, medium-bedded, medium-gray; weathers 

light gray; overlain by soft shaly limestone _____ _ 30 

6. Limestone, thin- to medium-bedded, dense, mediumgray -------------------------------------------- 94 

5. Limestone, medium- to thin-bedded, dense, medium- to 

yellowish-gray ---------------------------------- 124 

4. Limestone, thin- to medium-bedded, yellowish-gray, 

rubbly ------------------------------------------ 8 

3. Limestone, papery to thin-bedded, yellowish-gray_____ 32 

2. Limestone, mostly medium to thin-bedded, dense, cliffforming, yellowish-gray; some beds massive________ 47 

1. Limestone, thick-bedded, dense to oolitic, pink to 

medium-gray, cliff-forming; upper surface bears 

many shells of Camptonectes; rests sharply on red to 

green soft siltstone______________________________ 10 

Total thickness_________________________________ 345 

FOSSILS, AGE, A.NU CORRELATIONS 

The Watton Canyon Member is not very fossiliferous 

(table 8) and 1nost of the species present have fairly 

long ranges (fig. 9) . One pelecypod of age significance 

is .Myopholas hardyi Imlay, which was obtained a little 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 43 

above the middle of the member near the Duchesne 

River, Utah. Elsewhere it has been found in the upper 

600 feet of the Twelvemile Canyon Member of the 

Arapien Shale in central Utah, throughout the Rierdon 

Formation in Montana, and in the lower part of the 

"Lower Sundance" Formation in north-central Wyo~ 

1ning. All occurrences are in beds of Callovian age. 

Other fossils indicative of a Callovian age include 

small striate Gryphaeas that appear to be immature 

forms of Gryphaea nebrascensis Meek and Hayden. 

These were noted in the uppermost part of the Watton 

Canyon Member at Slideroek Creek northeast of Coke~ 

ville and at Cottmnvood Creek east of Smoot, Wyo. 

TABLE B.-Geographic distribution of marine Jurassic megafossils in the Watton Canyon Member of the Twin Creek Limestone 

[Gastropod identifications by N. F. Sohl. All one- and two-digit numbers refer to localities shown in figs. 16 and 17. Higher numbers are USGS Mesozoic locality numbers identifying individual collections] 

Wyoming Utah 

42 50 88 89 96 

1---.---.------.--1- ----

------------1-·1--------

~~~~~~E~~~~r,~;:~~~~~-::_:: :~: -~- =~~~ _::~ ~=:~ ;~; :~: ~~~~ :~: sp ____________________________________ X ---- ---- ---- X X ---- ---- ---- Lima (Plagiostoma) occidentalis Hall and Whitfield _______________________________________________ X X ___________ _ 

Ctenostreon cf. C. gikshanensis McLearn.__ ____ ____ ____ ____ X _______________ _ ~st_rea ~Liostrea~ st_rigilecula White _________________________ X X X _______ _ ngonta elegant~ss~ma Meek_______________ ____ ____ ____ ____ ____ ____ X _______ _ Vaugonia conradi (Meek and Hayden)_____ X ____ ____ ____ ____ X X 

Myophorella (Promyophorella) montanaent~~fi~E:Y:~~:~ll;;;;=~:;:::: ~~:l ~:~~ :~: :~: :~:: ~~: -:: ~~~ ~~~: I 

The Watton Canyon Member is traceable lithologically and stratigraphically into the "Lower Sundance" 

Formation exposed along the north side of Lower Slide 

Lake, in the eastern part of Jackson Hole, Teton 

County, Wyo. At this place it is represented by a sequence of thick- to thin-bedded oolitic limestone and 

interbedded shaly limestone and calcareous shale cmnprising units 16-21 of the published section (Imlay, 

1956a, p. 70). This sequence overlies brownish-red siltstone and olive-green shaly limestone which are correlated with the Boundary Ridge Membe~; the sequence 

underlies calcareous shale (unit 22) that is correlated 

with the Leeds Creek Member. The sequence is significant because of its lithologic resemblance to the W atton Canyon Member in the Twin Creek Limestone as 

exposed nearby on the western side of Jackson Hole, 

because similar limestone at the same position is traceable far eastward and northward and because its middle 

and upper parts contain fossils of early Callovian ageThese include Oadoceras, Warrenoceras (Frebold, 1963, 

p. 13, 14), and Gryphaea sp. juv. cf. G. nebrascensis 

Meek and Hayden in unit 19 and typical G. nebrascensi8 

in unit 20. 

Near Green River Lakes in the Wind River Mountains the probable equivalents of the Watton Canyon 

Member include 20 feet of shaly and oolitic limestone 

that overlies 40 feet of red claystone and underlies about 

230 feet of gray shale in which Gryphaea nebrascensis 

Meek and Hayden is abundant (Richmond, 1945). In 

the western part of the Wind River Basin the probable 

equivalents of the Watton Canyon Member consists o£ 

oolitic limestone, sandy limestone, and siltstone that constitute the third member above the base of the "Lower 

Sundance" Formation as defined by Love and others 

(1945). For example, at Red Creek in the Owl Creek 

Mountains the member consists of 35 feet of interbedded 

oolitic limestone and sandy siltstone that has furnished 

the ammonites Oadoceras and Warrenoceras. It underlies soft gray claystone containing Gryphaea nebrascensis in abundance, and it overlies red beds that appear 

to be an eastern extension of the Boundary Ridge 

Member. 

These oolitic and sandy beds pass eastward (fig. 10) 

along the south side of the Bridger Range and the Bighorn Mountains into an oolitic sandstone, known as the 

Canyon Springs Sandstone Member of the Sundance 

Formation (Tourtelot, 1953). This sandstone persists 

southeastward to the Hartville Uplift (Love and others, 

1949) and eastward to the northern part of the Black 

Hills (Imlay, 1947, p. 247-251) at the base of the Sundance Formation. It persists northward along the 

flanks of the Bighorn Mountains and Pryor Mountains 

as oolitic sandstone and oolitic limestone (Imlay 1956b, 

p. 585, 586, 588~ 589) that rests unconformably on the 

Gypsun1 Spring Formation (Imlay, 1956b, p. 579, 580). 

West ward in the Bighorn Basin and in the Pryor 

Mountains the basal sandy beds are replaced by oolitic 

limestone that persists at the same stratigraphic position as far as the Cody area, Wyoming (Imlay, 1956b, 

p. 592, 593) and across southern Montana (Imlay and 

others, 1948) as far as the Tendoy Range west of Dillon. In southern Montana this oolitic limestone forms 

a ledge as 1nuch as 20 feet thick at the base of the 

Rierdon Formation. In south-central Montana it rests 

sharply on red beds at the top of the Piper Formation 

that are similar to the eastern facies of the Boundary 

Ridge Formation in Wyoming. In southwestern Montana west of the Madison Range it rests on silty to sandy 

limestone, siltstone, and sandstone at the top of the 

Sawtooth Formation that are similar to the western 

facies of the Boundary Ridge Formation in Idaho. The 
44 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

oolitic li1nestone appears, therefore, to be the stratigraphic and lithologic equivalent of the Watton Canyon Member of the Twin Creek Limestone. It differs 

mainly by being thinner. Similar calcareous beds persist northward in the subsurface of eastern Montana in 

the basal part of the Rierdon Formation and have been 

correlated by Peterson ( 1957 a, p. 408, 428) with that 

part of the Twin Creek Limestone that is now called 

the Watton Canyon Member. 

In central Utah the Arapien Shale probably includes 

beds equivalent to the Watton Canyon Member of the 

Twin Creek Limestone. This correlation is suggested 

by the sequence on the south side of Red Canyon about 

2 miles northeast of Nephi (Imlay, 1964a, p. C6). At 

this place the lower 50-80 feet of limestone resembles the 

Sliderock Member. The overlying 100 feet or Inore, of 

soft shaly limestone rese~nbles the Rich Member. The 

next overlying 100 feet of yellowish-gray sandy oolitic 

to dense limestone near the top of the ridge resembles 

the Boundary Ridge Member. The highest 100 feet or 

more of limestone capping the ridge consists of thin- to 

medium-bedded medium-gray dense to granular limestone that resembles the Watton Canyon Member. The 

overlying soft splintery limestone south of the ridge 

resembles the lower part of the Leeds Creek Member. 

The sequence basally at one place includes a few feet 

of red siltstone comparable to that in the Gypsum 

Spring Member. 

In central Utah on the west side of the San Rafael 

Swell the highest part of the li1ny lower part of the 

Carmel Formation is comparable lithologically and 

stratigraphically with the Watton Canyon Member. 

This part consists generally of a cliff-forming oolitic to 

sandy poorly fossiliferous limestone such as represented 

by unit 88 of the section on Muddy River 1neasurecl by 

Guilluly and Reeside (1928, p. 87). It overlies sequences 

resembling the Boundary Ridge Member and the Rich 

Member and und~rlies a thick sequence of gypsiferous 

beds that compare in softness, thickness, and stratigraphic position with the Leeds Creek and Giraffe Creek 

Members of the Twin Creek Limestone. The overall 

appearance of the Carmel Formation on the west side of 

the San Rafael Swell resembles that of the westernmost 

exposures of the Carmel Formation in the Uinta Mountains at Whiterocks River (Imlay, 1953a, p. 58). At 

that place the Rich, Boundary Ridge, and Watton Canyon Members of the Twin Creek Limestone are recognizable lithologically and stratigraphically, but the 

overlying beds composing three-fourths of the formation consist of gypsum, red and green siltstone, claystone, and sandstone that are very different than the 

upper two Inembers of the Twin Creek Limestone as 

exposed in sections to the west. 

In southern Utah southwest of the San Rafael Swell, 

a cliff-forming limestone similar to the Watton Canyon 

Member is well developed near Teasdale (Smith and 

others, 1963, p·. 25, 92, 93) and Antimony Canyon 

(J. C. Wright, written commun., 1960). Near Teasdale 

it consists of about 72 feet of yellowish-gray cliff-forming sandy limestone and calcareous sandstone (units 26 

and 27 of Smith and others, 1963, p. 92). One thin 

bed of oolitic fossiliferous limestone occurs about 42 feet 

above the base. This cliff-forming limestone is underlain by about 250 feet of white gypsum, fine-grained 

gray sandstone, light-green to reddish-brown siltstone, 

gray claystone, and some very thin bedded limestone 

that is correlated tentatively with the Boundary Ridge 

Member. Below is 15-18 feet of interbedded very fossiliferous limestone and fine-grained sandstone (units 

8 and 9 of Smith and others, 1963, p. 93). These beds 

have furnished the pelecypod Vaugonia oonradi (Meek 

and Hayden) and Myophorella montanaensw (Meek) 

(USGS Mesozoic loc. 25669), which in the Twin Creek 

Limestone have not been found below the Rich Member. 

Between these fossiliferous limestone beds and the 

Navajo Sandstone are 42 feet of unfossiliferous soft 

brownish-red to yellowish-gray siltstone and claystone 

and yellowish-gray sandstone. 

In Antimony Canyon the lower limy part of the 

Carmel Formation likewise is marked at the top by 

about 70 feet of cliff-forming oolitic limestone th31t 

could be equivalent to the Watton Canyon Member. 

This limestone is underlain by about 90 feet of .gypsiferous red beds and then by about 110 feet of beds 

that are mostly limestone. Gervillia tnontanaensw 

Meek, a fossil characteristic of the Rich Member, was 

obtained from 20 to 30 feet above the base of the formation (USGS Mesozoic loc. 26308). 

In southwest Utah the only limestone beds that might 

correspond in stratigraphic position with the Watton 

Canyon Member of the Twin Creek Limestone are in_ 

the upper part of the Curtis Formation as used by 

Gregory (1950a, p. 96, 125.:._127) which extends from 

the Paria River west to near Cedar City (Gregory, 

1951, p. 29, 30, 59-60; 1950b, p. 41, 80, 83, 85, 89). If 

this correlation is correct, then the underlying 100-200 

feet of red beds that Gregory ( 1950a, p. 94, 1951, p. 27) 

called Entrada Sandstone would be equivalent to the 

Boundary Ridge Member. Such a correlation is supported by the presence in the upper part of the limestone underlying the Entrada Sandstone as used by 

Gregory, of certain species that are either characteristic 

of .the Rich Member of the Twin Creek Limestone, or 

that do not range above that member. 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 45 

ORIGIN 

The Watton Canyon Member of the Twin Creek 

Limestone was deposited in a shallow sea of early Callovian age that transgressed eastward across the area 

of Wyoming over the truncated edges of the Middle 

Jurassic Gypsum Spring Formation and extended far 

beyond. (Compare figs. 3 and 4 in Imlay, 1950b, opposite p. 80 and 82.) Evidence for truncation is 

conclusive in the area bordering the Bighorn and Pryor 

Mountains in north-central Wyoming and south-central 

Montana (Imlay, 1956b, p. 579, 580, figs. 4--7) and in 

the northern part of the Black Hills in Wyoming and 

South Dakota (Mapel and Bergendahl, 1956, p. 87, 88). 

Evidence for trm1ca,tion is weak for areas west of the 

Bighorn Mountains. The abrupt change from red 

beds at the top of the Boundary Ridge Member to 

marine limestone at the base of the Watton Canyon 

M~mber is proof of an environmental change but is not 

proof of an unconformity at the contact. Even an 

abrupt environmental cha~ge is not indicated in those 

places where the upper beds of the Boundary Ridge 

Member consist of oolitic limestone, as at Devils Slide 

and Burr Fork, or of calcareous gray shale, as at Birch 

Creek. Also, the fact that many species of mollusks 

range throughout most of ·the Twin Creek Limestone 

(fig. 9) indicates that marine sedimentation was continuous in parts of the western interior region from 

Bajocian into Callovian time. 

The shallowness of the sea in which the Watton Canyon Member was deposi.ted is indicated by an abundance 

of oolites and by the presence of such fossils as Ostrea 

and Gryphaea. The equivalent Canyon Spring Sandstone Member of the Sundance Formation in central 

and eastern Wyoming is considered to be of littoral to 

shallow marine origin on the basis of its fossils (Imlay, 

1950b, p. 94). 

The sourr:e of the silt and sand grains in the 'Vatton 

Canyon Member is not known. Some silt and sand 

occur sporadically throughout the area of outcrop of 

the member, but the amount does not increase in any 

particular direction. A northern source from Belt 

Island in Montana is unlikely, considering that the 

oolitic limestone at the base of the Rierdon Formation 

in southern Montana is not sandy. A source from the 

west, locally is suggested by the fact that the Watton 

Canyon Member at Birch Creek in northern Utah contains more sand tl1an occurs in the member to the north ' south, or east. A southeastern source from the area of 

southeastern Wyoming or northern Colorado is another 

possibility. That area appears to have been the source 

of some of the sand in the Canyon Springs Sandstone 

Member of the Sundance Formation (Imlay, 1947, 

p. 251; Pipiringos, 1957, p. 19-21), which is an eastern 

extension of the Watton Canyon Member. 

LEEDS CREEK MEMBE,R 

DEFINITION 

The Leeds Creek Member is the thickest and most conspicuous member of the Twin Creek Limestone. It supports scant vegetation and consequently forms extensive 

light-gray bare slopes that are visible for a considerable 

distance. It ranges in thickness from about 260 to 1,600 

feet (table 2), and consists mainly of soft dense lightgray shaly limestone that generally weathers into lighter 

colored splintery fragments, but in places weathers into 

chunky fragments. Its lower part in most sections is 

softer than its upper part and is generally marked by a 

ravine or valley. At wide intervals the 1nember contains hard thin beds of fossiliferous limestone. In some 

areas it also contains thin to thick limestone beds or units 

of which 'Some are oolitic and others are silty to sandy 

and ripple marked. Its upper part is gradational into 

the silty ~to sandy ripple Inarked cliff-forming limestone 

of the Gira.ffe Creek Member. Overall, the member 

greatly resembles the Rich Mmnber but differs by being 

much thicker and much less fossiliferous. The Leeds 

Creek Mmnber has previously been called member F 

(Imlay, 1950a, p. 40; 1953a, p. 59). The type section is 

designated as the north side of Leeds Creek and is described along with the type section of the Twin Creek 

Limestone (p.16). 

DISTRIBUTION AND THICKNESS 

The Leeds Creek Member crops out throughout the 

same areas as the Twin Creek Limestone as a whole. In 

northwestern Wyoming it terminates eastward at the 

Darby-Absaroka line of overthrusting. In the Uinta 

Mountains, Utah, it is recognizable lithologically as far 

east as Lake Fork. In the Wasatch Range it becomes 

Inore clayey and silty southward but is recognizable at 

least as far south as the Thistle a.rea. Its southenunost 

exposures at Monks Hollow and Thistle are lithologically similar to the upper shaly part of the Twelvemile 

Canyon Member of the Arapien Shale above the basal 

500-800 feet of medium- to thin-bedded lin1estone such 

as is exposed on the south side of Red Canyon about 2 

miles northeast of Nephi (Johnson, 1959, p. 20-25, 41; 

Imlay 1964a, p. C6). 

The Leeds Creek Member thickens westward considerably (table 2) . In its northen1 occurrences it thickens 

westward from a Ininimum of 330 feet in the Jackson 

Hole area, Wyoming, ~to about 1,145 feet at Willow 

Creek east of Idaho Falls, Idaho (figs. 2, 3). Farther 

south it thickens westward frmn about 260 feet at South 

Piney Creek to about 1,600 feet at Thomas Fork Canyon 

(figs. 4, 5). In northern Utah it thickens westwa.rd 
46 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

from about 115 feet at Lake Fork to 1,290 feet at Devils 

Slide and about 1,500 feet at Burr Fork (fig. 8). It apparently thins southward in the Wasatch Range, but the 

thickness of only 275 feet measured at Monks Hollow is 

not in line with regional thickness trends and may reflect 

eli1nination of beds by faulting. Si1nilarly in Cottonwood Creek east of Smoot, Wyo., the measured thickness of the Leeds Creek Member is several hundred feet 

less than it should be on the basis of regional thickening 

westward. 

LITHOLOGIC AND STRATIGRAPHIC FEATURES> 

The Leeds Creek ~{ember is characterized throughout 

its extent by soft light-gray shaly limestone !that com· 

1nonly weathers into long, pencillike splinters and contains some hard thin beds of limestone (figs. 2-8) . 

These features are n1odified in certain areas, however, 

by increases in clay or sand content or in the number of 

hard beds of limestone. Thus the 1nember becomes distinctly softer, more chunky, and 1nore clayey northward 

in Idaho and eastward in northwestern Wyoming. In 

the eastern part of the Jackson Hole area it passes into 

a very soft claystone facies that is included in the "Lower 

Sundance" Formation. This claystone facies, which is 

exposed at Lower Slide Lake, Teton County, Wyo., as 

well as the more chunky clayey facies of the Leeds Creek 

Member, contains thin beds of nodular limestone in 

which are abundant Gryphaea nebra:3censis Meek and 

Hayden. This species and the nodular limestone in 

which it occurs are common in nortlnvestern Wyoming 

as far south as South Piney Creek and Greys River and 

in Idaho as far south as Big Elk Mountain and McCoy 

Creek. 

The Leeds Creek Member like,vise becomes more 

clayey southward in the Wasatch Range, Utah, as compared with sections of the member to the east in the 

Uinta Mountains and to the north at Watton Canyon 

and Birch Creek. At Burr Fork, for example, the 

Leeds Creek Membe1· contains many units of soft calcareous claystone interbedded wiH1 calcareous gray silt~ 

stone, fine-grained sandstone, sandy limestone, smne red 

si1tstone, and 1nuch shaly limestone. Similar lithologic 

types occur in the member exposed at Devils Slide. This 

clayey to sandy facies is lithologically intermediate between the member as exposed to the north at Birch Creek 

and the upper part of the Twelvemile Canyon Member 

of the Arapien Shale exposed to the south near Nehpi 

in central Utah (Hardy, 1952, p. 25, 26). In contrast 

with the clayey facies in Jackson Hole, Wyo., and 

adjoining areas, it does not contain thin beds of nodular 

li1nestone and associated Gryphaeas and does contain 

considerable sandstone and siltstone. 

Another facies change occurs in the Leeds Creek Memher in Lincoln County, Wyo., south of the latitude of 

Big Piney. This change consists of a marked eastward 

increase in the number of hard beds of limestone and of 

silty and sandy 1naterial The limestone beds commonly 

consist of oolites associated with crinoid columnals~ 

echinoid spines, and shells of Oamptonectes. Many thin 

beds of limestone, however, are silty or sandy and show 

crossbedding and ripple marks. No eastward change in 

clay content is apparent. In the easternmost sections 

as exposed at Slate Creek, Fontenelle Gap,- La Barge 

Creek, and South Piney Creek the hard limestone beds 

form ledges from 1 to 9 feet high separated by thicker 

intervals of soft shaly limestone. Because of these 

ledges, the topographic appearance of the member differs considerably from that near the western boundary 

of Wyoming as at Leeds Creek, Sliderock Creek, and 

Thomas Fork Canyon. In fact the ledgy character of 

the n1ember makes the boundary with the Watton Canyon Member n1ore difficult to select than in any other 

area in which both members are exposed. 

The facies changes in the Leeds Creek Member in the 

Uinta Mountains are different from those just described. 

Near Peoa the member consists mostly of soft shaly 

limestone but above its middle contains some beds of 

hard sandy limestone. Near the Duche~ne River the 

1nember consists entirely of soft shaly limestone. At 

Lake Fork it is mostly covered but includes 10 feet of 

greenish-gray sandy shale basally. At Whiterocks 

River the probable equivalents consist of gypsum, red 

sandstone and siltstone, and soft green claystone that 

by definition are included in the Carmel Formation. 

These beds overlie 17 feet of sublithographic and shaly 

limestone that is identical lithologically and stratigraphically with the 'Vatton Canyon Member. 

The Leeds Creek Member is overlain gradationally 

by the silty to sandy beds of the Giraffe Creek Member 

and consequently their boundary must be selected arbitrarily in 1nost sections. Generally the boundary is 

placed at the base of a hard bed of silty to sandy ripplemarked limestone that underlies a sequence of similar 

hard ridge-forming limestone. 

Many sections of the Leeds Creek Member in WyoIning and Idaho have been described previously as member F (Imlay, 1950a, p. 42-44; 1953a, p. 60-62). Certain other sections are described below because they 

illustrate facies changes and suggest sources of clastic 

sediment. The section at Fontenelle Gap by comparison 

with the Leeds Creek section indicates a marked shallowing of the sea eastward and an eastern source of 

clastic sediments. The section at Burr Fork -by comparison with that at Birch Creek, Utah, indicates a 

source of clastic sediment west or southwest of the 

1V asatch Range. The section at Willow Creek, Idaho, 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 47 

contains smne sandstone and sandy beds that were not 

noted farther east in Idaho. 

Leeds Greek Member on north side of Fontenelle Gap in NE¥4 

sec. 5, T. 24 N., R.115 W., Lincoln County, Wyo. 

[Fig. 1, Ioc. 30] 

Feet 

46. Covered. Overlain by sandy, oolitic thin-bedded rubbly limestone forming low ridge at base of Giraffe 

Creek Member__________________________________ 12 

45. Limestone, medium- to thin-bedded, oolitic, darkgray; forms low ledge___________________________ 472 

44. Limestone, shaly, soft, poorly exposed______________ 17 

43. Limestone, oolitic, dark-gray; forms low ledge______ 3 

42. Limestone, shaly, soft, medium-gray---------------- 32 

41. Limestone, thick-bedded, oolitic, finely sandy, darkgray; shows some crossbedding; weathers dark 

brownish gray; forms cliff_______________________ 9 

40. Limestone, medium- to thin-bedded, silty to finely 

sandy, yellowish-gray, crossbedded; weathers 

brownish gray---------------------------------- 4 

39. Limestone, oolitic, dark-gray; weathers dark gray; 

forms base of cliff______________________________ 5 

38. Limestone, shaly, soft, medium-yellowish-gray, poorly 

exposed --------------------------------------- 65 

37. Limestone, medium-bedded, oolitic, dark-gray; consists of three hard beds 1-2 ft thick separated by 

softer beds_____________________________________ 9 

36. Limestone, shaly, soft_____________________________ 3 

35. Lime s to n e, medium-bedded, oolitic, dark-gray; 

weathers dark brownish gray; forms ledge________ 7 

34. Limestone, shaly, soft, poorly exposed______________ 15 

33. Limestone, oolitic, dark-gray_______________________ 172 

32. Limestone, shaly, soft, yellowish-gray; weathers light 

gray ------------------------------------------- 30 

31. Limestone, oolitic, dark-gray; weathers medium gray_ 172 

30. Limestone, thin-bedded to shaly, silty, yellowish-gray; 

shows some crossbedding; weathers brownish gray; 

froms ledge_____________________________________ 9 

29. Limestone, shaly, soft, medium-gray---------------- 6 

28. Limestone, oolitic, dark-gray, weathers dark gray; 

surface bears Oamptonectes, echinoid spines, and 

crinoid columnals------------------------------- 1% 

27. Limestone, shaly, sof,t, medium-gray---------------- 5 

26. Limestone, thin-bedded, silty to finely sandy, yellowish-gray, crossbedded, ripple-marked; weathers 

brownish gray---------------------------------- 10 

25. Limestone, shaly, silty, very soft, yellowish-gray_____ 10 

24. Limestone, oolitic, silty, hard, yellowish-gray; 

weathers yellowish gray; forms top of ledge______ 2 

23. Limestone, shaly, silty, yellowish-gray-------------- 1% 

22. Limestone, oolitic, silty, hard, medium-brownishgray; weathers same ; contains crinoid columnals 

and echinoid spines______________________________ 3 

21. Limestone, shaly, silty, soft, yellowish-gray; weathers 

same ----------------------------------------- 12 

20. Limestone, thin-bedded to shaly, silty, yellowish-gray, 

ripple-marked; one 6-in. bed of oolite about 1 ft 

below top; weathers brownish gray_______________ 4 

19. Limestone, shaly, soft, medium-gray ; in upper 10 ft 

occur dark-gray oolitic beds 4-12 in. thick_________ 20 

Leeds Greek JJI ember on north side of Fontenelle Gap, 

Lincoln County, Wyo.-Continued 

Feot 

18. Limestone, shaly, silty, soft, yellowish-gray, crinkly, 

ripple-marked ---------------------------------- 5 

17. Limestone, thin-bedded to shaly, silty to finely sandy, 

yellowish-gray, ripple-marked; some crossbedding 

present; weathers light brownish gray; forms 

low ledge at top_________________________________ 20 

16. Limestone, medium- to thin-bedded, oolitic, dark-gray; 

weathers medium gray ; upper surface bears many 

Oamptonectes and oysters________________________ 2% 

15. Limestone, thin-bedded, silty to finely sandy, yellowish-gray, ripple-marked; some crossbedding, weathers yellowish brown ; forms ledge________________ 6 

14. Limestone, shaly, soft, medium-gray; forms recess 

between ledges__________________________________ 2 

13. Limestone, shaly to thin-bedded, silty, yellowishgray, ripple-marked, weathers same; forms ledge___ 6 

12. Limestone, shaly, soft, medium-gray ; weathers yellowish gray; forms recess___________________________ 5 

11. Limestone, shaly, medium-gray, platy; soft at base, 

becomes harder upward, weathe.rs same__________ 18 

10. Limestone, silty to finely sandy, light-yellowish gray; 

shaly and soft at base, becoming thin bedded and 

ledge forming at top; some ripple marks and crossbedding present ; weathers brownish gray_________ 13 

9. Limestone, thin-bedded, silty, medium-gray; weathers 

yellowish gray; forms upper part of ledge_________ 4 

8. Limestone, shaly to thin-bedded, oolitic, medium-gray; 

weathers yellowish gray ; contains echinoid spines 

and crinoid columnals___________________________ 3 

7. Limestone, shaly, soft, yellowish-gray______________ 3 

6. Limestone, thin-bedded, silty to finely sandy, yellowish-gray ; small ripple marks present______________ 5 

5. Limestone, shaly, soft, medium-gray---------------- 2 

4. Limestone, finely sandy, ha.rd, yellowish-gray_______ 1 

3. Limestone, shaly to thin-bedded, silty to finely sandy, 

yellowish-gray, ripply-marked; weather same_____ 3 

2. Limestone, shaly to thin-bedded, finely laminated, 

medium gray; contains some silty streaks, becomes 

shalier at top; weathers light gray; forms ledge____ 13 

1. Limestone, shaly, soft, medium-gray; weathers same; 

forms recess. Overlies medium- to thin-bedded, 

even-bedded limestone at top of w·atton Canyon 

Member 6 

Total thickness-------------------------------· 420 

Leeds Greek Member along south side of Watton Canyon in sees. 

24-26, T. 9 N., R. 5 E., Rich County, Utah 

'[Fig. 1, Ioc. 3•3] 

2. Limestone, shaly, soft, medium-yellowish-gray; weathers into light-gray splinters. Contact with Giraffe 

Creek Member concealed _________________________ _ 

1. Limestone, shaly, silty, medium-yellowish-gray; contains some thin beds ; grades downward into harder 

thin-bedded limestone at top of 'Vatton Canyon MemFeet 

943+ 

ber --------------------------------------------- 75 

Total thickness _______________________________ 1,018+ 
48 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Leeds Ot·eek Member on north side of Emigrant Oanyon between 

Burr Fork and Brigham Fork from NE¥.! sec. 21 to NW. cor. 

sec. 27, T.1 N., R. 2 E., Salt Lake Oounty, Utah 

[Modified after A. E. Granger (19:53-, p. 10, 11). Fig. 1, loc. 35,] 

19. Limestone, thin-bedded to shaly, light-gray, locally 

fissile; weathers orange to yellowish-gray; grades 

upward into ridge-forming silty to sandy thin- to 

medium-bedded limestone forming the Giraffe Creek 

~ember ---------------------------------------- 18. Claystone, calcareous, light-yellowish-gray, splintery ; 

weathers a little darker ________________________ _ 

17. Limestone, shaly, light-gray, splintery; weathers pale 

gray ------------------------------------------- 16. Claystone, calcareous, pale-olive, splintery _________ _ 

15. Limestone, shaly to platy, light-gray, splintery ______ _ 

14. Claystone, calcareous, light-gray, splintery _________ _ 

13. Limestone, shaly, light-gray, splintery _____________ _ 

12. Limestone, sandy, yellowish-brown; shows ripple 

1narks; weathers darker ________________________ _ 

11. Claystone, calcareous, locally sandy, pale brown, 

splintery --------------------------------------- 10. Limestone, shaly, light-yellowish-gray, splintery ____ _ 

9. Sandstone, thin-bedded, calcareous. pale-brown; 

weathers yellowish gray-------------------------

8. Claystone, locally sandy, pale-brown, splintery _____ _ 

7. Limestone, shaly to thin-bedded, light-gray, splintery; 

weathers yellowish gray-----------------------~-

6. Silt;:;tone, soft, red, greenish~ and yellowish-gray ____ _ 

5. Limestone, shaly to thin-bedded, light-yellowish-gray, 

platy ------------------------------------------

4. Limestone, shaly, splintery, and splintery claystone; 

light-yellowish-gray, weathers light gray ________ _ 

3. Limestone, thin-bedded, finely sandy, yellowish-gray __ 

2. Siltstone, soft, red and yellowish-gray--------------

1. Limestone, silty, yellowish-gray, partly brecciated; 

overlies medium-bedded, even-bedded limestone 

typical of the Watton Canyon Member ____________ _ 

Feet 

170 

520 

55 

33 

27 

27 

71 

32 

115 

95 

6 

120 

125 

15 

30 

60 

8 

7 

4 

Total thickness ______________________________ 1,520 

Leeds Oreek Member along north side of Willow Oreek in SW1,4 

sec.17, T.1 N., R. 40 E., Bonneville Oounty, Idaho 

[Fig. 1, loc. 4] 

9. Limestone, shaly, soft, medium-yellowish-gray; overlain gradationally by greenish-gray thin bedded to 

shaly sandy ripple-marked limestone at base of 

Giracs Creek Member _____________ _ 

8. Limestone, shaly, soft, medium-gray; ~~t~i~-s--;-fe; 

beds of silty limestone and yellow fine-grained sandstone ------------------------------------------- 7. Limestone, shaly, soft, medium-gray-----------------

6. Sandstone, thin-bedded, fine-grained, yellowish-gray __ _ 

5. Limestone, shaly, silty, yellowish-gay----------------

4. Sandstone, thin-bedded, very fine grained, light-yellowish-gray; at top a thin bed of limestone bears Ostrea 

Oamptonectes, and crinoid columnals ______________ ~ 

3. Limestone, shaly, soft, medium-gray, splintery _______ _ 

2. Limestone, thin-bedded to shaly, dense, medium-gray __ 

1. Limestone, shaly, soft, medium-gray, splintery, poorly 

exposed ; some hard ledges of thin-bedded limestone 

crop out 30-50 ft above base; overlies thick-bedded 

oolitic limestone at top of 'Vatton Canyon Member __ 

Foot 

235 

126 

19 

3 

12 

4 

513 

107 

126 

Total thickness ________________________________ 1,145 

FOSSILS, AGE, AND CORRELA..TIONS 

The Leeds Creek Member has furnished few specifically identifiable fossils (table 9). The units of soft 

shaly limestone do not contain megafossils. The beds 

of oolitic limestone contain many fragments of crinoids, 

echinoids, and Oamptonectes, and rarely other fossils. 

Most of the identifiable species are represented by only 

a few specimens at one or two localities. Gryphaea 

nebrascensis Meek and Hayden is the most abundant 

molluscan species, and it is found only in the northern 

part of the area of outcrop of the Leeds Creek Member. 

Most of the identifiable species range upward from the 

Sliderock Member or the Rich Member of the Twin 

Creek Limestone -and have similar long ranges else~ 

where in the western interior region (fig. 9). 

Only two pelecypod species found in the Leeds Creek 

Member have any known age significance. One of 

these, Platymya roaky1nontana Imlay, n. sp., has been 

fom1d at one locality near the middle of the member. 

Outside of the area of the Tw:in Creek Limestone it has 

been found only in beds of Callovian age. In Montana 

it is present throughout the Rierdon Formation and is 

associated with such ammonites as Oadoceras and 

Kepplerites. At Lower Slide Lake in northwestern 

Wyoming it occurs in unit 19 of the published sequence 

(Imlay, 1956a, p. 70) in association with Oadoceras. 

The other pelecypod species of age significance is 

Gryphaea nebrascensis Meek and Hayden which occurs 

throughout the Leeds Creek Member. Outside of the 

area of the Twin Creek Limestone it is found through 

1nost of the Rierdon Formation of Montana and most 

of the "Lower Sundance" Formation of noDthern 

Wyoming (Imlay, 1948, p. 19; 1953b, p. 7, 8). G. 

nebrascensis is absent in the basal beds of both of 

these formations. In Wyoming its lowest occurrence 

is in the uppermost beds of the oolitic limestone and 

oolitic sandstone that comprise the Canyon Springs 

Sandstone Member of the Sundance Formation and in 

equivalent beds (Imlay, 1956b, p. 587, 591, 592). In 

northwestern Montana in the Sawtooth Range west of 

Great Falls it is preceded in the lower part of the 

Rierdon Formation by G. impressimarginata McLeam 

(fig. 12). 

Gryphaea nebrasaensis Meek and Hayden has been 

found a.t many places associated with ammonites of 

Callovian age, but has not been found in older beds. 

It has been found rarely in the lower part of the Swift 

Formation in Montana, but the specimens could have 

been reworked from underlying beds. Its presence 

throughout the Leeds Creek Member is excellent evidence for the Callovian age of that member and for 

correl31tions with part of the Rierdon Formation and 

the "Lower Sundance" Formation. 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 49 

TABLE 9.-Geographic distribution of marine Jurassic megafossils in the Leeds Creek Member of the Twin Creek Limestone 

[Gastropod identifications by N. F. Sohl. All one- and two-digit numbersrefer tolocalitiesshowninfi.gs.16and17. Highernumbersare USGS Mesozoic locality numbers 

identifying individual collections] 

Idaho Wyoming Utah 

2 9 11 22 24 25 28 29 30 32 40 42 44 50 55 65 74 76 94 95 

---- - - - --- - ----- - - - - ""' r:- 00 0> 0 g C'l C'l r:- 00 C"' ...... c:o 0> 0 ""' e-.1 r:- ~ ...... § ~ ~ ""' 00 ~ 00 00 0> 

~ 00 f?5 0 ...... C"' C"' c:o 0 .... 00 e-.1 0> 8 ..... c:o 0> 00 lQ lQ lQ lQ lQ @ c:o e-.1 r:- r:- 00 e-.1 e-.1 0 0 e-.1 ""' ~ 8 8 ~ 8 C"' 8 e-.1 e-.1 c:o 00 00 00 00 00 00 e-.1 ~ 00 r:- r:- 00 00 00 00 c:o 00 C"' r:- 0 0 00 0 0> 00 r:- r:- C'l C'l C"' C"' C'l C'l ..... c:o e-.1 C"' C"' C'l e-.1 <:Q e-.1 ...... e-.1 ..... C"' ..... C"' C'l C'l C'l C'l C'l C"' ...... ..... 

------·---------------------------------------------

Worm tubes___________________________________________ ___ ___ ___ X___ X ____________________________________ --- --- --- --- --- --- --- --- --- --- --- --- --- --- Echinoid fragments_______________________________________ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ X ____________ --- ___ --- --- --- --- --- Crinoid columnals_____________________________________ ___ ___ ___ ___ ___ X ___ X X X ___ ___ ___ ___ ___ ___ ___ X X ___ ___ ___ ___ X --- --- --- --- --- --- --- --- Nucula sp ___________________________________________________________________________________________________________________________ --- --- --- --- --- X 

Modiolus subimbricatus (Meek) __ ---------------------- ________________________________________________________________________ --- --- ___ --- --- --- --- X 

Camptonectes &tygiuo White ____________________________________________________________________________________________________ --- X--------- X------

ff~~7~~i;~~~:]~'=~~~~:::~==~~~~~=~==~~~:~~~~:::: ~=~ ~=~ ~=~ ~~= ~=~ =~~ =~ ;~ ~=~ ~== ~== ~~~ ~~~ ~~= :~ ~:~ ~~: ~:~ ~=~ ~~~ =~ ;~ ;~ :~ ;~ ~=~ ~~~ ;~ ~~~ ~=~ ;~ ~:: Ctenostreon cf. C. gikshanensis McLearn __________________________________ X _________________________________ ------------------------------------------ Ostrea (Liostrea) strigilecula White ____________________________________ X ________________________________________________ X X------------------------

Lopha sp _________________________________________________________________ X ________________________ ---------------------------------------------------

Gryphaea nebrascensis Meek and Hayden ______________ X X X X X ______ X ______ X X X ___ X X X X------------------------------------------ Trigonia americana Meek ___________________________________________________________________ : ____ X ___________________________ --- ___ ------------------

elegantissima Meek ____________________________________________________________________________________________________________ ------------ X------

Astarte Vau:~~~~:~~-a-~i-~~~~~~~~-~~:~_e_~~~================ meeki Stanton _________________________________________________ === === === === === === X === _______________________________________ === === === === === === === === === === === === === X === ____________ -~ === === ::: ------------------------ === === ·x ::: ::: ::: === Pronoella cinnabarensis (Stanton) _____________________________________ X ______________________________ ------------------------------------------------

Quenstedtia ~~~~a_h_e_n:_i~-~~~~================================= sublevis (Meek and Hayden) ____________________________________ === === === === === ·x === -x X === _____________________________________________ === === === === === === === === === === -x === === === ·x X=== ______ ===------------------ === === ·x ·x ~== ~ 

Pro?fcaitii;icf."P."schuchert{l\:fcieam_-~~=============== === === === === === ·x === ·x === === === === === =~= === === === === === === === === === _: === === === === === === === === 

f/e~ft:;g~ Gastropods r~~-v-~~-~t~-~~-r-~~~!:_~~-s~=================== undet ___________________________________________________________ === === === === === === === === X === ___ === ___ === ___ ___ === === ___ === ___ === ___ === ___ === ___ ___ ___ === === _: ___ === ___ === ___ === ___ === ___ === X === --- === X === --- --- --- === === === --- -x --- Cadoceras? sp ________________________________________________________________________________ X ______________________________ ------ ___ ---------------

A Callovian age for the Leeds Creek Member is 

suggested also by the presence of a fragment of the 

body whorl of a large globose ammonite (Mesozoic loc. 

28862) obtained about 70 feet above the base of the 

1nember on Cabin Creek, Teton County, Wyo. This 

ammonite has a fairly high, vertical umbilical wall that 

rounds abruptly into the flanks in a manner that is 

characteristic of the genus Oadoceras. Similar-appear~ 

ing fragments of ammonites belonging to Oadoceras 

are common in the Rierdon Formation-in Montana and 

in the "Lower Sundance" Formation in Wyoming 

(Imlay, 1953b, p. 23-25, pis. 7-14). 

The Leeds Creek member of the Twin Creek Limestone is correlated on the basis of lithologic continuity 

and the presence of Gryphaea nebrascensis with the 

Stockade Beaver Shale Member of the "Lower Sundance" Formation of \Vyoming and South Dakota and 

with the middle part of the Rierdon Formation of 

Montana (fig. 10). Southward in the Wasatch Range, 

Utah, the member becomes shalier and shows lithologic 

similarities to the upper shaly part of the Twelvemile 

Canyon Member of the Arapien Shale such as included 

by Hardy (1952, p. 15, 16) in his units A-D. Eastward 

in the Uinta Mountains, Utah, the Leeds Creek Member 

changes between the Duchesne River and the Whiterocks River (Imlay, 1953a, p. 58, 59) into red beds and 

gypsum typical of the upper part of the Carmel Forma-

-

221-691 0-66~-4 

tion. This facies change is much greater than that 

between the southernmost exposures of the Leeds Creek 

Member near Thistle and the northernmost exposures 

of the Ara pi en ~Shale as exposed east and northeast of 

Nephi. 

On the basis of the facies change in the Uinta Mountains, the Leeds Creek Member may be correlated roughly with the upper gypsiferous siHstone sequence of the 

Carmel Formation exposed on the west side of the San 

Rafael Swell. This correlation is substantiated by the 

presence in the lower fourth of the Carmel Formation 

of a sequence of units that are similar li!thologically, 

stratigraphically, and in part faunally to the Rich, 

Boundary Ridge, and Watton Canyon Members of the 

Twin Creek Limestone. 

ORIGIN 

The Leeds Creek Member was deposited in very shallow water along its eastern margin in Wyoming and its 

northern margin in Idaho. All the specimens of Ostrea 

and Gryphaea and most of the other benthonic mollusks 

that have been found in the member are from those marginal areas (compare table 9 and figs. 16, 17) where the 

member is thinnest, most oolitic, and most clayey or silty. 

In the ,Jackson Hole area !the member passes eastward 

and northward into a highly fossiliferous calcareous 

claystone facies of shallow water origin. West ward in 
50 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Wyoming and southward from the Snake River area of 

Idaho the member thickens, becomes less fossiliferous, 

less oolitic, and the fossils present are mostly pelagic. 

Presmnably these thicker sections were deposited in 

somewhat deeper waters. Nevertheless even the thicker 

sections contain a few beds of silty ripple-marked limestone that bear fragments of Oamptonectes, crinoids, and 

echinoids. Such beds could have been formed during 

storins, but their presence sugge·sts that the bottom of 

·the sea was not very deep. 

The shaly limestone that forms the bulk of :the Leeds 

Creek Member was probably deposited rapidly as soft 

calcareous 1nud in which most benthonic organisms 

could not find food or a suitable substratum. A chemical origin for most of the limestone is suggested by its 

fine texture, general lack of organic Inatter, and scarcity 

of fossils. Rapid subsidence is indicated by the considerable thickness of sediment that accumulated during 

only a part of early Callovian time (Imlay, 1950b, p. 83). 

The clastic material in the Leeds Creek Member appears to have been derived from several directions. To 

the north and northeast the member passes into a soft 

claystone-siltstone facies that is included in the Rierdon 

F9rmation in southern Montana and in the "Lower Sundance" Fornmtion in northern Wyoming. The clastic 

sediments in these formations apparently came from 

far to the east and southeast (Imlay, 1950b, p. 77, fig. 4; 

Peterson, 1954, p. 474). 

Toward the east in Lincoln County, Wyo., the Leeds 

Creek Member gradually becomes siltier and sandier. 

Toward the west in Idaho the 1nember exposed at Willow Creek contains beds of sandstone that were not 

noted in sections farther east in Idaho. A western 

source for this sandstone is suspected because the overlying Giraffe Creek Member is much sandier here than 

farther east. Toward the south in the Wasatch Range, 

Utah, the Leeds Creek Member becomes much more 

clayey, silty, and sandy and passes southward in central 

Utah into a similar facies in the upper part of the 

Twelvemile Canyon Member of the Arapien Shale. The 

source of this clastic material is not known. Presumably part of it could have been derived from the same 

eastern source as the clay and silt in the upper part of 

the Carmel Formation, but a western source cannot be 

excluded. 

GIRAFFE CREEK MEMBER 

DEFINITION 

The Giraffe Creek Member of the Twin Creek Limestone ranges in thickness from about 25 to 295 feet. It 

c?nsists mostly of yellowish- greenish- or pinkish-gray 

silty to finely sandy ripple-marked thin-bedded limestone that is interbedded with sandstone, but includes 

some shaly limestone and some medium-bedded limestone. The thicker beds are commonly oolitic and 

sandy. Sandy beds generally show crossbedding. Many 

beds are a coquina of crinoid and echinoid fragments 

whose upper surfaces are matted with Oamptonectes. 

The upper part of the member is generally harder and 

thicker bedded than the lower part and in many places 

forms low cliffs or ridges. The member grades upward 

into much softer red siltstone and sandstone at the base 

of the Preuss Sandstone. The type section is designated 

as the north side of Thomas Fork Canyon and is described below. The member 1s named after Giraffe 

Creek, which flows into Thomas Fork Canyon about 1 

mile east of the type section. 

Giraffe Creek Member (type section) on north side of Thomas 

Fork Canyon in NW1A,SW* sec. 20, T. 28 N., R.119 W., Lincoln 

County, Wyo. 

[Fig. l, loc. 21] 

4. Limestone, thin-bedded to shaly, silty to finely sandy, 

grayish-yellow; grades abruptly into soft red siltstone at base of Preuss Sandstone _______________ _ 

3. Sandstone, thin-bedded, yellowish- to dark-brownishFeet 

12 

gray, glauconitic________________________________ 50 

2. Sandstone, thick-bedded, medium-gr.ay, glauconitic; 

weathers brownish gray_________________________ 6 

1. Limestone, silty, yellowish-gray ; interbedded with 

yellowish-gray sandstone; grades downward into 

gray shaly limestone of the Leeds Creek Member___ 43 

Total thickness--------------------------------- 111 

DISTRIBUTION AND THICKNESS 

The Giraffe Creek Member is recognizable lithologically in Utah as far south as Monks Hollow near Thistle 

and as far east as the Duchesne River in the Uinta 

Mountains. Farther east at Lake Fork and the Whiterocks River the probable correlative beds consist of 

greenish-gray siltstone, claystone, and sandstone. In 

north western Wyoming the member is present in the 

western part of the Jackson Hole area but is absent at 

Lower Slide Lake. 

The Giraffe Creek Member thickens westward fairly 

regularly (table 2) from about 25 feet in the Jackson 

Hole area of northwestern Wyoming to 295 feet at 

Willow Creek near Idaho Falls (figs. 2, 3). Farther 

south in Wyoming and Idaho it thickens irregularly 

southward (fig. 6) but not westward (figs. 4, 5). In 

northern Utah it thickens irregularly westward from 

about 50 feet at Lake Fork to 200 feet near Burr Fork 

(fig. 8). Part of the irregularity in thickness may be 

depositional, part may be due to difficulties in selecting 

boundaries between gradational members, and part may 

be a result of minor faulting. 

LITHOLOGIC AND STRATIGRAPHIC FEATURES! 

The Giraffe Creek Member is one of the most distinctive members of the Twin Creek Limestone because 

of its lithologic characteristics and because it generally 
STRATIGRAPHY OF THE TWIN CREEK LIMESTONE 51 

forms cliffs or ridges between soft red siltstones at the 

~ase of the Preuss Sandstone and soft light-gray shaly 

limestone at the top of the Leeds Creek Member (figs. 

2-8). Lithologically it is characterized by thin-bedded 

limestone th~t is wavy bedded, ripple marked, silty to 

sandly and Is mostly yellowish, greenish, or pinkish 

gray. 

In the easternmost exposures of the member in 

Wyoming such limestone is interbedded with some thin 

units of soft red siltstone, thin- to medium-bedded sandsto~~' and medium-bedded limestone that is generally 

oohtic, sandy, and crossbedded. Westward in Wyoming and Idaho the membe.r becomes more sandy and 

glauconite becomes common. At Thomas Fork Canyon, Wyo., the member is more than half sandstone, and 

at Preuss Creek, Idaho, it is mostly sandstone. 

In northern Utah, similar westward changes occur in 

the Gir,affe Creek Member. Near the Duchesne River 

the member consists mostly of shaly limestone but contains interbeds of soft red siltstone thin-bedded sandy 

l . ' Imestone, and gray to yellow ripple-marked thin-bedded sandstone. Near Peoa the member consists of soft 

gray shaly limestone that contains many thin beds of 

sandy limestone. At Burr Fork the member consists 

of thin- to medium-bedded limestone that is ripple 

~arked,, silty to finely sandy, crossbedded in part, and 

hght ohve gray to yellowish gray, and that weathers 

orange to gray. South of Burr Fork at Monks Hollow 

and north of Burr Fork at Watton Canyon the member 

consists mostly of sandstone and siltstone. At Watton 

Canyon it includes pebbles of chert, limestone, sandstone, and quartzite as much as an inch across. 

The Giraffe Creek Member grades upward into the 

red Preuss Sandstone. In most sections the boundary 

can be selected within less than 10 feet and in some sections within a few inches. Difficulty in selecting the 

boundary arises in only a few sections near the eastern 

margin of outcrops of the Twin Creek Limestone in 

Wyoming where the highest limestone unit assigned to 

the member overlies soft red siltstone that resembles 

beds at the base of the Preuss Sandstone. Such examples occur near La Barge Creek (Imlay, 1950a, p. 42) 

and on Cabin Creek (Imlay, 1953a, p. 62). At South 

Piney Creek the member also contains two ridge-forming limestone units that are separated by a covered interval of 65 feet. This interval is marked by a ravine, is 

covered by red soil, and evidently is under lain by beds 

much softer than the outcropping limestone units. In 

these sections, the highest limestone units have arbitrarily been assigned to the Twin Creek Limestone inasmuch as a marine limestone is apt to be more persistent 

than a red-bed unit (Imlay, 1953a, p. 59) and is more 

apt to mark a time plane. 

It is possible, however, that the highest limestone 

units at Cabin Creek and South Piney Creek are eastward extensions of a marine member of the Preuss 

Sandstone such as occurs a little below the middle of the 

Preuss Sandstone in the Ammon and Cranes Flat quadrangles, Idaho (Imlay, 1952a, p. 1739-17 43). Furthermore, crinoid columnals and oysters (USGS Mesozoic 

loc. 28638) occur in pinkish-gray sandstone about a 

hundred feet above the base of the Preuss Sandstone on 

the north side of Crow Creek, SW%NE:% sec. 17, T. 

31 N., R. 119 W., Crow Creek quadrangle, Lincoln 

County, Wyo. Such occurrences show that the Preuss 

Sandstone passes westward into normally saline marine 

beds and suggest that the top of the Twin Creek Limestone may not represent a time plane throughout the 

entire area of its outcrop. 

M·any sections of the Giraffe Creek Member in Wyoming and Idaho have been described previously as member G (Imlay, 1950a, p. 42-45; 1953a, p. 60-62; 1952a, 

p. 17 40) . The following sections show features of special interest. The section at South Piney Creek contains two prominent limestone units and is unusually 

thick as compared to sections to the west and south. 

The upper unit is possibly a repetition of the lower 

unit owing to faulting, but it is lithologically somewhat 

different, and it may be younger. The section at the 

Duchesne River is near the eastern limit of the Giraffe 

Creek Member in Utah and contains interbeds of red 

siltstone suggestive of the Carmel Formation farther 

east. The section in W att.on Canyon contains more 

clastic material than any other section of the Giraffe 

Creek Member and is the only one that contains pebbles. 

Giraffe Oreek Membet· on north side of South Piney Oreek in sec. 

11, T. 29 N., R. 115 W., Sublette Oounty, Wyo. 

r[Fig. 1, loc. 19] 

9. Limestone, sandy, slightly oolitic, medium-brownishgray ; locally crossbedded; weathers light brownishgray; contains a few interbeds of red siltstone and 

pink sandstone; overlain gradationally within a foot 

Feet 

by red soft silrtstone at base of Preuss Sandstone__ 50 

R Covered with red soiL______________________________ 65 

7. Limestone, thin-bedded, oolitic, slightly silty, darkgray; weathers dark brownish gray---------------- 2 

6. Covered with red soiL______________________________ 5 

5. Limestone, thin-bedded, oolitic, silty, gray----------- 5 

4. Limestone, shaly to thin-bedded, medium-gray; 

weathers light gray------------------------------ 18 

3. Limestone, thin-bedded, oolitic, slightly silty, dark-gray, 

brittle; forms ridge; weathers brownish gray______ 43 

2. Limestone, shaly to thin-bedded, silty, light-yellowishgray to light-olive, ripple-marked, poorly exposed___ 32 

1. Limestone, medium- to thin-bedded, dark-gray; forms 

low cliff; weathers dark brownish gray; beds near 

top contain many fragments of crinoids and echinoids; grades downward within several feet into 

shaly limestone at top of Leeds Creek Member______ 12 

Total thickness--------------------------------- 232 
52 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Giraffe Greek jJJ ember on sotdh side of Duchesne River in SW* 

sec. 4, T. 1 S., R. 8 lV., Duohesne County, near Hanna, Utah 

[Fig. 1, loc. 38] 

4. Limestone, shaly, soft, gray, ripple-marked; alternates 

with red soft siltstone and some ~-in. layers of 

light-gray fine-grained sandstone; overlain by brownish-red sandstone and siltstone of the Preuss 

Feet 

Sandstone -------------------------------------- 45 

3. Sandstone, fine-grained, yellow---------------------- 4 

2. Siltstone, soft, red ; interbedded with gray shaly 

limestone and thin beds of sandy limestone________ 60 

1. Limestone, shaly, soft, light-gray ; contains some very 

thin beds ( %-1 in.) of light-gray fine-grained sandstone and yellowish-gray finely sandy limestone ; 

Gamptonectes shells present; overlies soft shaly 

limestone of Leeds Creek Member_________________ 56 

Total thickness-----------------------~--------- 165 

Giraffe Greek Member on 11orth 8idc of Watton Canyon abou-t 

2% miles above confluence of lVatton Canyon tcith BiJ·ch 

Greek in sec. 26, T. 9 N., R. 5 E., Rich County, Utah 

[Fig. 1, loc. 33] 

10. Siltstone, sandy, and silty sandstone. Unit is very 

thin bedded, very soft, medium- yellowish-brown 

and yellowish-gray; has minor crossbedding; conFeet 

tains green claystone in upper 6 in. that is overlain 

abruptly by red siltstone of the Preuss Sandstone__ 24 

9. Siltstone, sandy; interbedded with a few thin beds of 

brown to yellowish-gray fine-grained ripple-marked 

sandstone. At top is one 4-in. bed of dark-gray 

sandy limestone_________________________________ 13 

8. Covered ----------------------------------------- 16 

7. Sandstone, fine- to medium-grained, yellowish-gray; 

mostly thin bedded but in upper 10 ft occur beds 

6-12 in. thick; beds ripple marked; some cross 

bedding; unit gritty near top ; weathers yellowish 

gray ------------------------------------------- 85 

6. Sandstone, shaly to thin-bedded, yellowish- to pinkishgray, ripple-marked; some beds lenticular; unit 

weathers darker; contains furrowed trails________ 32 

3. Sandstone, massive, crossbedded; gritty to pebbly in 

lower 6 ft, becoming fine grained at top; yellowish 

gray at base, becoming pinkish gray upward ; 

weathers into large rounded pinkish-brown boulders; lower contact sharp_______________________ 12 

4. Sandstone, thin-bedded, fine-grained, medium-brownish-gray, ripple-marked; weathers to a lighter 

brown ----------------------------------------- 9 

3. Sandstone, oolitic, yellowish-gray, slightly pebbly ; 

beds 1-12 in. thick; interbedded with very thin 

beds of yellow sandstone________________________ 9 

2. Sandstone, thick-bedded, oolitic, yellowish-gray, pebbly ; forms cliff; weathers brownish gray. Pebbles 

abundant, some as large as 1 in. in diameter, 

consist mostly of yellow, brown, o.r black chert, but 

include some limestone, sandstone, and quartzite. 

Unit thickens and thins along outcrop from 6 to 

10 ft___________________________________________ 10 

1. Limestone, thin-bedded. sandy, yellowish- to pinkishgray, ripple-marked ; some crossbedding; contact 

with Leeds Creek Member covered_______________ 7 + 

Total thickness ________________________________ 221+ 

TABLE 10.-Geographic distribution of marine Jurassic megafossils 

in the Giraffe Creek Member of the Twin Creek Limestone 

[Gastropod identifications by N. F. Sohl. Two-digit numbers refer to localities in figs. 16, 17 ; higher numbers are USIQS Mesozoic 

locality numbers identifying individual collections] 

Idaho Wyoming Utah 

14 17 37 55 94 

9309 9315 1 12034 20346 17678 

---·---------1---------------

Crinoid columnals_ ------- X X X X 

Camptonectes sp __________ ------ ------ X ------------ Ostrea (Liostrea) strigicula 

White _________________ ------ X ------ ------ ------ Pronoella? sp ___________________ ------ X ______ ------

Neritina? SP------------- ------------------------ X 

1 Mesozoic loc. 9315 is possibly from the Leeds Creek Member. 

FOSSILS, AGE, AND CORRELATIONS 

Fossil fragments of crinoids, echinoids, and Oamptonectes are fairly common in the Giraffe Creek Member, but few specimens are worth collecting (table 10), 

and none is of n1uch age value. The age of the member 

is determined as Callovian because it lies gradationally 

above a sequence of early Callovian age and passes eastward in northwestern Wyoming into calcareous shale 

at the top of the "Lower Sundance" Formation (fig. 10). 

The Giraffe Creek Member is similar lithologically 

and occupies the same stratigraphic position as the 

Hulett Sandstone Member of the Sundance Formation 

in central and eastern Wyoming and western South 

Dakota (Imlay, 1947, p. 255-257; 1952b, p. 966; 1956b, 

p. 589-593, Tourtelot., 1953; Peterson, 1954, p. 476-479, 

487) and as the equivalent ridge-forming sandstone in 

southern Montana (Imlay, 1954, p. 58, 59; Richards, 

1955, p. 40, 41; Peterson, 1954, p. 479; 1957a, p. 431; 

1957b, p. 78, 79) . It differs mainly by being more 

glauconitic in its western sections. These sandstone 

members apparently were formed concurrently near the 

end of early Callovian time just prior to the deposition 

of red beds represented on the west by the Preuss Sandstone and on the east by the Lak Member of the Sundance Formation. They differ mainly in the source of 

the sand because the Giraffe Creek Member coarsens 

westward and the Hulett Sandstone Member coarsens 

southeastward. In central Montana the sandstone beds 

equivalent to the Hulett Sandstone Member pass northward into oolitic and chalky limestone that contains 

ammonites of early Callovian age (Imlay and others, 

1948; Imlay, 1953b, p. 7-10). 

In northwestern Wyoming the Giraffe Creek Member 

is exposed along the western and southern margins of 

the Jackson Hole area (fig. 1, locs. 6, 9-11, 14) but thins 

eastward, and is missing in part of western Wyoming. 

It is not recognizable at Lower Slide Lake (fig. 1, loc. 
PALEOGEOGRAPHY OF THE WESTERN INTERIOR 53 

7). Equivalent beds, if present, at that place probably 

occur in the highest part of the 160 feet of calcareous 

claystone and siltstone that contain On;phaea nebra8-

censis Meek and Hayden and are correlated with the 

Leeds Creek Member. Such a possibility is suggested 

by the fact that the Oryphaea-bearing beds (unit 22 of 

Imlay, 1956a, p. 70; unit 12 of Foster, 1947, p. 1565) 

are overlain conformably by 26 feet of greenish-gray to 

grayish-pink soft sandy siltstone that lithologically resembles the Preuss Sandstone or the Lak Member of 

the Sundance Formation, both of which are younger 

than the Giraffe Creek Member. (Observations by 

George Pipiringos and R. W. Imlay, July 1964.) 

Conformably above this siltstone is 15 feet of soft to 

ledgy very fine grained greenish-gray nonglauconitic 

silty sandstone, 1 foot of yellowish-white bentonite, and 

then 7 feet of glauconitic gray siltstone that contains 

siliceous nodules and ~nany belemnites. These units are 

overlain conformably by high cliffs consisting of glauconitic sandstone typical of the "Upper Sundance" 

Formation, or the Stump Sandstone as used by Foster 

(1947, p. 1566, 1567). None of the units in the entire 

section shows any resemblance to the Giraffe Creek 

Member. Furthermore, if the Preuss Sandstone has 

been correctly identified, the Giraffe Creek Member 

must have changed easbYard into soft clay to silt facies. 

In the Uinta Mountains, Utah, the Giraffe Creek 

Member changes eastward between the Duchesne River 

and Lake Fork into poorly exposed beds consisting 

mostly of soft greenish-gray siltstone and sandstone. 

Neither the Carmel Formation of central and southern 

Utah nor the Arapien Shale of central Utah contains 

any unit comparable lithologically or stratigraphically 

with the Giraffe Creek Member. 

ORIGIN 

The Giraffe Creek Member of the Twin Creek Limestone is so similar lithologically and stratigraphically 

to the Hulett Sandstone Member of the Sundance Formation that both must have had a similar origin (Imlay, 1953b, p. 9, 10 ; Peterson, 1957a, p. 431). Both 

were formed in littoral to shallow marine waters during 

a retreat of the sea near the end of early Callovian time. 

A shallow-water origin for ·the Giraffe Creek Member is 

indicated by such features as an abundance of oscilla1tion 

ripple marks, crossbedding, oolites, marked thinning 

and t hickening of -individual 'beds, and t he presence of 

some oysters. Bottom conditions in most places were 

unfavorable for benthonic organisms as shown by the 

fact that most fossils consist. of free-swimming or freefloating forms. The £ragmen tary condition of most of 

the fossils suggests deposition in much-agitated water. 

A western source for the clastic material is shown by a 

marked increase westward in the amount of sandy materia.! ·and by the presence of pebbles in one of the westernmost sections. The easternmost section of the 

Giraffe Creek Member resembles t he westernmost sections of the Hulett Sandstone Member in the Wind 

River and Bighorn Basins. The westemmost sections 

of the Giraffe Creek Member resemble the Hulett Sandstone Member in the Black Hills except for being much 

more glauconitic and a little more fossiliferous. The 

presence of more glauconite in the western a.rea is possibly related to the presence of metamorphic and igneous 

rocks to the west (Imlay, 1950b, p. 91) or to a greater 

amount of organic material. 

PALEOGEOGRAPHY OF THE WESTERN INTERIOR REGION DURING DEPOSITION OF THE TWIN CREEK 

LIMESTONE 

Marine Jurassic rocks of middle Bajocian to Callovi:an ages ·are widespread in the western interior of the 

United States and Canada (figs. 13-15). These were deposited in an embayment that spread eastward from the 

Pacific coast region across the northemmost part of the 

United States and adjoining parts of Cana,da. At one 

time it was thought that the embayment in the westem 

interior of Ca,nada was partly sepamted from the Pacific Ocean by a landmass (Schuchert, 1923, p. 226, fig. 

14; Crickmay, 1931, p. 89, map 10) whose position coincided approxima;tely with 't11e present Rocky Mountain 

trench and that a marine connection with the Arctic 

Ocean existed through the Yukon territory (Schuchert, 

1923, p. 226, fig. 14). Frebold (1954, p. 1, 2; 1957, p. 

37-41; 1958, p. 29) has presented evidence however, that 

both of these ideas are without foundation as far as 

Canada is concerned. In the United States by contrast, 

at least part of the embayment in the western interior 

was bounded on the west by low land masses or islands, 

or both, that furnished some sand and pebbles from late 

Bajocian until Oxfordian time (Imlay, 1950a, p. 37-42; 

1953a, p. 54-59). 

Major marine transgressions in the western interior 

region occurred during the middle to late Bajocian and 

early Callovian. Regressions occurred during the Bathonian and middle Callovian. Sedimentation was interrupted marginally during the Bathonian (Imlay, 

1956b, p. 564, 579-580; Mapel and Bergendahl, 1956, p. 

88, 89) and regionally during the late Callovian (Imlay, 

1950b, p. 76; 1952a, p. 1747-52; 1956b, p. 591). In 

Canada there may have been a hiatus, also, during the 

late Bajocian (Frebold, 1957, p. 13, 17). 

The first Middle Jurassic marine transgression in the 

western interior region began during the middle Bajocian and reached its climax during the late Bajocian. 

During the middle Bajocian, normal fossiliferous marine siltstone and sandstone were deposited as far east 
54 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

1000 MILES 

100~ 

FIGURE 13.-Regional distribution of Bajocian marine rocks in North America. Major occurrences based mainly on ammonites 

and Inoceramt~s a re indic<ated by X. Distribution of the Twin Creek Limestone is indicated by the rectangle ; land areas 

stippled. 
PALEOGEOGRAPHY OF THE WESTERN INTERIOR 55 

160 ' 

120' 100 " 

FIGURE 14.-Regional distribution of Bathonian and Bathonian ( ?) marine rocks in North America. Major occurrences based 

on ammonites are indicated by X. Distribution of the Twin Creek Limestone is indicated by the rectangle; land areas 

stippled. 
56 

150' 

TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

0 

0 

~ 

FIGURE 15.-Regional distribution of Callovian marine rocks in No·rth America. Major occurrences based on ammonites and 

Gryphaea are indicated by X. Distribution of Twin Creek Limestone is indicated by the rectangle; land areas stippled. 
PALEONTOLOGY, TWIN CREEK LIMESTONE AND CONTEMPORARY FORMATIONS 57 

as western Alberta and western Montana. Apparently, 

at the same time, large areas to the east and south were 

covered with highly saline waters in which were formed 

soft red beds and gypsum and minor amounts of limestone. Conditions were unfavorable for the growth 

and preservation of organisms. During the late Bajocian, the sea deepened considerably as shown by the 

deposition of normal marine highly fossiliferous limestone above red beds and gypsum as far east as the Dakotas and as far south as southwestern Utah. Within 

the Twin Creek trough, limestone deposition began a 

little earlier than in areas to the east. The basal limestone beds in the Twin Creek trough become sandier 

westward, a feature that indicates the presence of a 

landmass in that direction. 

During the Bathonian the sea became much shallower 

than it was during the late Bajocian. Erosion occurred 

along the eastern margin, and red beds and gypsum 

were deposited in much of its eastern part. An eolian 

sandstone (Thousand Pockets Tongue of the Navajo 

Sandstone) encroached on the southern margin. Some 

sand, derived apparently from the northeast, was deposited along its northern margin in Montana and Alberta (upper mmnber of Sawtooth Formation). Deposition of limestone occurred 1nostly in its western part, 

and conditions favoring the growth of mollusks existed 

only in an area that now includes western Montana and 

western Alberta. 

During the early Callovian marine transgression, the 

western interior sea spread farther south and southeastward than during the late Bajocian. Deposition of 

normally saline fossiliferous marine limestone, siltstone, 

claystone, and sandstone was widespread except in the 

eastern part of the Colorado Plateau. The 1nain source 

of sand was to the east and southeast. The Twin Creek 

trough subsided rapidly and accumulated a much 

greater thickness of limestone and claystone than areas 

to the east. The northern part of the trough near the 

end of early Callovian time received some sand derived 

from the west. 

During the middle Callovian regression the western 

interior sea was partly cut off by a rising landmass in 

central Montana and northern Wyoming (Imlay, 1952:a, 

p. 1751-1753). Deposition continued south and southeast of the landmass for at least part of late Callovian 

tilne and may have conti1iued into Oxfordian ti1ne. 

Apparently 1narine connections persisted along the 

west side of the landmass somewhere through western 

Montana or northern Idaho during deposition of the 

Preuss Sandstone of 1niddle to late Callovian age. ·Such 

is implied for that formation by the presence of bedded salt, some marine fossils, and a westward increase 

in the amount of fossiliferous marine beds. 

PALEONTOLOGY OF THE TWIN CREEK LIMESTONE AND 

SOME CONTEMPORARY FORMATIONS 

BIOLOGIC ANALYSIS 

The marine Jurassic pelecypods described herein 

(fig. 9) were obtained from the upper six members of 

the Twin Creek Limestone. They are of Bajocian and 

Callovian ages, number about 1,580 specimens and include 43 genera and subgenera and 50 species and subspecies. These pelecypods are all that have been collected by many geologists during about 60 years. Their 

distribution by genera, subgenera, and families is shown 

in table 11. The genus Gryphaea is by far the most 

common, but the genera Oamptonectes, Ostrea (Liostrea), Pronoella, and Pleurmnya are represented by 

100 specimens, or more, at many localities. ~The genus 

Oorbu1a is also represented by about 100 specimens, but 

all were obtained from a few localities in one small 

area in western Wyoming. The other genera are much 

less com1non. Genera or subgenera numbering from 

21 to 41 specimens in decreasing order include Astarte, 

Trigonia-, Vaugonia, Grammatodon, Pinna., Myophorella (Promyoplwrella), andAfodiolus. Genera or subgenera numbering from 10 to 19 specimens in decreasing order include Plagiostorna, Plicatula, Quenstedtia, 

0 oelastarte, Prorokia, M actromya, and Otenostreon. 

The other genera are represented by nine or fewer specimens and, therefore, may be considered rare. 

Newly described species and subspecies of pelecypods 

include Gryphaea planoconvewa fraterna Imlay, Pro-

'l'okia fontenellensi-<s Imlay, and Platymya rockymontana Imlay. The 50 species described do not include 

Afodiolus isone1nus (Meek) (1877, p. 132, 133, pl. 12, 

figs. 4, 4a) or },fyophoria lineata (Munster) of Meek 

(1877, p. 133, 134, pl. 12, figs. 3, 3a) which probably 

belongs to Opis ( Trigonopis). Both of these were obtained in Weber Canyon, Utah, and presumably from 

the Twin Creek Limestone near Devils Slide. Neither 

have been found sttbsequently in the 'J'win Creek Limestone, but Modiolus isonmnus (Meek) has been found in 

equivalent beds in the Arapien Shale (Imlay, 1964a, 

p. C24, pl. 1, fig. 22). 

'The marine Jurassic cephalopods described herein 

(fig. 11) were obt~ained fron1 the Sliderock and Rich 

Members of the Twin Creek Limestone and from approximately equivalent beds of Bajocian age elsewh~re 

in the western interior region. These cephalopods Include 142 specimens frmn the Twin Creek Limestone 

and 140 specimens fron1 other formations (table 12). 

This number is all the generically identifiable Bajocian 

cephalopods that have been collected by geologists of 

the U.S. Geological Survey in the western interior 

region since 1892. 
58 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

TABLE H.-Biological relationships and relative abundance of 

Bajocian and Callovian pelecypods in the Twin Creek Limestone 

Family Genus or subgenus Specimens 

N uculidae ________________ _ JVucula_______________ 1 

Parallelodontidae _________ _ Grammatodon_ _ ___ _ _ _ _ _ 33 

Arcidae __________________ _ Barbatia?_____________ 2 

Cucullaeidae _____________ _ I donearca_ _ _ _ _ _ _ _ _ _ _ _ _ 6 

Mytilidae ________________ _ Modiolus_____________ 21 

Musculus_____________ 1 

Isognornonidae ___________ _ M ytilus _ _ _ _ _ _ _ _ _ _ _ _ _ _ 3 

Gervillia? _ _ _ _ _ _ _ _ _ _ _ _ _ 3 

Isognomon_ _ _ ___ ____ __ 5 

Pinnidae _________________ _ Pinna________________ 28 

Pectinidae _______________ _ Camptonectes_ ___ _ _ _ _ _ _ 160 

Chlamys? _ __ __ _ __ _ _ __ _ 1 

Velata? _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 2 

Plicatulidae ______________ _ Plicatula_ _ _ _ _ _ _ _ _ _ _ _ _ 17 

Limidae _________________ _ Plagiostoma_ _ _ _ _ _ _ _ _ _ _ 19 

Ctenostreon_ _ _ _ _ _ _ _ _ _ _ 10 

Ostreidae ________________ _ Liostrea_ _ _ _ _ _ _ _ _ _ _ _ _ _ 100 

Trigoniidae ______________ _ 

Lopha________________ 3 

Gr~pha~a_____________ 576 Tr~gon~a______________ 40 

Vaugonia___ _ __ _ _ _ _ _ _ _ 34 

Prom yophorella_ _ _ _ _ _ _ _ 21 

Haidaia_ _ _ _ _ _ _ _ _ _ _ _ _ _ 3 

Astartidae _______________ _ Astarte_______________ 41 

Coelastarte_ _ _ _ _ _ _ _ _ _ _ _ 15 

Trigonopis__ _ _ _ _ _ _ _ _ _ _ 1 

Prorokia_ _ _ _ _ ________ 12 

Cyprinidae _______________ _ Pronoella_ _ _ _ _ _ _ _ _ _ _ _ _ 105 

Lucinidae ________________ _ Lucina?______________ 2 

Mactrornyidae ____________ _ Tancrediidae _____________ _ Mactromya? _ _ _ _ _ _____ 12 

Tancredia? _ _ _ _ _ _ _ _ _ _ _ 7 

Corbicella? _ _ _ _ _ _ _ _ _ _ _ _ 2 

Quenstedtiidae ___________ _ Quenstedtia_ _ _ _ _ _ _ _ _ _ _ 17 

Cardiidae ________________ _ Protocardia_ _ _ _ _ _ _ _ _ _ _ 6 

Pleurornyacidae __________ _ Pleuromya_ _ _ _ _ _ _ _ _ _ _ _ 135 

Corbulidae _______________ _ Corbula_ _ _ _ _ _ _ _ _ _ _ _ _ _ 100 

Pholadomyidae ___________ _ 

Pholadidae _______________ _ Laternulidae _____________ _ 

Pholadomya___ _ _ _ _ _ _ _ _ 9 

Homomya_ _ _ _ _ _ _ _ _ _ _ _ 1 

Goniomya____ _ _ _ _ _ _ _ _ _ 7 

Myopholas____________ 1 

Cercomya_____________ 4 

Platymya_ _ _ _ _ _ _ _ _ _ _ _ _ 1 

Thraciidae _______________ _ Thracia_ _ _ _ _ _ _ _ _ _ _ _ _ _ 14 

The distribution of the Bajocian cephalopods by 

genera and fa.milies is shown in table 12. The Stephanoceratidae is the dominant ·:family and is represented 

by the genera Stepha.nooeras, Ste'fl1!inra.tooeras, Sohlites, 

n. gen., and questionaJble N ormannites. The 1Sphaeroceratidtae is less than half ·as common and is represented 

by the genera Ohondrooeras, Paraohondrooeras, n. gen., 

and M egasphaerooeras. The Cardioceratidae is even 

less common and is represented by Eooephalites, n. gen. 

The Spiroceratidae is represented hy two fragments of 

Spirooeras. The most common and characteristic genera in the· Sliderock Member are Stemmatooeras, Eooephalites, n. gen., and M ega.rsphaerooeras. The ·only 

ammonites found in the Rich Member consist of one 

specimen of Sohlites, n. gen., three of Paraohondrooeras 

n. gen., and one question~bly assigned to Stephanooeras. 

Both Sohlites and Paraohondrooeras are much more 

common in equivalen't beds in Montana. These new 

genera have not been recorded outside the western interior of the United States. The genus Megasphaerooeras has been found elsewhere only in the Cook Inlet 

region, Alaska (Imlay, 1962b, p. A2, AlO, ~11) .. The 

genus Spirooeras has not been rooorded preVlously In t~e 

western interior region, although i~t is :fairly oommon m 

beds of late Bajocian to late Bathonian :age in many 

parts of the world. The genera St~phano~era:, Sterr:-

matooeras, and Ohondrooeras are widely distributed In 

beds of Bajocian age throughout the world. 

Among the amonite genera described as new, Sohlites shows resemblances to Reineokeia, Zemistephanus, 

and Ermooeras. The characteristics of its suture line 

suggest that it is probably most closely related to Ermooeras. Paraohondrooeras shows resemblances to iboth 

the Bajooian family Otditid:ae and the Bathonran to 

Callovian family TuHtidae. It differs from the described genera of those families hy having a narrowly 

rounded venter. Eooephalites shows considerruble resemblance to Paraoephalites and Oranooephalite$ and is 

possibly ancestral to both. 

Newly described species of ·anun:onites include Stemmatooeras aroioostum Imlay, Sohlites spinosus Imlay, 

Pa.raohondrooeras andrewsi Imlay, P. filioostatum Imlay, and Eooephalites primJUS Imlay. 

TABLE 12.-Biological relationships and. rela:tive a?undance of 

Bajocian cephalopods in the western wtenor reg~on 

Specimens 

Family Genus Twin Else· 

Creek where 

Lime- in stone western 

interior 

---

Nautilidae ______ -------- Eutrephoceras _ - - - -- - 1 ------ Spiroceratidae ____ ------- Spiroceras __________ 2 ------ Stephanoceratidae_------ Stephanoceras- ______ 2 ------ Stephanoceras?- - - - - - 1 ------ Stemmatoceras ___ - -- - 44 15 

Sohlites __ _______ - _-- 1 85 

Sohlites? ___ _____ ---- ------ 2 

JVormannites? __ ----- 4 1 

Sphaeroceratidae ________ Chondroceras __ -- - --- ------ 1 

Parachondroceras ___ __ 3 36 

M egasphaeroceras __ - _ 29 ------ Cardioceratidae ___ - _- _- _ Eocephalites ___ ____ -- 43 ------ Eocephalites? ____ - _-- 12 ------

Pelecypods are much more common than other megafossils in the Twin Creek Limestone; they outnumber 

cephalopods and gastropods about 13 to 1. Pelecypods 

attain their greatest variety in the Rich Member, 1have 

considerruble variety in the Sliderock 'Member, and are 

more common than other mega;fossils in all the upper 

six melllbers. Cephalopods are moderately common 

only in the Sliderock Member, are rare in the Rich Member and ·are represented by one fragment in the W•atton ' 0 Canyon Member. Gastropods are most a:bundant 1n the 

Sliderock Mem·ber but have been found in all the upper 
SOME FAUNAL ZONES IN THE WESTERN INTERIOR REGION 59 

six members. Crinoid ·and echinoid fragments occur 

throughout the Twin Creek Limestone but generally 

have not been collected. Worm tubes have been noted 

in the Sliderock, Rich, and Leeds Creek ·Members. One 

coloni:al coral, probably belonging to Actinastrea, was 

obtained from an ool'i'tic limestone in the lower part of 

the 'formation, presumably from the Sliderock ·Member. 

SOME FAUNAL ZONES IN THE WESTERN INTERIOR 

REGION 

CHONDROCERAS·STEMMATOCERAS ZONE 

The lower part of the middle shale 1nemher of the 

Sawtooth Formation in northern Montana has furnished one specimen of Ohondroceras (Iml1ay, 1948, p. 

19, pl. 5, figs. 1-5) and three fragments of Stemmatoceras resembling S. arcicostum Imlay, n. sp., described 

herein. The genus Ohondroceras has been found in 

many parts of world in beds of middle Bajocian age 

(Arkell and dthers, 1957, p. L292), is particularly common in lthe zone of Stephanoceras hu1nphriesianwm zone, 

and is not known aJbove that zone. Ste7l1ff'n.,atoceras is 

also characteristic of the middle Bajocian but is transitional 'between Stephanoceras and Teloceras which 

range upward into the upper Baj'Ocian. 

Ohondroceras has not been round elsewhere in the 

western intel'lior region of the United States, ·although 

it is fairly common in Oanada from the foothills of the 

Rocky Mountains westward. The time during which 

Ohondroceras lived could be represented in the Twin 

Creek trough by the lower third of the Sliderock Member <)r by the Gypsum Spring Member of the Twin 

Creek Limestone. This possibility is sugges'ted hy the 

presence of ammonites of early late Bajocian ·age in the 

middle and upper parts of the Sliderock Member. 

MEGASPHAEROCERAS ZONE 

The middle and upper parts of the Sliderock Member 

of the Twin Creek Limestone are characterized by a rotund species of M egasphaeroceras that is possibly identical with lf/ egasphaeroceras rotundum Imlay (1962b, 

p. A9-11) from beds of late Bajocian age in the Cook 

Inlet region, Alaska (fig. 12). Other characteristic 

fossils include the ammonites (fig. 11) Stephanoceras, 

Stemmatoceras, Spiroceras, Eocephalites, n. gen., and 

the pelecypod Gryphaea planoconvexa fraterna (fig. 9). 

Of these, Stephanoceras is probably represented also in 

the overlying Rich Member. Associated fossils that 

range above the Sliderock Member include species of 

pelecypods and some species of gastropods. The Megasphaeroceras zone has not been identified faunally outside the area of the Twin Creek Limestone. Its age on 

the basis of the ammonite genera is early late Bajocian. 

PARACHONDROCERAS ANDREWSIZONE 

The Rich Member of the Twin Creek Limestone is 

faunally the most distinctive member in the formation. 

It is characterized by the ammonite genera Parachondroceras and Sohlites (fig. 11) and by the pelecypod 

species (tig. 9) Gervillia? montanaensis Meek, Myophorella (Haidaia) yellowstonensis Imlay, and Gryphaea planoconvexa Whitfield. It contains the highest 

recorded occurrences of the pelecypods Gryphaea planoconvexa fraterna Imlay, n. subsp., Prorokia fontenellensis Imlay n. sp., Goniomya montanaensis Meek, and ' . Thracia weedi Stanton. The Rich Member contains 

the lowest recorded occurrences of the pelecypods lsognomon cf. /. perplana Whitfield, Trigonia elegantissima 

Meek, V augonia conradi (Meek and Hayden), M yophorella montanaens·is (Meek), Quenstedtia sub levis Meek 

and Hayden, and Oercomya punctata (Stanton). 

This distinctive association of molluscan gener~ and 

species occurring in the Rich Member is widespread in 

the western interior region. It has been found in the 

lowest limestone sequence of the Carmel Formation 

along the west side of the San Rafael Swell in central 

Utah, in the middle and upper parts of the lowest limestone sequence in southwest Utah, in the middle and 

upper parts of the limestone member of the Gypsum 

Spring Formation in north-central Wyoming, and 

throughout the middle limestone member of the Piper 

Formation in Montana. Irts age is probably late Bajocian, as indicated by its stratigraphic position above beds 

of early late Bajocian age and the greater resemblances 

of the ammonites to genera of Bajocian than of 

Bathonian age. 

PARACEPHALITES S·A WTOOTHENSIS ZONE 

The Paracephalites sawtoothensis zone (fig. 12) has 

been identified in the United States only in the upper 

siltstone member of the Sawtooth Formartion exposed 

in the front ranges of the Rocky Mountains in westcentral and northwestern Montana. In that area it is 

associated with the ammonite Oobbanites and the pelecypod Gryphaea impressimarginata McLearn (Imlay, 

1962a, p. 021). The zone is probably equivalent to the 

Paracephalites glabrescens zone in the Rocky Mountains 

of Alberta, Canada (Frebold, 1963, p. 28, 29). In Alberta the ammonite Paracephalites was obtained from 

35 feet of silty claystone and sandstone underlying a 

3:1f2-foot bed of coquina containing Gryphaea impressimarginata McLearn. The generic name Paracephalites 

Buckman ( 1929) is now used by Frebold ( 1963, p. 8-13, 

28, 29) for certain ammonites from Alberta and Montana rthat were previously referred to A rctocephalites 

(Imlay, 1948, p. 14, 19), Arctocephalites ( Oranocephalites) (Imlay, 1962a, p. 021, C23-C25), and Oadoceras 

(Frebold, 1957, p. 60, 62; 1963, p. 8-12, 28). 
60 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

WARRENOCERAS COUYENSE ZONE 

. The generic name W.arrenoceras ( Frebold, 1963, p. 13) 

IS now used for certain ammonites previously referred 

to Arct-icoceras (Imlay, 1948, p. 20, 21; 1953b, p. 5, 19-

22) and Lilloettia (Frebold, 1957, p. 56) from the western interior of the United States and Canada. 

A. zon~ characte~ized by W arrenoceras codyense (fig. 

12) IS widespread In northern Wyoming, western South 

Dakota, and Montana (Imlay, 1953b, p. 5) . Associated 

ammonites include Oadoceras and Oobbanites. In western Montana, species of W arrenoceras occur in the upper 

part of the range of Gryphaea hnpressimarqinata 

~c~arn. ' 

The W arrenoceras codyense zone in the United States 

~nay not be equivalent ~to the W arrenoceras henryi zone 

m Alberta, Canada, according to Frebold ( 1963, p. 27-

20). Th~ evidence presented ( Frebold, 1963, p. 24, pl. 

9, fig. 2) Is meager, but is supported by the fact that in 

the United States W. henryi (Meek and Hayden) has 

been found only above beds containing W. codyense 

(Imlay). 

The W arrenoceras codyense zone has not been identified by ammonites in the Twin Creek Limestone proper. 

The zone probably occurs, however, on the north side 

of Lower Slide Lake in northwestern Wyoming, in a 

sequence that by definition is included in the "Lower 

Sundance" Formation although the lower six members 

of the Twi~ Cree~ Limestone are recognizable lithologically, stratigraphiCally, and in part faunally. In this 

~equence, both lV arrenoceras ( = Arcticoceras n. sp. 

Inde!'· of !Inlay, 1953b, p. 20) and Oadoceras sp. were 

obtained (USGS) l\iesozoic loc. 2097 4) from unit 19 

(Imlay, 1956a, p. 70) in association with several immature striate speci~ens of Gryphaea that probably belong 

to G. nebr•ascenszs.Meek and Hayden. This unit is part 

of a member (units 16-21) consisting of oolitic limestone, granular limestone, and claystone that is correlated stratigraphically with the 'iV atton Canyon Member of the Twin Creek Limestone. 

GOW;E&ICERAS AND KEPPLERITES ZONES 

No new information concerning the GowericeraB and 

Jfepple?'ites z?nes (Imlay, 1953b, p. 7, 8) in the United 

~tates Is available. The zones with Kepplerite.rs have 

now been recognized in Canada, and a still younger 

zone has been added (Frebold 1963 I) 30 31) E · 1 · ' ' · , . quiVa- ents In the Twin Creek Limestone apparently include 

the Leeds Creek and Giraffe Creek Members. This correlation is indicated for the Leeds Creek Member by 

the presence throughout of Gryphaea nebra.scensis Meek 

and Hayden, which is the most common fossil in those 

zones and does not occur lower. It is indicated also by 

the presence of a large globose ammonite, probably belonging to 0 adoceras, obtained near the base of the 

Leeds Creek Member on Cabin Creek, Wyo. The Giraffe 

Creek Member probably corresponds to the K epplerites 

mclearni zone which has been identified at the top of 

the Rierdon Formation in central Montana. This correlation is based on the fact that the Giraffe Creek 

Member occupies the same stratigraphic position as the 

Hulett Sandstone Member of the Sundance Formation 

and that the Hulett Sandstone Member passes northward in central Montana into oolitic limestone at the 

top of the Rierdon Formation. 

COMPARISONS WITH OTHE.R FAUNAS 

CENTRAL AND SOUTHERN UTAH 

The 1nolluscan fauna of the lower limy part of the 

Oarmel Formation of central and southern Utah (Imlay, 1964a, p. C4) includes many of the same species 

that occur in the Twin Creek Limestone and some species 

that are characteristic of the Rich Member. The most 

noticeable difference is the absence in the Carmel Formation of the pelecypods Gryphaea, Opis, Prorokia, Protocardia, Oorbula, and Platy1nya, a near absence of ammonites, and a much greater abundance of Ostrea. These 

differences suggest that the Jurassic sea in central and 

southern lJtah was shallower and less favorable for the 

growth of a variety of mollusks than the sea farther 

north in which the Twin Creek Limestone was deposited. 

Shallowing of the Jurassic sea southward is shown by a 

marked southward increase in the relative abundance 

of 0 strea within the Twin Creek Limestone, by the 

complete absence of Gryphaea south of the Thistle ar~a, 

Utah, and by an absence of ammonites south of Thistle 

and the northwest end of the San Rafael Swell. Apparently the southern part of the seaway w1

as to shallow, 

or too agitated, or too saline for the survival of Gryphaea, of ammonites, and of.some other organisms. 

Most of the fossils found in the Twelvemile Canyon 

Member of the Arapien Shale in central UtJah (Imlay, 

1964a, p. C6) are likewise identical specifically with 

fossils that range through most of the Twin Creek Limestone. Astarte ( Ooelastarte) livingstonen.<sis Imlay, 

however, from the lower part of the Twelvemile Canyon 

Member has been found in the Twin Creek Limestone 

only in the Sliderock, Rich, and Boundary Ridge Memhers. M yopholas hardyi Imlay from the upper 600 feet 

of the Twelvemile Canyon Member also occurs in the 

Watton Canyon Member of the Twin Creek Limestone. 

MONTANA, WYOMING, AND SOUTH DAKOTA 

The characteristic Gryphaea and ammonites present 

in the Sliderock Member have not been found elsewhere 

in the western interior region. In contrast the species 

characteristic of the Rich Member occur in the lower 
COMPARISONS WITH OTHER FAUNAS 61 

part of the Carmel Formation in Utah, in limestone 

beds in the middle of the Gypsum Spring Formation in 

Wyoming, in the middle limestone member of the Piper 

Formation in Montana, in the lower and middle parts 

of the Sawtooth Formation in southwestern Montana, 

and in the limestone member of the Sawtooth (Piper) 

Formation exposed in the Bearpaw Mountains and the 

Sweetgrass Hills in north-central Montana. This 

faunal distribution is so much greater than that of the 

older fauna of the Sliderock Member as to suggest an 

abrupt deepening and spreading of the sea at the beginning of deposition of the Rich Member. 

The Rierdon Formation of Montana and the "Lower 

Sundance" Formation in Wyoming and South Dakota 

contain certain species in common with the Watton 

Canyon Member or the Leeds Creek Member of the 

Twin Creek Limestone. These species include M yopholas hardyi Imlay, Platymya rocky1nontana Imlay, 

n. sp., and Gryphaea nebrascensis Meek and Hayden. 

The lowest appearance of small striate Gryphaeas near 

the top of the Watton Canyon Member appears to coincide in time with the lowest apearance of Gryphaea 

nebrascensis at the top of the Canyon Springs Sandstone Member of the Sundance Formation. In addition 

the Leeds Creek Member near its base has furnished one 

globose ammonite that probably belongs to Oadoceras, 

a genus that is common in the Rierdon Formation and 

has not been found lower. 

WESTERN INTERIOR OF CANADA 

The described megafossils of Middle Jurassic age 

from the Fernie Formation in the western interior of 

Canada have very little in common with the megafossils from the Twin Creek Limestone (fig. 12). The 

pelecypods (Warren, 1932) and ammonites (McLearn, 

1928, 1932b; Warren, 1947; Frebold, 1957) of Bajocian 

age belong to different species than occur in the Sliderock and Rich Mmnbers of the Twin Creek Limestone. 

The Canadian Bajocian fossils, however, are dated as 

middle Bajocian (Frebold, 1957, p. 13; 1964, p. 5), 

whereas the Sliderock Member of the Twin Creek 

Limestone contains the ammonites M egasphaeroceras 

and Spiroceras of late Bajocian age. So the faunal 

differences may reflect differences in age. Inasmuch as 

the late Bajocian faunas in the western interior of the 

United States must have entered from the west or from 

the north, the absence of any fossil evidence for the late 

Bajocian in Canada suggests that the faunas entered 

from the west via the northern part of the United 

States. This suggestion agrees very well with Frebold's 

(1957, p. 3, 39-41) argun1ents against a landmass extending n~rthward from Nevada into British Columbia during Jurassic time, as had been suggested by 

Schuchert ( 1923, p. 226) and Crickmay ( 1931, p. 36, 

84-89). 

The Grey Beds in the upper part of the Fernie Formation (Frebold, 1957, p. 20; 1963, p. 1, 27) have 

furnished many pelecypods (McLearn, 1924) and ammonites (McLearn, 1928, 1929b; Frebold, 1957, 1963) 

that are identical with species in the upper part of the 

Sawtooth Formation and in the Rierdon Formation in 

Montana. Some of the pelecypod species are also identical with long-ranging species in the Twin Creek Limestone. These include in particular I donearca haguei 

(Stanton) and 0 ercomya punctata (Stanton) . 

Most of the described pelecypods were obtained from 

the lower 103 feet of the Grey Beds exposed on Grassy 

Mountain near Blairmore, Alberta. At this place the 

basal99 feet, called the Oorbula munda beds (Freehold, 

1957, p. 76; 1963, p. 4, 5), are lithologically similar to 

the upper siltstone member of the Sawtooth in nort~­

western Montana and in their upper 35 feet contain 

species of Paracephalites (Buckman, 1929, p. 10, 11; 

Frebold, 1963, p. 8-13) that are in part identical with 

species in the upper siltstone member of the Sawtooth 

Formation (Imlay, 1962a, p. C14, C15, C24, C25). If 

this correlation is valid, the 0 orbula 1nunda beds on 

Grassy Mountain are equivalent approximately to the 

Boundary Ridge Member of the Twin Creek Limestone 

on the basis of regional stratigraphic correlations. The 

Boundary Ridge Member has furnished so few fossils~ 

however that a faunal correlation is not possible. ' . The 0 orbula 1nunda beds on Grassy Mountain are 

overlain by 2 feet of sandstone and then by 1% feet of 

a sandy coquina that together are called the Gryphaea 

bed (Frebold, 1957, p. 20, 21, 76; 1963, p. 5) because 

of the abundance of Gryphaea impressimarginata 

McLearn. This bed contains the same species of pelecypods as the underlying 0 orbula munda beds but .ineludes such ammonites as W arrenoceras ( = Arctwoceras of Imlay, 1953b. p. 19-22) and Kepplerites which 

in Montana occur in the Rierdon Formation. The 

association of these ammonites with Gryphaea impressimarginata McLearn is normal for the lower part of 

the Rierdon Formation, although in Montana that 

species of Gryphaea occurs also in the upper siltstone 

member of the Sawtooth Formation. 

On the basis of the presence of W arrenoceras, the 

Gryphaea bed on Grassy Mountain may be oorrelated 

approximately with the Canyon Springs Sandstone 

Member of the Sundance Formation in central and 

eastern Wyoming (Imlay, 1947, p. 250), the basal bed 

of the Sundance Formation near Cody, Wyo. (Imlay, 

1956b, p. 59·2), -and unit 19 of the section described by 

Imlay ( 1956a, p. 70) at Lower Slide Lake in the Jackson 

Hole area, Wyoming. This unit is part of a sequence 
62 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

that is correlated on a stratigraphic basis with the 

Watton Canyon Member of the Twin Creek Limestone 

as exposed in ,the Jackson Hole area. It appears probable, therefore, that the bed containing Gryphaea 

impressimarginata McLearn near Blairmore, Alberta, 

correlates with some part of the Watton Canyon Member below the lowest occurrence of Gryphaea cf. 

G. nebrascen.si8 Meek and Hayden. 

The upper part of the Grey Beds in outcrops in 

Alberta and in the subsurface in Saskatchewan has 

furnished a few ammonites (Frebold, 1963, p. 4, 6, 7, 

23-26, 30, 31) that are similar or identical with species 

of K osmoceras and K epplerites in the middle and upper 

parts of the Rierdon Formation of Montana. These 

parts of the Rierdon Formation are correlated with 

the Leeds Creek Member of the Twin Creek Limestone 

on the basis of stratigraphic position and the presence of Gryphaea nebrascensis Meek and Hayden. 

Strangely, this species has not been recorded from 

outcrops of the Grey Beds in Canada, although present 

in the subsurface (Frebold, 1957, p. 26) in beds of 

Callovian age. It has been recorded also from outcrops 

of the Green Beds which overlie the Grey Beds and 

which contain Oardioceras of early Oxfordian age 

(Frebold and others, 1959, p. 10). 

COOK INLET REGION, ALASKA 

The Jurassic 1negafossils from the Cook Inlet region, 

Alaska, have very little in common faunally with those 

from the Twin Creek Limestone of the western interior. 

The only significant faunal element in common is the 

ammonite Megasphaeroceras (fig. 12) which in both 

areas is associated with ammonites of early late 

Bajocian age. In Alaska (Imlay, 1962b, p. A2) it is 

associated with Leptosphinctes, Sphaeroaeras, N 01'mannites ( = Dettermanites), and a questionable Spiroceras. 

In the Sliderock Member of the Twin Creek Limestone 

it is associated with Spiroceras, Stephanoceras, Stemmatoceras, and questionable N ormannites. The fact 

that M egasphaeroceruA has been found only in these 

·two areas indicates that marine oonnections existed 

during late Bajocian time between the Pacific Ocean 

and the western interior sea. 

GEOGRAPHIC DISTRIBUTION 

The occurrences of Bajocian and Callovian mollusks 

from the Twin Creek Limestone described herein are 

shown by State and locality in tables 3-5 and 7-10 and 

in figures 16 and 17. The occurrences of the Bajocian 

ammonites from the Gypsum Spring Formation of 

Wyoming and from the Piper and Sawtooth Formations of Montana are shown in table 6 and figure 18. 

Detailed descriptions of all these fossil occurrences are 

given in the following table in which index numbers 

1-97 in the left-hand oolumn refer to localities in the 

Twin Creek Limestone, numbers 98-102 refer to the 

Sawtooth Formation, numbers 103-109 refer to the 

Piper Formation, and numbers 110 and 111 refer to the 

Gypsum Spring Formation. The Bajocian ammonite 

occurrences from the Carmel Formation of central Utah 

have been published previously (Imlay, 1964a, table 6 

and fig. 1 under index numbers 45, 46, and 50). 
GEOGRAPHIC DISTRIBUTION 

------,-

I 

I 

I 

1 

~-1 

I 

l 

I 

Boulder Creek 

Ll CARIBOU 

BANNOCK 

I 

I 

l L(_ 

I 

\ 

\,~ ( I \ ,_ ,-----/ 

' I ~ 

< \ __ j ~~EAR : J l 

~ I FRANKLIN) 

C) I ~~ 

___ L ___ J~~~ _____ __l __ UTAH 

10 0 10 20 MILES 

63 

iL 

~ 

0 0 Jackson TETON 

HOLE 

oKemmerer 

FIGURE 16.-Index map of Jurassic fossil localities in the Twin Creek Limestone in southeastern Idaho and westem 

Wyoming. Numbers: on map show approximate .positions of fossil localities given in tables 3-5 and 7-10. 
64 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

112° 111° 110° 

42° 

10 0 10 20 MILES 

~~~ _ _L ______ i_~ 

40° 

UTAH 

FIGURE 17.-Index map of Jurassic fossil localities in the Twin Creek Limestone in northern Utah. Numbers on 

map show approximate pos:itions of fossil localities given in tables ~5 and 7-10. 
GEOGRAPHIC DISTRIBUTION 

49° 

48° 

46° 

45° 

44° I D AH 0 

25 0 25 50 MILES 

FIGURE 18.-Index map of certain Jurassic foosil localities in Montana and northern Wyoming outside the area of 

the Twin Creek Limestone. Numbers on map are fossil-collecting localities given in table 6. 

65 
66 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Descriptions of some Jurassic localities in the Twin Creek Limestone and contemporary formations in Montana, Wyoming, Utah and 

southeastern Idaho 

Locality on figs. USGS 16-18 Mesozoic 

locality 

1 ___________ 28584 

2 ___________ 28585 

28586 

28587 

28588 

28589 

28590 

28591 

3 ___________ 13491 

28500 

28501 

28502 

28505 

28506 

28507 

28516 

28635 

28636 

4 ___________ 21000 

5 ___________ 8621 

6 ___________ 8620 

7 ___________ 8622 

8 ___________ 8623 

9 ___________ 17892 

Collector's field No. 

HA-28-6L _________ 

~HS-89-61 ________ 

~HS-90-61 ________ 

~HS-97-61 ________ 

~HS-98-61 ________ 

~HS-99-61 ________ 

~HS-10Q-61 _______ 

~HS-101-61 _______ 

~-323A, L-154 _____ 

--------------------

--------------------

--------------------

--------------------

--------------------

------------------

--------------------

I-62-5-23A _________ 

I-62-5-23C _________ 

I-47-8-5 ___________ 

~t. 169 ____________ 

~t. 167 ____________ 

~t. 171 ____________ 

198 ________________ 

9-37 _______________ 

Collector, year of collection, description of locality, and stratigrapbi~ assignment 

H. F. Albee, 1961. North-central part of SE~ sec. 5, T. 4 N., R. 44 E., Garns ~ountain 15-min quad., Teton County, Idaho. Twin Creek Limestone, Leeds 

Creek ~ember. 

~.H. Staatz, 1961. Sequence measured southwest from top of Red ~ountain from NW~ to SW7~ sec. 33, T. 4 N., R. 44 E.,Garns Mountain 15-min quad., Teton 

County, Idaho. Twin Creek Limestone, near middle of Rich ~ember, about 186 

ft above top of Nugget Sandstone. ~. H. Staatz, 1961. Same location as 28585. Twin Creek Limestone, Sliderock ~ember, from 42 ft of fine-grained medium-gray limestone whose base is 39 ft 

above top of the Nugget Sandstone. ~. H. Staatz, 1961. Same location as 28585. Twin Creek Limestone, Leeds Creek ~ember, about 145 ft above base of member. ~.H. Staatz, 1961. Same location as 28585. Twin Creek Limestone, Leeds Creek ~ember, about 176 ft above base of member. ~.H. Staatz, 1961. Same location as 28585. Twin Creek Limestone, Leeds Creek ~ember, about 240ft above base of member. . ~.H. Staatz, 1961. Same location as 28585. Twin Creek Limestone, Leeds Creek ~ember, 295 ft above base of member. ~.H. Staatz, 1961. Same location as 28585. Twin Creek Limestone, Leed§l Creek ~ember, 365 ft above base of member. 

G. R. ~ansfield and W. B. Lang, 1925. NE%NE~ sec. 19, T. 1 N., R. 40 E., Am- mon 15-min quad., Bonneville County, Idaho. Twin Creek Limestone, Sliderock ~ember, upper part. , R. W. Imlay and Yaacov Nir, 1961. North side of Willow Creek, NE~ sec. 19, 

T. 1 N., R. 40 E., Bonneville County, Idaho. Twin Creek Limestone, Sliderock ~ember, 53ft below top of member and 15ft below 4ft of quartzitic sandstone. 

Yaacov Nir, 1961. North side of Willow Creek. On west side of and about 6ft 

above bed of small gulch, SE. cor. sec. 18, T. 1 N., R. 40 E., Ammon 15-min 

quad., Bonneville County, Idaho. Twin Creek Limestone, Sliderock ~ember, 

float from nodular limestone at top of member. 

Yaacov Nir, 1961. North side of Willow Creek, NE~ sec. 19, T. 1 N., R. 40 E., 

Bonneville County, Idaho. Twin Creek Limestone, Sliderock ~ember, 'from 4 ft 

of papery claystone 30 ft below top of member. 

R. W. Imlay and Yaacov Nir, 1961. North side of Willow Creek, NE~ sec. 19, 

T. 1 N., R. 40 E., Bonneville County, Idaho. Twin Creek Limestone, Sliderock ~ember, from 6~ ft of soft fissile claystone 86 ft below top of member. 

R. W. Imlay and Yaacov Nir, 1961. About~ mile north of road on north side of 

Willow Creek, NE~ sec. 19, T. 1 N., R. 40 E., Bonneville County, Idaho. Twin 

Creek Limestone, Sliderock ~ember, 79 ft below top of member. 

R. W. Imlay and Yaacov Nir, 1961. About~ mile north of Willow Creek on east 

side of a southward-trending gulch, SW. cor. sec. 17, T. 1 N., R. 40 E., Bonneville 

County, Idaho. Twin Creek Limestone, Rich ~ember, from thin beds of nodular 

limestone near middle of member and about 170-190 ft above its base. 

Yaacov Nir, 1961. Just west of bridge across Willow Creek in NE. cor. sec. 19, 

T. 1 N., R. 40 E., Bonneville County, Idaho. Twin Creek Limestone, float 

probably from Rich ~ember. 

R. W. Imlay, 1962. North side of Willow Creek, SE~ sec. 18, T. 1 N., R. 40 E., 

Bonneville County, Idaho. Twin Creek Limestone, Sliderock ~ember from 

nodular gray limestone directly above soft papery claystone and 20-25 ft below 

top of member. 

R. W. Imlay, 1962. North side of Willow Creek, NE~ sec. 19, T. 1 N., R. 40 E., 

Bonneville County, Idaho. Twin Creek Limestone, Sliderock ~ember, from 7 

ft of soft shaly limestone 79 ft below top of member. 

R. W. Imlay, 1947. West side of Fall Creek, in NW% sec. 18, T. 1 N., R. 43 E., 

Irwin 30-min quad., Bonneville County, Idaho. Twin Creek Limestone, Sliderock ~ember, from upper 13 ft. 

J. W. ~erritt for G. R. ~ansfield, 1913. NWXNW~ sec. 26, T. 2 S., R. 37 E., 

Fort Hall 30-min quad., Indian Reservation, Bingham County, Idaho. Twin 

Creek Limestone lower part. 

J. W. ~erritt for G. R. ~ansfield, 1913. SE7~SW% sec. 36, T. 2 S., R. 37 E., Fort 

Hall 30-min quad., Indian Reservation, Bingham County, Idaho. Twin Creek 

Limestone, member unknown. · J. W. ~erritt for G. R. ~ansfield, 1913. SW~SE% sec. 7, T. 3 S., R. 37 E., Fort 

Hall 30-min quad., Indian Reservation, Bingham County, Idaho. Twin Creek 

Limestone, member unknown. 

G. R. ~ansfield and G. H. Girty, 1913. Northwest of Higham Peak, in east-central 

part ofT. 3 S., R. 37 E., Fort Hall 30-min quad., Indian Reservation, Bingham 

County, Idaho. Twin Creek Limestone, lower part. 

W. W. Rubey, 1937. On hill just east of bench mark 7204, southeast junction of 

Deadman Creek and Bear Creek, SW% cor. sec. 4, T. 2 S., R. 44 E., Irwin 30-min 

quad., Bonneville County, Idaho. Twin Creek Limestone, Leeds Creek ~ember, 

near top. 
GEOGRAPHIC DISTRIBUTION 67 

Descriptions of some Jurassic localities in the Twin Creek Limestone and contemporary formations in Montana, Wyoming, Utah and 

southeastern Idaho-Continued 

Locality on figs. 16-18 

10 __________ 

1L _________ 

12 __________ 

13 _________ _ 

14 _________ _ 

15 _________ _ 

16 _________ _ 

17 _________ _ 

18 _________ _ 

19 _________ _ 

20 _________ _ 

2L ________ _ 

USGS 

Mesozoic 

locality 

18181 

18182 

16016 

21632 

21633 

22459 

22460 

28795 

28796 

9090 

28797 

9309 

9096 

12116 

9315 

7430 

7432 

7438 

6370 

4081 

28637 

Collector's field No. 

39-F-18 ____________ 

39-F-19 ____________ 

AW lOL ___________ 

I-49-9-7A _________ _ 

I-49-9-7B _________ _ 

I-5D-8-16 _________ _ 

I-5Q-8-16 _________ _ 

163-8-1A __________ _ 

I63-8-1B __________ _ 

56 ________________ _ 

1-63-7-5 __________ _ 

~.29 _____________ _ 

JW .1 _____________ _ 

BB ________ -- _-----

Ry .134 ____________ _ 

R.8 _______________ _ 

R. 17 _____________ _ 

T.6 _______________ _ 

11 ________________ _ 

42 ________________ _ 

I-62-5-31A ________ _ 

Collector, year of collection, description of locality, and stratigraphic assignment 

L. S. Gardner, 1939. On Big Elk ~ountain, sec. 23, T. 2 S., R. 44 E., Irwin 30-

min quad., Bonneville County, Idaho. Twin Creek Limestone, Sliderock ~ember. 

L. S. Gardner, 1939. Near the mouth of ~·cCoy Creek, SW~ sec. 6, T. 3 S., R. 46 

E., Irwin 30-min quad., Bonneville County, Idaho. Twin Creek Limestone, 

Leeds Creek ~ember. 

W. W. Rubey, G. R. M·ansfield, and J. S. Williams, 1931. On hillside west of North 

Fork of Stump Creek about ~ mile north of its junction with Stump Creek. 

About half way between natural gate (cliff) across the North Fork and its mouth 

at Stump Creek, NW~ sec. 27, T. 6 S., R. 45 E., Freedom 15-min quad., Caribou 

County, Idaho. Twin Creek Limestone, Sliderock ~ember, upper part. 

R. W. Imlay and John ~clntyre, 1949. Branch of Stump Creek in SE~ sec. 28, 

T. 6 S., R. 45 E., Freedom 15-min quad., Caribou County, Idaho. Twin Creek 

Limestone, about 110ft below top of Sliderock ~ember, in yellow gray, sandy, 

shaly limestone about 8 ft above a 6-ft unit of white-weathering limestone. 

R. W. Imlay and John ~clnty.re, 1949. Branch of Stump Creek in SE~ sec. 28, 

T. 6 S., R. 45 E., Freedom 15-min quad., Caribou County ,Idaho. Twin Creek 

Limestone, from upper 10ft of Sliderock ~ember and lower 40ft of Rich ~ember. 

R. W. Imlay, 1950. About ~~ mile above mouth of North Fork of Stump·Creek, 

south-central part of sec. 22, T. 45 E., R. 6 S., Freedom 15-min qua<_l., Caribou 

County, Idaho. Twin Creek Limestone, 10ft below top of Rich Member. 

R. W. Imlay, 1950. About ~mile above mouth of North Fork of Stump .Creek, 

south-central part of sec. 22, T. 45 E., R. 6 S., Freedom 15-min quad., Caribou 

County, Idaho. Twin Creek Limestone, 100ft below top of Rich·Member and 

160 ft stratigraphically below cliff-forming Watton Canyon Member that is 

marked by a cataract. 

R. W. Imlay, 1963. North side of Stump Creek about ~ mile east of the North 

Fork and just east of axis of an anticline, NE~ sec. 27, T. 6 S., R. 45 E., Caribou 

County, Idaho. Twin Creek Limestone, Sliderock Member, 2Q-30 ft below top 

in medium- to thin-bedded limestone. 

R. W. Imlay, 1963. About 200 ft east of Mesozoic loc. 28795 on north side of 

Stump Creek. Twin Creek Limestone, Rich Member, slightly above middle. 

Caribou County, Idaho. 

G. R. ~ansfield, 1914. Boulder Creek Canyon, west-central part of sec. 1, T. 7 S., 

R. 45 E., Freedom 15-min quad., Caribou County, Idaho. Twin Creek Limestone, Sliderock ~ember. 

R. W. Imlay, 1963. Northwest side of Boulder Creek about 1 mile southwest of 

Stump Creek, west-central part sec. 1, T. 7 S., R. 45 E., Freedom 15-min quad., 

Caribou County, Idaho. Twin Creek Limestone, Rich Member, about 9Q-100 

ft above base and a little below middle of member. 

G. R. ~ansfield and P. V. Roundy, 1915. SE~ sec. 15, T. 8 S., R. 46 E., 2 miles 

south of Draney Ranch, 1 mile east of Tygee Creek, Crow Creek 15-min quad., 

Caribou County, Idaho. Twin Creek Limestone, Giraffe Creek Member. 

E. L. Jones, Jr., for G. R. Mansfield, 1914. SW~SE~ sec. 34, T. 8 S., R. 46 E., 

Crow Creek 15-min quad., Caribou County, Idaho. Twin Creek Limestone, 

lower part. E.~. Parks and C. S. Lavington, 1923. West of center sec. 10, T. 9 S., R. 46 E., 

Crow Creek 15-min quad., Caribou County, Idaho. Twin Creek Limestone, 

Sliderock Member. 

P. V. Roundy for G. R. ~ansfield, 1915. SE~ sec. 23, T. 10 S., R. 45 E., 2~~ miles 

north of Half Way House, old stage route from Montpelier to Afton, Crow Creek 

15-min quad., Caribou County, Idaho. Twin Creek Limestone, upper part, 

probably Giraffe Creek ~ember. 

R. W. Richards, 1911. Sec. 25, T. 11 S., R. 44 E., Montpelier 30-min quad., 

Bear Lake County, Idaho. Twin Creek Limestone, probably in Sliderock ~ember. 

R. W. Richards, 1911. Sec. 24, T. 11 S., R. 44 E., ~ontpelier 30-min quad., 

Bear Lake County, Idaho. Twin Creek Limestone, Sliderock Member. 

E. L. Troxell for R. W. Richards, 1911. Sec. 25, T. 11 S., R. 44 E., ~ontpelier 

30-min quad., Bear Lake County, Idaho. Twin Creek Limestone, probably Sliderock Member. 

G. H. Girty, 1909. Near middle of sec. 17, T. 12 S., R. 45 E., ~ontpelier 30-

min quad., Bear Lake County, Idaho. Twin Creek Limestone, Sliderock ~ember. 

F. B. Weeks, 1906. About 1 mile north of ~ontpelier Canyon on east side of 

Star Valley road in NE. cor. sec. 27, T. 12 S., R. 45 E., ~ontpelier 30-min quad., 

Bear Lake County, Idaho. Twin Creek Limestone, Sliderock Member. 

R. W. Imlay, 1962. North side of U.S. Highway 89 in NW7~ sec. 35, T. 12 S., 

R. 45 E., Bear Lake County, Idaho. Twin Creek Limestone, Sliderock Member, 

shaly limestone in upper· half. 
68 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Descriptions of some Jurassic localities in the Twin Creek Limestone and contemporary formations in Montana, Wyoming, Utah and 

southeastern Idaho-Continued 

Locality on figs. 16-18 

22 _________ _ 

23 _________ _ 

24 _________ _ 

25 _________ _ 

26 _________ _ 

27 _________ _ 

28 _________ _ 

29 _________ _ 

30 _________ _ 

3L ________ _ 

32 _________ _ 

33 _________ _ 

34 _________ _ 

35 _________ _ 

36 _________ _ 

37 _________ _ 

38 _________ _ 

39 _________ _ 

USGS 

Mesozoic 

locality 

6359 

6360 

6638 

6639 

3831 

27727 

27728 

28862 

7217 

3848 

3830 

3834 

3808 

16031 

16036 

3839 

3859 

9102 

9101 

12018 

12034 

12119 

16058 

Collector's field No. 

131 _______________ _ 

133 _______________ _ 

1554 ______________ _ 

1555 ______________ _ 

1103/C ____________ _ 

A W .172 ___________ _ 

A W .180 ___________ _ 

Ry. 62 ____________ _ 

Ry. 61_ ___________ _ 

N.E ______________ _ 

T _________________ _ 

G.G ______________ _ 

A W 233 ___________ _ 

Collector, year of collection, description of locality, and stratigraphic assignment 

C. L. Breger, 1909. NE% sec. 25, T. 12 S., R. 45 E., Montpelier 30-min qua~., 

Bear Lake County, Idaho. Twin Creek Limestone, Leeds Creek Member, m upper part of the formation about 100ft above a red-bed unit. 

C. L. Breger, 1909. SW}~ sec. 30, T. 12 S., R. 46 E., Montpelier 30-min qua~., 

Bear Lake County, Idaho. Twin Creek Limestone, Leeds Creek Member, m upper part of the formation, probably below a red-bed unit in white-weathering 

shaly limestone. 

T. W. Stant<?p_, 1910. North side of Indian Creek, NE. cor. sec. 30, ?'· 15 S., 

R. 45 E., lVlOntpelier 30-min quad., Bear Lake County, Idaho. Tw1n Creek 

Limestone, Sliderock Member, lower 100ft. 

T. W. Stanton, 1910. Same location as 6638. Twin Creek Limestone, near top 

of Sliderock Member and about 200 ft stratigraphically higher than Mesozoic 

loc. 6638. 

H. C. Schluter, 1906. 330 feet north of SE. cor. sec. 13, T. 39 N., R. 115 W., 

Jackson 15-min quad., Teton County, Wyo. Twin Creek Limestone, Leeds 

Creek Member. 

R. W. Imlay and N. F. Sohl, 1960. Cabin Creek, S% sec. 17, T. 38 N., R. 116 W_., 

Teton County, Wyo. Twin Creek Limestone, Leeds Creek Member, near ba'se 

of unit 16 of published section (Imlay, 1953a, p. 62). 

R. W. Imlay and N. F. Sohl, 1960. Cabin Creek, SH sec. 17, T. 38 N., R. 11~ W., 

Teton County, Wyo. Twin Creek Limestone, Leeds Creek Member, near middle 

of unit 16 (Imlay, 1953a, p. 62). 

R. W. Imlay, 1964. Cabin Creek, same locatio11 as 27727. Twin Creek Limestone, 

Leeds Creek Member, about 70 ft above base. 

Eliot Blackwelder, 1911. Foot of Shoal Creek Falls, south side Gros Ventre 

Mountains, T. 38 N., R. 114 W., Jackson 30-min quad., Sublette County, Wyo. 

Twin Creek Limestone, member unknown. 

E. E. Smith, 1906. West of Cliff Creek, 550ft north of SW. cor. sec. 22, T. 38 N., 

R. 114 W., Jackson 30-min quad., Sublette County, Wyo. Twin Creek Limestone, member unknown. 

B. A. Iverson, 1906. North-central part of sec. 33, T. 38 N., R. 115 W., Jackson 

30-min quad., Lincoln County, Wyo. Twin Creek Limestone, Leeds Creek 

Member. 

E. E. Smith, 1906. 4,125 ft north of SE. cor. sec. 33, T. 38 N., R. 115 W., Jackson 

15-min quad., Lincoln County, Wyo. Twins Creek Limestone, Leeds Creek 

Member. 

E. E. Smith, 1906. 1,200 ft east of NW. cor. sec. 31, T. 37 N., R. 114 W., Sublette 

County, Wyo. Twin Creek Limestone, probably Leeds Creek Member. 

W. W. Rubey and J. S. Williams, 1931. On ridge about 174 miles northwest of 

peak of Lookout Mountain, in SE. cor. sec. 21, T. 35 N., R. 115 W., Afton 30-min 

quad., Lincoln County, Wyo. Twin Creek Limestone, Leeds Creek Member. 

W. W. Rubey and J. S. Williams, 1931. Hill about 3% miles southeast of peak of 

Lookout Mountain. Elevation about 9,200 ft, in NE%SW%SW7~ sec. 6, T. 34 N., 

R. 114 W., Afton 30-min quad., Lincoln County, Wyo. Twin Creek Limestone, 

Sliderock Member, about 15-20 ft above red siltstone. 

E. E. Smith, 1906. 2,200 ft east and 1,100 ft north of NE. cor. sec. 24, T. 34 N., 

R. 115 W., in sec. 18, T. 34 N., R. 114 W., Sublette County, Wyo. Twin Creek 

Limestone, Leeds Creek Member. 

W. H. Longhurst, 1906. North side of South Fork of Cottonwood Creek, T. 32 

N., R. 115 W., Sublette County, Wyo. Twin Creek Limestone. 

G. R. Mansfield, 1914. East side of south end of The Narrows in Star Valley, 

SE% sec. 13, T. 33 N., R. 119 W., Lincoln County, Wyo. Twin Creek Limestone, 

Sliderock Member. 

G. R. Mansfield, 1914. About 2% miles east of The Narrows of Star Valley, 

SWX sec. 20, T. 33 N., R. 118 W., Lincoln County, Wyo. Twin Creek Limestone, lower part. 

E. M. Parks and C. S. Lavington, 1923; % mile up Swift Creek about 1}~ miles 

east of Afton, Lincoln County, Wyo. Twin Creek Limestone, Sliderock Member, near base. 

E. M. Parks and C. S. Lavington, 1923. Near road along Crow Creek on west 

limb of anticline. Southwest of center of sec. 17, T. 31 N., R. 119 W., Crow 

Creek 15-min quad., Lincoln County, Wyo. Twin Creek Limestone, Giraffe 

Creek Member, within 50 ft of top. · E. M. Parks and C. S. Lavington, 1923; % mile northeast of Smoot, Star Valley, 

Afton 30-min quad., Lincoln County, Wyo. Twin Creek Limestone, Sliderock 

Member, 150 ft above base. . W. W. Rubey and J. S. Williams, 1931. Cottonwood Creek about% mile west of 

Cottonwood Lake, about 5 miles northeast of Smoot, Afton 30-min quad., Lincoln 

County, Wyo. Twin Creek Limestone, lower part, vertical beds as much as 5 

ft thick. 
GEOGRAPHIC DISTRIBUTION 69 

Descriptions of some Jurassic localities in the Twin Creek Limestone and contemporary formations in Montana, Wyoming, Utah and 

southeastern Idaho--Continued 

Locality on figs. USGS 16-18 Mesozoic 

locality 

40 __________ 12016 

12019 

4L _________ 3842 

3860 

42 __________ 17297 

17298 

17299 

17319 

20960 

20980 

28482 

28485 

28486 

43 __________ 3817 

44 __________ 17294 

45 __________ 20971 

20988 

20990 

27729 

28483 

46 __________ 6364 

47 __________ 21628 

48_-------- .. 20961 

20968 

Collector's field No. 

B _________________ 

c _________________ 

--------------------

--------------------

36-25 ______________ 

36-26 ______________ 

36-27 ______________ 

36-49 ______________ 

I-47-8-15A _________ 

I-47-8-15B _________ 

--------------------

--------------------

--------------------

--------------------

36-22 ______________ 

I-47-8-21A~--------

I-47-8-21B _________ 

I-47-8-22A _________ 

--------------------

--------------------

228 ________________ 

I-49-9-2 ___________ 

I-47-7-28C _________ 

I-47-8-28B _________ 

Collector, year of collection, description of locality, and stratigraphic assignment 

E. M. Parks and C. S. Lavington, 1923. Entrance to Cottonwood Canyon 2 miles 

southeast of Smoot, Afton 30-min quad., Lincoln County, Wyo. Twin Creek 

Limestone, Leeds Creek Member, less than 500 ft stratigraphically bel?w top of 

formation in well-bedded splintery blue shale containing thin lenses of hmesto?-e. 

E. M. Parks and C. S. Lavington, 1923. Entrance of Cottonwood Canyon, 2 miles 

southeast of Smoot, Afton 30-min quad., Lincoln County, Wyo. Twin Creek 

Limestone, Sliderock Member, within 200 ft of base. Thick-bedded coarsely 

crystalline limestone containing pyrite and calcite veins. 

B. A. Iverson, 1906; 1,100 ft east of NE. cor. sec. 30, T. 31 N., R. 114 W., Sublette 

County, Wyo. Twin Creek Limestone, member unkno":"n· . E. E. Smith and B. A. Iverson, 1906. North of North Pmey Creek, east Side of 

anticline, T. 31, N., R. 114 W., Sublette County, Wyo. Twin Creek Limestone. 

W. W. Rubey, 1936. South Piney Creek, sec. 11, T. 29 N., R. 115 W., Afton 30-

min quad., Sublette County, Wyo. Twin Creek Limestone, Rich Member, 118 ft 

above base. 

W. W. Rubey, 1936. South Piney Creek, sec. 11, T. 29 N., R. 115 W., Afton 

30-min quad., Sublette County, Wyo. Twin Creek Limestone, Watton Canyon 

Member, 320 ft above base of formation. 

W. W. Rubey, 1936. Gorge of Piney Creek, sec. 11, T. 29 N., R. 115 W., Afton 

30-min quad., Sublette County, Wyo. Twin Creek Limestone, basal part of 

Watton Canyon Member, 390 ft above base of formation. 

W. W. Rubey, 1936. South Piney Creek, sec. 11, T. 29 N., R. 115 W., Afton 

30-min quad., Sublette County, Wyo. Twin Creek Limestone from 50 to 75ft 

above loc. 17299 in Leeds Creek Member. 

R. W. Imlay, 1947. South Piney Creek, sec. 11, T. 29 N., R. 115 W., Sublette 

County, Wyo. Twin Creek Limestone, Leeds Creek Member, from 65 to 88 

ft above base of member. 

R. W. Imlay, 1947. South Piney Creek in sec. 11, T. 29 N., R. 115 W., Sublette 

County, Wyo. Twin Creek Limestone, upper part of Rich Member, about 

200-240 ft above base of formation. 

J. C. Wright and R. W. Imlay, 1961. South Piney Creek on north side of road, sec. 11, T. 29 N., R. 115 W., Sublette County, Wyo. Twin Creek Limestone, 

top of Watton Canyon Member. 

J. C. Wright, Yaacov Nir, and R. W. Imlay, 1961. South Piney Creek, sec. 11, 

T. 29 N., R. 115 W., Afton 30-min quad., Sublette County, Wyo. Twin Creek 

Limestone, lower half of Watton Canyon Member. 

J. C. Wright, Yaacov Nir, and R. W. Imlay, 1961. South Piney Creek, sec. 11, 

T. 29 N., R. 115 W., Afton 30-min quad., Sublette County, Wyo. Twin Creek 

Limestone, top of Rich Member. 

B. A. Iverson, 1906. East of La Barge Creek, 180ft north of NE. cor. sec. 24, T. 

29 N., R. 116 W., Lincoln County, Wyo. Twin Creek Limestone, member un- known. 

W. W. Rubey, 1936. Sec. 2, T. 28 N., R. 116 W., Afton 30-min quad., Lincoln 

County, Wyo. Twin Creek Limestone, lower part of Leeds Creek Member, 

about 500 ft above base of formation. 

R. W. Imlay, 1947. Thomas Fork Canyon, north-central part of sec. 24, T. 28 

N., R. 120 W., Montpelier 30-min quad., Lincoln County, Wyo. Twin Creek 

Limestone, from upper part of Sliderock Member. 

R. W. Imlay, 1947. North-central part of sec. 24, T. 28 N., R. 120 W., Montpelier 

30-min quad., Lincoln County, Wyo. Twin Creek Limestone, from upper part 

of Sliderock Member. • R. W. Imlay, 1947. North side of Thomas Fork Canyon in north-central part of sec. 19, T. 28 N., R. 119 W., Montpelier 30-min quad., Lincoln County, Wyo 

Twin Creek Limestone, middle part of Boundary Ridge Member, from unit 10 

of published section (Imlay, 1950a, p. 44). 

N. F. Sohl, 1960. Thomas Fork Canyon, NYz sec. 24, T. 28 N., R. 120 W., Montpelier 30-min quad., Lincoln County, Wyo. Twin Creek Limestone, Sliderock 

Member. 

J. C. Wright, Yaacov Nir, and R. W. Imlay, 1961. Thomas Fork Canyon, N% sec. 24, T. 28 N., R. 120 W., Montpelier 30-min quad., Lincoln County, Wyo. 

Twin Creek Limestone, upper part of Sliderock Member. 

G. H. Girty, 1909. West side of Sublette Range, NW% NE?~ sec 17, T. 27 N., 

R. 119 W., Lincoln County, Wyo. Twin Creek Limestone, Sliderock Member. 

R. W. Imlay and John Mcintyre, 1949. Ferney Gulch, SW7~ sec. 1, T. 27 N., R. 

117 Yz W., Cokeville 30-min quad., Lincoln County, Wyo. Twin Creek Limestone, upper part of Sliderock Member. 

R. W. Imlay, 1947. Head of Middle Fork of Devils Hole Creek in SE. cor. of sec. 15, T. 27 N., R. 117 W., Cokeville 30-min quad., Lincoln County, Wyo. Twin 

Creek Limestone, from upper 10ft of Sliderock Member. 

R. W. Imlay, 1947. Head of North Fork of Devils Hole Creek in NE 7~ sec. 15, T. 

27 N., R. 117 W., Cokeville 30-min quad., Lincoln County, Wyo. Twin Creek 

Limestone, from upper 3 ft of Sliderock Member. 
70 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Descriptions of some Jurassic localities in the Twin Creek Limestone and contemporary formations in Montana, Wyoming, Utah and 

southeastern I daho-C ontin ued 

Locality on figs. USGS 16--18 Mesozoic 

locality 

20972 

49 __________ 22017 

50 __________ 20963 

20983 

20992 

28639 

5L ________ .:. 22796 

52 __________ 3815 

53 __________ 21629 

54 __________ 20979 

20985 

55 __________ 20344 

20346 

20347 

20348 

20970" 

20973 

28484 

28481 

56 __________ 28794 

57 __________ 20978 

58 __________ 20345 

59 __________ 25261 

Collector's field No. 

I-47-7-28A _________ 

C2-47 ______________ 

I-47-8-12A _________ 

I-47-8-12C _________ 

I-47-8-12B _________ 

l-62-6-9a __________ 

39-3 _______________ 

--------------------

l-49-9-5a __________ 

I-47-7-13A _________ 

I-47-7-26D ________ 

I -46-9-14D ________ 

I-46-9-14A _________ 

I-46-9-14B _________ 

I-46-9-14C _________ 

1-47-8-3 ______ ~----

--------------------

------------------

--------------------

I-63-7-4A __________ 

1-47-7-13 __________ 

I-46-9-15B _________ 

R2-54 _____________ 

Collector, year of collection, description of locality, and stratigraphic assignment 

R. W. Imlay, 1947. Same location as 20968. Devils Hole Creek, Wyo. Twin 

Creek Limestone, from 18 ft of medium-bedded limestone 10 ft below top of 

Sliderock Member and 80 ft above base. 

G. E. Lewis, 1947. South-central sec. 16, T. 27 N., R. 116 W., Cokeville 30-min 

quad., Lincoln County, Wyo. Twin Creek Limestone, 25 ft above base of 

Sliderock Member. 

R. W. Imlay, 1947. West side of Sheep Creek near La Barge Creek in NW~~ se.c. 16, T. 27 N., R. 115 W., Cokeville 30-min quad., Sublette County, Wyo. Twm 

Creek Limestone, upper 30 ft of Rich Member, from 305 to 335 ft above base of 

formation. 

R. W. Imlay, 1947. North side La Barge Creek, NE% sec. 17, T. 27 N., R. 115 

W., Cokeville 30-min quad., Sublette County, Wyo. Twin Creek Limestone, 

158 ft below top of Leeds Creek Member, from upper part of unit 28 of published 

section (Imlay, 1950a, p. 42). 

R. W. Imlay, 1947. North side of La Barge Creek, NW H sec. 16, T. 27 N., R. 

115 W., Sublette County, Wyo. Twin Creek Limestone, about 150 ft above 

base of Watton Canyon Member and 16 ft below top of unit 12 of published 

section (Imlay, 1950a, p. 42). 

R. W. Imlay, 1962. Sheep Creek about ~~ mile north of La Barge Creek in NW% sec. 16, T. 27 N., R. 115 W., Cokeville 30-min quad., Sublette County, Wyo. Twin 

Creek Limestone, float from upper 40 ft of Rich Member. 

W. W. Rubey, 1939. SE ~~ sec. 13, T. 27 N., R. 115 W., Cokeville 30-min quad., 

Sublette County, Wyo. Twin Creek Limestone, 300± ft above base near top 

of Rich Member. 

B. A. Iverson, 1906. About 600ft east of NW. cor. sec. 4, T. 26 N., R. 115 W., 

Lincoln County, Wyo. Twin Creek Limestone, Rich Member. 

R. W. Imlay and John Mcintyre, 1949. South Fork Fontenelle Creek, NW% sec. 33, T. 26 N., R. 116 W., Cokeville 30-min quad., Lincoln County, Wyo. 

Twin Creek Limestone, about middle of Rich Member and about 100ft below 

its top. 

R. W. Imlay, 1947. Near head of Chapple Creek, NE~~SE% sec. 35, T. 26 N., 

R. 118 W., Lincoln County, Wyo. Twin Creek Limestone, upper part of Sliderock Member. 

R. W. Imlay, 1947. NW. cor. sec.· 36, T. 26 N., R. 118 W., Cokeville 30-min 

quad., Lincoln County, Wyo. Twin Creek Limestone, upper part of Sliderock Member. 

J. B. Reeside, Jr. and R. W. Imlay, 1946. North side of Sliderock Creek on cen- ter of line of sees. 10 and 11, T. 25 N., R. 118 W., Cokeville 30-min quad., Lincoln County, Wyo. Twin Creek Limestone, upper part of Sliderock Member 

from 200 to 215 ft above base of formation. 

J. B. Reeside, Jr., 1946. Sliderock Creek, west half of sec. 10, T. 25 N., R. 118 W., 

Cokeville 30-min quad., Lincoln County, Wyo. Twin Creek Limestone, from 

very top of Giraffe Creek Member. 

J. B. Reeside, Jr., 1946. Sliderock Creek, near center of sec. 10, T. 25 N., R. 118 

W., Lincoln County, Wyo. Twin Creek Limestone, about middle of Leeds Creek 

Member. 

R. W. Imlay, 1946. Sliderock Creek, near center of sec. 10, T. 25 N., R. 118 W., 

Lincoln County, Wyo. Twin Creek Limestone, about middle of Leeds Creek 

Member, slightly below 20347. 

R. W. Imlay, 1947. Same locality as 20344. Twin Creek Limestone, top of Sliderock Member. 

R. W. Imlay, 1947. Same locality as 20344. Twin Creek Limestone, upper 15ft 

of Sliderock Member. 

J. C. Wright, Yaacov Nir, and R. W. Imlay, 1961. North side of Sliderock Creek on center of line of sees. 10 and 11, T. 25 N., R. 118 W., Cokeville 30-min quad., 

Lincoln County, Wyo. Twin Creek Limestone, Sliderock Member, about 30-40 

ft below top. 

Yaacov Nir, 1961. Sliderock Creek area, center of sec. 10, T. 25 N., R. 118 W., 

Lincoln County, Wyo. Twin Creek Limestone, near middle of Leeds Creek 

Member. 

R. W. Imlay, 1963. North side of Dry Hollow, west of Cokeville, NW~~ sec. 12, 

T. 24 N., R. 120 W., Cokeville 30-min quad., Lincoln County, Wyo. Twin Creek 

Limestone, Sliderock Member, middle and upper parts. 

R. W. Imlay, 1947. NW% sec. 5, T. 24 N., R. 118 W., Lincoln County, Wyo. 

Twin Creek Limestone, about 20 ft below top of Sliderock Member. 

J. B. Reeside, Jr. and R. W. Imlay, 1946. North side of Pine Creek, SEXSWH sec. 34, T. 25 N., R. 118 W., Cokeville 30-min quad., Lincoln County, Wyo. Twin 

Creek Limestone, from upper 30 ft of Sliderock Member. 

W. W. Rubey, 1954. NE~ sec. 5, T. 24 N., R. 116 W., Cokeville 30-min quad., 

Lincoln County, Wyo. Twin Creek Limestone, top bed of Gypsum Spring Member. · 
GEOGRAPIDC DISTRffiUTION 71 

Descriptions of some Jurassic localities in the Twin Creek Limestone and contemporary formations in Montana, Wyoming, Utah and 

southeastern Idaho--Continued 

Locality on figs. USGS 16-18 Mesozoic 

locality 

60 __________ 3800 

6L _________ 3822 

28789 

28790 

62 __________ 3821 

17815 

28785 

28786 

28787 

28788 

63 __________ 20969 

64 __________ 20984 

65 __________ 20965 

20966 

20975 

20977 

20981 

20987 

20991 

28798 

66 __________ 3391 

3392 

20350 

67 __________ 6373 

68 __________ 6363 

8181 

21618 

Collector's field No. 

--------------------

--------------------

I-63-7-12A _________ 

I-63-7-13A _________ 

--------------------

--------------------

1-63-7-11 __________ 

I-63-8-3A __________ 

I-63-8-3B __________ 

I-63-8-3C __________ 

I-47-8-19A _________ 

I-47-7-12A _________ 

I-47-7-24B _________ 

I-47-7-24A _________ 

I-47-7-12C ____ -----

I-47-7-12B _________ 

--------------------

I-47-7-24B _________ 

I-47-7-24C ____ -----

I-63-7-4B __________ 

--------------------

--------------------

1-46-9-13 __________ 

--------------------

188 ________________ 

Ry. 133a ___________ 

I-49-7-15 __________ 

Collector, year of collection, description of locality, and stratigraphic assignment 

E. E. Smith, 1906. About 900ft east and 600ft north of NW. cor. sec. 13, T. 25 

N., R. 115 W., Lincoln County, Wyo. Twin Creek Limestone, probably from 

Sliderock Member. 

A. R. Schultz, 1906. South bank of Fontenelle Creek, T. 24 N., R. 115 W., Lincoln 

County, Wyo. Twin Creek Limestone, lower part. . . R. W. Imlay, 1963. North side of Fontenelle Gap, about X mile north of creek m north-central part of sec. 4, T. 24 N., R. 115 W., Fort Hill 7%-min quad., Lincoln 

County, Wyo. Twin Creek Limestone, near base of Rich Member. 

R. W. Imlay, 1963. North side of Fontenelle Gap, SWXNEX se~. 5, T. 24. N., 

R. 115 W., Fort Hill 7%-min quad., Lincoln County, Wyo. Twm Creek Limestone, upper 40 ft of Rich Member. 

A.R. Schultz, 1906. Slate Creek, halfway through canyon, T. 23 N., R. 115 W., 

Lincoln County, Wyo. Twin Creek Limestone, Rich Member. 

R. W. Brown and C. W. Mumm, 1938. About % way up north slope of Slate 

Creek Canyon about 12 miles northeast of Kemmerer, in sec. 21, T. 23 N., R. 

115 W., Lincoln County, Wyo. Twin Creek Limestone, Rich Member. . . R. W. Imlay, 1963. North side of Slate Creek Canyon, near center of ant1clme 

in NE7~ sec. 28, T. 23 N., R. 115 W., Lincoln County, Wyo. Twin Creek Limestone, Rich Member, from lower 50 of 130 ft of exposed part of member. 

R. W. Imlay, 1963. North side of Slate Creek Canyon in NW7~ sec. 27, T. 23 N., 

R. 115 W., Lincoln County, Wyo. Twin Creek Limestone, Rich Member, float 

from upper 30ft. 

R. W. Imlay, 1963. Same location as 28786. Twin Creek Limestone, Rich 

Member, from ledge 65 ft below top. 

R. W. Imlay, 1963. Same location as 28786. Twin Creek Limestone, Rich 

Member, from lower 60 ft of 130 ft of exposed part of member. 

R. W. Imlay, 1947. Head of Underwood Canyon, NE. cor. sec. 6, T. 23 N., R. 118 

W., Cokeville 30-min quad., Lincoln County, Wyo. Twin Creek Limestone, 

UJ>per part of Sliderock Member. 

R. W. Imlay, 1947. SEXSEX sec. 7, T. 23 N., R. 118 W., Lincoln County, Wyo 

Twin Creek Limestone, Sliderock Member. 

R. W. Imlay, 1947. North side of Leeds Creek Canyon, center of S% sec. 30, T. 23 

N., R. 118 W., Lincoln County, Wyo. Twin Creek Limestone, 38-40 ft above 

base of Rich Member and 210ft above base of formation. 

R. W. Imlay, 1947. Leeds Creek. Same locality as 20965. Twin Creek Limestone, Sliderock Member, upper 20 ft. 

R. W. Imlay, 1947. Near Leeds Creek, center S% sec. 30, T. 23 N., R. 118 W. 

Lincoln County, Wyo. Twin Creek Limestone, from upper 20 ft of Sliderock 

Member. 

R. W. ·Imlay, 1947. Near Leeds Creek, west center of NEX sec. 30, T. 23 N., R. 

118 W., Lincoln County, Wyo. Twin Creek Limestone, upper part of Sliderock 

Member. 

R. W. Imlay, 1947. Head of Leeds Creek Canyon, S% of sec. 30, T. 23 N., R. 

118 W., Lincoln County, Wyo. Twin Creek Limestone, near top of Sliderock 

Member. 

R. W. Imlay, 1947. Same location as 20965. Twin Creek Limestone, 35-40 

ft above base of Rich Member. 

R. W. Imlay, 1947. North side of Leeds Canyon, SW7~ sec. 30, T. 22 N., R. 119 

W., Lincoln County, Wyo. Twin Creek Limestone, Leeds Creek Member, 

from 6 ft of silty limestone 97 ft above base of member. 

R. W. Imlay, 1963. Head of North Fork of Leeds Creek in west-central part SW7~SE7~ sec. 30, T. 23 N., R. 118 W., Sage 7%-min quad., Lincoln County, 

Wyo. Twin Creek Limestone, Sliderock Member, upper part. 

A. C. Veatch, 1905. 2 miles northwest of Nugget Station, Lincoln, County, Wyo. 

Near NW. cor. sec. 6, T. 21 N., R. 118 W. Twin Creek Limestone, Sliderock 

Member. 

A. C. Veatch, 1905. About 9 miles west of Fossil, Lincoln County, Wyo. NE7~ 

sec. 1, T. 21 N., R. 119 W. Twin Creek Limestone, lower part. 

J. B. Reeside, Jr. and R. W. Imlay, 1946. North side of road, about 4 miles east 

of Sage in north-central part of sec. 2, T. 21 N., R. 119 W., Lincoln County, 

Wyo. Twin Creek Limestone, Rich Member. 

C. L. Breger, 1909. North Eden Canyon, near Bear Lake, Rich County, Utah 

Twin Creek Limestone, lower part. 

C. L. Breger, 1909. SEX sec. 27, T. 14 N., R. 6 E., South Eden Canyon, 2 miles 

east of Bear Lake, Rich County, Utah. Twin Creek Limestone, Rich Member, 

in light-weathering softer shaly beds. 

P. V. Roundy for G. B. Richardson, 1912. Near center sec. 27, T. 14 N., R. 6 

E., north side South Eden Canyon about 2 miles from Bear Lake, Randolph 

30-min quad., Rich County, Utah. Twin Creek Limestone, probably from Rich 

Member. · 

R. W. Imlay and John Mcintyre, 1949. North side of road, South Eden Canyon, sec. 27, T. 14 N., R. 6 E., Randolph 30-min quad., Rich County, Utah. Twin 

Creek Limestone, from upper 30 ft of Sliderock Member. 
72 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Descriptions of some Jurassic localities in the Twin Creek Limestone and contemporary formations in Montana, Wyoming, Utah and 

southeastern Idaho-Continued 

Locality on figs. 16-18 

69 __________ 

70 __________ 

7L _________ 

72 __________ 

73 __________ 

74 __________ 

75 __________ 

76 __________ 

77 __________ 

78 __________ 

79 __________ 

80 _________ _ 

8L ________ _ 

USGS 

Mesozoic 

locality 

8177 

6362 

28644 

28645 

28646 

28647 

28648 

22638 

28642 

2906 

28478 

28641 

28643 

21619 

6948 

985 

28476 

28791 

3218 

3219 

3220 

3221 

Collector's field No. 

Ry. 77 ______________ 

167 ________________ 

I-62-5-16A _________ 

I-62-5-16B _________ 

I-62-5-16C _________ 

I-62-5-17A _________ 

I-62-5-17C _________ 

l-51-7-3a __________ 

--------------------

901 ________________ 

--------------------

--------------------

--------------

I-49-8-25A _________ 

7 __________________ 

-------------

63 MC-28 _________ _ 

116 _______________ _ 

117 _______________ _ 

118 _______________ _ 

119 _______________ _ 

Collector, year of collection, description of locality, and stratigraphic assignment 

P. V. Roundy for G. B. Richardson, 1912. About 12 miles west of Woodruff, 

probably NE7~ sec. 23, T. 9 N., R. 5 E., Randolph 30-min quad., Rich County, 

Utah. Twin Creek Limestone, Sliderock Member. . . C. L. Breger, 1909; 8 miles due west of Woodruff, 1 nnle northeast of the.reserv01r on Birch Creek, probably sec. 18, T. 9 N., R. 6 E., Randolph 30-mm quad., 

Rich County, Utah. Twin Creek Limestone, Rich Member, in the light-weathering shaly beds. · R. W. Imlay, 1962. Near top of hill just north of confluence of Birch qreek and 

Watton Canyon in NE7~NW% sec. 20, T. 9 N., R. 6 E., Randolph 30-nnn quad., 

Rich County, Utah. Twin Creek Limestone, Sliderock Member, upper part, 

from 62 ft of medium- to thin-bedded limestone. 

R. W. Imlay, 1962. About 7~ mile up Birch Creek from Watton Canyon con- fluence on north side of road just west of crest of ridge in NW%NW7~ sec. 20, 

T. 9 N., R. 6 E., Rich County, Utah. Twin Creek Limestone, Sliderock Member, 

about 45 ft below top. . . R. W. Imlay, 1962. About 7~ mile west of loc. 28645, JUSt north of road m NEH sec. 19, T. 9 N., R. 6 E., Rich County, Utah. Twin Creek Limestone, Sliderock 

Member, rubbly limestone in upper part. 

R. W. Imlay, 1962. Top of hill north of Birch Creek, in NW. cor. sec. 20, T. 9 N., 

R. 6 E., Rich County, Utah. Twin Creek Limestone, Sliderock Member. Fr?m 

top of rubbly linestone overlying 16ft of ledge-forming sandstone and underlymg 

5 ft of shaly linestone. About 135ft below top of member. 

R. W. Imlay, 1962. Same location as 28645 but a little higher stratigraphically. 

Twin Creek Limestone, Sliderock Member. Mostly from 32-37 ft above base of 

62 ft of medium-bedded linestone forming upper part of member but partly from 

float directly below. 

R. W. Imlay, 1951. On Woodruff Creek, about % mile below mouth of Sugar 

Pine Canyon and 10 miles southwest of Woodruff, SE7~ sec. 3, T. 8 N., R. 5 E., 

Rich County, Utah. Twin Creek Limestone, upper 6ft of Slidero~k Member. 

R. W. Imlay, 1962. North side of Weber River Canyon, north of Devils Slide, sec. 24, T. 4 N., R. 3 E., Morgan County, Utah. Twin Creek Limestone, Sliderock 

Member, from medium-bedded linestone 30-40 ft above basal oolitic limestone. 

T. W. Stanton, 1903. Low hills on north side of Weber Canyon, 1 mile abo~e 

Croydon Station just below mouth of Lost Creek, Morgan County, Utah. Twm 

Creek Limestone, talus from upper part of Leeds Creek Member. 

R. W. Imlay, 1961. On south side of U.S. Highway 308, sec. 24, T. 4 N., R. 3 E., 

Weber River Canyon, Morgan County, Utah. Twin Creek Limestone, near top 

of Rich Member. In shaly linestone about 40ft below oolite forming the western 

ledge of Devils Slide. 

R. W. Imlay, 1962. On south side of Weber River Canyon, a little west of Devils 

Slide, sec. 24, T. 4 N., R. 3 E., Morgan County, Utah. Twin Creek Limestone, 

Sliderock Member, float found 30 ft above basal oolitic limestone of member. 

R. W. Imlay and T. E. Mullens, 1962. In railroad cut east of railroad bridge across Weber River at town of Devils Slide, Morgan County, Utah. Twin Creek 

Limestone, Leeds Creek Member. 

John Mcintyre, 1949. NE7~ sec. 1, T. 1 S., R. 6 E., Weber Canyon, Summit 

County, Utah. Twin Creek Limestone, Sliderock Member, about 11 ft above 

thin bed of green tuff at top of Gypsum Spring Member. 

J. W. Boutwell, 1902. West end of Uinta Mountain uplift on nose of spur between 

White Creek and Whites Basin, north of Weber Canyon, SW~ sec. 10, T. 1 S., 

R. 6 E., Summit County, Utah. Twin Creek Limestone. 

T. W. Stanton, 1892; 1 mile north of Peoa, on east side of U.S. Highway 189 in NWH sec. 14, T. 1 S., R. 5 E., Summit County, Utah. Twin Creek Limestone, Rich 

Member. From 225 ft of shaly limestone forming lower exposures in the formation. 

R. W. Imlay, 1961; 1 mile north of Peoa, on east side of U.S. 189, sec. 14, T. 1 S., 

R. 5 E., Summit County, Utah. Twin Creek Limestone, Rich Member, 15-35 

ft above base of lowest exposed limestone. 

M. D. Crittenden, Cal Bromfield, and R. W. Imlay, 1963. SW. cor. sec. 30, T. 1 S., 

R. 4 E., Park City West 77~-min quad., Summit County, Utah. Twin Creek 

Limestone, Boundary Ridge Member, about 15-20 ft below top. 

H. T. Boutwell and J. M. Boutwell, 1904. South side of ridge, NW7~SE7~SW% 

sec. 2, T. 2 S., R. 3 E., Park City West 7~-min quad., Summit County, Utah. 

First limestone above Nugget Sandstone. Twin Creek Limestone, Sliderock 

Member. 

J. M. Boutwell and H. T. Boutwell, 1904. Same location as 3218 but on north 

side of divide. Twin Creek Limestone, Sliderock Member, 45 ft above Nugget Sandstone. 

J. M. Boutwell and H. T. Boutwell, 1904. Same location as 3218. Twin. Creek 

Limestone. 

J. M. Boutwell and H. T. Boutwell, 1904. Same location as 3318. North side 

of divide, 200-225 ft above Nugget Sandstone in Twin Creek Limestone, lower 

part. . 
GEOGRAPHIC DISTRIBUTION 73 

Descriptions of some Jurassic localities in the Twin Creek Limestone and contemporary formations in Montana, Wyoming, Utah and 

southeastern Idaho-Continued 

Locality on figs. USGS 16-18 Mesozoic 

locality 

82 __________ 17964 

83 __________ 17961 

84 __________ 17985 

85 __________ 17982 

86 __________ 17983 

17984 

87 __________ 17976 

17977 

17978 

88_ -- -- -- -- -· 21623 

21624 

89 __________ 21620 

21621 

21622 

90 __________ 17048 

20349 

28793 

9L ____ -.- ___ •28792 

92 __________ 21468 

93 __________ 17674 

94 __________ 17683 

17678 

22.1-691 Ot--66---5 

Collector's field No. 

38-61 ______________ 

38-40 ______________ 

38-174 _____________ 

38-171 _____________ 

38-172 _____________ 

38-173 _____________ 

38-152 _____________ 

38-153 _____________ 

38-154 _____________ 

I-49-8-27A _________ 

I-49-8-27B _________ 

I-49-8-26A _________ 

I-49-8-26B _________ 

I-49-8-26C _________ 

--------------------

I-46-9-12A _________ 

I-63-8-12A _________ 

I-63-8-10 __________ 

1 __________________ 

A9 _________________ 

A27 ________________ 

A14 ________________ 

Collector, year of collection, description oflocality, and stratigraphic assignment 

A. A. Baker, 1938. On east side of new railroad grade on north line of sec. 15, 

T. 4 S., R. 4 E., Strawberry Valley 30-min quad., Wasatch County, Utah. Twin 

Creek Limestone, Sliderock Member, about 10ft above base. 

A. A. Bak~!; 1938. Near summit of hill about 1% miles due west of Charleston 

in the S w% sec. 16, T. 4 S., R. 4 E., at west line of Strawberry Valley 30-min 

quad., Wasatch County, Utah. Twin Creek Limestone, Sliderock Member, 

about 60 ft above base. 

A. A. Baker, 1938. North of Lake Creek road in SE7~ sec. 33, T. 3 S., R. 5 E., 

Coalville 30-min quad., Wasatch County, Utah. Twin Creek Limestone, Sliderock Member, basal part. 

A. A. Baker, 1938. From ledges along Lake Creek road at northeast end of main 

outcrop, about 3 miles due east of center of town of Heber and 7f mile north of 

north boundary of Strawberry Valley 30-min quad., SW7~SW7~ sec. 35, T. 3 S., 

R. 5 E., Wasatch County, Utah. Twin Creek Limestone, lower part. 

A. A. Baker, 1938. In SE7~ sec. 3, T. 4 S., R. 5 E., north of Lake Creek road, 

Strawberry Valley 30-min quad., Wasatch County, Utah. Twin Creek Limestone, lower part. 

A. A. Baker, 1938; 1,000 ft west of loc. 17983, on west end of knoll on north boundary of quad:;:angle, near center of sec. 3, T. 4 S., R. 5 E., Strawberry Valley 30-min 

quad., Wasatch County, Utah. Twin Creek Limestone, lower part. 

A. A. Baker, 1938. On top of ridge in NW% sec. 23, T. 1 S., R. 12 W., northwest 

of Right Fork of Strawberry River, Strawberry Valley 30-min quad., Wasatch 

County, Utah. Twin Creek Limestone, lower part. 

A. A. Baker, 1938. On top of ridge in NW7~ sec. 23, T. 1 S., R. 12 W., Strawberry 

Valley 30-min quad., Wasatch County, Utah. Twin Creek Limestone, lower 

part. 

A. A. Baker, 1938. An isolated outcrop on north end of ridge in SW% sec. 14, 

T. 1 S., R. 12 W., Strawberry Valley 30-min quad., Wasatch County, Utah. 

Twin Creek Limestone, lower part. 

R. W. Imlay and John Mcintyre, 1949. Northwest side of Sand Creek in northcentral part of sec. 7, T. 1 S., R. 8 W., near north section line, Duchesne County, 

Utah. Twin Creek Limestone. From 4ft of oolitic limestone about 44ft below 

top of Watton Canyon Member. 

R. W. Imlay and John Mcintyre, 1949. Northwest side of Sand Creek, in northcentral part of sec. 7, T. 1 S., R. 8 W., Duchesne County, Utah. Twin Creek 

Limestone, Watton Canyon Member, 3 ft above base. 

John Mcintyre, 1949. Southwest side of Duchesne River, in SW7~ sec. 4, T. 1 

S., R. 8 W., Duchesne County, Utah. Twin Creek Limes~one, Rich Member, 

from shaly limestone in lower 40ft of member. 

R. W. Imlay and John Mcintyre, 1949. Near top of cliff on southwest side of 

Duchesne River at same locality as 21620, Duchesne County, Utah. Twin 

Creek Limestone, Boundary Ridge Member. From brown limestone 3-4 ft 

below massive oolitic limestone at base of Watton Canyon Member. 

R. W. Imlay, 1949. Near top of ridge on southwest side of Duchesne River at same locality as 21620, Duchesne County, Utah. Twin Creek Limestone, 

float from cliff-forming limestone in upper 44 ft of Watton Canyon Member. 

E. M. Spieker and J. B. Reeside, Jr., 1935. Gap of Duchesne River about 3 miles 

north of Tabiona on road from Duchesne to Kamas, SW. cor. sec. 3, T. 1 S., 

R. 8 W., Duchesne County, Utah. Twin Creek Limestone, about 60 ft above 

base of Rich Member and 100ft above top of Nugget Sandstone. 

J. B. Reeside, 1946. North side of road in SW. cor. sec. 3, T. 1 S., R. 8 W., Duchesne 

County, Utah. Twin Creek Limestone, Rich Member, from shaly limestone 

below highest oolite unit. 

R. W. Imlay, 1963. North side of Ducheshe River in SW. cor. sec. 3, T. 1 S., R. 

8 W., Duchesne County, Utah. Twin Creek Limestone, Rich Member, near middle in thin-bedded limestone. 

R. W. Imlay, 1963. East side of Farm Creek, south-central part of SE7~ sec. 31, 

T. 1 N., R. 7 W., Duchesne County, Utah. Twin Creek Limestone, Rich Member, near middle. 

J. W. Huddle, 1948. Lake Fork, east side of canyon in sec. 3, T. 1 N., R. 5 W., 

Duchesne County, Utah. Twin Creek Limestone. Float about 50 ft above the 

base of formation on lower part of the Rich Member. 

A. A. Baker and M. N. Bramlette, 1937. East side of Red Hollow in NW7~ sec. 28, T. 8 S., R. 5 E., Strawberry Valley 30-min quad., Utah County, Utah. Twin 

CreekLimestone, Sliderock Member 50-75ft above Navajo Sandstone. 

A. A. Baker and M. N. Bramlette, 1937. East side of Monks Hollow near mouth 

in SE% sec. 32, T. 8 S., R. 5 E., Strawberry Valley 30-min quad., Utah County, 

Utah. Twin Creek Limestone, Leeds Creek Member, near base. 

A. A. Baker and M. N. Bramlette, 1937. Monks Hollow, sec. 5, T. 9 S., R. 5 E., 

Strawberry Valley 30-min quad., Utah County, Utah. Twin Creek Limestone, 

Giraffe Creek Member, about 75ft below base of Preuss Sandstone. 
74 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Descriptions of some Jurassic localities in the Twin Creek Limestone and contemporary formations in Montana, Wyoming, Utah and 

southeastern Idaho--Continued 

Locality on figs. 16-18 

95 __________ 

96 __________ 

97 __________ 

98 __________ 

99 __________ 

100 _________ 

lOL ________ 

102 _________ 

103 _________ 

04 _________ 

105 ________ _ 

USGS 

Mesozoic 

locality 

3548 

17064 

17673 

22657 

27637 

28457 

28458 

28459 

28460 

17019 

19192 

28825 

28826 

27337 

21391 

22170 

28758 

28670 

28800 

8096 

19153 

19158 

15635 

19214 

25866 

Collector's field No. 

R-5-50 ____________ 

F.19 _______________ 

AS _________________ 

--------------------

--------------------

--------------------

--------------------

--------------------

--------------------

S31F10 ____________ 

--------------------

FE27-6L __________ 

FE27a-61 __________ 

MK-58-T1-1L _____ 

8-1157 _____________ 

S-1109 _____________ 

S-1206 _____________ 

VV9-406 ____________ 

I-63-7-30A _________ 

53 _________________ 

FE-2a _____________ 

FE-1 _____________ _ 

Y-55-18 __________ _ 

Collector, year of collection, description of locality, and stratigraphic assignment 

G. B. Richardson, 1905. Thistle Junction, Utah County, Utah. Twin Creek 

Limestone. 

E. M. Spieker and J. B. Reeside, Jr., 1935. Spanish Fork Canyon opposite mouth 

of Lake Fork Canyon, not far above Thistle, Utah County, Utah. Twin Creek 

Limestone, Leeds Creek Member. 

A. A. Baker and M. N. Bramlette, 1937. Top of high cliff n?rtheast of ~histle on north side of Spanish Fork, Utah County, Ut.ah. Twm Creek Limestone, 

Sliderock Member, about 75ft above top of NavaJO Sandstone. 

Collector unknown, 1951(?). Cut south of railroad tracks. 50 yd east of automobile bridge at Thistle, Utah County, Utah. Twin Creek Limestone, from lower 

few hundred feet exposed in cut. 

J. C. Wright and R. P. Snyder, 1960. From ridge north of highway just east of 

Thistle, sec. 28, R. 9 S., R. 4 E., Utah County, Utah. Twin Creek Limestone, 

VV at ton Canyon Member. From 15ft of medium- to thin-bedded limestone about 

42-57 ft above base of member. 

R. VV. Imlay and N. F. Sohl, 1961. 2 miles southwest of Thistle, near mouth ?f 

Crab Creek Canyon, SVV% sec. 5, T. 10 S., R. 4 E., Utah County, ·u~ah. Twm 

Creek Limestone, float from top of Sliderock Member (unit 2 of Harns, 1954, p. 199). 

R. VV. Imlay and N. F. Sohl, 1961. Same location as 28457 except fossils o·btained 

from outcrop at top of the Sliderock Member. 

R. VV. Imlay and N. F. Sohl, 1961. Same data as 28457 except from middle of 

Sliderock Member. . R. VV. Imlay and N. F. Sohl, 1961. From Crab Creek, 2 miles southwest of Thistle, 

Utah County, Utah. Twin Creek Limestone, base of Watton Canyon Member 

(equals unit 7 of Harris, 1954, p. 198). 

E. M. Spieker, 1931. On left fork of Lake Fork, near Smith's Reservoir, in northwest part ofT. 11 S., R. 5 E., Utah County, Utah. Twin Creek Limestone. 

R. VV. Imlay and H. C. Yingling, 1944. North side of Swift Reservoir in Birch 

Creek, sec. 27, T. 28 N., T.10 VV., Pondera County, Mont. Sawtooth Formation, 

about 20ft above base in lower part of shale member (equals unit 6 of Cobban, 

1945, p. 1295) directly above several inches of conglomerate. · H. VV. Frebold, 1961. Swift Reservoir, NE~i sec. 27, T. 28 N., R. 10 VV., Pondera 

County, Mont. Sawtooth Formation. From conglomerate bed (equals unit 5 of 

Cobban, 1945, p. 1295) in lower part of shale member. 

H. VV. Frebold, 1961. Same location as 28825 but 14 ft higher stratigraphically 

(e_guals unit 10 of Cobban, 1945, p. 1295) in the Sawtooth Formation. 

M. E. Kaufman, 1958. From gully southeast of Tigh Creek about 3 miles west of 

Drummond, east-central part of sec. 34, T. 11 N., R. 13 W., Drummond 30-min 

quad., Granite County, Mont. Sawtooth Formation, near middle of basal unit 

in brownish-gray siltstone. 

R. VV. Swanson, 1939. Madison Range, Jack Creek at road, NE~SW~i sec. 35, 

T. 5 S., R. 1 E., Madison County, Mont. Sawtooth Formation. 

R. VV. Swanson, 1938. Madison Range, Jack Creek at road, NE~iSW% sec. 35, 

T. 5 S., R. 1 E., Madison County, Mont. Sawtooth Formation. 

R. VV. Swanson and VV. H. Bucher, 1941. Madison Range. Crest of narrow spur 

halfway up south slope of main ridge at elevation of 7,150 ft, in SE~iSW~ sec. 30, 

T. 6 S., R. 4 E., about 2 miles west of Gallatin River, Gallatin County, Mont. 

Sawtooth Formation. 

I. J. VVitkind, 1962. Near head of east fork of Red Canyon Creek, north-central 

part of NVV~i sec. 24, T. 11 S., R. 4 E., Tepee Creek 15-min quad., Gallatin 

County, Mont. Sawtooth Formation, near base. 

R. VV. Imlay and I. J. VVitkind, 1963. Same location as 28670. Sawtooth Formation, lower 20 ft. 

C. F. Bowen and Harvey Bassler, 1912. East end of Bearpaw Mountains, SVV% SVV7~ sec. 1, T. 28 N., R. 20 E., Blaine County, Mont. Piper Formation (in part). 

J. B. Reeside, Jr., VV. A. Cobban, and R. VV. Imlay, 1944. North side of Rocky 

Creek on north side of U.S. Highway 10 about 3. 7 miles southeast of site of Fort 

Ellis in sec. 19, T. 2 S., R. 7 E., float from top of unit 8 of described section (Cobban, Imlay, and Reeside, 1945, p. 452). Piper Formation, from shale and shaly limestone about 59 ft below top of formation. 

J. B. Reeside, Jr., VV. A. Cobban, and R. VV. Imlay, 1944. Same location as 19153. 

From 14~~ ft of shale and shaly limestone forming unit 6 of described section 

(Cobban, Imlay, Reeside, 1945,. p. 452). Piper Formation, about 28 ft above 

base and 97 ft below top of formation. 

D. A. Andrews, 1930. Yellowstone Canyon, 2H miles south of Livingston, Park 

County, Mont. Piper Formation, middle limestone member. 

A. C. Peale, 1872. Near lower canyon of Yellowstone River, Mont. Piper Formation, middle limestone member. 

A. E. Roberts, 1955; 1,250 ft north and 2,150 ft east of SVV. cor. sec. 35, T. 2 S., 

R. 9 E., Park County, Mont. Piper Formation, middle limestone member, 

from 59ft of silty limestone. 
SYSTEMATIC DESCRIPTIONS 75 

Descriptions of some Jurassic localities in the Twin Creek Limestone and contemporary formations in Montana, Wyoming, Utah and 

southeastern I daho-C ontin ued 

Locality on figs. USGS 

16--18 Mesozoic Collector's field No. 

locality 

105 ________ _ 27628 

106 ________ _ 19630 I-45-7-5B _________ _ 

27624 --------------------

107 _________ 

108 _________ 

109 _________ 

no _________ 

11L _______ _ 

27035 

5738 

19621 

17662 

20864 

17096 

17105 

20962 

20967 

22112 

27724 

R-58-3 ____________ 

C23 ________________ 

I-45-7-6A __________ 

F-1 ________________ 

--------------------

A182 _______________ 

A184 ______________ _ 

I-47-8-28A ________ _ 

I-47-8-28B ________ _ 

K8-F 1 ____________ _ 

Collector, year of collection, description of locality, and stratigraphic assignment 

N. F. Sohl and R. W. Imlay, 1960. On west side of mouth of Canyon of Yellowstone River 27~ miles southwest of Livingston in sees. 26 and 35, T. 2 S., R. 9 E., 

Park County, Mont. Piper Formation, middle limestone member, from middle 

of 7 ft of light-gray massive limestone whose base is 136 ft above the Quadrant 

Quartzite. Collections from locs. 15625, 19214, and 25966 are either from this 

unit or from overlying 14ft of limestone. 

L. S. Gardner, C. P. Rogers, William Saalfrank, and R. W. Imlay, 1945. West 

side of Boulder River about 20 miles south of Big Timber in sec. 23, T. 3 S., 

R. 12 E., Sweet Grass County, Mont. Piper Formation, middle limestone 

member. From 6 to 9 ft of shaly limestone whose base is 23 ft above the top 

of the lower red bed. 

N. F. Sohl, Grant Bartlett, Stanley Hashimoto, and R. W. Imlay, 1960. West 

side of Boulder River in sec. 23, T. 3 S., R. 12 E., Sweet Grass County, Mont. 

Piper Formation, middle limestone member. From 9 ft of dark-gray thinbedded limestone whose base is 52 ft above the top of the lower red-bed mem- ber, 165ft below top of formation, and 20ft above loc. 19630. 

A. E. Roberts and R. W. Imlay., 1958. About % mile west of Yellowstone River 

in NE,%SE,% sec. 7, T. 4 S., R. 9 E., Park County, Mont. Piper formation, 

middle limestone member, lower part. 

W. R. Calvert, 1908; 7~ mile northwest of Electric, Park County, Mont. Piper 

Formation. 

R. W. Imlay and William Saalfrank, 1945. Cinnabar Mountain, 6 miles northwest of Gardner, sec. 36, T. 8 N., R. 7 E., Park County, Mont. Piper Formation, from 11 ft of dark shaly limestone, 54 ft below main diabase sill near middle 

of formation. 

D. A. Andrews, 1937. West side of Pryor Mountains, south side of Red Dome, NW7~ sec. 28, T. 7 S., R. 24 E., Carbon County, Mont. Piper Formation, from 

upper 11ft of middle limestone member just below upper red-bed member. 

D. A. Andrews, 1937(?). Southeast part of Red Dome in NW% sec. 27, T. 7 S. 

R. 24 E., Carbon County, Mont. Piper Formation, middle limestone member, 

6ft above coral bed and just below base of upper red-bed member. 

D. A. Aildrews, 1935; 6 miles south of Cody, NE. cor. sec. 6, T. 51 N., R. 101 W., 

Park County, Wyo. Gypsum Spring Formation, from upper part of coquinitic 

limestone about 76 ft above a massive gypsum unit and 120ft below top of highest 

red bed at top of formation. 

D. A. Andrews, 1935; 6 miles south of Cody in NE. cor. sec. 6, along line of sec. 5, 

T. 51 N., R. 101 W., Park County, Wyo. Gypsum Spring Formation, from 

limestone below upper red-bed member and probably from same unit as Mesozoic 

loc. 17096. 

R. W. Imlay, 1947. North side of Lower Slide Lake on Gros Ventre River, sec. 4, T. 42 N., R. 114 W., Teton County, Wyo. "Lower Sundance" Formation, 20ft 

above base of unit 10 of published section (Imlay, 1956a, p. 70). 

R. W. Imlay, 1947. Same location as 20962. From 12 to 13ft above base of unit 

10. 

W. P. Keefer and L. L. Garrow, 1948. North side of Lower Slide Lake, N%NW% sec. 3, T. 42 N., R. 114 W., Teton County, Wyo. "Lower Sundance" Formation, 

from 20 to 30 ft above base. 

N: F. Sohl, R. W. Imlay, and Grant Bartlett, 1960. Same location as 20962. From 

10 to 12 ft above base of unit 10. 

SYSTE~TIC DESCRIPTIONS 

Class PELECYPODA 

The species is small, inflated, subrhomboidal in outline, and obliquely elongated. The hinge line is less 

than the greatest length of the valves. The umbones 

are prmninent, incurved, and a little anterior to the 

center of the hinge. The postumbonal slope is subangular to cM'inate and delimits a concave posterior area. 

The ornamentation consists of fine, closely spaced, fairly 

regular concentric ribs, concentric undulations, and very 

fine radial striae. The striae on the anterior part of 

the shell are widely spaced, are curved anteriorly, and 

generally extend to the ventral margin. The striae on 

the remainder of the shell are closely spaced and on most 

specimens are confined to the umbones. On a few specimens the striae extend below the 1niddle of the shell. 

Genus GRAMMATODON Meek and Hayden, 1860 

Grammatodon haguei (Meek) 

Plate 1, figures 2-6 

Oucullaea haguei Meek, 1877, U.S. Geol. E'xplor. 40th Parallel 

(King), v. 4, pt. 1, p. 134, 135, pl. 12, figs. 1, la, lb. 

Oucullaea haguei Meek. Stanton, 1899, U.S. Geol. Survey Mon. 

32, pt. 2, p. 618, pl. 73, fig. 1. 

Oucullaca hagttei Meek. Veatch, 1907, U.S. Geol. Survey Prof. 

Paper 56, pl. 6, fig. 10. 

Oucullaea haguei Meek. Butler, 1920, U.S. Geol. Survey Prof. 

Paper 111, pl. 10, figs. 12, 13. 

This species is represented by 34 specimens frmn the 

Twin Creek Limestone. 
76 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Narrow parallel, nearly horizontal teeth are present at 

both ends of the hinge plate. On several1nolds there are 

two teeth at the anterior end and four teeth at the posterior end. There is no evidence of short, transverse 

teeth in the center of the hinge prate, such as occur in 

the genus o~wullaea. The cardinal area, poorly exposed 

on one internalinold, is long and fairly narrow. LigaInental grooves are not preserved. The molds show no 

trace of a posterior 'adduction septum. 

This species is assigned to Granwnatodon ra.ther than 

Parallelodon because of the Inesial position of its mnbones, the presence of a postumbonal ridge, and the lack 

of a sinus on the ventral nrargin. Its general appearance is similar to that of a small O?lcullaea, but the characters of its hinge plate and its narrow cardinal area 

favor an assignment to Gram/)Jwtodon. The presence 

of nearly horizontal teeth at the anterior end of the 

hinge line does not bar such ·an assignment (Cox, 1940, 

p. 40; Arkell, 1930, pl. 15, figs. la, 7b, 8b) . Furthermore, as noted by Meek ( 1877, p. 135) , G. haguei closely 

resen1bles species similar to Oucul1aea concinna Phillips, 

which Arkell (1930, p. 303-306, pl. 15) and Cox (1940, 

p. 41-43) place in Grmnnwtodon. In particular it resembles G. bathonicus Cox and Arkell ( 1948, p. 2; 

Morris and Lycett, 1853, p. 50, pl. 5, fig. 7) from the 

Great Oolite of England, but is distinguished by the 

presence of radiating striae on the umbonal region. In 

shape and orna1nentation G. haguei is more similar to 

G. jur£anu8 Cox (19·40, p. 43, pl. 2, figs. 15-20) from 

India but has stronger radial striae. G. concinnus Phillips (Arkell, 1930, p. 481, pl. 15, figs. 2-4) frmn the Oxford Clay of England appears to lack radial striae on 

its un1bonal region. 

The only other described species of Gra'l7Wnatodon 

from the western interior region is G. inornatus Meek 

and Hayden (1858, p. 51; 1865, p. 90, pl. 8, figs. 9, 9a, 

9b) which is distinguished from G. hagu.ei (Meek) by 

having a nearly smooth surface and a much less prominent postumbonal ridge. G. inornalu.rs occurs only in 

stratigraphi~ally higher beds of early Oxfordian age, 

such as the Redwater Shale Member of the Sundance 

Formation. It was erroneously recorded by ArkelJ 

(1930, p. 341) as of Early Jurassic age. 

Among Canadian species, one of the n1ost similar is 

Granunatodon sonninianus Crickm'ay (1930, p. 47, pl. 

3, figs. h, i) from beds of middle Bajocian age in British 

Colun1bia. It differs from G. haguei (Meek) by having 

weaker less extensive radial striae and probably a more 

rounded postumbonal ridge. The associated G. sendornat~ts Crickmay (1930, p. 46, pl. 3, fig. g) has more 

closely spaced radial striae on its anterior surface and 

none on its mnbo. 

Types: Holotype USNM 12546 (apparently lost); plesiotypes 

USNM 28955,132717-132720. 

Occ11rrences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 3218, 3219, 21632, 28459, 28483, and 28645 ; 

probably from the Sliderock Member at Mesozoic locs. 3548 and 

12019; Rich Member at Mesozoic locs. 20963, 20965, 20987, 21620, 

28785, and 28788 ; Sliderock or Rich Member at Mesozoic loc. 

17977. The holotype of the species was collected in Weber Canyon, Wasatch Range, Utah, presumably from the Twin Creek 

Limestone exposed near Devils Slide. 

Grammatodon ltaguei (Meek) has been :f:iound also, at Lower 

Slide Lake, Teton County, ·wyo., in units 10, 19, and 22 of the 

described section (Imlay, 1956a, p. 70). These units correlate 

with the Rich, 'Vrutton Canyon, and Leeds Creek Members of 

the Twin Creek Limestone. In Montana the species occum in 

the Piper Formation south of Big Timber and Livingston, Mont., 

and in the Sawtooth :mormation of western Montana. It has not 

been found in the Piper Forma.Jtion in the Pryor Mountains, 

Mont., or in the Gypsum Spring and ''Dower Sundance" Formations of· north-central Wyoming. It occurs rarely in the Rierdon Formation in Montana, but most of the specimens of Gram-

'inatodon from that formation belong to a larger and smoother 

species. 

Genus IDONEARCA Conrad, 1862 

Idonearca haguei (Stanton) 

Plate 1, figures 11-13, 15 

Oypricardia? haguei Stanton, 1899, U.S. Geol. Survey Mon. 32, 

pt. 2, p. 623, pl. 73, figs. 11-13. 

?Oucullaea livingstonensis McLearn, 1924, Royal Soc. Canada 

Proc. and Trans., 3d ser., v. 18, sec. 4, p. 39, 40, pl. 2, fig. 7. 

This species is represented by six specimens from the 

Twin Creek Limestone. Of these, five are from the 

upper part of the Sliderock Member and one is from the 

middle of the Rich Member. The generic identification 

was questioned by Stanton (189·9, p. 623) because all the 

specimens available to him were internal molds that did 

not show the ornamentation or the hinge. He compared 

the species on the basis of form with Oyprioardia bathonioa d'Orbigny of Morris and Lycett ( 1853, p. 75, pl. 

7, figs. 8, Sa-c) which is the genotype of Pseudo trapezium (Fisher 1887, p. 1075; Cox 1947, p. 144,145, 175). 

Most of the specimens now available are likewise internal molds, but one specimen from the Twin Creek 

Limestone (USGS Mesozoic loc. 28792) retains some 

shell material that bears both concentric marks and radial striae, such as occur on Ououllaea and I donearoa 

but not on Pseudotrapez·iun&. Another specimen from 

the Piper Formation of Montana (USGS Mesozoic loc. 

19613) shows that the hinge bears long, horizontal 

lateral teeth as in I donearoa. 

The specimen illustrated by Stanton (1899, pl. 73, figs. 

11, 12) is herein designated lectotype of I donearoa 

hague·i (Stanton). 

Types: Lectotype USNM 132721; syntype USNM 28941; 

plesiot.ype USNM 132722. 

Occun·ences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 20344, 20345, and 28645 ; Rich Member at 

Mesozoic loc. 28792. 
SYSTEMATIC DESCRIPTIONS 77 

The species occurs also in ·the ''Lower Sundance" Formation 

at Lower Slide Lake, Teton County, Wyo., in unit 10 (Imlay, 

1956a, p. 70) which is correlated with the lower part of the Rich 

Member of the Twin Creek Limestone. It occurs in the upper 

siltstone member of the Sawtooth Formation in northwestern 

Montana (Imlay, 1962a, p. 021), at several places in the Piper 

Formation in south-central Montana, and in the Sawtooth Formation in southwestern Montana. On Cinnabar Mountain, 

Mont., just north of Yellowstone National Park, the species 

occurs (USGS Mesozoic loc. 19618) from 24 to 26 feet above the 

base of the Rierdon Formation in association with the ammonite 

Wan·enoccms and about 30 feet below the lowest occurrence of 

Gryphaea nebrasccnsis Meek and Hayden. On Sykes Mountain 

in the Bighorn Basin, Wyo., the species occurs from 8 to 12 feet 

above the base of the "Lower Sundance" Formation, a few feet 

below the lowest occurrence of Gryphaea nebmscensis Meek and 

Hayden (Imlay, 1956b, p. 591). 

Genus MODIOLUS Lamarck, 1799 

Modiolus subimbricatus (Meek) 

For synonymy, characterization, comparisons, and illustrations see Imlay, 1964a, p. C23, pl. 1, figs. 16-20. 

Sixteen specimens of this species have been obtained 

fro1n the Twin Creek Limestone. At lower Slide Lake, 

Teton County, Wyo., the species ranges from unit 10 

to unit 22 of the "Lower Sundance" Formation (Imlay, 

1956a, p. 70). In unit 10 it is associated with ammonites of Bajocian age. In units 19-22 it is associated with ammonites of early Callovian age. 

Occurrences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 20345, 28645, and 28794; Rich Member at 

Mesozoic loc. 17297, 28639; Watton Canyon Member at Mesozoic 

loc. 21642 ; Leeds Creek Member at Mesosoic loc. 17064. 

Subgenus MUSCULUS Bolten, 1798 

Modiolus (Musculus) sp. 

Plate 1, figures 14, 19 

One laterally crushed specimen from the Twin Creek 

Limestone is assigned to the subgenus lll1.b8culu.s because of the presence of fine radiating striae on the 

posterodorsal and posterior parts of the shell. It differs 

from Modio"Vus jor1nosu.s (Meek and Hayden) (1865, p. 

86, 87, figs. A and B on p. 87; Whitfield 1880, p. 361, 

pl. 5, fig. 15) from the Upper Jurassic of the Bighorn 

Mountains, Wyo., by having n1uch finer radiating striae, 

coarser more irregular concentric ribs, and apparently 

a less elongate form. It differs from 111_. ( M'usculus) 

p1.tlchra Phillips (Arkell, 1929, p. 58, pl. 3, fig. 1-3) 

from Callovian and Oxfordian beds in Europe by having finer radial striae that do not form a reticulate pattern with the concentric ribs. 

Figt!4red specimen: USN)1132723. 

Ocettrrence: Twin Creek Limestone, Rich Member at USGS 

Mesozoic loc. 28788. 

Genus MYTILUS Linne, 1758 

Mytilus cf. M. whitei Whitfield 

The genus jjf ytilus is represented in the Twin Creek 

Limestone by three smooth internal molds. Their 

s1noothness contrasts with the rugose shells of M. 1JJhitei 

Whitfield (1877, p. 18, 1880, p. 360, pl. 5, figs. 9-12) 

fron1 the Jurassic beds of the Black Hills, but resembles 

that of specimens of M ytilus from the Carmel Formation of southern Utah (Imlay, 1964a., p. C24, pl. 1, figs. 

4-6). 

Occurrence: Twin Creek Limestone, lower part, at USGS 

Mesozoic loc. 3392. 

Genus G ER VIL.LIA Defrance, 1820 

Gervillia t montanaensis Meek 

Plate 1, figures 1, 7-10 

Gervillia montanaen8is Meek, 1873, U.S. Geol. Survey Terr. 

(Hayden), 6th Ann. Rept., 1872, p. 472. 

Gervillia sparsilirata Whitfield, 1876, in Ludlow, 'Villiam, Report of a reconnaissance from Carroll, Montana Territory, * * * to the Yellowstone National Park, * * * p. 

142, pl. 2, fig. 8. 

Gervillia montanaensis Meek. White, 1880, U.S. Geol. Survey 

Terr. (Hayden) 12th Ann. Rept., 1878, p. 145, pl. 37, 

figs. la,b. 

Gervillia montanaensis Meek. Stanton, 1899, U.S. Geol. Survey 

Mon.32,pt.2,p.617. 

Gervilli.a rnontanaensis Meek. Imlay, 1964a, U.S. Geol. Survey 

Prof. Paper 483-C, p. 024, pl. 1, fig. 21. 

This species is represented in the Twin Creek Limestone by three specimens. 

The original description by Meek (1873, p. 472) is 

as follows: 

A medium-sized, very oblique species, with posterior ear flattened and of moderate size, angular ·at the extremity, and 

equaling on the hinge line about half the length of the valves ; 

body portion of the valves rather slender, nearly straight, or a 

little arched, ranging at an angle of 28° to 30° below the hinge 

line, in the left valve convex, in the right flattened, or less 

convex than the other. Surface of both valves marked by fine 

concentric striae, and a few stronger furrows of growth, crossed 

on the body part of the left valve, by a few slender radiating 

costae, separated by wider spaces. 

The original collection studied by Meek was collected 

by A. C. Peale from the west side of the Lower Canyon 

of the Yellowstone River about 2 miles south of Livingston, Mont. The type lot contains 11 specimens. 

Of these, two were illustrated by White (1880, p. 145, 

pl. 37, figs. 1a,b) who described the species in detail. 

The specimen illustrated by White on his plate 37, figure 1b is herein designated the lectotype of Gervillia.C? 

nwntanaernsi.s Meek and is reillustrated. 

Gervillia sparsilirata Whitfield ( 1876, p. 142, pl. 2, 

fig. 8) was obtained fron1 60 feet of blue compact limestone exposed near the south end of the east side of the 
78 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Bridger Range (Dana and Grim1ell, 1876, p. 118-121). 

Inasmuch as the species was associated with Gryphaea 

planoconvexa Whitfield in compact limestone, it must 

have been obtained from the middle limestone member 

of the Piper Formation, which is well exposed at the 

south end of the range. 

The holotype of Ger·villia sparsilirata Whitfield is a 

small fragment of a left valve that shows parts of the 

body of the shell and of the posterior ear. Whitfield 

considered that the species differed from G. montanaensis Meek by having a larger hinge and a s1naller angle 

between the body of the shell and the hinge line. These 

distinctions are clearly invalid, however, because the 

hinge line of the holotype is not well preserved and the 

parts of the shell that are preserved agree very well in 

shape and ornamentation with small specimens in the 

type lot of G.? montanaensis Meek, as illustrated herein. 

Ger·villia? 1nontanaensis Meek is readily distinguished 

fron1 other described species of Gervillia from the 

Jurassic of the western interior of North America by the 

presence of radi,al riblets that are generally conspicuous 

on the lefit valves and weak to faint on the right valves. 

This species was assigned by Hayami (1961, p. 315) tto 

Bakevellia, presumably because of its Pteria-like shape. 

Until the hinge characters of the species are known, 

however, its generic position is questionable. 

Types: Lectotype, USNM 132724; syntypes, USNM 7795, 

132836-132838. 

Occurrences: Twin Oreek Linws,bone, upper part of Rich Member a:t USGS Mesozoic loos. 20980 ·and 28793 ; Slide:r:ock or Rich 

Member a:t Mesozo:i!c loc. 17977. 

In the Sundance F1ormation at Lower Slide Lake, Teton 

Oounty, Wyo., the .species occurs in un:it 10 (Imlay, 1956a, p. 70) 

which is correlalted with the lower pavt of the Rich Member. 

In southern Montana the 'Species is fairly eommon in :the limestone member of the Piper :F'ormation. It ha'S not been found in 

the "Lower Sundance" Formation in north--c·entral Wy'oming or 

in the Rierdon F·ormation in Montana. It appears to be restricted to beds of Bajoc'ian age. 

Genus ISOGNOMON Solander, 1786 

Isognomon cf. I. perplana (Whitfield) 

M yalina ( Gervillia) perptana Whitfield, 1876, in Ludlow, William, Repo·I'It ;of a reconnaisance from Carroll, Montana 

Territory, * * * ~to the Yellow:stone Nat:ional ~ark * * * 

p. 143, pl. 1, fig. 8. ' 

The genus lsognomon is represented in the Twin 

Creek Limestone by five specimens of w hieh one was 

erroneously assigned hy White (1880, p. 147, pl. 37, fig. 

9a) tJo Mytllus tohitei Whitfield. All the specimens are 

poorly preserved bu't are si1nilar in shape and ornamenta1Jion to lsognmnon perplana (Whitfield), which species has been described and illustrated by Imlay (1964a, 

p. 024, pl. 1, figs. 3'2-34). 

The holotype of Isognmnon perplana (Whitfield) is 

from the limestone ·member of the Piper Form·ation exposed near the south end of the east side of the Bridger 

Range, northeast of Bozeman, Mon:t. The species is 

moderately common in the same member in southern 

Montana and in equivalent beds in northwestern Wyoming. It occurs also in the lower part of the "Lower 

Sundance" Forma·bion in the Bighorn Basin (USGS 

Mesozoic locs. 17670, 19381). 

Occurrences: Twin Creek Limestone, Rich Member at USGS 

M·esozoic loes. 21629, 28639, and 28790; undetermined position 

at Mesozoic loc. 17019; the 'Specimen illustrated by W'htte its 

labeied "near Fontenelle Canyon, west side Green River Basin, 

Wyoming." Presumably it was obttained near Fontenelle Gap 

north of Kemmerer, Wyo. 

Genus PINNA Linne, 1758 

Pinna kingi Meek 

Plate 1, figures 20-22 

For synonymy see Imlay, 1964'a, p. C25. 

'This species is represented by 28 specimens from the 

Twin Creek Limestone. All are fr01n the Sliderock ·and 

Rich 1M embers. At Lower Slide Lake, 'Teton County, 

Wyo., the species ranges from units 10 to 19 of the described section (Imlay, 1956,a, p. 70). 'These units are 

correlated with :the Rich, Boundary Ridge, and Watton 

Canyon Members of the Twin Creek Limestone. In 

Montana, Pinna kingi Meek has been recorded from the 

upper siltstone member of the Sawtooth Formation 

(Imlay, 1962a, p. C21) and from 'the middle limestone 

1nember of the Piper Formation (Imlay, 1956b, p. 576). 

It is uncommon in the Rierdon Formation in Montana 

and in the "Lower 'Sundance" Formation in northcentral Wyoming. 

Occurrences: Twin Oreek Limestone, S'liderock Member aot 

USGS Mesozoi•c l:ocs. 3219, 3391, 6364, 17964, 20345, 20977, 20984, 

28459, 28483, 28645, and 28648 ; Rich Member a.t Mesozoic loos. 

17297, 20963, 20965, 28785, 28788, :and 28793 ; Sliderock or Rich 

Member at Mesozoic l:oc. 17977; member undetermined !at Mesozoic loos. 3220 and 6948. 

Genus CAMPTONECTES Meek, 1864 

Camptonectes stygius White 

For synonymy, description, and ·illustrations see Iml•ay, 1964a, 

p. 025, pl 2, figs. 1-10. 

This species is represented in the Twin Creek Limestone by 41 specimens; it ranges from the upper part of 

the Slideroek Member to as high ·as the 1niddle of the 

Leeds Cr,eek Member. 

Occurrences: Twin Creek Lirm·estone, SLiderock Membe·r at 

USGS Mesozoic loc. 20345; Rich Member at Mesoz;oic loc. 3821; 

Wa1tton Canyon Mem!ber at Mesozoic Iocs. 17299, 21624, and 

28460; Leeds Creek Mernber at Mesozoic locs. 20348 and 28643 ; 

from undetermined member ·at Mesozoic loos. 3548 aud 4 798. 
SYSTEMATIC DESCRIPTIONS 79 

Camptonectes platessiformis White 

Pla1te 2, figures 1, 2 

For synoynmy, descriptions, and illustrati·on:s •see Imlay, 1964a, 

p. C26, pl. 2, figs. 11-14. 

'This species is represented in the Twin Creek Limestone iby 32 specimens, of which nl'Ost are frmn the Sliderock and Rich ·Members. There is one specimen from 

the Boundary Ridge Member and one frmn near the top 

of the Leeds Creek Member. Elsewhere in the western 

interior region the same species occurs in the Piper, Sawtooth, and Rierdon Formations in Montana, in the 

"Lower Sundance" Forrna'tion in Wyoming, and in the 

lower limy unit of the Carmel Formrution in Ut:ah. 

Types: Holotype, USNM 8179; plesiotypes, USNlVI 131939, 

131940, 132725. 

Occurrences: Twin Oreek Limestone, SHderock l\Iember tat 

USGS Mesozoic locs. 16016, 17674, 17985, 20345, 20972, 21628, 

28459; 28506,"'28795; Rich Member at Mesozoic locs. 17297, 21468, 

21629; Sliderock or Ri~h Member at :\Iesozoic loc. 17977 ; Boundary Ridge Member at Meso~oic loc. 20990 ; Leed•s Creek Member 

at Mes'OzQiic loc. 17892; lower part of formation at Mes·o~oic locs. 

6373, '17983, 17984; undetermined position att M•esozoic loc. 3817. 

Genus PLICATULA Lamarck, 1801 

Plicatula sp. 

Plate 1, figure 16 

The genus Plicatula is represented by 16 specimens 

from the Twin Creek Limestone. These show considerable variation in fineness of ribbing and spinosity similar to that shown in speci1nens from the Carmel Formation of Utah (!Inlay, 1964a, p. C26, pl. 1, figs. 13-15). 

Figucd specimen: USNM 132726. 

Occut·rences: Twin Creek Limes1tone, Sliderock Member at 

USGS Mesozoic locs. 17674, 17985, and 20345; Rich Member 'at 

Mesozoic locs. 21620, 28785, 28786, and 28788; Leeds Creek Member at Mesozoic locs. 3830 and 20983. 

Genus LIMA Bruguiere, 1797 

Subgen-us PLAGIOS.TOM:A, J. Sowerby, 1814 

Lima (Plagiostoma) occidentalis Hall and Whitfield 

For synonymy, description, 'and illustrations see Imlay, 1964a, 

p. 027, pl. 2, figs. 17-20. 

This species is represented by 13 specimens from the 

Twin Creek Limestone. These are mostly frmn the 

Watton Canyon Mmnber, but two are probably frmn the 

Sliderock Member. The species at Lower Slide Lake, 

Teton County, Wyo., lras been found in unit 17 (Imlay, 

1956a, p. 70) . In Montana the species occurs in the middle limestone Inember of the Piper Forn1ation (Imlay, 

1956b, p. 576, 577) and rarely in the lmver part of the 

Rierdon Formation. 

Occurrences: Twin Creek Limestone, Watton Canyon Member 

at Mesozoic locs. 20992 and 21623 ; lower part of Twin Creek 

Limestone at Mesozoic locs. 3800, 7438, and 17982. 

Genus CTENOSTREON Eichwald, 1862 

Ctenostreon cf. C. gikshanensis McLearn 

Plate 2, figures 4, 12,13 

OtenoBtreon is represented in the Twin Creek Limestone by 10 fragmentary specimens. These resemble 0. 

gikBhanenBis McLearn (1926, p. 91, pl. 19, figs. 3, 4) 

from the Bajocian of British Columbia in shape and 

in the presence of six to eight radiating ribs. They 

differ, however, by having appreciably wider ribs on 

their right valves. Wherever the shell is preserved, the 

ribs are about two-thirds as broad as the interspaces. 

This feature plus the snrall number of 1najor ribs distinguishes the American species from the common European Jurassic species of OtenoBtreon (Morris and Lycett, 

1853, pl. 6, fig. 9; Lycett, 1863, pl. 39, fig. 1; Cox, 1930, 

p. 302; Arkell, 1932, p.144-148, pl.15, fig. 3). 

The genus is not common in the western interior 

region but has been found in beds of late Bajocian to 

early Callovian age. Besides the occurrences in the 

Twin Creek Limestone, it has been found 12 feet above 

the base of the "Lower Sundance" Formation in the 

Bighorn Basin, "\Vyo. (USGS Mesozoic loc. 17651), 25 

feet above the base of the Rierdon Formation in the 

Pryor Mountains, Mont. (USGS Mesozoic loc. 21636), 

above the 1niddle of the limestone member of the Piper 

Formation near Livingston, Mont. (USGS Mesozoic 

loc. 19622), and at the top of the limestone member of 

the Piper Formation on Belt Creek, Mont~ (USGS 

Mesozoic loc. 19159). All the specimens of OtenoBtreon 

at these localities appear to belong to the same species 

as the specimens from the Twin Creek Limestone, but 

most of the specimens are so poorly preserved that specific identification is uncertain. 

Fig?tred specimen: USNM 132727. 

Occurrences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 20345, 20971, 20972, 20984, 22017, 28459, 

2848±, and 28648; probably from Sliderock Member at Mesozoic 

loc. 7430; 'Vatton Canyon Member at Mesozoic loc. 20992; upper 

part of Leeds Creek Member at Mesozoic loc. 17892. 

Genus OSTREA Linne, 1758 

Subgenus LIOSTREA Douville, 1904 

Ostrea (Liostrea) strigilecula White 

For synonymy, descriptions, and illustration see Imlay, 

l964a, p. C27, pl. 1, figs. 26-29. 

About a hundred small nondescript oysters have been 

collected at about 30 localities from the Twin Creek 

Limestone throughout its area of distribution (tables 

:3-5 and 8-10). Most of the specimens are from the 

Sliderock, Rich, and Watton Canyon Members or from 

uncertain stratigraphic positions in the lower part of 

the fonnation. A few are frmn the Leeds Creek and 

Giraffe Creek Members. More than half of the specimens are f;rom north-central Utah. This distribution 
80 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

suggests that bottom conditions were most favorable 

for the growth of oysters prior to the. deposition of the 

Leeds Creek Member and were more favorable in Utah 

than farther north. 

Genus GRYPHAEA Lamarck, 1801 

Gryphaea planoconvexa Whitfield 

Plate 3, figures 1-13 

Gryphaca planoconrcJXt 'Vhitfield, 1876, in Ludlow, 'Villiam, 

Report of a reconnaissance from Carroll, Montana Territory, * •:• •:• to the Yellowstone Nationa'l Park, * •:• * p. 

142,pl. 1,figs. 9,10,pl.2,figs.9,10. 

Gryphaca tJlanoconrcJ'a 'Vhitfield. Stanton, 1899, U.S. Geol. 

Survey l\Ion. 32, pt. 2, p. 611, pl. 72, figs. 9, 10. 

Gryplwcct tJlanoconrc.rct 'Vhitfield. Imlay, 1948, U.S. Geol. Survey Prof. Paper 214-B, p. 17, 18, pl. 5, fig. 6. 

This species is of medium size for the genus. The 

right valve is subcircular in outline, concave, and 

marked only by coneentric grmvth lines. The left valve 

is subeircular, longer than high, and depressed convex 

to moderately eon vex~ it bears eoneentric growth lines, 

and its posterior side on many speeimens bears a weak 

furrmv that separates a triangular, winglike lobe from 

the remainder of the shell. The umbo of the left valve 

is narrmv to pinehed, lmv to moderately prominent, 

gently ineurvecl, and is not defleeted posteriorly. The 

beak is generally pointed, but on some specimens is terminated by an attaelnne.nt scar. 

The speeies, as typieally developed, is readily distinguished frmn any other species of Gryphaea in the 

\Vestern interior region by its subcireular outline and 

its narrow, moderately in curved umbo. Some of the 

more inflated variants of G. planoconvexa vVh:itfield, 

however, are similar to the le.ss inflated variants of 

Gryphaea, planoconvexa fraterna Imlay, n. subsp., that 

oecur in the Slideroek and Rich Members of the Twin 

Creek Limestone. The inflated variants of G. planocon-

·veJJa generally ean be distinguished by their relatively 

greater length, a lack of radial ribs, and a less strongly 

incurved umbo that is not deflected posteriorly. 

The speeies is widespread in the lo·wer part of the 

Sa\vtooth Formation in southwestern Montana and in 

the middle limestone member of the Piper Formation 

in southern and central Montana (Imlay, 1948, p. 18) 

in association \vith the ammonites herein deseribed as 

Sohlites, n. gen., and Pm'achondroce1·as, n. gen. It has 

been found, also, in the subsurface of north-central 

Montana in the Texas Co. Bowdoin Unit 1 (822) in 

the center SW-%BW'% sec. 8, T. 32 N., R. 32 E., Phillips 

County, at the depth of 2,896-2,906 feet. 

The holotype and paratype specimens of Gryphaea 

planocmwexa Whitfield \vere colleeted from about 60 

feet of "firm, blue, compact limestone" exposed on the 

east side of the Bridger Range near its south end northeast of Bozen1an, Mont. (Dana and Grinnell, 1876, p. 

119-121). This lin1estone is now known as the rniddle 

limestone member of the Piper Formation. 

The species has not been found in the Twin Cre~k 

Limestone in Utah or Wyoming. It has been found rn 

Wyoming, however, in the lower 20 feet of unit .10 of 

the Sundance Fonnation exposed on the north srde of 

Lower Slide Lake, Teton County (Imlay, 1956a, p. 70) · 

This unit is correlated with the Rich Member of the 

Twin Creek Limestone on the basis of lithologic similarity, stratigraphic position, and the occurrence in 

comn1on of the amn1onite Parachondroceras as well as 

the typieal form of G1·yphaea planocornvexa Whitfield. 

Types: Holotype, Yale University 6774; paratype, Yale University 6778 ; plesiotypes, USNM 12369, 28931, 104138, 132728-

132730. 

OcclttTences : Twin Creek Limestone, middle and upper parts 

of the Rich Member at USGS -Mesozoic locs. 28507, 28585, and 

28796. These localities are all in southeastern Idaho toward 

the northern part of the Twin Creek Limestone outcrops. 

Gryphaea planoconvexa fraterna Imlay, n. subsp. 

Plate 4, figures 1-17 

This subspecies is represented in collections from the 

Twin Creek Limestone by about 430 specimens; it ranges 

from a little below ~the middle of the Sliderock Member 

to near the 1niddle of the Rich Member. ~lost of the 

specimens are from the upper part of the Sliderock 

Member. 

The subspecies typically differs from Gryphaea planoconvexa Whitfield (1876, p. 142, pl. 2, figs. 9, 10), which 

succeeds it stratigraphically, by having a much more 

convex left valve, a subovate 1to subtrigonal instead of a 

subcircular outline, a higher more strongly incurved 

umbo, and a posteriorly direeted beak, and by weak 

radial riblets on smne specimens. 

The lowest known stratigraphic occurrence of the subspecies is on Bireh Creek about 9 miles west of Woodruff, Utah, at USGS Mesozoic locality 28647, which is 

about the top of the lower two-fifths of the Sliderock 

Member. On specimens from this locality (pl. 4, figs. 

1-3 6 10 13) the right valve is concave and subovate in 

outline. ' ' ' The left valve is 1noderate in size for the 

genus, is strongly convex, subtrigonal in outline, and 

higher than long. Its basal1nargin is extended posteriorly, or anteriorly, or in both directions. Its umbo is 

strongly incurved. Hs beak is slender and deflected 

strongly posteriorly. Its posterior side bears a sulcus 

that varies in strength from faint to deeply impressed. 

Its surface is marked mainly by weak growth lamellae, 

but a few speeimens bear faint radial riblets on the beak. 

Most specimens of the subspeeies from the upper part 

of the Slideroek Member and from the lower part of 

the Rich Mmnber are identieal in appearance with those 
SYSTEMATIC DESCRIPTIONS 81 

just described except for averaging larger in size (pl. 

4 fiO's 7 9 12 14 15-17). They are associated, how- ' b. ' ' ' ' 

ever, and are connected gradationally with other specimens ·whose left valves are much less convex, are subovate instead of subtrigonal in outline, are about as long 

as hiO'h 'and have less strongly incurved mnbones (pl. b ' • 4, figs. 8, 11). In addition, about one-third of the specimens, regardless of their shape, bear weak radial riblets 

on the umbonal part of the shell (pl. 4, figs. 4, 5, 8, 11, 

16) and broad ribs and furrows toward the ventral margin (pl. 4, figs. 4, 5, 11, 16). 

The subspecies fraterna bears resemblances to several 

species of Gryphaea in the 'vest ern interior of North 

America. Specimens that are highly convex, are subtrigmral in outline, and bear radial riblets are similar 

in appearance to Gryplwea cadominen8is Warren (1932, 

p. 17, pl. 3, figs. 1--4) from beds of middle Bajocian age 

in Alberta. They differ by having much weaker radirt1 

ribs and by lacking broad radiating furrmvs on the postmnbonal slope. Such ribbed specimens also resemble G. 

hnpres8imarginata McLean1 (1924, p. 44, pl.-±, figs. 1-5; 

!Inlay 1948, p. 18, pl. 5, figs. 9-11) frmn beds of Bathonian to Callovian age in Montana and Albert'a, but 

differ by having a less robust shape, a narrower umbo, 

and much weaker ribbing. 

Specimens of the subspecies that are Inoderately convex, subovate in outline, and lack distinct radial ribs 

or furrows greatly resemble the more convex variants 

of G'ryphaea planoconvexa Whitfield, as discussed under 

the description of that species. For example, the speciInen shown on plate 4, figure 11, differs from the convex 

variant of G. planocon1'exa vVhitfield only by having 

faint broad ribs near its ventral margin. It '-vas obtained (USGS Mesozoic loc. 21618), however, in the 

upper 11art of the Sliclerock Member in association with 

many Gryphaeas showing the characteristics of the subspecies. Similarly some Gryphaeas obtained near the 

middle oft he Rich Member on the North Fork of Stump 

Creek, Idaho (USGS Mesozoic loc. 22460), resemble 

the convex variant of G.zJlanoconvexa. Whitfield, except 

that they have posteriorly curved beaks as in the subspecies fnderna. 

Such associations of variable gryphaeas plus the fact 

that typical specimens of Gryphaea planoconve:ra Whitfield occur in the Iniddle and upper parts of the Hich 

Member indicate that all the Gryphaeas in the Sliderock and Rich l\1embers belong to a single gradational 

series. Recognition of the subspecies fraterna is useful 

stratigraphically, however, because it has been found 

only in the lower part of the Twin Creek Limestone, 

whereas the typical depressed subcircular form of G. 

planocon1•exa has a nnlCh '-vider geographic distribution in northern Wymning and in Montana at a slightly 

higher stratigraphic position. 

Type: Holotype, USNM 132731; paratypes, USNM 132732-

132741. 

Occurrences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 3219, 6364, 7432, 8177, 9102, 12116, 13491, 

16016,18181,20344,20345,20961,20966,20968,20970-20973,20975, 

20977, 20988, 2.1000, 21618, 21633, 22638, 2-7729, 28483, 28484, 

28502, 28505, 28635-28637, 28644, 28645, 28647, 28648, 28795; 

Rich Member at Mesozoic locs. 2:2460 and 28797. 

Gryphaea nebrascensis Meek and Hayden 

For synonymy ancl diagnosis see Imlay, 1948, p. 18, 19. 

This species is represented by 135 specimens from the 

Leeds Creek Member of the Twin Creek Limestone. It 

has been found in westernmost Wyoming from South 

Piney Creek northward to the Jackson Hole area and in 

easternmost Idaho in the Inotmtains bordering the Snake 

River. It ranges throughout the Leeds Creek Member 

but is most comn1on at or a little below the middle of the 

1nmnber. 

In addition, small striate Gryphaeas that are pro~­

ably immature specimens of Gryphaea nebrasaenszs 

Meek and Hayden have been found in the uppermost 

part of the \V atton Canyon Member of the Twin Cr~ek 

Limestone at Sliderock Creek northeast of Cokeville 

and in Cottonwood Creek east of Smoot, Wyo. Similar 

iinmature specimens that probably represent G. nebras~ 

censi8 occur in unit 19 of the Jurassic section at Lower 

Slide La.ke, Teton County, \Vyo. (Imlay, 1956a, p. 70). 

This unit is considered to be equivalent of the upper part 

of the Watton Canyon Member. Such occurrences of 

Gryphaea sp. juv. cf. G. nebrasaensis Meek and Hayden 

in the Watton Canyon Member or equivalent beds are 

probably at about the saine stratigraphic position as the 

· lowest occurrences of the species at the top of the basal 

member of the "Lower Sundance" Formation in northcentral Wymning (Imlay, 1956b, p. 591). 

Occzwrences: Twin Cree,k Limestone, Leeds Creek Member at 

USGS Mesozoic locs. 3808, 3830, 3831, 3834, 16031, 18182, 27727, 

27728, 28584 and 28587-28590. 

Genus TRIGONIA Bruguiere, 1789 

Trigonia americana Meek 

For synonymy, descriptions, and illustrations see Imlay, 1964a, 

p. C28, pl. 3, figs. S--13. 

This species is represented by 18 speci1nens in the 

Twin Creek Limestone and ranges from the Sliderock 

to the Leeds Creek Mmnber. In the section on the nmth 

side of Lower Slide Lake, Teton County, Wyo. (Imlay, 

1956aJ p. 70), it ranges from units 10 to 20. 

Occurrences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 16036, 17674 and 28798; lower part of formation at Mesozoic loc. 3392!; Rich Member at Mesozoic locs. 21620, 

28785, 28788 ancl 28793 ; Leeds Creek Member at Mesozoic loc. 

3830: stratigraphic position unknown at Mesozoic loc. 6948. 
82 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Trigonia elegantissima Meek 

For synonymy, description, and illustrations see Imlay, 1964a, 

p. C29, pl. 3, figs. 1-7. 

This species is represented by 19 specimens from the 

Twin Creek Limestone, ranging from the Rich to the 

Leeds Creek Member. At Lower Slide Lake, Teton 

County, ¥1yo., it occurs in units 10 and 19 (Imlay, 1956a, 

p. 70), which are correlated with the Rich, Boundary 

Ridge, and \V atton Canyon Members of the Twin Creek 

Limestone. In Montana the species occurs in the middle 

limestone member of the Piper Formation and in the 

lower part of the Rim·don Formation (Imlay, 1956b, p. 

576, 577, 588). In north-central Wymning it occurs in 

equivalent beds ( 1956b, p. 583, 591) in the Gypsum 

Spring and Sundance Formations. 

Occurrences: Twin Creek Limestone, Rich Member at USGS 

Mesozoic locs. 20963, 20965, 28639, 28785, and 28788 ; Sliderock 

or Rich Members at Mesozoic loc. 3822; Boundary Ridge Member at Mesozoic loc. 21621; Wattou Oanyon Member at Mesozoic 

loc. 21624; Leeds Creek Member at Mesozoic loc. 28643. 

Genus VAUGONIA Crickmay, 1930 

Vaugonia conradi (Meek and Hayden) 

For synonymy, description, and illustrations see Imlay, 1964a, 

p. C30, pl. 3, figs. 14-21. 

This species is represented by 33 spe~i1nens from the 

Twin Creek Limestone in W yon1ing and north -central 

Utah. It is fairly co1nmon in the Rich and Watton 

Canyon Members and uncommon in the lower part of the 

Leeds Creek Mmnber. At Lower Slide Lake Teton 

County, ' 'Vyo., it ranges from the lower part of unit 10 

(USGS Mesozoic loc. 20967) to near the top of unit 19 

(Imlay, 1956a, p. 70). These units correspond to the 

Rich and Watton Canyon Members of the Twin Creek 

Limestone. 

Occurrences: Twin Creek Limestone, Rich Member at USGS 

Mesozoic locs. 3821, 17297, 20349, 22796, and 28793; ·watton Canyon Member at Mesozoic locs. 17298, 21623, and 21624 ; Leeds 

Creek Member at Mesozoic loc. 20960; member unknown at 

Mesozoic locs. 3292, 3820, 3842, 3860. 

Genus MYOPHORELLA Bayle, 1878 

Subgenus PROMYOPHORELLA Kobayashi and Tamura, 1955 

Myophorella (Promyophorella) montanaensis (Meek) 

11-,or synonymy, descriptions, and illustrations see Imlay, 

1964a, IJ. C31, pl. 3, figs. 33-36, 38--41. 

This speeies is represented by 21 speeimens from the 

Twin Creek Limestone and ranges from the middle of 

the Slideroek Member to near the top of the \V atton 

Canyon Member. In Montana the speeies oeeurs in the 

middle limestone member of the Piper Formation 

(Imlay, 1956b, p. 577), in the middle and upper members of the Sawtooth For1nation (l1nlay, 1962a, p. C21) ~ 

and in the lower part of the Rierdon Formation (Cob 

ban,1945,p. 1281). 

Occurrences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 20344, 20973, 28483, 28644, 28645, 28648, 

and 28798 ; Rich Member at Mesozoic locs. 17815, 20350, 20963, 

21468, and 28792; Watton Canyon Member at Mesozoic locs. 

21622 and 21624 ; unknown member at Mesozoic locs. 3548. 

Subgenus HAIDAIA Crickmay, 1930 

ltlyophorella (Haidaia) yellowstonensis Imlay 

For synonymy, descriptions, and illustrations see Imlay 1964a, 

p. C33, pl. 3, figs. 27-32. 

This species is represented by three specimens from 

the Twin Creek Limestone in Wyoming. Two specimens (USGS Mesozoic locs. 17815 and 20963) are from 

the upper part of the Rich Member and one (USGS 

Mesozoic l9c. 3822) is either from the Sliclerock or the 

Rich Member. Elsewhere in Wyoming the species has 

been found on the north side of Lower Slide Lake, 

Teton County (USGS Mesozoic loc. 20967) in unit 10 

of the described section (Imlay, 1956a, p. 70). In southern Montana on the Boulder River the species has been 

found (USGS Mesozoic loc. 27624) near the middle of 

the limestone member of the Piper Formation in association with the ammonites Sohlites and Parachondroceras, which in the Twin Creek Limestone have been 

found only in the Rich Member. In southwestern 

Montana it has been found (1JSGS Mesozoic loc. 21385) 

i11 the Sawtooth Formation in association with Gryphaea planoconvexa Whitfield, which species in the 

Twin Creek Limestone has been found oniy in the Rich 

Member. The species has not been found in the Rierclon 

Formation in Montana or in equivalent beds of Callovian age in Wymning. It appears, therefore, to have a 

narrow stratigraphic range in a sequence of Bajocian 

age. 

Genus ASTARTE Sowerby, 1816 

Astarte meeki Stanton 

Plate 5, figures 1-6 

Astarte meeki Stanton, 1899, U.S. Geol. Survey Mon. 32, pt. 2, 

p. 620, pl. 73, figs. 3-5. 

This species is characterized by its subcircular outline, 

1noclerate eonvexity, 1neclimn size, and fine regular dense 

eoneentrie ribs. The beaks are situated from 1;3 to % 

of the length of the shell from the anterior encl. The 

shells are slightly longer than high, and some are relatively n1ore elongate than others. The concentric ribs 

are eoarsest on the umbones, become finer and denser 

ventrally, and in some specin1ens become faint near the 

ventral margins. The density of ribbing varies considerably frmn one specimen to another. The specimen 

illustrated by Stanton on his plate 73, figure 3, has 

about 42 ribs in a distance of 15 mm ( Inillimeters) from 
SYSTEMATIC DESCRIPTIONS 83 

the umbo. Others have as few as 30-35 ribs in about 

the same distance. 

Astarte 1neeki Stanton in shape and ribbing resembles 

Astarte depressa Blake and Huddleston (Arkell, 1934, 

p. 235, pl. 33, figs. 1-9) from the Oxfordian of England, 

but probably has even finer and denser ribbing. Among 

described American species the 1nost similar is Astarte 

packardi White (1880, p. 149, pl. 37, fig. 6). It differs 

from A. 1neeki Stanton by being smaller and more convex and by having rrinch coarser and sparser ribbing. 

The specimen figured by Stanton ( 1899) on his plate 

73, figure 3, is herein selected as lectotype of Astarte 

nweki Stanton. 

The species is represented by 40 specimens in collections from the Twin Creek Limestone. Most of these 

are frmn the Sliderock and Rich Members, but three 

are fron1 the Watton Canyon Member and two are from 

the Leeds Creek Member. At Lower Slide Lake, Teton 

County, Wyo., the species ranges from unit 10 to unit 22 

(I1nlay, 1956a, p. 70). In Montana the species is fairly 

com1non in the middle limestone 1nember of the Piper 

Formation, in the middle and upper parts of the Sawtooth Formation, and occtirs rarely in the Rierdon 

Fonnation. 

Types: Lectotype, USNM 28936; syntype, USNM 132742; 

plesiotypes, USNM 132743-132746. 

OccutTences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 3218, 3219, 3391, 13491, 18181, 20984, 21618 

and 28648 ; Rich Member at Mesozoic locs. 3815, 20963, 20965, 

21620, 21629, 28507, 28785, and 28793 ; Sliderock or Rich Member at Mesozoic loc. 17977; Boundary Ridge Member at Mesozoic loc. 21621; Watton Canyon Member at Mesozoic loc. 21624; 

Leeds Creek Member at Mesozoic locs. 12016 and 28591. 

Subgenus COELASTARTE Boehm, 1893 

Astarte ( Coelastarte) livingstonensis Imlay 

For synonymy, des-criptions and 'illustrations see Imlay, 1964ra, 

p. C 33, pl. 4, figs. 1--4. 

Fifteen specimens of this species have been found in 

the Twin Creek Limestone. These are mostly from 

various parts of the Sliderock and Rich Members, bu1t 

two speciments were obtained from ~the upper part of 

the Boundary Ridge Member. 

The species occurs ·in unit 10 of the section at Lower 

Slide Lake, Teton County, Wyo. (Imlay, 1956a, p. 70). 

It is fairly common in beds of Bajocian age in the 

western interior region and has not been found in beds 

of Callovian age. 

Occurrences: Twin Creek Limestone, SHderock Member at 

USGS Mesozoic locs. 20344, 21619, :and 28645; Rich Member at 

:NiesoZioic locs. 17297," 20963, 20987; Boundary Ridge Member ·at 

MesoZioic loc. 21621. 

Genus OPIS Defrance, 1825 

Subgenus TRIGONOPIS (Munier-Chalmas) Fischer, 1887 

Opis (Trigonopis) sp. 

Plate 2, figure 3 

One large internal 1nold from the Twin Creek Limestone is assigned to Opis on the basis of its triangular 

shape and i~ts extremely prominent narrow ir:cu~ed 

umbones. A weak ridge extends fr01n the posteriOr side 

of the umbo to the postero-ventral angle. The ornamentation is not preserved. 

The genus Opis is probably represented also by a 

sn1all specimen described by Meek ( 1877, p. 133, 134, pl. 

12, figs. 3, 3-a) questionably as Myophoria Uneata 

(Munster). The specimen, now lost, is reported to have 

been obtained above the "quarry rock" in Weber Oanyon Waswtch Range, Utah. Presumably it was col-

' 'I lected frmn the Twin Creek Limestone near Dev1 s 

Slide. Meek's description and illustration indica:te that 

the specimen resembles Opis ( Trigonopis) corallina 

Damon (Arkell, 1934, p. 259, pl. 34, figs. 7-9) from 

England but has less prominent umbones and n more 

quadra;te outline. 

Elsewhere in the western interior region, Opis (Trigonopi8) ha:s been found in limestone in the upper part 

of the Gypsum Spring Formation near Mill Creek, FreInont County, Wyo. (USGS Mesozoic locs. 18939 and 

19357). The species present is identical in lateral view 

with the specimen described by Meek from W eher Canyon, Utah, except that the -concentric ribs are fairly 

strong dorsal to the postmnbonal ridge instead of terminat1ing or passing into fine growth lines. 

Figtu·e(l 8pevimen. USNM 132747. 

Occurrence: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic loc. 20344; unktwwn position near Devils Sl'ide. 

Weber Oanyon, Utah. 

Genus PROROKIA Boehm, 1883 

Prorokia fontenellensis Imlay, n. sp. 

Plate 5, figures 9-13 

This species is represented in the Twin Creek LimeJ 

stone by 12 molds, of which several retain some shelly 

material. The shell is small, moderately convex, o'b· 

long, longer :than high. The umbones are small, depressed, and anterior to the middle of the shell. The 

vent.ral 1nargin is gently curved and makes nearly 'a 

right angle with the posterior margin. The ·anterior 

margin is evenly rounded. The posterior margin is 

nearly straight in its lower pa1~t and gently convex in 

its upper part. A low ridge trends obliquely from the 

umbo to the posteroventral angle. The surface is cov J 

ered with fine concentric ribs of variable strength and 

spacing. The internal border of the shell is denticulate. 

The hinge characters are unknown. 
84 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

This species is assigned to Prorok£a, rather than 

Astarte because of its oblong shape, its depressed beaks, 

and the presenee of a weak postumbonal ridge. Ooela8-

tarte Boehm is flatter, more inequilateral, and more 

quadrate in outline. Among foreign species, P1·orokia 

rwstica Lycett ( 1863, p. 76, pl. 35, fig. 5, pl. 40, figs. 8, 

Sa; Cox and Ark ell, 1948, p. 28) frmn the Bathonian of 

England is nearly identical with American species described herein in shape, size, and ornamentation, but it's 

beaks are less prominent and are located more anteriorly. 

Prorokia fontenellensi8 Imlay, n. sp., has been found 

in the Twin Creek Limestone only in the Sliderock and 

Rich Members. In Montana it has been found in the 

middle limestone member of the Piper Fonnation south 

of Livingston, Mont., (USGS Mesozoic locs. 25866 and 

27035). 

Types: Holotype, USNM 132748; para types, USNM 132749-

132752. 

Occurrences: Twin Oreek JjimestOiw. Sliderock Member a't 

USGS Meso~oic loc. 21619; Rich Member a:t Mesozoic locs. 17297, 

28788, and 28790. 

Genus PRO'N'OELLA Fischer, 1887 

Pronoella spp. 

The collections from the Twin Creek Limestone contain 105 speci1nens that belong to Pronella, or to similar 

appearing genera of the family Cyprinidae. Most of 

the specimens are internal molds. Silicified sp.ecimens 

showing the ornamentation and shape of the shell occur 

only at two localities (USGS Mesozoic locs. 16058, 

28791). Inasmuch as the hinge of a right valve is preserved on only one specimen (USGS Mesozoic loc. 

16058), the generic identification is based n1ostly on resemblances to described species of Pronoella from .Jurassic beds elsewhere in the western interior region. These 

includePronoella cinnabarertusis (Stanton) (1899, p. 621, 

622, pl. 72, figs. 7, 8 ; Imlay, 1964a, p. C34, pl. 4, figs. 27-

30); P. uintahensis I1nlay (1964a, p. C34, pl. 4, figs. 15-

23), and P. ?iddingsi (Stanton) (1899, p. 622, pl. 73, fig. 

9; Imlay, 1964a, pl. 4, figs. 24, 25). Of these species, P. 

cinnabarensis (Stanton) is identified from the Twin 

Creek Limestone at USGS Mesozoic. localities 20971 and 

28790, P. uintahensis Imlay at Mesozoic localities 3221, 

3392, 17064, 17961, 28648, and 28791, and P.? cf. P.? 

iddingsi (Stanton) at Mesozoic localities 16058, and 

20977. Specimens of Pro,nella that a~·e too poorly preserved for identification with any described speeies occur 

at 1nany other localities. The knmvn stratigraphic 

range of P. cinnabarensis (Stanton) and P. uintahen8i8 

Imlay is from the middle of the Sliderock Member to 

the middle of the Leeds Creek Member. External molds 

of Pronoella? sp. have been found in the Giraffe Creek 

Mmnber at Mesozoic. locality 12034. 

Occurrences : Twin Creek Limestone, Sliderock Member at US 

GS Mesozoic locs. 3391, 16036, 17673, 17961. 17964, 18182. 20344. 

20345, 20970, 20971, 20973, 20977, 20984, 28459, 28483, 28644, 

28648; lower part, probably Sliderock Member at Mesozoic 

locs. 3392 and 16058; Rich Member at Mesozoic locs. 3221, 17297. 

20963,20980,22796,28507,28785,28787, 28788, and 28790; Boundary Ridge Member at Mesozoic locs. 20990, 21621, and 28791: 

Watton Canyon Member at Mesozoic loc. 21624; Leeds Creek 

Member at Mesozoic locs. 2906, 12016, 17064, 18182, 20983, 28591, 

28643, and questionably at Mesozoic loc.12034. 

Genus MACTROMYA Agassiz, 1842 

Mactromya ~ sp. 

Plate 1, figure 18 

Twelve internal molds fron1 the Twin Creek Linlestone are tentatively assigned to 111 actrmnya. They are 

ovate, inflated, and generally a little longer than high. 

The umbones are swollen, incurved, and slightly anterior 

to the 1niddle of the mold. The orna1nentation consists 

only of fine dense concentric growth limes. 

Figured specimen: USNM 132753. 

Occurrences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 3219, 20966 and 20977; Rich Member at 

Mesozoic loc. 28789 and 28793 ; Sliderock or Rich Member at 

Mesozoic loc. 17977; member unknown at Mesozoic loc. 3220. 

Genus QUENSTEDTIA Morris and Lycett. 1854 

Quenstedtia sublevis (Meek and Hayden) 

Plate 5, figures 7, 8, 14, 15 

Tllracia? sublevis l\feek and Hayden, 1860, Acad. Nat. Sci., Philadelphia Proc., Y. 12, p. 182. 

Thracia? sublcvis Meek and Hayden, 1685, Smithsonian Conti'. 

Knowledge, Y. 14, no. 172, p. 102, pl. 4, figs. 4a, 4b. 

?Thracia? 8ublevis Meek and Hayden. Whitfield, 1880, U.S. 

Geog. and Geol. Suryey Rocky Mtn. Region (Powell), p. 

375, pl. 5, fig. 34. 

Quenstedtia sublevis (Meek and Hayden), Imlay, 1947, Am. 

Assoc. Peteroleum Geologists Bull., v. 31, p. 253. 

?Thracia canadensis McLearn, 1924, Royal Soc. Canada Proc. 

and Trans., 3d ser., sec. 4, v. 18, p. 56, pl. 9, figs. 8, 9. 

This species is represented by seven specimens in the 

Twin Creek Limestone. The original description 

(Meek and Hayden, 1860, p. 182) is as follmYs: 

Shell elongate, or narrow oblong-oval, rather compressed. Anterior end narrowly rounded; base nearly straight along the 

middle, rounding up toward the ends ; posterior side longer than 

the other, rounded, or slightly truncate, and apparently gaping 

a little at the extremity; dorsal border straight or concave in 

outline, and nearly horizontal behind the beaks, declining more 

abruptly in front. Beaks moderately elevated, the right one 

being usually a little higher than the other; located in advance 

of the middle ; posterior umbonal slopes prominently rounded. 

Surface concentrically striate. 

Length, 1.19 inch ; height 0.63 inch ; breadth, about 0.32 inch. 

The two specimens illustrated by Meek are from a 

collection of five internal molds obtained from the southwest base of the Black Hills. Four of these are preserved in a fine-grained sandstone and one is preserved 

in a calcareous matrix. All can be matched with 
SYSTEMATIC DESCRIPTIONS 85 

specimens in the Geological Survey collections from the 

Stockade Beaver Shale Member of the Sundance Formation. The specimens preserved in a sandy matrix, 

however, -could have been obtained from the Hulett 

Sandstone Mmnber or the Canyon Springs Sandstone 

Me1nber of the Sundance Formation. The speci1nen 

illustrated by Meek and Hayden (1865, pl. 4, fig. 4a) is 

herein selected as lectotype cf. Qru.ernstedtia sublevis 

(Meek and Hayden). 

The speein1en deseribed by Whitfield (1880, p. 375, pl. 

5, fig. 34) as Thracia? sublevis Meek and Hayden differs 

frmn the type specimens of rthat species by having a 

faint posterior unhonal ridge and a slightly concave 

ventral margin. These are the features that are reported to distinguish Q~tenstedtia arcuata (Meek and 

H·ayden) (1860, p. 182; 1865, pl. 102, pl. 4, fig. 8) from 

Q. sub le·v is (Meek and Hayden) . Quenstedtia cana.-

densis (MeLearn) (1924, p. 56, pl. 9, figs. 8, 9) from 

beds of early Callovian age near Blairmore, Alberta., is 

reported to differ frmn Q. sublevis (Meek and Hayden) 

by having its beaks nearer the eenter of the shell. 

Types: Lectotype, USNM 132754; syntypes, USNM 197, 

132841 ; plesiotype, USNM 132755. 

Occurrences: Twin Creek Limestone, Rich Member at USGS 

Mesozoic locs. 17297 and 28787; Leeds Creek Member at Mesozoic locs. 18182 and 20983; Sliderock or Rich Member at Mesozoic loc. 3822. 

Genus PROTOCARDIA Beyrich, 1845 

Protocardia cf. P. schucherti McLearn 

This genus is represented by six poorly preserved internal molds from the Twin Creek Limestone. These 

molds are small and ova.te, have an angular postumbonal slope, from 9 to 12 radiating ribs on rthe posterior 

surface, and fine lines of growth elsewhere on the surface. They differ from Protocardia schucherti McLearn 

(1924, p. 57, pl. 9, figs. 4, 10) by being less stout and by 

ha.ving fewer radiating ribs on the postumbonal slope. 

Sin1ilar internal molds from the Yellowstone Park area 

were assigned by Stanton ( 1899, p. 621) toP. shun!Jardi 

(Meek and Hayden) (1860, p. 98, 99), but the ornamentation on the type speeimens of tim t species is not preserved sufficiently for positive identification of the 

species outside of the type locality. 

The types of Protocard'ia shu1nm·di (Meek and Hayden) were obtained from the southwestern part of the 

Black Hills in association with species 1that are characteristic of the Redwater Shale Member of the Sundance 

Formation. That 1nember contains the ammonite Oardioce'lYts of late Oxfordian age. The types of P. 

scl-uucherti MeLearn were obtained fr'Onl Grassy Mountain near Blairmore, Alberta (Frebold 1957 I) 21 76 · ' ' . ' ' 1963, p. 4, 5), in beds that are elated as early Callovian 

and possibly also late Bathonian (Frehold, 1963, p. 28-

30). Protocard£a. in the Twin Creek Li1nestone has 

been found only in the Rieh and Leeds Creek Members 

of Bajocian and early Callovian ages. 

Oacurrences: Twin Greek Limestone, Rich Member at USGS 

Mesozoic locs. 20963 and 28507 ; Leeds Creek Member a•t Mesomic loc:s. 18182 rand 28591. 

Genus PLEUROMYA Agassiz, 1843 

Pleuromya subcompressa (Meek) 

Platte 5, figures 16-21 

For synonymy ~and discussion of this species see Imlay, 1964-a, 

p.C35. 

Pleu'l·mnya snbcmnpressa (Meek) is represented by 

about 135 specimens from the Twin Creek Limestone. 

These were collected frorn the middle and upper parts 

of d1e Sliderock Member and fron1 ail parts of the Rich 

Member.· The species possibly includes P. ~veberensis 

(Meek) (1877, p. 137, pl. 12, figs. 11, 11a.) (pl. 5,_figs. 22, 

23) w hieh was obtained with P. subcmnpressa (Meek) 

from the Twin Creek Limestone in Weber Canyon, 

Wasatch Range, Utah. Similar-appearing speeimens 

of Pleurmnya have since been obtained (USGS Mesozoic loc. 28642) at about rthe same place from 30 to 40 

feet above the basal oolitic limestone of the Sliderock 

Member. The species is eommon elsewhere in the western interior in beds of Bajoci~an to early Callovian age. 

For example, in the Jurassic sequence on the north side 

of Lo,Yer Slide Lake, Teton County, Wyo., the speeies 

ranges from the upper part of unit 10 to about 40 feet 

above t.he base of unit 22 (Imlay, 1956a, p. 70-71). The 

associwted a.Inmoniters ·are of Bajocian nge in unit 10 ·and 

of Callovi·an age in units 19 and 22. 

Types: Lectoitype, USNM 132756; •syntypes, USNM 8061, 

132839, 132840; plesio1types, USNM 8180, 28958-28001, 132757. 

Ocaurrences: Twin Oreek Limestone, Slide rock Member ra.t 

USGS Mesowic locs. 3218, 3219, 3391, 6364, 8177, 17961, 20344, 

20345, 20966, 20968, 20969, 20970, 20975, 20977, 20984, 20988, 

21618, 21619, 21632, 27729, 28459, 28483, 28484, 28635, 28642, 

28644, 28645, 28648 and 28798; Rich Member at Mesozoic locs. 

985, 6362, 63:63, 1729•7, 20963, 20965, 21468, 21620, 21629, 28639, 

28785, 28788, 28792 and 28793 ; from Sliderock or Rich Mem'ber 

at Mesozoic locs. 3822, 9101, rand 17977. 

Genus CORBULA Bruguiere, 1797 

Corbula cf. C. munda McLearn 

Plate 2, figures 5-11 

c.rf. Oorbula munda McLearn, 1924, Royal Soc. Canada 1Proc. and 

Trans., 3d ser., sec. 4, v. 18, p. 58, pl. 9, figs. 1, 2, 5. 

Many 1nolds of Oorbula have been obtained from the 

Twin Creek Limestone between Labarge Creek and 

Slate Creek in western Wyoming. These molds are 

similar in size, shape, and in fineness of concentric ribbing to Oorbnla 1nunda McLearn (1924, p. 58, pl. 9, figs. 

1, 2, 5) from beds of late Bathonian or early Callovian 
86 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

age on Grassy Mountain near Blairmore, Canada (Frebold, 1963, p. 5, 29). 0. munda McLearn has been obtained elsewhere in the lower 38 feet of the Rierdon 

Formation (USGS Mesozoic loc. 18716) in northwestern Montana (Cobban, 1945, p. 1280, 1281; Imlay, 1945, 

p. 1024). That part is considered to be of earliest Callovian age on the basis of the occurrence of the ammonites Oadoceras and Kepplerites. The specimens of 

Om·bu)a in the Twin Creek Limestone occur in beds 

that are of Bajocian age. 

Figured specimens: USNM 132758-132760. 

Occurrences: Twin Creek Limestone, Rich Member at USGS 

Mesozoic locs. 20963, 21629, 28786-28788. 

Genus PHOLADOMYA G. B. Sowerby, 1823 

Pholadomya kingi Meek 

Plate 5, figures 36-38, 42 

For .synonymy see Imlay, 1964a, p. C460. 

This species bears from 8 to 14 strong radial ribs that 

occur on the middle and most of the anterior parts of 

the shell. 

It is represented in the Twin Creek Limestone by five 

specimens from the Sliderock Member, by one from the 

Rich Member, and by one fragment probably from the 

Rich Member. Elsewhere in the western interior region 

the species is moderately common in shale and shaly 

limestone of Bajocian and early Callovian age. 

Types: Holotype, USNM 7815; plesiotypes USNM 28957, 

132003. 

Occun·ences: Twin Creek Limestone ; Sliderock Member at 

USGS Mesozoic locs. 2034:1:, 20972, 20988, 21619, and 28645; Rich 

Member at Mesozoic locs. 28785 and 21620. 

Pholadomya inaequiplicata Stanton 

Plate 5, figures 24-26 

Phola(lomya inaequiplicata. Stanton, 1899, U.S. Geol. Survey 

Mon. 32, p. 625, pl. 74, fig. 4. 

Pholadomya inaequiplicata Stanton. Imlay, 1964a, U.S. Geol. 

Surv~y Prof. Paper 384-C, p. C36, pl. 4, figs. 37, 38. 

This species is characterized by having about 20 

radiating ribs that cover most of the shell. It differs 

from Phola.dmruya kin.gi Meek by being smaller, thicker, 

and shorter and by having more radiating ribs. 

It is represented by four specimens from the Twin 

Creek Li1nestone. 

Types: Holotype, USNM 28956; paratypes, USNM 30668; 

plesiotypes, USNM 132004, 132005. 

Occurrences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 17985 and 28635 ; Sliderock or Rich Member 

at Mesozoic locs. 3822 and 17977. 

Genus GONIO:MYA Agassiz, 1842 

Goniomya montanaensis Meek 

Plate 1, figure 17 

For synonymy and description see Imlay, 1964a, p. C36, pl. 4, 

figs. 34, 35. 

This species is represented by seven specimens from 

the Twin Creek Limestone; it ranges from the upper 

part of ·the Sliderock Member to the upper part of the 

Rich Member. 

Types: Holotype, USNM 7814; plesiotype, USNM 132761. 

Occurrences: Twin Creek Limestone, Sliderock Member at 

USGS Mesozoic locs. 20971, 20977, and 28798; Rich Member at 

Mesozoic locs. 20963 and 28785; Sliderock or Rich Member at 

Mesozoic loc.17977. 

Genus MYOPHOLAS H. Douville, 1907 

Myopholas hardyi Imlay 

Plate 5, figures 27-35 

Myopholas hardyi ·Imlay, 964a, U.S. Geol. Survey Prof. Paper 

483-C, 037, pl. 4, figs. 12-14. 

In addition to the primary types, this species is now 

represented by one specimen from the Watton Canyon 

Member of the Twin Creek Limestone in Utah, five 

specimens from the Rierdon Formation in central Montana, and 17 specimens from the "Lower Sundance" 

Formation in the Bighorn Basin, Wyo. On the basis 

of these specimens, the species may be redescribed as 

follows: 

The shell is rather large for the genus, elongate ovate, 

nearly twice as long as high, highly convex, and inequivalve. The umbones are broad, strongly incoiled, inclined forward, and are at about one-third of the length 

of the shell from the anterior end. The anterior margin 

is evenly rounded. The ventral margin is gently convex. 

The posterior 1nargin is narrowly rounded and gaping. 

Most of the gape is in the right valve. 

The ornamentation consists of strong radial ribs, 

slightly weaker widely spaced concentric ribs, and a 

radial sulcus that extends steeply backward from the 

posterior side of the umbo to the ventral margin. The 

anterior part of the shell bears from five to seven strong 

sharp widely spaced radial ribs that curve anteriorly as 

they approach the ventral margin. The medial part of 

the shell bears from six to eight less sharp closely spaced 

radial ribs. These are bounded posteriorly by a sulcus 

that is much 1nore pronounced on the right valve than 

on the left and that marks the beginning of the gape and 

of an abrupt change in the curvature of the shell. Posterior to the sulcus follow six or seven rather fine radial 

ribs, the most posterior of which is stronger than the 

others and marks another change in the curvature of 

the shell. Posterior to these ribs the surface is marked 
SYSTEMATIC DESCRIPTIONS 87 

by concentric ribs and by a weak sulcus near the dorsal 

Inargin. This sulcus on the right valve extends farther 

anteriorly than on the left valve, contains small pits, 

and apparently marks the position of the external ligament. The intersection of the concentric ribs with the 

radial ribs produces a reticulate appearance that is most 

pronounced posterior to the sulcus. 

This species greatly resembles ffifyopholas nana H. 

Douville (1907, p. 110, pl. 2, fig. 1; Lissajous, 1932, p. 

198, pl. 32, figs. 8, Sa) from beds of Bathonian age in 

Europe but differs by having somewhat finer radial ribs 

on the medial and posterior parts of the shell and by attaining a much larger size. Compared with }}/, 

acuticostata (Sowerby) (1829, pl. 546, fig. 1; Arkell, 

1935, p. 349, pl. 45, figs. 14-16), it has a broader and 

deeper radial sulcus, a broader posterior maTgin, and 

stronger concentric ribbing. The genus 111 yopholas is 

possibly represented by N eaera. longirord1·a Whitfield 

(1880, p. 376, pl. 5, fig. 35) from the Jurassic of the 

Black Hills, but the type specimen is a poorly preserved 

mold that retains mere traces of ornamentation. 

All occurrences of Myopholas lwrdy·i Imlay are in 

beds of Callovian age as indicated by associated ammonites or by high stratigraphic position. In central 

Utah the species occurs within the upper 500-600 feet 

of the Twelvemile Canyon Member of the Arapien 

Shale (USGS Mesozoic locs. 21448 and 21449). In 

northeastern Utah near Hanna it occurs a little above 

the middle of Watton Canyon Member of the Twin 

Creek Limestone (USGS Mesozoic loc. 21623). In 

central Montana near Lewistown it occurs in the lower 

15 feet of the Rierdon Formation (USGS Mesozoic loc. 

19170) associated with Oadoceras and G-ryphaea nebra.rscens·i.g Meek and Hayden. In north -central Montana in the Little Rocky Mountains it occurs near the 

top of the Rierdon Formation (USGS Mesozoic locs. 

18750 ancl18754) associated with l{ epplerites and Gryphaea. nebra8censirs Meek and Hayden. 

In north-central Wyoming, Myopholas hardyi Imlay 

occurs in the lower part of the "Lower Sundance" 

Formation just below the lowest occurrence of Gryphaea nebra8censis Meek and Hayden. At one locality 

near Cody (USGS Mesozoic loc. 19633) it was found 

with Arcticoceras codyense Imlay (now referred to 

W arrenocera.g by Frebolcl, 1963, p. 13) 'in a bed 7 feet 

above the base of the formation. At another locality in 

the Mavericks Springs area (USGS Mesozoic loc. 

26032) north of Lander it was found with Oadoceras 

from 28 to 35 feet above the top of reel beds. At. these 

localities in Wyoming it occurs in oolitic beds that pass 

eastward into the Canyon Springs Sandstone Member 

of the Sundance Fonnation and westward into theW atton Canyon Member of the Twin Creek Limestone· (Imlay, 1950a, p. 40; 1953·a, p. 56). 

Types: Holotype, USNM 132007; para type, USNM 132008; 

plesiotypes, USNM 132762-132766. 

Occurrence: Twin Creek Limestone, Watton Canyon Member 

at USGS Mesozoic loc. 21623. 

Genus CERCOMYA Agassiz, 1843 

Cercompa. punctata Stanton 

Plate 5, figures 39-41 

For synonymy and discussion see Imlay, 1964a, p. C37, pl 4, 

fig. 31. 

This species is represented by four specimens from the 

Twin Creek Limestone. All were collected from the 

soft shaly limestone of the Rich Member in the easternmost occurrences of the form·ation. In the "Lower 

Sundance" Formation exposed north of the Lower 

Slide Lake, Teton County, Wyo., the species ranges from 

the lower part of unit 10 into unit 19 (Imlay, 1956a, p. 

70). Of these, unit 10 is correlated with the lower part 

of the Rich Member and unit 19 with the Watton Canyon Member of the 'Twin Creek Limestone. In northwestern Montana it has been found in the upper siltstone member of the Sawtooth Formation (Imlay, 

1962a, p. C21) as well ·as near the top of the Rierdon 

Formation USGS Mesozoic loc. 10729). 

Types: Holotype, USNM 28945; para type, USNM 30593 ; 

plesiotypes, USNM 132009, 132767, and 132768. 

Occurrences: Twin Creek Limestone, Rich Member at USGS 

Mesozoic locs. 3815, 20963, and 28788. 

Genus PLATYMYA Agassiz, 1838 

Platymya rockymontana Imlay, n. sp. 

Plate 5, figures 43-49 

This species is represented in the Geological Survey 

collections by 14 specimens of which 1 is from the Twin 

Creek Limestone. 

The shell is Inoderate in size for the genus, elongate, 

oblong, 1noderately inflated and gaping posteriorly. 

The umbones are wide, depressed, contiguous, directed 

slightly backward, rising 115-YtJ of the height of the 

shell above the hinge line, and are about % of the length 

of the shell from the anterior end. The anterior end 

is evenly rounded. The posterior end is subquadrate 

in outline and beeomes narrower during growth. The 

hinge is long and straight. The ventral margin is subparallel to the hingeline and concave posterior to its 

eenter. The post-umhonal slope is marked by a broadly 

rounded carina that trends from the umbones to the 

posteroventral angle. On most specimens this carina. 

is followed dorsally by a shallow sulcus which is followed in turn by a sharp narrow carina bordering the 

eseutcheon. In addition the shell bears a nearly vertical 
88 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

sulcus that trends frmn the mnbo to the ventralinargin 

a little anterior to the center of the shell and produces an 

mnargination of the ventral margin. 

The ornainentation consists of coarse widely spaced 

concentric growth lines on which are superimposed 

fine dense concentric growth lines. The coarse growth 

lines are strongest on the anterior part of the shell are . . ' farrly strong on the postumbonal ridge, and are weakest 

on the posterior part of the shell ventral to the postnmbonal ridge. Radial rows of fine punctae are preserved on several specimens. On one specimen they are 

visible on Inost of the ventral half of the shell anterior 

to the postumbonal ridge. The hinge line is not exposed. None of the specimens are preserved well 

enough for accurate measurements. 

This species is referred to Platy1nya rather than Oercomya because its posterior margin is subquadrate, and 

its posterior part is not extended as a long, narrow rostrum. Platy1nya was once considered by Arkell ( 1936, 

p. 351) to be a synonyin of Oercmnya but later was recognized by Cox and Arkell (1950, p. 96) as a distinct 

genus. This conforms with the views of Gillet ( 1924, 

p.154). 

The species shows considerable resemblance to 

"Plectontya" rugosa (Romner) as figured by de Loriol 

and others (1872, p. 193, 194, pl. 12, figs. 6, 7) from the 

Portlandian of Europe, but its beaks are more anterior, 

its ventral Inargin is concave, and it has a distinct 

median sulcus. Platy1nya scarburgensig (Morris and 

Lycett) (1855, p. 138, pl. 15, fig. 13) from the Inferior 

Oolite of England is similar in shape and concentric ribbing but lacks a median sulcus. "Anatina" agassizi 

d'Orbigny (1845, pl. 369, figs. 3, 4) from the Neocomian 

of Europe differs from Platyntf!Ja rocky1nontana Imlay, 

n. sp., by having a broader truncated posterior margin. 

"A natina" rastieriana cl'Orbig~1y ( 1845, pl. 370, figs. 4, 5) 

of N eocomian age has a less distinct sulcus and sinaller 

less elevated umbones. 

Another similar species is "Pleuromya" postcubninata McLearr1 (1924, p. 55, pl. 9, fig. 6) from the Oorb7tla 1nunda beds on Grassy Mountain near Blairmore, 

Alberta, (McLearn, 1929c, p. 86; Frebold, 1957, p. 76; 

1963, p. 5, 29). It differs from Platy1nya rockymontana 

Imlay, n. sp., by having Inore prominent umbones that 

are situated more anteriorly; and its anterior dorsal 

margin is concave. 

Platynuya rockynwntana Imlay, n. sp., has been found 

in the Twin Creek Limestone only in the Leeds Creek 

Member. 'This member is considered to be of Callovian 

age because its eastern and northerninarginal parts contain the pelecypod Gryphaea nebrascensis Meek and 

Hayden, which elsewhere in the western interior is associated with aminonites of Callovian age and has not 

been found in older beds. Furthermore, within the 

Jackson Hole area in northwestern Wyoming, the Leeds 

Creek Member passes into shaly beds that contain the 

Callovian ammonites Oadoceras and Xenocephalites 

(Imlay, 1956a, p. 70). 

Other occurrences of Platy1nya rockymontana !Inlay, 

n. sp., outside of the Twin Creek Limestone are all from 

beds that contain Gryphaea nebrascensis Meek and 

Hayden and that generally contain Callovian ammonites. For example, at Lower Slide Lake, Teton 

County, Wyo., the species occurs (lJSGS Mesozoic loc. 

20974) in unit 19 (Imlay, 1956a, p. 70) in association 

with Oadoceras. In the Big Snowy Mountains, Mont., 

it occurs (USGS Mesozoic loc. 19170) in the basal 15 

feet of the Rierdon Formation associated with Oadoceras. In the Little Rocky Mountains, Mont., it occurs (USGS Mesozoic locs. 9834, 18735, 19583) near 

the top of the Rierdon Formation associated with K epplerites. In Park County, Mont., about 8 miles west 

southwest of Livingston, it is associated with W arrenoceras (USGS Mesozoic loc. 25868). These occurrences 

are evidence that the species is a good indicator for a 

Callovian age. 

Types: Holotype, USNM 132769; para types, USNM 132770-

132773. 

Occurrences: Twin Creek Limestone, Leeds Creek Member at 

USGS Mesozoic loc. 12016; Rierdon Formation at Mesozoic 

locs. 9834, 10729, 18735, 18750, 19170, 19,583 and 25868; "Lower 

Sundance" Formation at Mesozoic loc. 20974. 

Genus THRACIA Leach, 1823 

Thracia weedi Stanton 

For synonymy, description, and illustrations see Imlay, 1964a, 

p. 038, pl. 4, figs. 32, 33. 

Thracia rweedi Stanton is represented by 14 specimens 

from the Rich Member and one from the upper part of 

the Sliderock Member of the Twin Creek Limestone in 

Wyoming and Utah. The species is fairly common in 

shaly liinestone of Bajocian age near Yellowstone National Park. For example, it occurs from 8 to 20 feet 

above the base of unit 10 of the section on the north side 

of Lower Slide Lake, Teton County, Wyo. (Imlay, 

1956a, p. 70.) It is· common in the middle limestone 

mmnber of the Piper Formation in southern Montana. 

Oce1wrences: Twin Creek Limestone, Slide rock Member at 

USGS Mesozoic loc. 20977; Rich Member at USGS Mesozoic 

locs.17297, 20963,21468,21629,28785,28788, 1

and28793. 

Class CEPHALOPODA 

Genus SPIROCERAS Quenstedt, 1858 

Spiroceras cf. S. orbignyi Baugier and Sauze 

Plate 6, figures 4-6, 9, 11 

cf. Toxoceras orbignyi Baugier and Sauze. d'Orbigny, 1850, 

PaU~ontologie fran~aise: Terrains jurassiques, v. 1, 

Cephalopodes, p. 593, pl. 231, pl. 232, figs. 1, 2. 
SYSTEMATIC DESCRIPTIONS 89 

cf. Patoceras orbignyi (Baugier and Sauze). Fallot and 

Blanchet, 1923, Inst. C'atalana Hist. Nat., Trabajos, v. 

1921-22,pt.11,p.131,pl.3,figs.8,9. 

cf. Spi1·ocera8 orbignyi Baugier and Sauze. Potonie, 1929, 

Preuss. geol. Landesanstalt Jahrb., v. 50, p. 247, pl. 18, 

figs. 2!J-32. 

cf. Spit·oceras orbignyi Baugier and Sauze. Roman and Petourand, 1927, Lyon Univ., Lab. Geologie Travaux Mem. 9, 

pt. 11, p. 36, pl. 4, figs. 14, 14a,b,c. 

The genus Spiroceras is represented by two 1nolds. 

One is a small external n1old that shows the ventral 

region, and one is a large internal mold that is part of 

a slightly curved shaft. The internal n1old is subovate 

in section and higher than wide. The on1an1entation 

consists of two rows of tubercles on each side of the 

venter and of simple, low, broad, gently curved ribs. 

The n1ost ventral row of tubercles is the stronger. The 

ribs incline slightly forward on the upper part of the 

flan4:s, cross the venter transversely, and are smnewhat 

weakened along the midline of the venter. The ribs on 

the lower part of the flanks curve toward the aperture, 

weaken dorsally, and are not apparent on the dorsum. 

Neither 1nold shows any trace of the suture line. 

These specimens compare closely in size, shape, and 

ornamentation with the illustrations of Spiroceras 

orbignyi Baugier and Sauze, as given above and could 

be within the range of variation of that species. The 

specimen showing the closest resemblance is that illus~ 

trated by Potonie (1929, pl. 18, figs. 29, 30). The large 

speci1nen from the Twin Creek Limestone differs apparently only by having a straighter shaft. The fact 

that the shaft is only slightly curved does not preclude 

its assignment to Spiroceras whose shaft i1~ places and 

in smne specin1ens may be straight (compare d'Orbigny, 

1950, pl. 226, fig. 1, pl. 228, fig. 1; Potonie, 1929, pl. 17, 

fig. 24, pl. 18, fig. 30). 

Fig1wed specimens: USNM 132774, 132775. 

Occurrences: _Twin Creek Limestone, upper part of Sliderock Member at USGS Mesozoic locs. 20969 and 20985. 

Genus STEPHANOCERAS Waagen, 1869 

Stephanoceras cf. S. skidegatensis (Whiteaves) 

Plate 6, figure 10 

('f. Stephanoceras cf. skidegatensis (Whiteaves). 'Varren, 1947, 

Research Council Alberta Rept. 49, p. 72, pl. 5, fig. 2, pl. 7, 

fig. 1. 

cf. Stephanoceras ex. gr. skidegatensi8 (Whiteaves). Frebold, 

1957, Canada Geol. Survey Mem. 287, p. 49, pl. 21, fig. 1. 

One crushed body whorl bears about 38 weak primary 

ribs that curve forward gently on the lower part of the 

flanks. At the adapical end of the whorl the ribs terminate in weak tubercles. On most of the whorl the primary ribs are nontuberculate and pass indistinctly into 

weaker secondary ribs that outnumber the primary ribs 

about three to one. The body chamber occupies about 

two-thirds of a whorl and is probably nearly complete. 

The specimen has considerably weaker ribbing than 

~the holotype of Stepha.noceras skidegatensis (Whiteaves) ( 1876, p. 34, pl. 3, figs. 8, 9; McLearn, 1932a, p. 

54, pl. 1, fig. 2, pl. 2, fig. 3, pl. 3, figs. 8, 9) from the 

Queen Charlotte Islands and .shows more resemblance 

to the specimens of Stephanoceras from the Rock Creek 

Member of the Fen1ie Formation as illustrated by Warren (1947, pl. 5, fig. 2, pl. 7, fig. 1) and Frebold (1957, 

pl. 21, fig. 1). 

Figured specimen: USNM 132776. 

Occurrence: Twin Creek Limestone, about 110 feet below the 

top and 17 feet above the base of the Sliderock Member at USGS 

Mesozoic loc. 21632. 

Stephanoceras aff. S. nodosum (Quenstedt) 

Plate 7, figures 4, 5 

The species is represented by one distorted internal 

1nold that shows the umbilicus, part of the penultimate 

whorl, and most of the adult body whorl. The shell 

is compressed and evolute. The septate whorls embrace the preceding whorls about one-half. The body 

whorl embraces the penultimate whorl less than onethird. The whorls are ovate in section and a little 

wider than high. The flanks, venter, and umbilical 

wall round evenly into each other. The umbilicus is 

fairly wide and shallow. The umbilical wall is steeply 

inclined on the inner whorls, but becomes gently inclined o~n the outer whorls. The body chamber occupies about three-fourths of a whorl. The aperture on 

the internal 1nold is marked by a forwardly inclined 

constriction that is followed by a collar. 

The whorls exposed in the umbilical wall bear rather 

weak moderately spaced primary ribs that incline 

slightly forward and teTininate in acute tubercles near 

the middle of the flanks. Ado rally, the prima.ry ribs 

gradually become widely spaced and weaker and are 

barely visible on the body chamber. The tubercles 

remain acute and persist a little below the middle of 

the flanks to within 50 mm of the aperture. The body 

whorl bears 15 tubercles. Secondary ribs, exposed on 

the adapical end of the penultimate whorl, are fine and 

closely spaced and outnumber the tubercles about six 

or seven to one. On the body whorl the secondary 

ribs are broader and outnumber the tubercles four or 

five to one. 

The specimen at a 1naximmn diameter of 140 mm, 

has a whorl height of 38 mm, a whorl thickness of 41 

1nm, and an umbilical width of 70 mm. The suture 

line cannot be traced. 

This species is characterized by the presence of 

widely spaced acute tubercles on its body whorl, by 

marked weakening of its primary ribs adorally, and 

by the presence of fine closely spaced secondary ribs on 
90 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

the penultin1ate whorl. It resembles Stephanoceras 

nodosum ( Quenstedt) ( 1887, p. 532, pl. 65, fig. 17; 

vVeisert, 1932, p. 136-138, pl. 15, figs. 1, 2) in tuberculation, involution, and whorl shape, but its body whorl 

has sparser and weaker primary ribs, and its septate 

whorls have finer and denser secondary ribs. It shows 

a closer resemblance in these respects to a fragmentary 

specimen described by Maubeuge ( 1951, p. 59, pl. 6, 

fig. 6) asS. aff. S. nodosu1n (Quenstedt). 

Figm·c(l specimen: USNM 132777. 

Occm~rence: Twin Creek Limestone, upper 30 feet of Sliderock Member in Utah at USGS Mesozoic loc. 21618. 

Genus STEMMATOCERAS Mascke, 1907 

Stemma toceras arcicostum Imlay, n. sp. 

Plate 8, figures 1, 2; plate 9, figures 1-11 

The species is represented by eight fragmentary 

speci1nens from one locality on Birch Creek in northcentral Utah and seven specimens from Tigh Creek near 

Drun1mond, Mont. 

The shell is a stout serpenticone. The whorls are 

subova:te in section, a little wider than high, and embrace the preceding whorls about one-half. The flanks 

round evenly into the broadly rounded venter and into 

the steeply inclined umbilical wall. The umbilicus is 

fairly wide and does not appear to widen on the adult 

whorl. The adult body chamber occupies about threefifths of a whorl. The aperture, preserved only in part 

on the holotype, is slightly contracted. The curvature 

of growth lines near the adoral end of the holotype show 

that the aperture was prolonged ventrally. 

The small septate whorls are marked by fairly sharp 

moderately spaced primary ribs that arise near the 

umbilical sea1n, curve slightly fonvard on the flanks, 

and ter1ninate in small acute tubercles just below the 

1niddle of the flanks. From the tubercles pass bundles 

of three to four sharp secondary ribs that arch gently 

forward on the venter. Other ribs arise freely at or 

above the zone of tuberculation, resulting in about five 

secondary ribs for each primary rib. 

On the larger septate whorls the primary ribs become 

broader and more widely spaced; tuberculation becomes 

less distinct, and the secondary ribs arch forward 1nore 

strongly on the venter. 

On the adult body cha1nber the primary ribs are broad 

and low and terminate weakly at about one.:third of 

the whorl height 'vithont forming tubercles. The upper 

part of the flanks and the venter are marked by weak 

secondary ribs that arch forward strongly on the 

venter, outnumber the primary ribs nearly six to one, 

and are mostly indistinctly connected with the primary 

ribs. Adorally, on the body chamber, the primary ribs 

become broader and fainter and are replaeed near the 

aperture by forwardly curved striae. The seeondary 

ribbing persists on the venter almost to the aperture. 

The body whorl bears 23 primary ribs. 

The suture line has broad saddles that descend evenly 

to the unbilical seam. The first lateral lobe is trifid, 

moderately wide, and is shorter than the ventral lobe. 

The first auxiliary lobe is inclined obliquely. 

The holotype, 1neasured near its aperture, has a maximunl dia1neter of 205 mm, has a whorl height of 68 mm, 

a whorl thickness of 89 mm, and an unbilical width of 

85 mm. One-half whorl adapical from the aperture 

the same dimensions are 170, 55, 78, and 69 mm, 

respectively. 

This species is assigned to Sten1/lnatoceras rather than 

Teloceras because of its weak tuberculation. It is much 

stouter and more involute than Stephanoceras. It is 

characterized by its fine, closely spaced secondary ribs, 

by the forward arching of those ribs, and by its early 

loss of tuberculation. 

The septate whorls of this species resembles the septate holotype of Ste1nmatoceras palliseri McLearn 

(1932b, p. 114, pl. 2, pl. 5, fig. 1) from Canada in wl~orl 

shape and umbilical width. They differ by having 

weaker tubercles and ribs and by their secondary ribs 

being closer spaced and ·arching forward on the venter 

instead of crossing the venter transversely. The adult 

body whorl of S. palliseri JJ! cLearn, as illustrated by 

Warren (1947, p. 68, pl. 3, fig. 1), differs from the adult 

body whorl of S. arcicostum Imlay, n. sp., by becoming 

higher than wide adorally and by contracting considerably from the penultimate whorl. S. mclearni Warren 

( 194 7, p. 68, 69, pl. 1, fig. 1, pl. ·1, fig. 1) differs from S. 

arc·icmsturrn Imlay, n. sp., by developing a higher whorl, 

by being more evolute, and by having only about three 

secondary ribs for each primary rib. Teloceras allani 

Warren (1947, p. 70, pl. 2) is more evolute, has sparser 

secondary ribs, and appears to have coarser primary 

ribs on its septate whorls. All the various described 

species of Ste1nr1natoceras from central Europe (Weisert, 

1932) have sparser and coarser secondary ribbing. 

Types: Plastoholotype, USNM 132778; para types, USNM 

132779-132783. 

Occurrences: Twin Creek Limestone, about 45 feet below 

top of Sliderock Member at USGS Mesozoic loc. 28645. Sawtooth Formation near Tigh Creek 3 miles west of Drummond, 

Mont., at USGS Mesozoic loc. 27337 (equals Marvin Kauffman's 

locs. MK-58-T-3 and 4, MK-60-TC, and MK-61-TC). The 

holotype is the property of J. M. Rufus at Drummond, Mont., and 

was obtained by him at, or near, the same locality as the specimens collected by Marvin Kauffman. Three fragments of Stemmatoceras resembling S. arcicostum Imlay, n. sp., were collected 

by Hans Frebold from Swift Reservoir west of Dupuyer, Mont. 

Two of these fragments (USGS Mesozoic loc. 28825) are from 

a conglomeratic bed near the base of the shale member of the 

Sawtooth Formation (unit 5 of Cobban, 1945, p. 1295) and one 

(USGS Mesozoic loc. 28826) is from 14 feet higher (unit 10 

of Cobban, 1945, p. 1295) . 
SYSTEMATIC DESCRIPTIONS 91 

Stemmatoceras, n. sp. aff. S. albertense McLearn 

Plate 10, figures 1-S; plate 16, figures 5-7 

cf. Stemmatocet·as albertcruw l\IcLearn, 1928, Canada Natl. Mus. 

Bull. 49, Geol. ser., no. 48, p. 20, pl. 5-7. 

cf. Stemmatoccras albcrtcnse McLearn. Warren 1947, Alberta 

Research Council Rept. -19, p. 67, 68, pl. 5, fig. 1. 

cf. Stemmatoccras albertcn8c McLearn. Frebold, 1957, Canada 

Geol. Suryey Mem. 287, p. 50, 51, pl. 21, figs. 2a, b, pl. 23, 

figs. la-c. 

Forty-six fragmentary ammonites, herein compared 

with 8ten1/lnatoceras alberteruse McLearn, are characterized by having a depressed whorl section that is much 

wider than high. On the inner whorls the primary ribs 

are sharp, fairly closely spaced, curved forward, and 

terminate ventrally in acute tubercles just below the 

middle of the flanks. Adorally the primary ribs become 

stronger and more widely spaced and the tubercles become n10re pronounced. Secondary ribs arise by two's 

and three's on the inner whorls and by three's and four's 

on the outer whorls. The secondary ribs are fairly 

sharp, are separated by much wider interspaces, incline 

forward on the upper parts of the flanks, and cross the 

venter almost transversely. One large, mostly nonseptate, internal 1nold, probably representing part of the 

adult body whorl, bears strong widely spaced primary 

ribs and Inuch 1veaker secondary ribs that arch forward 

slightly on the venter. The ventral ends of the primary 

ribs bear blunt tubercles. 

The specimen shown on plate 10, figures 1 and 3, 

at an estimated diameter of 98 mm, has a whorl height 

of 35 1nm, a whorl thickness of 45 mm, and an umbilical 

width of 43 n1m. The next outer whorl of the same 

specimen (pl. 10, figs. 4, 7) at a whorl height of 50 n1n1 

has a whorl thickness of 62 111m. The largest figured 

fragment (pl. 10, fig. 5) at a whorl height of about 67 

m1n has a whorl thickness of 10± mm. Evidently, the 

whorl section becmnes lower during growth. 

These fragmentary ammonites differ from the holotype of 8tenwuttoceras albertense McLearn by havinO' 

a slightly higher whorl section, more widely spaced 

pr~n1ary ribs on their outer whorls, more closely spaced 

primary ribs on their inner whorls, and somewhat 

stronger secondary ribs. Their secondary ribs are no 

stronger, however, than those on the specimen of 8. 

albertense McLearn illustrated by Warren ( 194 7, pl. 5, 

fi~. 1). Their whorl height and the spacing of their 

primary ribs is similar to that on 8tenunwtoceras 

'nwlearni Warren (1947 p. 68 69 I)l 1 fio· 1 pl 4 fig 1) ' ' ' • ' b' ' • ' • ' but that species, as illustrated, has much weaker primary ribs and tubercles. 

Figut·ed specimens: USNM 132784-132786, 132940, 132941. 

Occll1'rences: Twin Creek Limestone, upper part of Sliderock 

Member at USGS Mesozoic locs. 16016, 20344, 20345, 20988, 

21618, 22638, 28644-28646, 28483, 28794, and 28795. The species 

is questionably represented by one specimen from the Sawtooth 

Formation on Tigh Creek west of Drummond, Mont. (loc. MK61-TC). 

Stemmatoceras cf. S. palliseri McLearn 

Plate 7, figures ;t-3 

Four fragmentary internal molds from Tigh Creek 

near Drummond, Mont., resemble 8tenl/tnatoceras palliseri.llfcLem'n (1932b, p.114, pl. 2, pl. 5, fig.1) in whorl 

shape and ribbing. One of the fragments includes part 

of a septate whorl and part of the body chamber. The 

septate whorl bears four secondary ribs for each priInary rib. The body chamber bears broad low widely 

spaced forwardly inclined primary ribs that fade out 

near the middle of the flanks. The venter and the upper 

parts of the flanks are sinooth. These specimens differ 

from those herein assigned to 8. arcicostu1n Imlay, n. sp., 

by having coarser and sparser ribbing on the septate 

whorl and by the secondary ribs fading on the body 

chamber. The adult body chamber of 8. palli!seri. MeLearn, as illustrated by Warren (1947, pl. 3), shows 

similar weakening of the ribbing adorally. 

Figured specimen: USNM 132787. 

Occurrences: Sawtooth Formation on Tigh Creek, 3miles west 

of Drummond, Mont. at USGS Mesozoic loc. 27337 (locs. MK58-T1, MK-58-T2, MK-60-TC and MK-61-TO of Marvin 

Kauffman) . 

Genus SOHL~TES Imlay, n. gen. 

This genus is characterized by fairly involute coiling, 

by a moderately compressed whorl section, by prominent 

umbilical tubercles that during growth change from 

conical to radially elongate and frmn a position a little 

below the middle of the flanks to near the umblicus, by 

Inany sharp forwardly inclined ribs that arise from the 

tubercles or from the zone of tuberculation and are 

strongest on the venter, and by simple suture lines that 

have nonretracted suspensive lobes. 

The type species of 8ohlites is 8. spinosus Imlay, n. sp. 

The genus is named in honor of Norman F. Sohl of the 

U.S. Geological Survey. 

The ornan1entation of Sohlites resembles that of Reineckeia, Bayle, 1878 (see Arkell and others, 1957, p. 

L312), Zemisteplwnus McLearn (1927, p. 72; 1929a, p. 

18), and Ermocera.s H. Douville (1916, p. 17; Arkell, 

1952, p. 272-277). Sohlites differs from any described 

genus of the Reineckiidae, however, by being much more 

involute, by lacking a ventral band or sulcus, and by 

having a much simpler suture line. From Reineckeia, in 

addition, it differs also by its tubercles being nearer the 

umbilicus after an early growth stage. In this respect 

it shows more resemblance to Zentisteplwnus and particularly to s1nall specimens of Z. carlottensis (Whiteaves) 

(Imlay, 1964b, p. B52, pl. 28, figs. 1, 2). It is easily distinguished from Zentistephanus by greater involution 

and a much simpler suture line that has a straight in-
92 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

stead of a retracted suspensive lobe. It also shows some 

resemblance to Ernwceras but differs by lacking a ventral sulcus, by having a Reineckeia-like arrangmnent of 

tubercles and ribs on its inner whorls, and by having a 

slightly sin1pler suture line. 

The systemmtic position of Sohiites is uncertain, but 

it is tentatively assigned to the fmnily Stephanoceratidae. On the basis of its resemblance to Ernwceras, however, it 1night be assigned to the family Than1boceratidae 

W estennann ( 1964, p. 409). 

Sohlites spinosus Imlay, n. sp. 

Plate 11, figures 1-26; plate H, figures 7, 8 

This genus is represented by 86 molds from the western interior region. Of these, 37 were obtained in southern Montana south of Big Timber, Livingston, and 

Bozen1an; 30 were obtained near Cody, Wyo.; 1 was 

obtained from the northwest end of the San Rafael 

Swell in central lTtah; and one was obtained from the 

Duehesne River near Hanna in northeastern Utah. All 

are frag1nentary and 1nost are erushed or distorted. All 

appear to belong to a single species. 

The innermost whorls to a diameter of about 6% mm 

are smooth and evolute. The succeeding septate whorls 

are strongly ornamented and fairly involute, are ovate 

in section, and embrace each other about three-fifths. 

The body whorl is unknown. 

The ornmnentation appears abruptly after the initial 

smooth stage. It consists of prmninent umbilical tubercles and rather sharp forwardly inclined ribs that 

arch forwardly gently on the venter and are strongest 

on the venter. On the smallest ornamented whorls the 

tubercles are round, very high and very widely spaced, 

and arise at about two-"fifths of the height of the whorl. 

From them pass from three to four high thin ribs that 

incline gently forward. In addition, from one to two 

similar ribs arise freely a little above the zone of 

tuberculation. 

During subsequent growth to a diameter of about 

50 m1n the tubercles become radially elongate, somewhat less prominent, and extend frmn the very edge 

of the umbilicus to about one-fourth of the height of 

the flanks. From the tubercles arise bundles of four 

to five ribs that are faint near the tubercles, are sharp 

and forwardly inclined on the flanks, and that cross the 

venter with. a gentle forward arching. In addition, 

from one to hvo ribs arise freely on the flanks and 

result in a total of six to seven ribs for each tubercle. 

All ribs are highest and broadest on the venter. 

At dia1neters greater than 50 mm the tubercles become 

lower and more elongate radially and the ribs become 

fewer relative to the number of tubercles. On the largest specimens, from· two to three ribs arise from each 

tubercle and from one to two ribs arise freely on the 

flanks; thus, there are about four ribs for each tubercle. 

On these specimens the ribs become fairly broad ventrally and arch forward moderately on the venter. 

Some specimens show faint traces of fine radial striation parallel to the ribs. 

The specimens available are too poorly preserved for 

accurate 1neasurements, but the whorl section appears 

to be higher than wide. 

The suture line is simple and varies appreciably in 

shape from one specimen to another. The lobes are 

shallow, wide, and irregular in shape. The ventral lobe 

is about as long as the first lateral lobe. The first lateral 

lobe varies from irregularly trifid to almost bifid. The 

second lateral lobe is irregularly bifid in some specinlens, and in others merges with the suspensive lobe 

which trends nearly straight to the umbilical seam. The 

first lateral saddle is very broad and irregular in shape. 

The second lateral saddle, where recognizable, is 1nuch 

narrower than the first lateral saddle. 

In summary, Sohlites spinosus Imlay, n. sp., is characterized by an early evolute smooth stage that is followed 

at a small size by a fairly involute coronate strongly 

tuberculate stage. At first the tubercles are high and 

conical and are at about two-fifths of the height of the 

·flanks where they give rise tq bundles of ribs as in the 

Callovian genus Reineckeia. Later, the tubercles become radially elongate and occupy a position near the 

umbilicus as in the Bajocian genera Zernistephanus and 

E rmoceras. The secondary ribs at all growth stages 

incline forward on the upper part of the flanks, become 

higher and broader ventrally, and areh forward ?n _the 

venter without reduction in strength along the m1dhne. 

The larger specimens of Sohlites spinosus Imlay, n. 

sp., show considerable resemblance in ornamentation to 

the intermediate size whorls of Zernistephanus carlottensis (Whiteaves) (1876, p. 38, pl. 6; Imlay, 1964a., 

p. B52, pl. 24, fig. 6; pl. 28, figs. 1-3), but their umbilical 

tubercles are more elongate radially and their secondary 

ribs are sparser and stronger. The small whorls of S. 

spinosus Imlay, n. sp., differ frmn the smallest whorls 

of Z. carlottensis (Whiteaves) by being more involute 

and by having from five to seven ribs per tubercle instead 

of three to five. 

Within the genus Ermoceras, the type species E. 

nwgharense Douville (1916, p. 19, 20, pl. 2, ·figs. 5-9) 

is probably 1nost similar to Sohlites spinosus Imlay, n. 

sp., but differs by having a distinct ventral sulcus, by 

lacking Reineckeia-like ornamentation on its immature 

whorls, by its ribs bending forward more strongly on 

the venter, and by having numerous fine radial striations parallel to the major ribs. 
SYSTEMATIC DESCRIPTIONS 93 

Types: Holotype, USNM 132788; para types, USNM 132789-

132806. 

Occurrences: Carmel !formation in central Utah at USGS 

Mesozoic loc. 12555; Twin Creek Limestone, Rich Member in 

Utah at Mesozoic loc. 17048; Gypsum Spring Formation in 

north central Wyoming at Mesozoic locs. 17096 and 17105; Piper 

Formation in Montana at Mesozoic locs. 5738, 19621, 19214, 

25866, 27035, and 27624; Sawtooth Formation in Montana at 

Mesozoic loc. 21391 and 28758. Fragments assigned questionably to Sohlite.r.; were obtained from the Carmel Formation in 

central Utah at Mesozoic locs. 20351 and 25678 (pl. 11, figs. 19, 

22, 23). 

The occurrences of Sohlites in the western interior of 

the United States indicate that it is restricted to the 

upper part of a 1narine sequence of Bajocian age. In 

central Utah, it has been found from 6 to 12 feet above 

the base of the Carmel Formation exposed at the northwest end of the San Rafael Swell. In northeast Utah 

it has been found 100 feet above the base of the Twin 

Creek Limestone in the upper part of the Rich Member 

exposed along the Duchesne River near Hanna. In 

north-central Wyoming, it has been found about 6 miles 

south of Cody in shaly beds at or just above the middle 

of the middle limestone member of the Gypsmn Spring 

Formation (Imlay, 1956b, p. 582, 583). In southern 

Montana, it has been found 1nostly in the middle and 

upper parts of the 1niddle lin1estone 1nember of the 

Piper Formation south of Big Timber and Livingston. 

The fossils at lTSGS Mesozoic loe. 27035, however, appear to be from the lower part of the limestone 1nember. 

It has not been found in the Sliderock Member of the 

Twin Creek Limestone. 

Sohlites has been found with Paraclwndroceras only 

in Montana and only at USGS Mesozoic locs. 19214, 

25866, 27035, and 27624. At locality 27624 it was collected from the same bed. At the other localities, colleetions were made from an interval of at least 20 feet. 

It may be signifieant ecologically that Sohlites has been 

found with Paraclwndroceras at only fotl.r localities; 

its total occurrences are far fewer than those of ParaclwndJ•ocera8, but it has been eollected in 1nuch greater 

numbers. 

Genus NORMANNITES Munier-Cha.Imas, 1892 

Normannites? cf. N. crickmayi (M:cLearn) 

Plate 12, figures 1-4 

cf. Kanastephanus crickmayi McLearn, 1927, Royal Soc. Canada 

Proc. and Trans., 3d ser., v. 21, sec. 4, p. 73, pl. 1, figs. 5, 6. 

cf. Kanastephanus crickrnayi McLearn, 1929a, Canada Natl. 

Mus. Bull. 54, p. 22, pl.16, figs. 7, 8. 

The genus N O'J'lnWlnHe8 is probably represented by 

five fragments frmn the western interior region. One 

fragment obtained frmn the Sawtooth For1nation in 

western Montana, represents part of the body chamber 

but does not retain the lateral lappet. The other four 

fragments, obtained from four localities in the Twin 

Creek Limestone, are all septate but probably represent 

part of the penultimate whorl. The specimens all have 

a depressed whorl section, a wide umbilicus, and. coarse 

widely spaced ribs. There are two secondary r1bs for 

each primary rib. All the specimens are considerably 

corroded, but one is well enough preserved to show the 

presence of acute tubercles at the ventral ends of the 

primary ribs at about one-third of the height of the 

flanks. 

Figm·ed spec,irnens: USNM 132809, 132810. 

Occurrences: Twin Creek Limestone, upper part of Sliderock 

Member at USGS Mesozoic locs. 9090, 20978, 28645, and 28794; 

Sawtooth Formation at Mesozoic loc. 27337. 

Genus CHONDROCERAS Mascke, 1907 

Chondroceras cf. C. allani (McLearn) 

Plate 6, figures 1-3, 7, 8 

Detonticet·as oblatum (Whiteaves). Imlay, 1948, U.S. Geol. Survey Prof. Paper 214-B, p. 19, pl. 5, figs.l-5. 

cf. Samitoniceras allani McLearn, 1928, Canada Geol. Survey 

Bull. 49, p. 21, 22, pl. 8, figs. 1, 2. 

cf. Ohondroceras (Samitoni.ceras) allani McLearn. "\V'estermann, 

1956, Geol. Jahrb. Beihefte, no. 24, p. 107-108, pl. 12, fig. 

3a-c, text fig. 64. 

cf. Ohondroceras allani McLearn. Frebold, 1957, Canada Geol. 

Survey Mem. 287, p. 53, pl. 27, figs. 2a, h. 

One partly crushed ammonite from northwestern 

Montana was formerly assigned to Ohondrroce1Yl8 oblatum (Whiteaves) (Imlay, 1948, p. 19) because of its 

very simple suture line, an abrupt coarsening of ribbing 

at the adapical end of the body chamber, and a fairly 

broad depressed whorl section on the body chamber. 

The fineness of the ribbing on its outermost septate 

whorl, however, resembles that on 0. allani (McLearn) 

(1928, pl. 8, figs. 1, 2; Frebold, 195?, pl. 27, fig. 2a,b) 

1nore than that on 0. oblatum (Wh1teaves) (1876, pl. 

4, figs. 2a, b; McLearn, 1929a, pl. 15, fig. 1) . The 

available collections of 0. allatni (McLearn) from Canada (USGS Mesozoic locs. 14630 and 27642) include a 

few specimens that have a fairly broad whorl section and 

that show a fairly abrupt coarsening of ribbing on the 

body chamber. 

One difficulty in identifying the Montana specimen is 

that published descriptions of most of the Canadian 

species are based on single specimens and consequently 

the range of variation within the species is unknown. 

For example, Ohondroceras obla;tun1 (Whiteaves) is 

based on a single corroded specimen. It possibly includes other described species as suggested by Westermann (1956, p. 102), but such has not yet been demonstrated. 

The Montana specimen is of considerable interest because it is the only specin1en of Ohondroceras definitely 

identified in the western interior region of the United 
94 TWIN CREEK LIMESTONE IN THE· WESTERN UNITED STATES 

States. All other specimens from that region, previously assigned by the writer to Olwndroceras, are now 

included in the new genera. Parachondroceras and 

E ocephalites. 

Figured specimen: USNM 104134. 

Occurrence: Sawtooth Formation in Montana at'USGS Mesozoic loc. 19192. The specimen was collected from the spillway 

at Swift Reservoir about 21 feet above the base of the Sawtooth 

Formation. This occurrence is 2 feet above a thin conglomerate 

bed (unit 5 of Cobban, 1945, p. 1295) that has furnished Stemmatoceras and 12 feet below a shale bed (unit 10 of Cobban, 

1945, p. 1295) that has ·also furnished Stemm.atoceras. 

Genus PARACHONDROCERAS, n. gen. 

This genus is characterized by highly involute coiling 

on the septate whorls, by markedly eccentric coiling on 

the body chamber, by a high compressed whorl section, 

by a narrowly rounded to subacute venter, by fairly 

sharp ribbing on septate whorls, by disappearance of 

primary ribs on the body cha1nber, and by a rather 

simple suture line that has broad shallow lobes and saddles and a bifid second lateral lobe. 

The ribbing of the septate whorls resembles that of 

Ohondroceras lrf as eke (see illustrations in Westermann, 

1956, pls. 1-12), but differs by the thickening of the 

secondary ribs slightly ventrally and their arching forward on the venter. The suture line, also, is similar to 

that of Ohondroceras, although simpler. The genus 

differs fr01n Ohondrocera8 mainly by being discoidal instead of spherical, by having a vertical instead of a 

rounded umbilical wall, and by fading of its primary 

ribs on the body chamber. There is no evidence of a 

pronounced terminal constriction and collar as in 

0 lwndroceras. The fading of ribs on the body chamber 

is similar to that on Labyrinthoceras Buckman (1919, 

pl. 134). 

Parachondroceras resembles the Bathonian to Callovian family Tulitidae in its eccentric coiling and suture 

line, but has a 1nuch different whorl shape and on its 

septate whorls has much sharper ribbing. Overall, it 

shows 1nore resemblance to such genera as Ohondrocera8 

and Labyrinthoceras of the Otoitidae. 

The type.species of Parachondrocera~rs is P. andrmvsi 

Imlay, n. sp. 

P.arachondroceras andrewsi Imlay, n. sp. 

Plate 12, figures 5-18 

This species is represented definitely by 11 specimens 

and .Probably also by 10 fragmentary immature specimens. All the specimens except the holotype have been 

somewhat crushed. 

On the holotype the shell is compressed and involute. 

The venter is nearly acute on the penultimate whorl and 

is narrowly rounded on the body chamber. The whorl 

section is much higher than wide. The flanks are flattened and convergent. The umbilicus is very narrow 

on the septate whorl but widens abruptly on the adoral 

half of the body chamber. The umbilical wall is low 

and vertical, and rounds abruptly into the flanks. The 

body chamber is not completely preserved but occupies 

nearly three-fifths of a whorl and probably was not 

much longer. This probability is indicated by marked 

contraction of the adoral half of the body chamber and 

by the presence of a weak constriction at the adoral end 

of the body chamber. 

The immature whorls of this species are all somewhat 

crushed but appear to have had a high elliptical whorl 

section, a narrowly rounded venter, flat flanks, and a 

narrow umbilicus. 

The immature whorls bear sharp moderately spaced 

primary ribs that are nearly radial, or that incline forward· slightly on the lower two-fifths of the flanks. 

From the primary ribs arise two slightly weaker secondary ribs that become broader ventrally and arch forward gently on the venter. Most pairs of forked ribs 

are separated by single ribs that arise freely a little 

above the furcation zone. 

During further growth the primary ribs become 

broader and stronger relative to the secondary ribs. 

The secondary ribs become ·more common and more 

strongly arched forward. On largest septate whorls 

there are from three to four secondary ribs for each 

primary rib. 

On the body whorl the ribbing fades rather abruptly 

on the lower part of the flanks and is replaced by forwardly inclined striae. In contrast, the venter of the 

body whorl bears strong forwardly arched secondary 

ribs. 

The holotype at a diameter of 64 mm has a whorl 

height of 32 mm, a whorl thickness of 23 mm, and an 

umbilical width of 11.5 mm. At the diameter of 80 

mm the same dimensions are 40, 30, and 15 mm, respectively. At the maximum diameter of 90 mm the 

same dimensions are 90, 41, 29, and 18 mm. respectively. 

The suture line, well preserved on the holotype, is 

characterized by. broad, shallow lobes and saddles and 

particularly by the presence of a bifid second lateral 

lobe. It is similar in plan to that of Ohondrocera/3 

(Westermann, 1956, p. 53, 59, 71, 75, 81, 90, 101, 105, 107) 

but is less frilled. 

The ribbing on the larger septate whorls of this species greatly resembles that on some species of Ohrondroceras, such as figured by McLearn (1928, pl. 8, figs. 

1-4; 1929a, pl. 12, figs. 1-3, pl. 13, figs. 4, 5), and was 

the basis for some identifications of that genus in previous reports (Imlay, 1945, p. 1021; 1948, p. 14 in part; 

1950a, p. 39; 1956h, p. 577). 
SYSTEMATIC DESCRIPTIONS 95 

These identifications were supported by such features 

as the suture line and the small mnbilicus. The species, 

however, is clearly differentiated from any species of 

0 hdndrocera8 by having a high com pressed whorl section instead of a rounded whorl section and by the 

fading out of the prima,ry ribs on the body chamber. 

Small crushed fragments of the species may possibly be 

distinguished from similar fragments of Ohondroceras 

by the thickening ventrally and arching forward on the 

venter of the secondary ribs. 

The species is named in honor of David A. Andrews 

who collected the holotype specimen. 

Types: Holotype, USNM 132811; paratypes, USNM 12221, 

132812-132814. 

Occzwrences: Piper Formation, middle limestone member in 

Montana at USGS Mesozoic locs. 17662, 19153, 19214, 20864, 

27035, and 27628. Twin Creek Limestone, Rich Member in 

·wyoming at Mesozoic loc. 21629; Twin Creek Limestone, probably from the Rich Member, in Utah at Mesozoic loc. 8181; 

"Lower Sundance" Formation in Wyoming at Mesozoic locs. 

20967, and 27724. . Small fragmentary specimens of Parachond1'oceras, probably belonging to P. andrewsi Imlay, n. sp., 

occur in the Sawtooth Formation in Montana, at Mesozoic Iocs. 

22170 and 28670. 

This species in Montana occurs in the upper part of 

the middle limestone member of the Piper Formation 

(USGS Mesozoic locs. 19214, 208464, and 27628). In 

west-central Wyoming it occurs (USGS Mesozoic loc. 

21629) near the middle of the Rieh Member of the Twin 

Creek Limestone exposed on the South Fork of Fontenelle Creek. In north,vestern Wyoming it occurs 

(USGS Mesozoic locs. 20967 and 27724) from 10 to 13 

feet above the base of unit 10 of the described section 

of the "Lower Sundance" Formation north of Lower 

Slide Lake on the Gros Ventre River. This occurrence 

is in shaly limestone that is considered to be equivalent 

to the Rich Member of the Twin Creek Limestone of 

nearby areas. In north-central Utah it occurs in the 

lower part of the Twin Creek Limestone (USGS 

Mesozoic loc. 8181). If the location given is correct it 

should be from the Rich Member rather than from the 

Sliderock Member. It is associated with Sohlites, n. 

gen., in the Piper Formation at USGS Mesozoic loc. 

19214. ParachondrroceT~as cf. P. a'ndrew·s·i Imlay was 

obtained ·from the lmver 20 -feet of the Sawtooth Formation at USGS Mesozoic loc. 28670 and from the top of 

unit 8 (USGS Mesozoic loc. 19153) in the type section 

of the Ellis Group (Cobban and others, 1.945, p. 452). 

Parachondroceras :filicostatum Imlay, n. sp. 

Plate 13, figures 11-14, 16-25 

This species is represented by 16 specimens of which 

most are fragmentary and have been crushed laterally. 

The shell is compressed and involute. The whorl section is much higher than wide. The venter is narrowly 

rounded on the inner whorls, subacute on the largest 

septate whorl, and narrowly arched on the body chamber. The flanks are flattened, subparallel in immature 

specimens, and convergent on adult specimens. The 

umbilicus is very narrow on the septate whorls but 

widens abruptly on the adoral half of the body chamber. 

The umbilical wall is low and vertical and rounds 

abruptly into the flanks. The body chamber is incompletely preserved but occupies at least half a whorl. 

The ribbing on the immature whorls consist of fairly 

sharp 1noderately closely spaced primary ribs and very 

fine densely spaced secondary ribs. The primary ribs 

begin low on the umbilical wall and incline forward 

slightly on the lower two-fifths of the flanks. From the 

primary ribs arise ·two or three secondary ribs that are 

very weak near the zone of .furcation but become a little 

stronger ventrally. In addition, from one to two secondary ribs arise freely between successive forked ribs 

near the middle of the flanks. All secondary ribs are of 

equal strength ventrally, incline slightly forward on the 

flanks, and arch gently forward on the venter. 

During subsequent growth the fine dense forwardly 

inclined secondary ribs persist on the venter as far as 

the adapical end of the body chamber. The primary 

ribs gradually become broader and fainter and eventually fade out on the largest septate whorl. Most of 

the body chamber is smooth. 

The holotype at a diameter of 64 mm has a whorl 

height of 34 mm, a whorl thickness of 20 mm, and an 

umbilical width of 9.5 mm. At a diameter of 81 mm, 

the sa1ne dimensions are 40, 27, and 11 mm, respectively. 

At the maximum diameter of 90 mm, the same dimensions are 43, 30, and 161nm, respectively. 

The suture line has broad, shallow saddles and lobes. 

The first lateral lobe is irregularly trifid. The second 

lateral lobe is bifid. 

This species differs from PaTachondrocera8 aJ:ndrewsi 

Imlay, n. sp., mainly 'by its much finer and denser ribbing that disappears at a much earlier growth stage. 

Because it is associated with P. a'ndrew8i at several 

loca~ities, it might be considered to be a finely ribbed 

variant of that species. This possibility is unlikely, 

however, because the collections do not contain any 

specimens having ribbing intermediate in coarseness. 

The available specimens are either finely ribbed or fairly 

coarsely ribbed. 

Types: Holotype, USNM 132815; paratypes, USNM 132816-

132819. 

Occurrences: Piper Formation, middle limestone member, in 

Montana at USGS Mesozoic locs. 15635, 19158, 19214, 19630, 

20864, 25866, 27624, and 27628; Sawtooth Formation, lower part, 

in Montana at Mesozoic loc. 28800; unit 10 of "Lower Sundance" 

Formation at Lower Slide Lake in northwest Wyoming (Imlay, 

1956a, p. 70) at Mesozoic locs. 20962 and 22112; Twin Creek 
96 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

Limestone, Rich Member in Wyoming at Mesozoic loc. 20963. 

Fragments questionably assigned to this species have been found 

in the Piper Formation at Mesozoic loc. 17662 and in the lower 

part of the Twin Creek Limestone in Utah at Mesozoic loc. 

21468. 

This species in central and southern Montana ranges throughout most of the middle limestone member of the Piper Formation and through the lower part of the Sawtooth Formation. In 

west-central ·wyoming it was found in the upper 30 .feet of the 

Rich Member of •the Twin Creek Limestone. In northwestern 

'Vyoming on the north side of Lower Slidt' Lake it ·was found 

from 10 to 20 feet above the base of unit 10 of the published 

section (Imlay, 1956a, p. 70). This occurrence is near the middle 

of a shaly sequence that is considered equivalent to the Rich 

Member of the Twin Creek Limestone in nearby areas. 

Genus MEGASPHAEROCERAS Imlay, 1961 

Megasphaeroceras cf. M. rotundu:m. Imlay 

Plate 14, figures 1-6, 9-16; plate 16, figures 1-4 

Twenty-two internal 1nolds frmn the Twin Creek 

Limestone closely resemble lJ! egasphaeroceras r·otundum 

Imlay (1962b, p. A10, pl. 3, figs. 1, 4-12) frmn the Cook 

Inlet region, Alaska, in their stout ·to globose form, tiny 

umbilicus, and fine ribbing, and they are probably 

within the range of variation of that species. The body 

chanlher, preserved on three specimens, occupies about 

~three-fourths of a whorl and is slightly contracted a.t 

the apm~ture. The aperture curves forward and on the 

internal mold is 1narked by a broad constriction that is 

followed by a weak swelling. 

The small septate whorls bear fine sharp flexuous ribbing that curves forward on the flanks and arches gently 

:forward on the venter. Some primary ribs rem·ain 

simple, but most of ~them bifurcate at about the top of 

the lower fourth of the flanks. From one to two 

secondary ribs arise freely near the zone of furcation 

between adjoining primary ribs. The secondary ribs 

are nearly as sharp and strong as the primary ribs on 

the flanks and become stronger ventrally. On the body 

whorl of internal molds the ribbing fades out 'toward 

the aperture. Fading of the ribs ·occurs first on the 

lmver part of <the flanks and finally on the ventral region. 

F·ig1wed specimens: USNM 132820-132822, 132939. 

Occurrences: Twin Creek Limestone, upper part of Sliderock 

Member a't USGS Mes.ozoic locs. 12019, 2034:4, 21618, 28483, 

28644, 2·8645, 2'8648 and 28794. 

Megasphaeroceras a:ff. M. rotundum Imlay 

Plate 13, figures 6-10 

Six internal molds differ fron1 M egasphaer·oceras 

1'otwndun& Imlay (I1nlay, 1962b, p. A10, pl. 3, fig. 1, 

4-12) and from closely similar specimens herein compared with that speeies by having somewhat coarser 

ribbing thwt persists onto ·the adult body chamber. 

The umbilicus appears to he a little wider, also, on two 

specimens but on the others is very sm·all. This apparent difference in the mnhilicus is prdbably due to 

deformation and defeetive preservation. The ·adoral 

end of the ·adul1t body whorl is slightly contra~d from 

the preceding whorl. 

The suture line (pl. 13, fig. 10) grewtly resembles that 

of the compressed variant of M egasphaeroceras rotundurn Imlay herein illustrated (pl. 13, fig. 15). Both 

have a broader first lateral saddle than present on the 

holotype of that species (!Inlay, 1962b, pl. 3, fig. 9). 

Figured spec-imens: USNM 13282'3, 132824. 

Occurrenc-es: Twin Creek Limestone, Sliderock Member 1att 

USGS Mesozoic 1ocs. 16016,20978, 28483, and 28635. 

All oollections, except possibly those from Mesozoic loc. 16016, 

were obtained near 1the top of the member. 

Megasphaeroceras sp. undet. 

Plate 13, figures 4, 5 

One internal1nold of a. body chamber probably represents an adu~t as indicated by weakening of the ribbing 

at d1e adoral end of the body chamber and by the shape 

of the umbilicus. The specimen differs from those in 

the Twin Creek Limestone that are herein compared 

with M. rotundurn Imlay by having 'a compressed subquadrate whorl section, a much wider umbilicus, and 

fairly strong ribbing on the internal mold of the body 

chamber. The shape of the umbilicus suggests that the 

body chamber is more contracted. 

This specimen probably represents a new species; 

however, the upper Bajocian collections of lJf egaspha.-

eroceras from Al'aska contain three similarly compressed specim.ens that also bear ribs on the internal 

molds of the ·adult body chamber (USGS Mesozoic locs. 

20001, 21314, and 21318). These are associated with 

the typical stout to globose variant of Jf. rotundunu 

Imlay ( 1962b, p. A10, pl. 3, figs. 1, 4-12) as well as with 

others that are less stout. One of these compressed 

specimens, herein illustrated for comparative purposes 

(pl. 13, figs. 1-3, 15), has a wider umbilicus than the 

typical variant of M. rotwnd'ltnL Imlay, but is less wide 

than the compressed specimen from the Twin Creek 

Limestone. It bears a resemblance to some sm·all specimens of {fra:nocepka.lites from Greenland ( Callmnon, 

1959, pl. 17, figs. 3, 4; pl. 18, figs. 1, 2) but has weaker 

pri1nary ribs and less eccentric coiling. 

Figu1·ea spec·irnens: USNM 132825. 

Occurrence: Twin Creek Limestone, upper 20 feet of the 

Sliderock Member 'at USGS Mesozoic loc. 20975. 

Genus EOCEPHALITES, n. gen. 

This genus is characterized by fairly involute coiling; 

by the clevelop1nent of slightly eccentric coiling near the 

adult aperture; by a stout whorl section; by fairly sharp 

ribbing ·that inclines forward on the flanks, arches 

gently forward on the venter, and does not weaken on 
SYSTEMATIC DESCRIPTIONS 97 

the adult body chamber except slightly on internal 

molds; by the simple termination of the aperture; and 

by H1e suture line having broad saddles, trifid lobes, and 

a long ventral lobe. 

The 1type speeies of E ocephalites is E. prhnus I1nlay, 

n. sp. 

This genus differs from 111 egasplzaeroce·Pas Imlay 

(1961, p. 470; 1962b, p. A9-A10, pl. 3, figs. 1-12), with 

which it is associated in the Twin Creek Limestone, by 

having a wider and deeper mnbilieus, coarser ribbing 

that does not \veaken appreeiably on the molds of adult 

body chambers, and by the much broader saddles of the 

suture line. It differs from Ohondroceras by not becoming scaphitoicl, by its aperture terminating simply instead of bearing a marked constriction and flare, and by 

its second lateral lobe being trifid instead of bifid. It 

differs frmn Paracephalites (Buckman, 1929, p. 8; Frebold, 1963, p. 8, 9) by having a narrower adult umbilicus, 

by its ribbing remaining fairly strong on the adult body 

chmnber instead of fading, and by its suture line having 

much broader saddles. H. differs from Oranocephalites 

(Spath, 1932, p. 14-16) by not developing a scaphitoid 

body chamber and by having wider saddles (Spath, 

1932, pl. 1, figs. 4a, b; pl. 3, fig. 5), although some specimens of Oranocephalites have fairly wide saddles 

(Spath, 1932, p. 22, pl. 1, fig. 6 ~ pl. 4, fig. 8). It differs 

from Arctocephalites (Spath, 1932, p. 32-34) by not developing a smooth, or nearly sn1ooth, body ehamber and 

by having much broader saddles. 

Among these genera, Eocephalites probably shows the 

greatest resemblance to the small- and intennediatesizecl whorls of Paracephalites, such asP. sawthoothenshs (Imlay) (1962a, pl. 3, fig_. 1-5, 7, 8~ pl. 4, fig. 2). 

Inasmuch as it oecurs in the same fa.unal provinee as 

Para.cephalite:rs and in only slightly older beds, it could 

be the aneestor of that genus. 

The overall charaeteristics of E ocephalites suggest 

that it is in the same family as Paracephalites, Oranocephalites, and A1'ctocephalites which Spath (1932, p. 9) 

placed in the family Macroeephalitidae and which Arkell and others ( 1957, p. L-301) placed in the family 

Cardioceratidae and in the subfamily Cadoeeratinae. 

Callmnon ( 1959, p. 507-511) stated that 01'anocephalites 

is characteristic. of beds of Bathonian age in East Greenland, that the oldest occurrences of the genus are in beds 

of possible late Bajocian age, that it bears close resemblances to Ohondrocerms of middle Bajocian age, and 

that it is the earliest of the Cadoceratinae. The resemblance of Eocephalites to Oranocepharites suggests that 

the Cadoceratinae began in late Bajocian time. 

221-691 0-66--6 

Eocephalites primus Imlay, n. sp. 

Plate 15, figures 3-5, 9-13, 16-20; plate 16, figures 8-14 

This species is represented by 43 specimens of which 

1nost are fragmentary and distorted internal molds. 

The septate whorls are fairly stout, wider than high, 

and are widest at about one-fou!lth of the height of the 

flanks. The venter is fairly broad and rounds evenly 

into the flanks. The flanks round rather abruptly into 

a low steeply inclined umbilical wall that becomes vertical near its base. The tnnbilicus is narrow but not occluded. The adult body whorl is less stout, is a little 

wider 'than high, and becomes slightly contracted from 

the septate whorls. The body chamber occupies about 

three-fifths of a whorl. The aperture on the internal 

1nold is 1narked by a forwardly inclined weak constriction tha1

t is followed by a weak swelling. 

The ribbing on the inner septate whorls is fairly 

sharp. The primary ribs begin low on the unbilical wall 

and curve forward on the lowe.r third of the flanks 

where most of them pass into two slightly weake.r secondary ribs. A few primary ribs remain unbranched, 

and a few ribs arise freely along the zone of furcation. 

The secondary ribs ineline forward on the flanks and 

venter. 

The ribbing on the larger septate whorls gradually 

becmnes stronger and more widely spaced during 

growth. The primary ribs become stronger relative to 

the secondary ribs and develop a pronounced forward 

curve. The secondary ribs arise in pairs, incline considerably forward on the flanks, and arch gently forward on the venter. A few ribs arise freely at or above 

the zone of furcation resulting in the presence of about 

three secondary ribs for each prima.ry rib. 

The ribbing on the body whorl gradually becomes 

sparser adorally and on internal molds weakens considerably as eompared with the ribbing on the septate 

whorls. On body whorls 'that are represented by external1nolds or that retain some shell material, however, 

the ribbing is fairly strong. 

The suture line, partly preserved on three specimens, 

is characterized by broad saddles, a trifid second lateral 

lobe, and a long ventral lobe. 

One para type (pl. 15, figs. 3-5) at a diameter of 48 

m1n has a whorl height of 25 mm, a whorl thickness of 

32 m1n, and an umbilical width of 8.5 mm. It has been 

slightly distorted at the adapical end. On the holotype 

at a dia1neter of 76 mm, the same dimensions are 36, 41, 

ancl17 mm, respectively. 

This species differs from 111 egasphaeroceras rotundru,rn Imlay, 1962b, p. A10, pl. 3, figs. 1, 4-12) from beds 

of late Bajocian age in Alaska by having a little wider 

umbilicus and 1nuch stronger and sparser ribbing, by the 

persistence of ribs on the body chambers of external 
98 TWIN CREEK LIMESTONE IN THE WESTERN UNITED STATES 

· molds, and by the much broader saddles of its suture 

lines. 

The ribbing on t.he septate whorls of Eocephalites 

prinvws Imlay, n. sp., is similar to that on some species 

of Oranoc.ephalites from Greenland (Spath, 1932, pl. 1, 

figs. 3a, b ; pl. 2, figs. 3a, b; pl. 4, figs. 3a, b; pl. 5, figs. 2a, 

b, 7) except for having a 1nuch 1nore pronounced forward curvature. The adult of Oranocephalites is distinguished by developing a scaphitoid body chamber 

whereas the adult body chamber of E. primns is 

eontracted only slightly near the a.perture. 

Eocephalites prirruus Imlay, n. sp., shows even more 

resemblance in ribbing to species of Paracephalites (Frebold, 1963, p. 8-13, pl. 1, 2, 4; Imlay, 1962a, pl. 3-5) fron1 

Alberta, British Columbia, and Montana, but Hs ribs do 

not fade out nearly as much on the body chamber, its 

suture line has n1uch broader saddles, its adult un1bilicus 

is narrower and shallower, its umbilieal wall is more 

gently inclined, and its ribs arch gently forward on the 

venter instead of crossing the venter transversely. The 

s1nall septate whorls of Paracephalites sa,wthoothensis 

I1nlay (1962a, pl. 3, figs. 1-5, 7, 8) greatly resmnble the 

small whorls of Eocephalites prin&u8 Imaly, n. sp., in 

shape and ribbing, but are distinguished by 1nuch 

sharper ribbing that curves backward slightly on the 

flanks. 

Types: Holotype, USNM 132826; para types, USNM 132827-

132832, 132938, 132942. 

OccutTences: Twin Creek Limestone, upper part the Sliderock Member at USGS Mesozoic locs. 20344, 20345, 20961, 20966, 

20968, 20970, 20973, 20988, 21618, 21628, 22638, 27729, 28483, 

28484, 28644, 28645, and 2S648. Most occurrences are in the 

upper third of the member, ranging from the top to within 35 

feet of the top. 

Eocephalites 1 sp. undet. A 

Plate 15, figures 6-8 

One small speeimen from the Twin Creek Limestone 

represents an adult as shown by the presence of an apertural constrietion and by eontraetion of the body ehamber near the aperture. 

The shell is stout.. The whorl seetion is depressed sub- . ' mrcular, and wider than high. The flanks and venter 

are evenly rounded. The umbilieus is narrow but widens 

considerably near the aperture. The umbilieal wall is 

low and steep and rounds evenly into the flanks. The 

body ehamber oceupies about three-fifths of a whorl. 

The aperture is imperfectly preserved, but its lmver 

part inclines strongly forward and is constrieted. 

The ribbing is high and sharp. The primary ribs 

begin low on the umbilieal 'vall, incline fonvard on the 

lower two-fifths of the flanks, and generally divide into 

t;vo sli?"htly weaker secondary ribs. A fe,v seeondary 

ribs arise freely below the middle of the flanks. All 

secondary ribs ineline gently forward on the flanks and 

venter. 

The specimen at a diameter of 45 n1m has a whorl 

height of 22 mm, a whorl thiekness of 29 mm, and an 

umbilical width of 6.5 mm. The suture line cannot be 

traced. 

This species differs from Eocephalites prinLu.s Imlay, 

n. sp., by having much sharper and sparser ribbing, a 

more depressed whorl section, and by being much smaller 

at adult size. 

Figured specimen: USNM 132833. 

Ocmwrence: Twin Creek Limestone, Sliderock Member, upper 

25-30 feet, at USGS Mesozoic loc. 28648. 

Eocephalites1 sp. undet. B 

Plate 15, figures 14, 15 

Ten fragmentary distorted ammonites frmn the Twin 

Creek Limestone differ from E ocephalite8 pri1nns Imlay, n. sp., by ha.ving a stouter form and 1nueh coarser 

ribbing. The whorl section is subeireular and is wider 

than high. The flanks and venter are evenly rounded. 

The umbilieus, preserved only on one large specimen, is 

narrow and fairly deep. The umbilical wall is low and 

vertical and rounds evenly into the flanks. The adult 

body ehamber consists of about five-sixths lif a whorl 

and is scaphitoid near its adoral end. The aperture is 

not preserved. 

The ribbing is coarse and sparse. The primary ribs 

incline slightly forward on the lower fourth or third 

of the flanks where 1nost of them pass into pairs of 

slightly weaker seeondary ribs. A few pri1nary ribs 

remain simple, and -a few secondary ribs arise freely 

near t.he zone of furcation. The seeondary ribs ineline 

forward gently on the flanks and venter. 

The suture line cannot be traced. 

Figured specimen: USNM 132834. 

Occurrences: Twin Creek Limestone, upper part of the Sliderock Member at USGS Mesozoic locs. 20979, 21618, 28644, and 

28648. Most of the fossils are from the upper 30 feet of the 

member. 

Eocephalites 1 sp. undet. C 

Plate 15, figures 1, 2 

One fragmentary speeimen is po~sibly an immature 

form of the speeies herein deseribed as Eocephalites? 

sp. undet. B, but it appears to have 1nueh stronger 

primary ribs and a more depressed whorl seetion. Near 

the a pi cal end of the speeimen, one set of ribs is cut off 

obliquely by another set of ribs. This ehange corresponds roughly with the beginning of the body ehamber. 

Aside from this feature, the charaeter of the ribbing is 

similar to that on (} hond/rocera8 ( Praetulites) kroizingai 

Westermann (1956, p. 108-111, pl. 13 and text figs. 65, 

66; Kruizinga, 1926, p. 52, pl. 12, fig. 4, pl. 14, figs. 2, 3) 

from the Sula Islands in Indonesia. The holotype of 
REFERENCES 99 

W ester1nann's species was once considered by Arkell (in 

Arkell and Playford, 1954, p. 583) to belong to Ohondroceras and later (Arkell, 1956, p. 440) was given as 

"* * * a Sphaeroceratid at least closely related to 

Chondroceras * * *." 

Figured specimen: USNM 132835. 

Occurrence: Twin Greek Limestone, upper part of .the Sliderock Member at USGS Mesozoic loc. 28644. 

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INDEX 

[Italic page numbers indicate major references or fossil descriptions] 

A Page 

Actinastrea__ _ _ _____________ __________________ 35 

hyatti__ ____________ _______________________ 33, 35 

acuticostata, Myopholas________________________ 87 

agassizi, Anatina____________________________ __ 88 

albertense, Sternmatoceras _____ 25, 26, 34, 91; pls. 10, 16 

allani, Chondroceras _____________________ 34,98; pl. 6 

Chondroceras (Saxitoniceras) _ _ _ _ _______ ___ 93 

Saxitoniceras __ --------------------------- 93 Teloceras _ _ ____________ _ __________________ 90 

americana, Trigonia _________ 25, 26, 27, 32, 34, 35, 49,81 

Anatina agassizi_ _ _ _ __ _ __ _ ___ _ _ _ _ ____ ___ _ ___ _ _ 88 

astieriana_ _______ ___ _ ____ __ ____ ________ _ __ 88 

andrewsi, Parachondroceras __ 2, 32, 34, 58, 94, 95; pl. 12 

Arapien Shale ________________________________ 21,30 

arcicosturn, stemmatoceras_____________________ 2, 

21, 24, 25, 26, 34, 58, 59, 90, 91; pls. 8, 9 Arcidae_____ _ ___ __ _ ___ _ ______ __ __ ___ _ _ __ _ _ ____ 58 

Arcticoceras __ _________________________________ 60, 61 

codyense_ _ __ _______ __ _ _____ _ _ ________ __ __ _ 87 

Arctocephalites ______ __________________________ 59, 97 

arcuata, Quenstedtia __ ------------------------ 85 

Astarte ____ ------------------------ 2, 35, 57, 58, 82,84 depressa_ _ ___ _ __ _ __ _ ____ ______ ___ ____ _ ___ _ 83 

livingstonensis____ _ ____________ _________ ___ 35 

meekL _________ 25, 26, 27, 32, 39, 43, 49, 82, 83; pl. 5 

marion __ --------------------------------- 35 packardi__ ___ ____ ___________ __ _____ ____ ___ 83 

(Coelastarte) livingstonensis _ _ _____________ 3, 

24, 25, 26, 32, 34, 35, 39, 60, 83 

Astartidae_________ __ _ ___ _ __ _ _______ ____ _ ____ _ 58 

astieriana, Anatina __ ------------------------- 88 

B 

Bajocian ammonites _________________________ _ 

Bakevillia___ __ _____ _ _ _ _ ___ ___ _ ___ ___ _ _ ___ ___ _ _ 78 

Barbatia___ _ ___ ___ _ _ _ _ _ ___ _ __ _ __ __ __ _ ___ ____ _ _ 58 

sp_ _ ___ ____ ______ _ _ __ _ __ _ ___ _ _ __ _ ___ _ __ ___ 32 

bathonica, Cypricardia __ ---------------------- 76 

bathonicus, Grarnmatodon______________________ 76 

c 

Cadoceras ___ __ -----" 1, 39, 43, 48, 49, 59, 60, 61, 86, 87,88 sp ______________________________________ 3, 49,60 

cadominensis, Gryphaea_ _ __ _ ___ ___ _ __ _ _ __ _ __ __ 81 

Camptonectes __ --------------------------- ____ 2, 

3, 16, 19, 22, 23, 24, 35, 36, 38, 39, 46, 47, 

48, 50, 52, 57, 58, 78. 

distans____ ___ ___ ____ ___ ___ ___ _ __ _ _ __ __ _ _ __ 19 

platessiforrnis __ 25, 26, 27, 32, 34, 35, 39, 49, 79; pl. 2 

stygius ___________ _____________ 25, 26, 32; 43, 49,78 

sp_ ------------------ ____________ 25, 26, 27, 32,43 

canadensis, Quenstedtia_____ _ __ _ __ _ __ _ __ _ ___ _ _ _ 85 

Thracia_______________ _________ _________ __ 84 

Canyon Springs Sandstone Member of Sundance __________________________ 43, 45,61 

Cardiidae_ _ _ _ ___ _______ __________ ___ ___ ___ ___ 58 

Cardioceras ___________________________________ 62, 85 

Cardioceratidae_ _ _ _ _ ___________________ ______ 58 

carlottensis, Zernistephanus ___________________ 91,92 

Carmel Formation ______________________ 4, 21, 24,35 

Cephalopoda_________________________________ 88 

Cercomya __ ________________________________ 58, 87, 88 

punctata ________________ 31,32,33,59,61,87; pl. 5 

Ceritella sp ___ -------------------------------- 25,26 

Chlamys__ __ _ ____ ___ _ _ _ __ __ __ ___ _ _ __ _ __ _ _ __ _ __ 58 

sp ________________________________________ 25, 26 

Page 

Chondroceras _____ __ 21, 27, 31, 35, 58, 59, 93, 94, 95, 97,99 

allanL _____________________________ 34,93; pl. 6 

lYiascke ____ _______________________________ 3I, 94 

oblatum________ _ ___ ______ _ __ _ _ __ ___ _ _____ _ 93 

(Praetulites) kruizingai____________________ 98 

(Saxitoniceras) allani______________________ 93 

Chondroceras-Stemmatoceras zone______________ 59 

cinnabar ens is, Isocyprina __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 34 

Pronoella _______ ---------- _______ 25, 26, 32, 49,84 

Pseudolimea_ _ _ ___ _ ___ _ __ _ _ __ _ _ __ _ ___ __ __ _ 35 

Cobbanites _ _ ___ ___ __ __ ___ _ __ _ ___ _ _ _ __ __ _ __ _ _ __ 60 

codyense, Arcticoceras _ _ _ ___ __ ____ __ _ _ _ _ __ __ _ __ 87 

TVarrenoceras_ -------------------- ________ 60 

Coelastarte ______________________________ 57, 58, 83,84 

livingstonensis_____ _ ___ ______ __ _ _ _ _____ _ __ _ 34 

(Coelastarte) livingstonensis, Astarte___________ 3, 

24, 25, 26, 32, 34, 35, 39, 60, 83 

Comparisons with other faunas______________ 60 

concinna, Cucullaea___________________________ 76 

concinnus, Grammatodon______________________ 76 

conradi, Vaugonia_ 27, 3I, 32, 33, 34, 40, 43, 44, 49, 59, 82 

Cook Inlet region, Alaska____________________ 62 

Coral, undetermined_________________________ 27 

corallina, Opis (Trigonopis)___________________ 83 

Corbicella______ _ __ _ _ _ _ __ ____ __ _ __ _ _ __ _ ___ _ __ _ _ 58 

sp __ ----------- __ --- _____________ ----- __ _ _ 27 

Corbula _____________ --------------- 2, 57, 58, 60, 85,86 

munda _____________________ 32, 61, 85, 86, 88; pl. 2 

Corbulidae_ _ __ __ _ __ ____ __ _ __ _ ___ __ __ _ __ _ __ _ __ 58 

Cossmannea_ _ _ _ _ _ __ _ ___ _ __ _ __ _ __ _ _ __ _ _ __ _ __ _ _ 33 

sp __ -------- ____ --------- ______________ 25, 26, 27 

Cranocephalites ______________________ 27, 58, 96, 97,98 

crickmayi, Kanastephanus_____________________ 93 

Normannites _________________ 25,26,34,93; pl. I2 

Crinoid columnals ________________ 25, 26, 27, 32, 43,49 

Ctenostreon ___________________________ 2, 57, 58, 78,79 

gikshanensis ____ -------- 25, 26, 27, 43, 49, 79; pl. 2 

Cucullaea______ _ _ _ _ _ _ _ __ _ __ _ __ _ __ _ _ __ _ __ _ __ _ _ _ 76 

concinna___ _ ___ _ _ _ __ ___ _ __ _ __ _ _ __ _ ______ _ _ 76 

hagueL _ ______ __ _ _ _ ____ _ __ _ __ _ _ ___ ____ ____ 75 

livingstonensis_____________ ____ ___ _________ 76 

Cucullaeidae_ _ _ __ _ _ _ _ _ _ ___ _ __ _ __ ____ _ _ ____ __ _ 58 

Cylindrobullina sp ____________________________ 25,26 

Cypricardia bathonica_ _ _ _____ _______ _______ ___ 76 

hagueL _ -------------------------------- __ 76 

Cyprinidae___________________________________ 58 

D 

Defonticeras oblaturn _ _ __ _ _____ ____ _ __ _ _ __ _ _ __ _ 93 

depressa, Astarte______________________________ 83 

Dettermanites_ ___ _ __ _ __ _ __ _ __ _ __ _ _ __ _ ___ _ __ _ __ 62 

distans, Carnptonectes_________________________ 19 

Distribution of fossils ______________ ----------_ 62 

dolobrata, Gervillia ____ ------------------------ 35 

E 

Echinoid fragments _______________________ 32, 43,49 

Echinoid spines ______________________________ 25, 26 

elegantissima, Trigonia__ ______________ ________ 27, 

3I, 32, 33, 34, 39, 43, 49, 59, 82 

Eocephalites ____________________ 2, 27, 58, 59, 94, 96,97 

primus _____________ 2, 25, 26, 58, 97, 98; pls.I5, I6 

sp. A __________________________________ 25, 26,98 

sp. B ___________________________ 25, 26, 98; pl. I5 

sp. c ________ -------------------------- 25, 26,98 

sp_ -------------------------------- 25, 26; pl. I5 

Page 

Ermoceras ______________________________ 3I, 58, 9I, 92 

mogharense_ _ _ _ __ _ __ _ _ __ ___ _ __ ___ _ _ _ __ _ __ _ 92 

Eutrephoceras______ __ __ _ __ _ ___ _ _ _ _ __ _ __ _ __ _ __ _ 58 

sp ________________________________________ 25, 26 

F 

Faunal zones in the western interior region___ 59 

Faunas, comparisons, central and southern 

Utah_____________________________ 60 

comparisons, Cook Inlet region, Alaska__ 62 

comparisons, Montana, Wyoming, and 

South Dakota____________________ 60 

comparisons, western interior of Canada_ 61 

filicostaturn, Parachondroceras _ _ 2, 32, 34, 58, 95; pl. I3 

fontenellensis, Prorokia________________________ 1, 

2, 24, 25, 26, 3I, 32, 34, 57, 59, 83, 84; pl. 5 

forrnosus, Modiolus____________________________ 77 

fraterna, Gryphaea planoconvexa_______________ I, 

24, 25, 26, 3I, 32, 57, 59, 80; pl. 4 

G 

gallatinensis, Homornya _______________________ 25,26 

Gastropods _______________________ 25, 26, 27, 32, 43,49 

Geographic distribution of fossils_____________ 62 

Gervillia _________________________________ 2, 58, 77,78 

dolobrata ____ _____________ ------ _ __ _ ______ _ 35 

montanaensis _____ ---------------------- I, 3, 27, 

3I, 32, 33, 34, 35, 40, 44, 59, 77, 78; pl. 1 

sparsilirata _______________________________ 77, 78 

(Gervillia) perplana, Myalina__________________ 78 

gikshanensis, Ctenostreon ____ 25, 26, 27, 43, 49, 79; pl. 2 

Giraffe Creek Member_---------------------- 50 

glabrescens, Paracephalites__ __________________ 59 

Goniomya _____________ --------------------- 2, 58,86 

montanaensis ___ -------------------------- I, 

24, 25, 26, 27, 31, 32, 34, 59, 86; pl. I 

Gowericeras__ ______ _ _ __ ________ _ ______ _ ___ _ _ _ _ 60 

Gowericeras and Kepplerites zones __ ---------- 60 

Grarnmatodon ________________ _________ 2, 57, 58, 75,76 

bathonicus ______________________________ -_ 76 

concinnus __ -----------------------------·- 76 

haguei_ _________________ 25, 26, 27, 32, 75, 76; pl. I 

inornatus_ _______ ________ _________________ 76 

jurianus_ ----------------------------- ____ 76 serniornatus____ _ __ _ ___ _ __ _ _ ___________ __ _ _ 76 

sonninianus __________ -------------------- 76 

Gryphaea ____________________________ ------ 2, 23, 24, 

30, 36, 45, 49, 53, 54, 57, 58, 60, 6I, 80, 8I 

cadominensis __ ______________________ ------ 81 

impressimarginata ____________ 48, 59, 60, 61, 62,81 

nebrascensis __________________________ 1, 2, 3, 34, 

43, 46, 48, 49, 53, 60, 61, 62, 77, 81' 87, 88 

planoconvexa __ --------------------------- 1, 

2, 3, 21, 31, 32, 34, 59, 78, 80, 81, 82; pl. 3 

fraterna___ ______ ____ _ _________________ 1, 

2, 24, 25, 26, 31, 32, 57, 59, 80, 81; pl. 4 

sp_ --------------------------------------- 27 

Gypsum Spring Member__________________ 17 

H 

haguei, Cucullaea. ___ --------- ---------------- 75 

Cypricardia ____ _______ -------------------- 76 

Grammatodon _________ 25, 26, 27, 32, 75, 76; pl. 1 

Idonearca ___________ 25, 26, 32, 34, 35, 61, 76; pl. 1 

Haidaia _____ --------------------------------- 58,82 

103 
104 

Page 

(Haidaia) yellowstonensis, Myophorella________ 27, 

31, 32, 59, 82 

hardyi, Myopholas ______ 1, 3, 42, 43, 60, 61, 86, 87; pl. 5 

henryi, TVarrenoceras__________________________ 60 

Homomya_ _ _ ___ _ _ _ __ _ __ _ __ _ __ __ _ ___ __ _ _ __ __ __ 2, 58 

gallatinensis_ ------ ________________________ 25, 26 

Hulett Sandstone Member, Sundance Formation___________________________ 52 

humphriesianum, Stephanoceras _______________ 24, 59 

hyatti, Actinastrea ____ _________________________ 33, 35 

iddingsi, Isocyprina_ _ _ _____ __ ________ ___ ______ 35 

Pronoella ___________________________ 25, 26, 27, 84 

Idonearca ___ __________________________________ 58, 76 

haguei_ ____________ 25, 26, 32, 34, 35, 61, 76; pl. 1 

impressimarginata, Gryphaea ______ 48, 59, 60, 61, 62,81 

inaequiplicata, Pholadomya ______ 25, 26, 27, 86; pl. 5 

Inoceramus___________________________________ 54 

inornatus, Grammatodon_ _ _ _ _ __ _ __ __ _ __ _ ___ __ _ 76 

Isocyprina cinnabarensis_______________________ 34 

iddingsi_ _____ _ __ _ _ _ ___ _ __ _ ______ _ ___ ___ _ __ 35 

Isognomon __ ________________________________ 2, 58, 78 

perplana ______________________ 31, 32, 33, 34, 59,78 

Isognomonidae_ _ _ _ __ ___ _ __ _ __ _ __ _ __ _ __ _ __ _ __ _ 58 

isonemus, Modiolus___________________________ 57 

J 

jurianus, Grammatodon_______________________ 76 

K 

Kanastephanus crickmayi_ _ _ __________________ 93 

Kepplerites __ ------------------ 48, 60, 61, 62, 86, 87,88 

mclearnL___ _ _ _ ____ _ __ _ __ __ _ _ _ __ _ __ ___ _ _ _ 60 

kingi, Pholadomya _________________ 25, 26, 32, 86; pl. 5 

Pinna _________________ 25, 26, 27, 32, 34, 78; pl. 1 

Kosmoceras__ _ __ _ _____ _ _ _____ _ ____ ___ _ __ _ _ _ _ __ 62 

kruizingai, Chondroceras (Praetulites) _ _ _ _ _ _ ___ 98 

L 

Labyrinthoceras _______________________________ 31, 94 

Lak Member, Sundance Formation__________ 53 

Laternulidae_ _ _ ____________ _________ ________ _ 58 

Leeds Creek Member_________________________ 45 

Leptosphinctes ________________________________ 27, 62 

Lilloettia_ _ __ _ ___ __ ____ _ __ _ ___ _ __ ___ ___ _ ___ _ __ 60 

Lima_________________________________________ 2, 79 

(Plagiostoma) occidentalis ______________ 27, 43,79 

sp ______________________________ 25, 26, 32,49 

Limidae__ ____ __ __ __ __ __ _____ _ __ _ __ _ _ _ _ _______ 58 

lineata, Myophoria ____________________________ 57,83 

f1~:!~~:~)~-0~t;e-;~ ~~~~ ~~~~~ ~~~~~~ ~~~~~~~ ~~~~~~ 58'~~ strigilecula, Ostrea __ ·~--- ____ 25, 26, 27, 32, 43, 49, 79 

livingstonensis, Astarte________________________ 35 

Astarte (Coelastarte)_______________________ 3, 

24, 25, 26, 32, 34, 35, 39, 60, 83 

Coelastarte__ __ _ ___ _ __ _ _ ___ ___ __ _ __ _ _ __ _ ___ 34 

Cucullaea_ _ __ ___ _ ___ _ _ _ _ ____ __ __ _ __ _ __ __ _ 76 

longirostra, Neaera____________________________ 87 

Lopha_ _ ___ _ __ ___ _ ___ __ __ _ _ _ _ ___ ___ _ _ _ __ __ __ _ _ 2, 58 

sp_-------------------- ________________ 25, 26, 49 

Lucina _ _ __ _ __ _ ______ _ _____ _ _ ___ _ __ ___ __ _ _ ____ 58 

sp_--------------------------------- __ _ ___ 32 

Lucinidae_ _ __ ______ __ _ _ ______ _ __ _ __ _ ___ ___ ___ 58 

Lyosoma powelli_ _______________________ 25, 26, 27,32 

sp_ --------------------------------- __ _ __ _ 43 

M 

mclearni, Kepplerites_ _ _______________________ 60 

Mac~~:::::a~~:~~~~~~~~~~~~~~~ ~~~~~ ~~~ ~~~~ ~- 2~57, ~~: :~ sp ________________________ 25, 26, 27, 32, 84; pl. 1 

Mactromyidae__ ___________ _________ __________ 58 

Mascke, Chondroceras _________________________ 31,94 

meeki, Astarte ____ 25, 26, 27, 32, 39, 43, 49, 82, 83; pl. 5 

Megasphaeroceras _____ 1, 2, 3, 24, 27, 58, 59, 61, 62, 96,97 

rotundum ____________ 25, 26, 59, 96; pls. 13, 14, 16 

sp __________________________ 25, 26, 96, 97; pl. 13 

INDEX 

Page 

Megasphaeroceras zone________________________ 59 

Modiolus ________ ------------------------ 2, 57, 58,77 

formosus_______ _ __ ____ ___ ___ ____ ___ _ ___ ___ 77 

isonemus______________ ___ _ __ _ _______ ______ 57 

subimbricatus ______________ 25, 26, 32, 35, 43, 49,77 

(Musculus) pulchra_______________________ 77 

SP------------------------------- 32, 77; pl.1 

mogharense, Ermoceras________________________ 92 

montanaensis, Gervillia ______________________ 1, 3, 27, 

31, 32, 33, 34, 35, 40, 44, 59, 77, 78; pl. 1 Goniomya_____ _ __ _ _ _ ___ _ _ _______ _______ _ __ 1, 

24, 25, 26, 27, 31, 32, 34, 59, 86; pl. 1 

Myophorella __________________ 25, 26, 35, 36, 44,59 

(Promyophorella) _____________ 27, 31, 32, 34, 43,82 

marion, Astarte_______________________________ 35 

munda, Corbula ________________ 32,61,85,86,88; pl. 2 

Musculus _____________________________________ 58,77 

(Musculus) pulchra, Modiolus________________ 77 

sp., Modiolus _______________________ 32, 77; pl. 1 

Myalina (Gervillia) perplana__________________ 78 

Myopholas ______ _________________________ 2, 58, 86, 87 

acuticostata_ _ _ _ __ _ _ _ _ ______ ___ _ _ ____ ___ ___ 87 

hardyi__ _____________ 1, 3, 42, 43, 60, 61, 86, 87; pl. 5 

nana ____ ------------------------------- __ 87 

Myophorella_ _ _ ___ _ _ __ _ _ _ _ ___ _ __ _ __ _ _____ ___ _ _ 2, 82 

montanaensis __ --------------- 25, 26, 35, 36, 44, 59 

(Haidaia) yellowstonensis _________ 27, 31, 32, 59,82 

(Promyophorella) ______________________ 25, 26, 57 

montanaensis _____________ 27, 31, 32, 34, 43,82 

Myophoria lineata _____ ________________________ 57, 83 

Mytilidae_ _ ________ _________ ___ ______ ______ __ 58 

Mytilus _____________________________________ 2, 58,77 

whiteL __________________________ 25, 26, 27, 77,78 

N 

nana, Myopholas ____ ------------------------- 87 

Nautilidae____________________________________ 58 

Navajo Sandstone _________________________ 21, 40,57 

Neaera longirostra_____________ ___ _________ ____ 87 

nebrascensis, Gryphaea ____________________ 1, 2, 3, 34, 

43, 46, 48, 49, 53, 60, 61, 62, 77, 81, 87,88 

Nerinea sp ____________________________________ 39,43 

Neritina sp___________________________________ 49 

Nesson Formation____________________________ 21 

nodosum, Stephanoceras ___________ 25, 26, 89, 90; pl. 7 

Normannites _______________________ 2, 24, 27, 58, 62,93 

crickmayi_ ___________________ 25, 26, 34, 93; pl. 12 

Nucula_ _ __ _ __ _ __ _ _ _ _ _ __ _ __ _ ____ _ _ ___ _ ___ _ _ _ __ 2, 58 

sp_ _ _ _ _ ___ _ __ _ _ __ _ _ _ _ ___ _ __ _ __ _ _ _ _ __ _ _ _ _ __ 49 

Nuculidae__ ___ _ __ _ _ _ _ __ _ __ _ _ _ _ _ _ _____ __ _ __ _ __ 58 

Nugget Sandstone ____________________________ 17, 21 

0 

oblatum, 01wndroceras _____ _ _ _ _ __ _ __ _ __ _ _ _ __ ___ 93 

Defonticeras______ _ _ _ __ _ __ _ __ _ __ _ __ _ _ _ __ _ __ 93 

occidentalis, Lima (Plagiostoma) ____________ 27,43, 79 

Opis ______________ _________________________ 2, 60,83 

(Trigona pis) ______________________________ 57,83 

corallina____ _ __ _ _ _ __ _ __ _ __ __ _ _ _ _ ____ _ _ 83 

sp ___________________________ 25, 26, 83; pl. 2 

Oppelismilia sp _____ -------------------------- 35 

orbignyi, Patoceras____________________________ 89 

Spiroceras ____________________ 25, 26, 88, 89; pl. 6 

Toxoceras__________ _____________ __________ 88 

Ostrea __________________ 2, 3, 24, 30, 36, 45, 48, 49, 60,79 

(Liostrea) strigilecula _______ 25, 26, 27, 32, 43, 49,79 

(Liostrea)____ __ ____ __ ___ _ __ _ ___ _ __ _______ _ 57 

sp_ _ ____ ___ ____ _ _ __ ___ _ __ _ ___ _ __ __ ___ __ __ _ 35 

Ostreidae___ ______ _________________ _ __________ 58 

Otoites sauzei_ ________________________________ 24,27 

Otoites sauzei zone____________________________ 27 

Oxytoma__ ________________ __________ __________ 34 

p 

palliseri, Stemmatoceras ______________ 34, 90, 91; pl. 7 

Stemrnatoceras_ _ _ _ _ _ _ ______ _ ____ __ _ _ ____ __ 91 

Paracephalites ____________________ 27, 58, 59, 61, 97, 98 

glabrescens_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 59 

sawtoothensis ___________________________ 59, 97,98 

Paracephalites sawtoothemis zone •. ___________ • 59 

Page 

Parachondroceras_ 1, 2, 3, 31, 33, 58, 59, 80, 82, 93, 94,95 

andrewsi_ _______________ 2, 32, 34, 58, 94, 95; pl.12 

filicostatum ________________ 2, 32, 34, 58, 95; pl. 13 

sp_ _ _ _ _ _ _ __ __ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ 34 

Parachondroceras andrewsi zone_______________ 59 

Parallelodon_ _________ _ ___ _ ___ _____ __ ____ _____ 76 

Parallelodontidae____ _ _________ ____ ____ __ ____ _ 58 

packardi, Astarte______________________________ 83 

Patoceras orbignyi_____________________________ 89 

Pectinidae____ _______ ___ _ __ _ __ __ ___ ______ ____ _ 58 

Pelecypoda______ __ _ ___ ___ _ ____ _ ______ _____ _ _ _ 75 

Periseris_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 35 

perplana, Isognornon ______________ 31, 32, 33, 34, 59,78 

Myalina (Gervillia)________________________ 78 

Pholadidae_ ___ ___ _ __ _ __ _ __ _ _ _ _ ___ ______ __ ____ 58 

Pholadornya _____ ---------------------------- 2, 58,86 

inaequiplicata ________________ 25, 26, 27, 86; pl. 5 

kingi_ _______________________ 25, 26, 32, 86; pl. 5 

Pholadomyidae____ _____ _ __ _ __ _ _ ___ __ _________ 58 

Pinna ________________________________ 2, 35, 57, 58,78 

kingi_ _________________ 25, 26, 27, 32, 34, 78; pl. 1 

Pinnidae_ __ ___ _ __ _ __ _ __ _ ___ _ ____________ _ ____ 58 

Piper Formation__________________________ 21, 24, 35 

Plagiostorna _______________________________ 57, 58, 79 

(Plagiostoma) occidentalis, Lima ____________ 27, 43,79 

sp., Lima ___________________________ 25, 26, 32,49 

planoconvexafraterna, Gryphaea_______________ 1, 

24, 25, 26, 31, 32, 57, 59, 80; pl. 4 Gryphaea____ __ _ ___ _________ ________ __ ___ _ 1, 

2, 3, 21, 31, 32, 34, 59, 78, 80, 81, 82; pl. 3 

platessiformis, Camptonectes___________________ 25, 

26, 27, 32, 34, 35, 39, 49, 79; pl. 2 

Platyrnya _________________ ____________ 2, 58, 60, 87,88 

rockymontana _ _ _ _ _ _ _ 2, 3, 48, 49, 57, 61, 87, 88; pl. 5 

scarburgensis ____ ------------------------- 88 

Plectomya rugosa______________________________ 88 

Pleurornya _________ ________________ 2, 24, 38, 57, 58,85 

postculminata __ _____________ -------------- 88 

subcompressa _______________ 25, 26, 27, 32, 85; pl. 5 

weberensis. ---------------------------- 85; pl. 5 

Pleuromyacidae_ _ ___ __ _ _____ __ _ _____ ___ __ ____ 58 

Plicatula ___ __________________________ 2, 49, 57, 58,79 

sp __ ----------------------- 25, 26, 32, 49, 79; pl. 1 Plicatulidae. _ __ __ ___ ___ _ __ _ _ _ _ ___ ___ ___ _ _ _ _ __ 58 

postculrninata, Pleuromya_____________________ 88 

powelli, Lyosoma ________________________ 25, 26, 27,32 

(Praetulites) kruizingai, Chondroceras_ ________ 98 

Preuss Sandstone._-------------------------- 35,50 

prim us, Eocephalites ______ -------------------- 2, 

25, 26, 58, 97, 98; pls. 15, 16 

Promyophorella_ ------------------------------ 58,82 

(Promyophorella) montanaensis, Myophorella_ _ 27, 

31, 32, 34, 43, 82 

Myophorella ____________________________ 25, 26,57 

Pronoella _______ _____ -------------------- 2, 57, 58,84 

cinnabarensis ____________________ 25, 26, 32, 49,84 

iddingsi__ ___________________________ 25, 26, 27,84 

uintahensis __ ----------------- 25, 26, 27, 39, 49,84 

sp ___ ---------------------- 25, 26, 32, 39, 43, 49,84 

Prorokia ___________________________ 2, 57, 58, 60, 83,84 

fontenellensis____ __ ___ _ ___ __ _______ ____ __ _ _ 1, 

2, 24, 25, 26, 31, 32, 34, 57, 59, 83, 84; pl. 5 

rustica__ _____ _____ _ __ _ ___ _ __ _ ___ _ __ _ __ _ __ _ 84 

SP---------------------------------------- 25, 26 

Protocardia ___ __ ------------- ____________ 2, 58, 60,85 

schuchertL _____________________________ 32, 49, 85 

shumardi ___ ------------------------------ 85 

Pseudolimea cinnabarensis_____________________ 35 

Pseudornelania sp _____________________________ 25,26 

Pseudotrapezium______________________________ 76 

Pteria _ ___ _ ___ _ _ __ _ __ _ _ __ _ __ _ _ _ ___ ___ _ __ __ _ ___ 78 

punctata, Cercomya ______ ____ 31, 32, 33, 59, 61, 87; pl. 5 

Q 

Quenstedtia ___________________________ -" _ 2, 57, 58, 84 

arcuata_ _ _ ___ _ _ _ _ ____ __ _ __ __ __ _ ______ __ _ _ _ 85 

canadensis________________________________ 85 

sublevis ______________ 27, 31, 32, 49, 59, 84, 85; pl. 5 

sp _______ o __________________________ 25, 26, 32,49 

Quenstedtiidae. _ --------------------- ________ 58 
R Page 

Reineckeia _________________________________ 58, 91, 92 

Rhabdocolpus sp_ -------------------------- 25, 26,32 

Rich Member________________________________ 30 

Rierdon Formation ________________________ 43, 45,60 

rockymontana, Platymya_______________________ 2, 

3, 48, 49, 57, 61, 87, 88; pl. 5 

rotundum, Megasphaeroceras___________________ 25, 

26, 59, 96, 97; pls. 13, 14, 16 

rugosa, Plectomya_____________________________ 88 

rustica, Prorokia_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 84 

s sauzei, otoites _________________________________ 24, 27 

Sawtooth Formation ___________________ 21, 24, 34,35 

sawtoothensis, Paracephalites _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 59, 97, 98 

Saxitoniceras allani____________________________ 93 

(Saxitoniceras) allani, Chondroceras____________ 93 

scarburgensis, Platymya_______________________ 88 

schucherti, Protocardia ______________________ 32, 49,85 

schumardi, Protocardia________________________ 85 

semiornatus, Grammatodon____________________ 76 

skidegatensis, Stephanoceras __________ 25, 26, 89; pl. 6 

Sliderock Member____________________________ 22 

Sohlites ______ 1, 2, 19, 31, 33, 58, 59, 80, 82,91, 92, 93,95 

spinosus _________ 2, 3, 32, 34, 58, 91, 92; pls. 11, 14 

sp ____________________________________ 93; pl. 11 

Sonninia sowerbyL___________________________ 27 

Sonninia sowerbyi zone________________________ 27 

sonninianus, Grammatodon____________________ 76 

sowerbyi, Sonninia____________________________ 27 

sparsilirata, Gervillia __________________________ 77,78 

Sphaeroceras_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 62 

Sphaeroceratidae ___ -------------------- ______ 58 

spinosus, Sohlites _____ 2, 3, 32, 34, 58, 91, 92; pls. 11, 14 

Spiroceras _____________ 1, 2, 24, 58, 59, 61, 62, 88, 89 

orbignyi__ __________________ 25, 26, 88, 89; pl. 6 

sp ________ ------------ _____ ___ _ ____ ___ ___ _ 3 

Spiroceratidae____ _ ___ ___ _ __ _ __ _ ___ ______ _ ___ _ 58 

Starfish arm__________________________________ 27 

Stemmatoceras ____________ 1, 2, 3, 21, 24, 58, 59, 62, 90 

albertense _________________ 25, 26, 34, 91; pls. 10, 16 

arcicostum ____ 2, 21, 25, 26, 34, 58, 59, 90, 91; pls. 8, 9 

INDEX 

Stemmatoceras-Continued Page 

mclearni_ _________________________ ________ 90,91 

palliserL _________________________ 34, 90, 91;pl. 7 

sp_ ------------------------------- __ 25, 26, 27,34 

Stephanoceras _________ ___ 1, 2, 3, 24, 31, 58, 59, 62, 89,90 

humphriesianum __________________________ 24,59 

nodosum __________ _____________ 25, 26, 89, 90; pl. 7 

skidegatensis ______________________ 25, 26, 89;pl. 6 

sp_ ------------- ___ ---- _ _ ___ ____ __ ___ _ __ _ _ 32 

Stephanoceratidae____ _______ ____ _ __ _ _ __ __ ___ _ 58 

Stockade Beaver Shale Member______________ 49 

Strenoceras subfurcatum_______________________ 27 

Strenoceras subfurcatum zone__________________ 27 

strigilecula, Ostrea (Liostrea) ___ 25, 26, 27, 32, 43, 49,79 

Stump Sandstone____________________________ 53 

stygius, Camptonectes _____________ 25, 26, 32, 43, 49,78 

subcompressa, Pleuromya _______ 25, 26, 27, 32, 85;pl. 5 

subfurcatum, Strenoceras______________________ 27 

subimbricatus, Modiolus ________ 25, 26, 32, 35, 43, 49,77 

sublevis, Quenstedtia _____ __ 27, 31, 32, 49, 59, 84, 85; pl. 5 

Thracia ___ ________________________________ 84, 85 

Sundance Formation_------------------------ 27,36 

Swift Formation______________________________ 35 

T 

Tancredia _____________ ---------------- ______ __ 58 

sp ______ -------- ____________________ 25, 26, 27, 32 

Tancrediidae_ ___ __ _ ___ __ _ _ __ _ __ ___ ___ _ __ _ __ _ _ 58 

Teinostomopsis SP----------------------------- 25,26 

Teloceras ___ -------------- __ _ __ ____ _ __ _ __ _ __ _ _ 59 

allani___ __ _ ____ ______ _ __ _ ___ ___ _ __ _ __ _ _ __ _ 90 

(Zemistephanus) __ _____ _______ ___ _ ___ ___ __ 33 

Temple Cap Member of the Navajo Sandstone_---------------------------- 21 

Thamnasteria sp_ _ __ _ _ _ ___ ___ _ ___ _ _ __ ___ _ __ _ __ 35 

Thracia __________ ___________________________ 2, 58, 88 

canadensis __ -------------------___________ 84 

sublevis ___________________________________ 84, 85 

weedL __________ 1, 24, 25, 26, 30, 31, 32, 33, 34, 59,88 

Thraciidae. _ _ _ ____________________ ___________ 58 

Toxoceras orbignyi_____________________________ 88 

Trigonia __ _______________________________ 2, 57, 58, 81 

105 

Trigonia-Continued Page 

americana ____ ------------- 25, 26, 32, 34, 35, 49,81 

elegantissima ______ 27, 31, 32, 33, 34, 39, 43, 49, 59,82 

Trigoniidae __________________ . -----------· ____ 58 

Trigonopis ___________________________ _______ 2, 58, 83 

(Trigonopis) corallina, Opis___________________ 83 

Opis _____________ _________________________ 57, 83 

sp., Opis _________________________ 25, 26, 83; pl. 2 

Twelvemile Canyon Member of the Arapien 

Shale ___________________________ 4, 43,45 

u uintahensis, Pronoella _____________ 25, 26, 27, 39, 49,84 

v 

Vaugonia_ ------------------------ ____ 2, 16, 57, 58,82 

conradL _______ 27, 31, 32, 33, 34, 40, 43, 44, 49, 59,82 

sp __ --------------------- _____ __ ____ ___ ___ 49 

Velata_ _ ___ _ ___ ___ ___ _ __ ____ _ _____ __ __________ 58 

w Warrenoceras ____________________ 1, 43, 60, 61, 76, 87,88 

codyense __ -------------- __ ____ ___ _______ __ 60 

Warrenoceras codyense zone___________________ 60 

henryL _ ------------- ___ _ _ ___ _ __ _ ______ _ __ 60 

Watton Canyon Member_____________________ 41 

weberensis, Pleuromya ______________________ 85; pl. 5 

weedi, Thracia __ ____ 1, 24, 25, 26, 30, 31, 32, 33, 34, 59,88 

Western interior region faunal zones___________ 59 

whitei, Mytilus _______________________ 25, 26, 27, 77,78 

Worm tubes ____________________________ 25, 26, 27,49 

X 

Xenocephalites __ _ _ ___ ___ ____ ______ _ _ __ ____ _ __ _ 88 

y 

yellowstonensis, Myophorella (Haidaia)________ 27, 

31, 32, 59, 82 

z Zemistephanus __________________________ 31, 58, 91,92 

carloUensis ____ ____________________________ 91, 92 

(Zemistephanus), Teloceras_ --··--------------- 33 
PLATE 1 

[Figures natural size unless otherwise indicated] 

FIGURES 1, 7~10. Gervillia? montanaensis Meek (p. 77). 

1. Syntype USNM 132837. 

7. Syntype USNM 7795. 

8. Syntype USNM 132838. 

9. Syntype USNM 132836. 

10. Lectotype USNM 132724. All specimens shown are from the Piper Formation at USGS Mesozoic loc. 

19214, at mouth of lower canyon of the Yellowstone River. 

2-6. Grammatodon haguei (Meek) (p. 75). 

2. Internal mold of left valve of plesiotype USNM 28955 (X 2) from USGS Mesozoic loc. 1181, Piper Formation, near Sentinel Butte, Yellowstone National Park. 

3. Concave composite mold of left valve showing external radial ribs and internal dentition. Plesiotype 

USNM 132720 (X 2) from USGS Mesozoic loc. 20987, Twin Creek Limestone, Rich Member. 

4. Internal mold of left valve of plesiotype USNM 132718 (X 2) from USGS Mesozoic loc. 20974. "Lower 

Sundance" Formation, unit 19 of published section (Imlay, 1956a, p. 70). 

5. Internal mold of left valve of ples~otype USNM 132719 (X 2) from USGS Mesozoic loc. 20987, Twin Creek 

Limestone, Rich Member. 

6. Internal mold of left valve of plesiotype USNM 132717 (X 2) from USGS Mesozoic loc. 20964. "Lower 

Sundance" Formation, unit 22 of published section (Imlay, 1956a, p. 70). 

11-13, 15. I donearca haguei (Stanton) (p. 76). 

11. Internal mold of left valve of syntype USNM 28941 from USGS Mesozoic loc. 28941 from Piper Formation at head of Fawn Creek, Yellowstone National Park. 

12, 13. Internal mold of lectotype USNM 132721 from USGS Mesozoic loc. 28941. 

15. Internal mold showing hinge, plesiotype USNM 132722 from USGS Mesozoic loc. 19613. Lower 5 ft 

of limestone member of Piper Formation on Five Mile Creek, sec. 36, T. 5 S., R. 24 E., Carbon County, 

Mont. 

14, 19. Modiolus (Musculus) sp. (p. 77). 

Left valve (X 2 and X 3) of figured specimen USNM 132723 from USGS Mesozoic loc. 28788. Note fine 

radiating striae shown in fig. 19. Twin Creek Limestone, Rich Member. 

16. Plicatula sp. (p. 79). 

Figured specimen USNM 132726 from USGS Mesozoic loc. 17985, Twin Creek Limestone, Sliderock Member. 

17. Goniomya montanaensis Meek (p. 86). 

Plesiotype USNM 132761 from USGS Mesozoic loc. 20963, Twin Creek Limestone, Rich Member. 

18. Mactromya? sp. (p. 84). 

Figured specimen USNM 132753 from USGS Mesozoic loc. 20977, Twin Creek Limestone, Sliderock Member. 

20-22. Pinna kingi Meek (p. 78). 

Holotype USNM 20221 from Weber Canyon, Watatch Range, Utah. Presumably collected near Devils 

Slide from the Twin Creek Limestone. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE 1 

5 

GERVILLIA?, GRAMMATODON, IDONEARCA, MODIOLUS (MUSCULUS) 

PLICATULA, GONIOMYA, MACTROMYA?, AND PINNA 

6 
PLATE 2 

[Figures natural size unless otherwise indicated] 

FIGURES 1, 2. Camptonectes platessiformis White (p. 79). 

Left and right valves of plesiotype USNM 132725 from USGS Mesozoic loc. 19170. Rierdon Formation, 

lower 15ft, from one-half mile west of Bear Gulch, sec. 15, T. 14 N., R. 21 E., Fergus County, Mont. 

3. Opis (Trigonopis) sp. (p. 83). 

Internal mold of left valve of figured specimen USNM 132747 from USGS Mesozoic Joe. 20344, Twin Creek 

Limestone, S!iderock Member. 

4, 12, 13. Ctenostreon cf. C. gikshanensis MeL earn (p. 79). 

Left valve (mostly an internal mold), right valve retaining shell material, and anterior view of specimen 

USNM 132727 from USGS Mesozoic loc. 21636. Rierdon Formation, 25 ft above base, on divide onehalf mile west of Five Mile Creek, sec. 36, T. 5 S., R. 24 E ., Carbon County, Mont. 

5-11. Corbula cf. C. munda McLearn (p. 85). 

5, 8, 10, 11. Figured specimens USNM 132758 (X 3) from USGS Mesozoic loc. 28787, Twin Creek Limestone, Rich Member. 

6. Figured specimen USNM 132760 (X 3) from USGS Mesozoic loc. 18716. Rierdon Formation, lower 38 

ft, head of Rierdon Gulch, sec. 23, T. 24 N., R. 9 W., Teton County, Mont. 

7, 9. Figured specimens, USNM 132759 (X 3) from USGS Mesozoic Joe. 28788, Twin Creek Limestone, 

Rich Member. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE 2 

CAMPTONECTES, OPIS (TRIGONOPIS), CTENOSTREON, AND CORBULA 
PLATE 3 

[All figures natural size] 

FIGURES 1-13. Gryphaea planoconvexa Whitfield (p. 80). 

1, 4. Lateral and dorsal views of left valve of holotype, Yale University 6774. Piper Formation on east side of 

south end of Bridger Range, Mont. 

2. Left valve of plesiotype USNM 104138 from USGS Mesozoic loc. 19619. Sawtooth Formation in Madison 

Range, sec. 20, T. 8 S., R. 2 E., Madison County, Mont. 

3. Left valve of ·plesiotype USNM 132729 from USGS Mesozoic loc. 19158. Piper Formation on north side of 

Rocky Creek, sec. 19, T. 2 S., R. 7 E., Gallatin County, Mont. 

5, 10. Right valve of paratype, Yale University 6778 from same locality as holotype. 

6. Left valve of plesiotype USNM 12369 from USGS Mesozoic loc. 19214, Piper Formation in lower canyon of 

Yellowstone River. 

7, 11. Left and right valves of an immature specimen. Plesiotype USNM 132730 from USGS Mesozoic loc. 

28800. Sawtooth Formation, head of Red Canyon sec. 24, T. 11 S., R. 4 E., Gallatin County, Mont. 

8, 9. Interior and exterior of right valve of plesiotype USNM 132728 from USGS Mesozoic loc. 20989. "Lower 

Sundance" Formation, 8-20 ft above base of unit 10 (Imlay, 1956a, p. 70). 

12, 13. Exterior and interior of left valve of same specimen represented by right valve shown in figs. 8 and 9. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE ,3 

GRYPHAEA 
PLATE 4 

[All figures are natural size] 

FIGuRES 1-17. Gryphaea planoconvexa fraterna Imlay (p. SO) . 

1-3. Posterior; dorsal, and lateral views of left valve of paratype USNM 132732 from USGS Mesozoic loc. 28647 . 

4. Paratype USNM 132738 from USGS Mesozoic loc. 20344. Note radial riblets. 

5. Paratype USNM 132739 from USGS Mesozoic loc. 20344. Note radial riblets. 

6, 10. Laterial and posterior views of paratype USNM 132734 from USGS Mesozoic loc. 28647. 

7, 9. Right and left valves of paratype USNM 132736 from USGS Mesozoic loc . 21618. 

8. Paratype USNM 132737 from USGS Mesozoic loc. 21618. 

11. Paratype USNM 132735 from USGS Mesozoic Joe. 21618. This specimen resembles Gryphaea planoconvexa 

Whitfield in its pinched beak and nearly circular outline, but it bears radial furrows ventrally and is associated 

with t he specimens shown in figs. 7-9 which are typical of the variety fraterna . 

12, 14. Right and left valves of paratype USNM 132740 from USGS Mesozoic loc. 28483. Shows only a trace of 

a posterior sulcus. 

13. Right valve of paratype USNM 132733 from USGS Mesozoic loc . 28647. 

15, 16. Posterior and lateral views of holotype USNM 132731 from USGS Mesozoic loc. 28795. Note radial 

furrows and posterior sulcus. 

17. Paratype USNM 132741 from USGS Mesozoic loc. 16016. Shows pronounced posterior sulcus and broad, 

strongly incurved umbo. All specimens shown on this plate are from the Sliderock Member of the Twin Creek 

Limestone. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE 4 

GRYPHAEA 

PLATE 5 

[Figures natural size unless otherwise indicated] 

FIGURES 1-6. Astarte rneeki Stanton (p. 82). 

1. Plesiotype USNM 132743 from USGS Mesozoic loc. 25866. Piper Formation, west side of lower canyon 

of Yellowstone River, Park County, Mont. 

2. Plesiotype USNM 1327 44 from same locality as fig. 1. 

3. Lectotype USNM 28936 from USGS Mesozoic loc. 1164, Piper Formation on Cinnabar Mountain, 

Park County, Mont. 

4. Syntype USNM 28937 from USGS Mesozoic loc. 1153. Ridge west of head of Gardner River, Yellowstone National Park. 

5. Plesiotype USNM 132746 from USGS Mesozoic loc. 28507. Twin Creek Limestone, Rich Member. 

6. Plesiotype USNM 132745 from USGS Mesozoic loc. 21621. Twin Creek Limestone, Boundary Ridge 

Member. 

9-13. Prorokia fontenellensis Imlay, n. sp. (p. 83). 

9. Right valve of paratype USNM 132749 (X 2) from USGS Mesozoic loc. 25866. Piper Formation. 

10. Left valve of holotype USNM 132748 (X 2) from USGS Mesozoic loc. 25866. Piper Formation. 

11. Right valve of paratype USNM 132750 (X 2) from USGS Mesozoic loc. 28788. Twin Creek Limestone, 

Rich Member. 

12. Right valve of parat.ype USNM 132751 (X 2) from USGS Mesozoic loc. 21619. Twin Creek Limestone, Sliderock Member. 

13. Rubber cast of external mold of paratype USNM 132752 from USGS Mesozoic loc. 28788. Twin 

Creek Limestone, Rich Member. 

7, 8, 14, 15. Quenstedtia sublevis (Meek and Hayden) (p. 84). 

7. Plesiotype USNM 132755 from USGS Mesozoic loc. 3822. Twin Creek Limestone, lower part. 

8. Lectotype USNM 132754 from southwest base of the Black Hills, Wyo. Probably from Stockade 

Beaver Shale Member of the Sundance Formation. 

14. Syntype USNM 132841 from southwest base of the Black Hills, Wyo., Sundance Formation, lower 

part. 

15. Syntype USNM 197 from southwest base of the Black Hills, Wyo., Sundance Formation, lower part. 

16----21. Pleurornya subcompressa (Meek) (p. 85). 

16, 17. Lectotype USNM 132756 from USGS Mesozoic loc. 19214. Piper Formation. 

18. Syntype USNM 132839 from USGS Mesozoic loc. 19214. Piper Formation. 

19. Syntype USNM 8061 from USGS Mesozoic loc. 19214. 

20. Plesiotype USNM 132757 from USGS Mesozoic loc. 20344, Twin Creek Limestone, Sliderock Member. 

21. Syntype 132840 from USGS Mesozoic loc. 19214. Piper Formation. 

22, 23. Pleurornya weberensis (Meek) (p. 85). 

Lateral and anterior view of holotype USNM 13386 from Twin Creek Limestone, Weber, Canyon, Wasatch 

Range, Utah. Posterior end missing. Possibly a finely ribbed variant cf. P. subcompressa (Meek) 

with which it is associated. 

24--26. Pholadomya inaequiplicata Stanton (p. 86) .· 

Posterior, anterior, and right valve of holotype USNM 28956 of USGS Mesozoic loc. 1130. Rierdon 

Formation, basal part. 

27-35. Myopholas hardyi Imlay (p. 86). 

27. Posterior part of right valve of plcsiotype USNM 132766 from USGS Mesozoic loc. 21623, Twin Creek 

Limestone, Watton Canyon Member.· 

28, 29. Posterior end of left and right valves of plesiotype USNM 132763 from USGS Mesozoic loc. 18750, 

Rierdon Formation near top, Little Rocky Mountains, middle S% sec. 5, T. 24 N., R. 26 E., Phillips 

County1 Mont. 

30,.--32. Right valve, dorsal view, and left valve of plesiotype USNM 132764 from USGS Mesozoic loc. 

18754. Rierdon Formation, top bed, NEX sec. 11, T. 26 N., R. 24 E., Blaine County, Mont. 

33. Posterior end of right valve of plesiotype USNM 132765 from USGS Mesozoic loc. 19170. Rierdon 

Formation, lower 15ft, near center of sec. 15, T. 14 N., R. 21 E., Fergus County, Mont. 

34, 35. Left valve and dorsal view (anterior end to right) of plesiotype USNM 132762 from USGS Mesozoic loc. 18750. Rierdon Formation, near top, S}§ sec. 5, T. 24 N., R. 26 E., Phillips County, Mont. 
PLATE 5-Continued 

FIGURES 36-38, 42. Pholadomya kingi Meek (p. 86). 

Dorsal view, right valve, anterior end, and posterior end of holotype USNM 7815 from USGS Mesozoic 

loc. 19214, Piper Formation, middle limestone member. 

39-41. Cercomya punctata (Stanton) (p. 87). 

39. Right valve of plesiotype USNM 132767 (X 2) from USGS Mesozoic loc. 3815. Twin Creek Limestone, 

Rich Member. Shows fine radiating striae posterior to the sulcus near the middle of the valve. 

40. Right valve of plesiotype USNM 132768 (X 2) from USGS Mesozoic loc. 20963. Twin Creek Limestone, Rich Member. 

41. Left valve of holotype USNM 28945 from USGS Mesozoic loc. 1131, south slope of ridge south of Gray 

Mountain, Yellowstone National Park. Probably from Piper Formation. 

43-49. Platymya rock_ymontana Imlay, n. sp. (p. 87). 

43. Right valve of paratype USNM 132773 from USGS Mesozoic loc. 12016, Twin Creek Limestone, 

Leeds Creek Member. 

44, 48. Left valve of paratype USNM 132772 (X 2, X 3) from USGS Mesozoic loc. 20974. "Lower Sundance" Formation, 8 ft above base of unit 19 (Imlay, 1956a, p. 70). Oriented to show ventral margin 

(fig. 44) and radial striae (fig. 48). 

45. Right valve of paratype USNM 132770 from USGS Mesozoic loc. 10729, Rierdon Formation, upper 

part, 1 mile east-northeast of St. Paul Mission at northwest end of Little Rocky Mountains, Blaine 

County, Mont. 

46, 47. Left valve and dorsal view of holotype USNM 132769 from USGS Mesozoic loc. 18735, Rierdon 

Formation, upper part, SE% sec. 12, T. 25 N., R. 25 E., Phillips County, Mont. 

49. Left valve of paratype USNM 132771 from USGS Mesozoic loc. 18750, Rierdon Formation, upper part, 

S}~ sec. 5, T. 24 N., R. 26 E., Phillips County, Mont. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE 5 

ASTARTE, PROROK/A, QUENSTEDTIA, PLEUROMYA, PHOLADOMYA, 

MYOPHOLAS, CERCOMYA, AND PLATYMYA 
PLATE 6 

[All figures are natural size] 

FIGuREs 1-3, 7, 8. Chondrocems cf. C. allani (McLearn) (p. 93). 

Ventral, lateral, and apertural views and last suture line of specimen USNM 104134 from USGS Mesozoic 

loc. 19192. Sawtooth Formation. 

4-6, 9, 11. Spiroceras cf. S. orbignyi Baugicr and Sauze (p. 88). 

4- 6, 11. Lateral and ventral views and cross section of internal mold USNM 132774 from USGS Mesozoic 

loc. 20985. Twin Creek Limestone, Sliderock Member. Adoral end is pointed downward on figs. 4 and 5. 

9. Ventral view of external mold USNM 132775 from USGS Mesozoic loc. 20969. Twin Creek Limestone, 

Sliderock Member. 

10. Stephanoceras cf. S. skidegatensis (Whiteaves) (p. 89). 

Laterally cr'Jshed internal mold of nearly complete body chamber. USNM 132776 from USGS Mesozoic 

loc. 21632, Twin Creek Limestone, Sliderock Member. 
GEOLOGICAL SU RVEY PROFESSIONAL PAPER 540 PLATE 6 

~-~I 

8 I 

11 

CHONDROCERAS, SPIROCERAS, AND STEPHANOCERAS 

221-6910-66-----7 
PLATE 7 

[All figures natural size] 

FIGURES 1-3. Stemmatoceras cf. S. palliseri McLearn (p. 91). 

1, 2. Ventral and lateral views of part of last septate whorl of specimen 

USNM 132787 from USGS Mesozoic loc. 27337. Basal part of Sawtooth 

Formation near Tigh Creek west of Drummond, Granite Count, Mont. 

In fig. 1 the adoral end is pointed downward. 

3. Lateral view of part of body chamber of same specimen as shown in 

figs. 1 and 2. 

4, 5. Stephanoceras aff. S. nodosum (Quenstedt) (p. 89) . 

Lateral and ventral view of adult body whorl of specimen USNM 132777 

from USGS Mesozoic loc. 21618. Twin Creek Limestone, Sliderock 

Member. 
GEOLOGICAL SURVEY PROFESSIONAL PAPEH 540 PLATE 7 

STEMMATOCERAS AND STEPHANOCERAS 
PLATE 8 

[Figures slightly reduced] 

FIGURES 1, 2. Stemmatoceras arcicostum Imlay, n. sp. (p. 90). 

Ventral and lateral views of plastoholotype USNM 132778. Sawtooth 

Formation, basal part, near Tigh Creek, 3 miles west of Drummond in 

Granite County, Mont. Shows complete body chamber except for a 

ventral prolongation. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE 8 

STEMMATOCERAS 
PLATE 9 

[All figures are natural size] 

FIGURES 1-11. Stemmatoceras arcicostum Imlay, n. sp. (p. 90). 

1. Parts of three internal whorls and suture line of para type USNM 132780 from USGS Mesozoic Joe. 27337. 

Sawtooth Formation. 

2, 9. Lateral and ventral views of internal whorls of paratype USNM 132783 from USGS Mesozoic Joe. 28645. 

Twin Creek Limestone, Sliderock Member. In fig. 9 the adoral end is pointed downward. 

3, 10. Lateral and ventral views of septate whorl of paratype USNM 132782 from USGS Mesozoic Joe. 28645. 

Twin Creek Limestone, Sliderock Member. 

4. Cross section of holotype shown on pl. 8 measured about one-sixth of a whorl adapical from the aperture at 

whorl height of 68 mm. 

5, 6. Ventral and lateral views of paratype USNM 132781 from USGS Mesozoic Joe. 27337. Sawtooth Formation, 

basal part. 

7, 8, 11. Snture line, cross section, and lateral view of paratype USNM 132779 from USGS Mesozoic Joe. 27337 . 

Sawtooth Formation, basal part. 
GEOLOGICAL SURVEY 

\ I 

\{ 

\ 

\ 

STEMMATOCERAS 

/ 

I 

PROFESSIONAL PAPER 540 PLATE 9 
PLATE 10 

[All figures are natural size] 

FIGURES 1-8. Stemmatoceras n. sp. aff. S. albertense McLearn (p. 91) . 

1-4, 7. Lateral and ventral views and cross section of specimen USNM 132786 from USGS Mesozoic loc. 2864u. 

Twin Creek Limestone, Sliderock Member. Figs. 4 and 7 arc an outer whorl of the same specimen shown in 

figs. 1 and 3. 

5. Cross section of a large fragment measured at the adapical end of the body chamber, USNM 132785 from USGS 

Mesozoic loc. 28645. Twin Creek Limestone, Sliderock Member. 

G, 8. Ventral and lateral views of a septate fragment, USNM 132784 from USGS Mesozoic loc. 28644. Twin 

Creek Limestone, Sliderock Member. In fig. 8 the adoral end is pointed downward. 
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PROFESSIONAL PAPER 540 PLATE 10 
PLATE 11 

[Figures natural size unless otherwise indicated] 

FIGURES 1-18, 21, 24-26. Sohlites spinosus Imlay, n. sp. (p. 92). 

1. Paratype USNM 132794 from USGS Mesozoic Joe. 27624. Piper Formation. 

2. Paratype USNM 132795 from USGS Mesozoic Joe. 27624. Piper Formation. Note smooth 

nucleus. 

3. Paratype USNM 132803 from USGS Mesozoic Joe. 19214. Piper Formation. 

4. Rubber cast of an external mold, paratype USNM 132797 from USGS Mesozoic Joe. 27035. Piper 

Formation. 

5, 6. Suture lines (X 2) and lateral view of crushed paratype USNM 132790 from USGS Mesozoic Joe. 

27624. Piper Formation . Suture line drawn at whorl height of about 18 mm. 

7. Rubber cast of an external mold of paratype USNM 132796 from USGS Mesozoic Joe. 27035. Piper 

Formation. Specimen does not appear to be crushed. 

8, 9. Suture lines (X 2) and lateral view of paratypc USNM 132802 from USGS Mesozoic Joe. 25866. 

Piper Formation. 

10. Ventral view of paratype USNM 132793 from USGS Mesozoic Joe. 27624. Piper Formation. 

11, 12. Suture lines (X 2) and lateral view of paratypc USNM 132801 from USGS Mesozoic Joe. 25866. 

Piper Formation. 

13, 14. Suture lines (X 2) and lateral view of paratype USNM 132800 from USGS Mesozoic Joe. 27035. 

Piper Formation. 

15. Ventral view of paratype USNM 132792 from USGS Mesozoic Joe. 27624. Piper Formation. 

16. Lateral view of paratype USNM 132791 from USGS Mesozoic Joe. 27624. Piper Formation. 

Smoothness of adoral third of specimen is due to corrosion. 

17, 20. Suture lines (X 2) and lateral view of holotypc USNM 132788 from USGS Mesozoic Joe. 27624. 

Piper Formation. Specimen has been crushed laterally. 

18. Lateral view of laterally crushed paratype USNM 132789 from USGS Mesozoic Joe. 27624. Piper 

Formation. 

21, 25. Lateral and ventral views of paratype USNM 132805 from USGS Mesozoic Joe. 17096. Gypsum Spring Formation. In fig. 25 the adoral end of specimen is pointed downward. 

24. Lateral view of rubber cast of external mold of paratype USNM 132804 from USGS Mesozoic Joe . 

12555 on wedge about 2 miles south of Fullers Bottom along San Rafael River, Emery County, 

Utah. Carmel Formation, 6-12ft above base. 

26. Ventral view of paratype USNM 132806 from USGS Mesozoic Joe. 17096. Gypsum Spring Formation. 

19, 22, 23. Sohlites? sp. (p. 93). 

19, 22. Lateral views of rubber casts of two extemal molds of specimens USNM 132807 from USGS 

Mesozoic Joe. 20351. Head of Buckhorn Wash, sec. 13, T. 19 S., R. 10 E., Emery County, Utah. 

Carmel Formation, about 25 ft above base. 

23. Ventral view of internal mold of specimen USNM 132808 from USGS Mesozoic loc. 25678. From 

NE),~ sec. 18, T . 19 S., R. 11 E., Emery County, Utah. Carmel Formation, about 40ft above base. 
GEOLOGICAL SURVEY 

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PROFESSIONAL PAPER 540 PLATE ll 

SOH LITES 
PLATE 12 

[All figures are natural size] 

FIGUREs 1-4. Normannites? cf. N. crickmayi (McLearn) (p. 93). 

1, 2. Ventral and lateral views of specimen USNM 132809 from USGS Mesozoic loc. 27337. Sawtooth Formation, basal part. Adoral end is pointed downward. 

3, 4. Lateral and ventral views of specimen USNM 132810 from USGS Mesozoic loc. 28645. Twin Creek Limestone, Sliderock Member. 

5-18. Parachondroceras andrewsi Imlay, n. sp. (p. 94). 

5- 8, 13- 18. Lateral, apertural, and ventral views, cross section, and suture lines of holotype USNM 132811 from 

USGS Mesozoic loc. 20864. Piper Formation. Fig. 5 is from a rubber cast of an external mold and corresponds 

to the lowest part of fig. 15. It shows forwardly inclined striae. In figs. 6-8, part of the body whorl has been 

removed in order to show the changes in ornamentation and whorl shape. Figs. 13 and 14 are from a rubber 

cast of an external mold and correspond with the innermost part of the septate whorl shown in fig. 6. Figs 

15-18 show the nearly complete body chamber. The suture line was drawn at a whorl height of 31 mm near 

the adapical end of the last septate whorl. 

9. Internal mold of paratype USNM 132813 from USGS Mesozoic loc. 21629. Twin Creek Limestone, Rich 

Member. 

10. From rubber cast of external mold of paratype USNM 12221 from USGS Mesozoic loc. 19214. Piper Formation. 

11. Laterally crushed internal mold of paratype USNM 132812 from USGS Mesozoic loc. 19214. Piper Formation. 

12. Laterally crushed internal mold of paratype USNM 132814 from USGS Mesozoic loc. 8181. Twin Creek 

Limestone, probably from Rich Member. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE 12 

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NORMANNITES? AND PARACHONDROCERAS 
FIGURES 

PLATE 13 

[Figures natural size unless otherwise indicated] 

1-3, 15. Megasphaeroceras rotundum Imlay, var. (p. 96). 

Apertural, ventral, and lateral views and suture line of plesiotype USNM 132824 from USGS Mesozoic 

Joe. 20001, Cook Inlet region, Alaska. Twist Creek Siltstone (Detterman 1963, p. C31-C33; Imlay, 

1962b, p. A5). This specimen is considered to be a compressed variant of M . rotundum Imlay. 

4, 5. Megasphaeroceras sp. undet. (p. 96). 

Ventral and lateral views of specimen USNM 132825 from USGS Mesozoic lcic. 20975. Twin Creek Limestone, Sliderock Member. Note resemblance to specimen shown in figs. 1-3. 

6-10. Megasphaeroceras atf. M. rotundum Imlay (p. 96). 

6, 7, 9, 10. Ventral, aperturl!l, and lateral views and suture line of specimen USNM 132823 from USGS 

Mesozoic Joe . 20978, Twin Creek Limestone, Sliderock Member. Suture line drawn at diameter of 54 

mm. 

8. Lateral view of specimen USNM 132824 from USGS Mesozoic Joe . 28635. Twin Creek Limestone, 

Sliderock Member. About three-fifths of specimen represents body chamber. Note slight contraction 

of whorl at adoral end. 

11-14, 16-25. Parachondroceras filicostatum Imlay, n. sp. (p. 95). 

11, 16, 17, 21, 23- 25. Suture line, inner whorls, lateral, and ventral views and cross section of holotype 

USNM 132815 from USGS Mesozoic Joe. 20864. Piper Formation. Suture line drawn near adoral 

end of last septate whorl at whorl height of 34 mm. Figs. 16 and 17 are from a rubber cast of an external mold of the septate whorl corresponding to the ribbed part of the internal mold shown in fig. 

25. On this figure , part of the body chamber has been removed. 

12, 13, 22. Lateral and ventral views and suture line of internal mold of paratype USNM 132818 from 

USGS Mesozoic Joe. 20962. "Lower Sundance" Formation, 20ft above base of unit 10 (Imlay, 1956a, 

p . 70) of section at Lower Slide Lake, Teton County, Wyo. 

14. Rubber cast of an external mold of paratype USNM 132816 from USGS Mesozoic Joe . 19214. Piper 

Formation. 

18. Internal mold of paratype USNM 132817 from USGS Mesozoic Joe . 15635. Piper Formation. 

19, 20. Small undeformed internal mold of inner whorls of paratype USNM 132819 from USGS Mesozoic Joe. 28800. Sawtooth Formation, near base. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE 13 

MEGASPHAEROCERAS AND PARACHONDROCERAS 
PLATE 14 

[All figures are natural size] 

FIGURES 1-6, 9- 16. Megasphaeroceras cf. M. rotundum Imlay (p. 96). 

1, 2. Ventral and lateral views of internal mold USNM 13282la from USGS Mesozoic loc. 28483. 

3, 4. Ventral and lateral views of USNM 13282lb from USGS Mesozoic loc. 28483. Note apertural constriction. 

5, 6. Apertural and lat9ral views of internal mold of USNM 132820a from USGS Mesozoic loc. 28648. Note 

sharp ribbing similar to that on Alaskan species (Imlay, 1962b, pl. 3). 

9, 13. Lateral and ventral views of internal mold USNM 132820b from USGS Mesozoic loc. 28648. In 

fig. 9 the adoral end is pointed downward. Note fading of ribs on lower part of flanks. 

10-12. Apertural, lateral, and ventral views of adult specimen USNM 132820c from USGS Mesozoic loc. 

28648. Fig. 10 shows strong ribbing on shell material preserved at the adoral end of the last septate whorl. 

The body chamber does not bear any shell material and is almost smooth. Note the apertural constriction 

and t erminal swelling and the slight contraction of the body chamber near the aperture. 

14-16. Apertural and lateral views of adult specimen USNM 132822 from USGS Mesozoic loc. 28794. Note 

fading of ribs adorally on the body chamber. All specimens of Megasphaeroceras arc from the Sliderock 

Member of the Twin Creek Limestone. 

7, 8. Sohlites spinosus Imlay, n. sp. (p. 92). · 

7. Rubber cast of an external mold, paratypc USNM 132798 from USGS Mesozoic loc. 27035. Piper Formation. 

8. Rubber cast of an external mold, paratype USNM 132799 from USGS Mesozoic loc. 27035. Piper Formation. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE 14 

MEGASPHAEROCERAS AND SOHLITES 
FIGURES 

PLATE 15 

[All figures are natural size. All specimens from the S!iderock member of the Twin Creek Limestone.] 

1, 2. Eocephalites? sp. undet. C (p. 08). 

Ventral and lateral view of internal mold USNM 132835 from USGS Mesozoic loc. 28644. In fig. 

2 the adoral end is pointed downward. 

3-5, 9-13, 16-20. Eocephaliles primus Imlay, n. sp. (p. 07). 

3- 5. Lateral and ventral views of paratype USNM 132828 from USGS Mesozoic loc. 22638. 

9-11. Suture line, ventral, and lateral views of inner whorls of a larger specimen. Paratype USNM 

132831 from USGS Mesozoic loc. 28483. Suture line drawn at diameter of 32 mm. 

12, 13, 16. Lateral and ventral views and suture line of internal mold. Paratype USNM 132829 

from USGS Mesozoic loc. 21618. · 

17. Partial suture line showing first and second lateral lobe. Paratype USNM 132832 from USGS 

Mesozoic loc. 20973. 

18. Paratype USNM 132942 from USGS Mesozoic loc. 20961. Shows nearly complete body chamber whose adapical end is marked by a small fault. Note presence of part of an apertural constriction. The body chamber is an internal mold and has been corroded. The septate part of the oute r 

whorl bears some shell material and has fairly sharp ribbing. 

19. Internal mold of paratype USNM 132830 from USGS Mesozoic loc. 21618. 

20. Internal mold of laterally crushed adult body chamber of paratype USNM 132827 from USGS 

Mesozoic loc. 27729. 

6-8. Eocephalites? sp. undet. A (p. 98). 

Lateral and apertural views of specimen USNM 132833 from USGS Mesozoic loc. 28648. This 

specimen retains some shell material. 

14, 15. Eocephalites? sp. undet. B. (p. 98). 

Lateral and apertural views of internal mold of specimen USNM 132834 from USGS Mesozoic loc. 

20979. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE 15 

EOCEPHALITES 
PLATE 16 

[All figures are natural size. All specimens from the Sliderock Member of the Twin Creek Limestone). 

FIGURES 1- 4. Megasphaeroceras cf. M . rolundum Imlay (p. 96). 

Lat eral, apertural, and ventral views of septate internal mold USNM 132939 fromUSGS Mesozoic loc. 28645 . 

. 5- 7. St emmatoceras n. sp. cf. S . alberlense McLearn (p. 91). 

5, 7. Ventral and lateral views of immature specimen, USNM 132940 from USGS Mesozoic loc. 28645. Shows 

ribbing on internal whorls. 

6. Immature specimen USNM 132941 from USGS Mesozoic loc. 28645. 

8- 14. Eocephalites primus Imlay, n. sp. (p. 97). 

8-10. Lateral and ventral views of para type USNM 132938 from USGS Mesozoic loc. 20345. Shows ribbing on 

adult body chamber. 

11-13. Ventral and lateral views of holotype USNM 132826 from USGS Mesozoic loc. 20345. Shows ribbing on 

internal mold of adult body chamber and abrupt truncation at aperture. 

14. Lateral view of internal mold of paratype 132827 from USGS Mesozoic loc. 27729. See view of same specimen 

on pl. 15, fig. 20. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 540 PLATE 16 

MEGASPHAEROCERAS, STEMMATOCERAS, AND EOCEPHALITES 


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