Late Bajocian Ammonites 

From Southern Alaska 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1189 
Late Bajocian Ammonites 

From Southern Alaska 

By RALPH W. IMLAY 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1189 

In Alaska, upper Bajocian beds have been found 

only in the south. They have been dated by 

ammonites as early late Bajocian. Apparently, 

southern Alaska was not connected with the 

rest of Alaska during late Bajocian times 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1982 
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Imlay, Halph Willarrl, 1908-

Latc Bajocian ammonites from southern Alaska. 

(Geological Survey professional paper ; 1189) 

Bibliography: p. 

Ineludes index. 

Supt. of Docs. no.: I 19.16:1189 

1. Ammonoidea. 2. Pal<:•ontology--Jurassic. 3. Paleontology-Alaska. I. Tit!<:•. II. Series: United Stat<:•s. Geological 

Survey. Professional paper ; 1189. 

QE807.A51597 564'.53 80-607116 

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304 
CONTENTS 

Page 

Abstract ------------------------------------------------------------------- 1 

Introduction ---------------------------------------------------------------

Biologic analysis -----------------------------------------------------------

Stratigraphic summary -------------------------------------------------- ___ _ 

Talkeetna ~ountains --------------------------------------------------

VVest side of Cook Inlet -------------------------------------------------

Lower Kuskokwim-Bristol Bay area --------------------------------------

Age and correlation ---------------------------------------------------------

Ammonite faunal setting ------------------------------------------------ ___ _ 

Geographic distribution -----------------------------------------------------

Systematic descriptions ------------------------------------------------- ___ _ 

References cited ------------------------------------------------------------

Index ----------------------------------------------------------------------

ILLUSTRATIONS 

[Plates follow index] 

1 

2 

2 

2 

4 

6 

6 

11 

11 

14 

18 

21 

PLATE 1. M acrophylloceras and Calliphylloceras. 

2. Liroxyites, Sphaeroceras, and Lissoceras. 

3. Stephanoceras and Nonnannites. 

4. Nonn,annites, Leptosphinctes, Megasphaeroceras, and Cadomites. 

5, 6. Stemmatoce·ras. 

7. Cadomites and Stemmatoceras. 

Pa~te 

F'IGURES 1-6. Index maps showing: 

TABLE 

1. Principal occurrences of upper Bajocian marine rocks in southern Alaska ·----- 3 

2. Major areas in which late Bajocian ammonites occur in the Talkeetna 

Mountains ---------------------------------------------------------- 4 

3. Upper Bajocian ammonite localities in the Boulder Creek area of the 

Talkeetna Mountains ------------------------------------------------ 5 

4. Upper Bajocian ammonite localities in the Tuxedni Bay area ---------------- 6 

5. Upper Bajocian ammonite localities in the northern part of the Iniskin 

Peninsula ----------------------------------------------------------- 7 

6. Approximate occurrence of an upper Bajocian ammonite locality in the lower 

Kuskokwim-Bristol Bay area ----------------------------------------- 8 

7. Diagram showing European ranges of ammonite genera present in beds of late Bajocian 

Age in southern Alaska ----------------------------------------------------- 9 

TABLES 

1. Ammonite genera and subgenera from beds of late Bajocian Age in southern Alaska, 

showing biological relationships and relative numbers available for study _______ _ 

2. Geographic distribution of late Bajocian ammonites in southern Alaska by areas and 

localities _______________________________________________________ -----_____ _ 

3. Description of upper Bajocian ammonite localities in southern Alaska ---------------

Page 

2 

10 

12 

UI 
LATE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA 

By RALPH w. IMLAY 

ABSTRACT 

Early late Bajocian ammonites have been found in southern 

Alaska in the Talkeetna Mountains, on the west side of Cook 

Inlet between Tuxedni Bay and Iniskin Bay, and in the lower 

Kuskokwim-Bristol Bay area northeast of Tagiak Bay. This 

dating is based mostly on the association of Normannites, 

StephanoceTas, and StemmatoceTas, which are unknown or 

rare above beds of that age, with Cadomites and Leptosphinctes, which occur rarely just below beds of that age 

but range much higher. The dating is also based on the 

presence of a fragmentary ammonite that in lateral view 

resembles Spi?·oceTas, a genus not known in beds older than 

early late Bajocian. 

Such an association of ammonites occurs west of Cook Inlet 

in the Twist Creek Siltstone, which rests conformably on the 

Cynthia Falls Sandstone of late middle Bajocian age and is 

overlain with marked unconformity by the Bowser Formation 

of early to late Bathonian Age. Such an association of 

ammonites is also found in the Talkeetna Mountains in unnamed beds that are nearly identical lithologically with the 

Twist Creek Siltstone, that rest conformably on beds of late 

middle Bajocian Age, and that are overlain with probable 

unconformity by beds of early Bathonian Age. The presence 

of such an unconformity is suggested by the fact that beds 

of early late Bajocian Age have been found only in two areas 

in the Talkeetna Mountains, that still younger Bajocian beds 

have not been found anywhere in southern Alaska, and that 

still younger lower Bathonian beds are fairly widespread in 

the Talkeetna Mountains as well as west of Cook Inlet. 

Most of the ammonite genera and subgenera present in the 

Twist Creek Siltstone and in equivalent beds in southern 

Alaska are characteristic of the Tethyan Realm. The exceptions include M e,qasphae?·oce?·as and the subgenus Oppelia 

(LiToa:yites). Of these, M egasphae?·oce?·as constitutes 42 

percent of the ammonite specimens collected and occurs elsewhere as far south as eastern Oregon and the western interior 

region of the United States. Li?·oxyites constitutes 27 percent 

of the ammonite specimens collected and has been found 

elsewhere only in northern Alaska in beds of middle Bathonian 

Age. Failure to find any ammonites of late middle Bajocian 

to middle late Bajocian Age in arctic Canada and in northern 

Alaska suggests that those areas were undergoing erosion at 

that time and that all ammonites of early late Bajocian Age 

found in southern Alaska entered from the Pacific Ocean. 

Megasphae?·oceras definitely and Li?·oxyites probably were 

derived from the nearby Pacific Realm rather than from the 

Tethyan Realm, judging by their rather limited geographic 

distribution as compared with the other genera present. The 

fact that some of those ammonites are identical with species 

found in eastern Oregon and along the Idaho-Wyoming border 

suggests that the Bajocian beds now exposed in southern 

Alaska may have originally been deposited much farther 

south. Such a possibility is supported by paleomagnetic data. 

INTRODUCTION 

The late Bajocian ammonites in southern Alaska, 

some of which were described by the writer in 1962, 

are redescribed herein on the basis of additional 

collections from the Talkeetna Mountains and the 

Iniskin Peninsula west of Cook Inlet and one small 

collection from the Kuskokwim area in southwestern 

Alaska. Restudy of these ammonites permits more 

precise dating in terms of the standard Bajocian 

zones of western Europe and the evaluation of their 

geographic origin. 

This study is based on biostratigraphic data compiled by Detterman and Hartsock in 1966 (p. 34, 35) 

for the Twist Creek Siltstone west of Cook Inlet 

and obtained by R. L. Detterman and the writer in 

1972 and 1974 from equivalent beds in the Boulder 

Creek area of the Talkeetna Mountains. In addition, the writer took part in field studies west of 

Cook Inlet with D. J. Miller in 1948 and in the 

Talkeetna Mountains with Arthur Grantz in 1952. 

Most of the geologists who have collected ammonites 

of late Bajocian age in southern Alaska have been 

listed previously (Imlay, 1962, p. A1). All are listed 

herein in table 3 under locality descriptions. Many 

thanks are due to these geologists for collecting the 

fossils and for furnishing locality and stratigraphic 

data. 

This study includes well-preserved ammonites 

from two localities in the Twist Creek Siltstone on 

the Iniskin Peninsula. These ammonites are part 

of a much larger collection from 28 localities made 

partly or entirely by Carleton Beal in 1937, and 

possibly during adjoining years, from beds of 

Bajocian Age on the peninsula. Most of the fossils 

1 
2 LATE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA 

do not have written labels, but all specimens bear 

an inked five-digit, number, which ranges from 

28952 to 30143 according to the locality and which 

presumably represents a numbering system at some 

museum, university, or oil company. In addition, 

19 of the 28 localities are represented by two-digit 

field labels that are preceded by the letter "B" and 

that are written on paper as well as inked on the 

specimens. 

I believe that the fossils from these two localities 

were collected by Carleton Beal because 1) the letter "B" precedes the 19 two-digit field labels ranging 

from B24 to B52; 2) Beal was associated with a 

company that drilled the first well on the peninsula 

in 1936-1938 (Detterman and Hartsock, 1966, p. 

72, 73) ; and 3) one of his field labels was written 

as follows: "29515 (B24). About lJJ. mile upstream 

from 29513. Fossil Creek, Iniskin District. Alaska 

Peninsula. Field no. 24. Carleton Beal. Coli. 1937". 

BIOLOGIC ANALYSIS 

Southern Alaskan Jurassic ammonites of late 

Bajocian Age that are discussed or described herein 

number 254 specin1ens, of which 154 specimens have 

been described previously (Imlay, 1962). Their 

distribution by genera, subgenera, subfamilies, and 

families is shown on table 1. The table shows that 

the Sphaeroceratidae comprises about 421/2 percent, the Oppeliidae, 27 percent, the Stephanoceratidae, 111/2 percent, the Phylloceratidae, 10 percent, 

and the remaining families, 9 percent of the total 

number of specimens. The most common ammonite 

genera are Megasphaeroceras, Liroxyites, and 

Nonnannites. These genera constitute about '76 percent of the total number of specimens. 

STRATIGRAPHIC SUMMARY 

TALKEETNA MOUNTAINS 

Beds lithologically and faunally nearly identical 

with those in the Twist Creek Siltstone west of 

Cook Inlet have been found in the Talkeetna Mountains in two areas (figs. 1-3 and tables 2 and 3). 

One area is near the head of Sheep Creek in the 

northwestern part of the Anchorage (D-2) quadrangle. The other is 24 to 26 km farther west along 

the northwest side of Boulder Creek in the Anchorage ( D-3) and ( D-4) quadrangles. The occurrence 

near Sheep Creek is based on a single collection 

(USGS Mesozoic loc. 24821) obtained near the top 

of the Tuxedni Group. The ocurrence northwest of 

Boulder Creek is based on several exposures and on 

nine fossil collections. _ 

Beds exposed on the ridge in the NW cor. SE 1;4 

sec. 26, T. 27 N., R. 7 E. in the Anchorage (D-3) 

quadrangle consist of orange-gray siltstone that 

contains limestone concretions and the ammonite 

MegasphawroceTas and that is at least 9 m thick. 

Above is a covered interval that represents a thickness of 21 m. Above that is 21 m of graywacke that 

contains C1·anocephalites costidensus Imlay of 

probable earliest Bathonian Age. Above that is 61.5 

m of gray to brown siltstone that contains the Bathonian ammonites C1·anocephalites, Cadomites, and 

Cobbanites. Below the 9 m of orange-gray siltstone 

is 21 m of brown and gray siltstone that has not 

TABLE !.-Ammonite genm·a and subgenera from beds of late Bajocian Age in southern Alaska, showing biological ~relationFamily 

ships and relative numbers available for study · 

Subfamily 

Genus 

or 

subgenus 

Number 

of 

specimens 

Phylloceratidae __ ___________________ Phylloceratinae _____________________ Macrophylloceras _ --------------··· 13 

Calliphylloceratinae _________________ Calliphylloceras -----------------··- 18 

Lytoceratidae -----------------··----- Lytoceratinae ______________________ Lytoceras --------·--------------··- 1 

Spiroceratidae _________ _____________ _ ___________________________________ Spiroceras? --------------------··- 1 

Haploceratidae _____________________ _ __ --------------------------------- Lissoceras ________ --------------··- 11 

Oppeliidae _________________________ Oppeliinae ------------------·------- Liroxyites ----------------------··- 68 

Stephanoceratidae ___________________ ------------------------------------- Normannites -------------------··- 20 

------------------------------------ Stephanoceras -------------------- 1 

------------------------------------ Stemmatoceras ___ --------------··- 7 

---------------------------- ________ Cadomites ----------------------··- 1 

Sphaeroceratidae ___________________ _________ ______________ _____________ M egasphaeroceras --------------··- 106 

------------------------------------ Sphaeroceras _____________________ 2 

Perisphinctidae _____________________ Leptosphinctinae __ ------------------ Leptosphinctes ------------------·-- 8 

------------------------------------ L. (PToTsisphinctes?) -----------··- 2 
STRATIGRAPHIC SUMMARY 3 

152" 148" 140" 136" 132" 128" 124. 

68" 

GULF 

p A C F I C 0 c E4/11 

50 100 150 200 MILES 

0 50 100 150 ?00 250 KILOMETERS 

FIGURE !.-Principal occurrences of upper Bajocian (Jurassic) marine rocks in southern Alaska. 1, Eastern part of the 

Talkeetna Mountains; 2, Tuxedni Bay area; 3, northern part of the Iniskin Peninsula area; 4, lower Kuskokwim-Bristol 

Bay area. 

furnished any ammonites. Below the brown and 

gray siltstone is 23 m of similar siltstone that contains the ammonite Chondroceras of late middle 

Bajocian Age. Evidently the beds characterized by 

M egasphaer·oceras could be as much as 51 m thick 

and are probably considerably thicker than 9 m. 

Sirnilar orange to gray siltstone that contains 

many fossiliferous limestone concretions and is at 

least 40 m thick is well exposed on another ridge 

2 to 3 km to the southwest in the SE l)t. sec. 34, T. 

22 N., R. 7 E., and in the NE 1;4 sec. 3, T. 21 N. R. 

7 E., in the Anchorage (D-4) quadrangle. Still other 

exposures are found in a slumped area between that 

ridge and the tributary of Boulder Creek in the 

NW cor. sec. 2, T. 21 N., R. 7 E In addition, just 

east of the slumped area and west of that tributary 

are some well-exposed beds of late Bajocian Age 

that do not appear to be slumped and that are at 

least 15 m thick as those beds are about 300 m 

lower in altitude than beds of the same age on the 
4 LATE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA 

61°45' rr=============~=======================±======t=========:==n 10 MILES 

0 

I 

10 

II 

15 KILOMETERS 

FIGURE 2.-Major areas in which late Bajocian ammonites occur in the Talkeetna Mountains. Numbers on map refer to 

listing in tables 2 and 3. 

ridge, as they also dip eastward fairly steeply, and 

as they are nearly 1 km farther east, they could 

be an eastern extension of the beds on the ridge. 

The thickness of about 40 m for the upper 

Bajocian beds exposed on the ridge west of the tributary is only 11 m less than the maximum possible 

thickness for the beds exposed on the ridge east 

of the same tributary in sec. 26. T. 22 N., R. 7 E. 

Nonetheless, both Robert L. Detterman and the 

writer, who examined the lithologic units of late 

Bajocian Age on both ridges, consider that the 

unit on the ridge west of the tributary is probably 

at least 100 m and may be as much as 150 m thick. 

Such possible differences in thicknesses of the 

upper Bajocian beds in the two ridges in the Boulder 

Creek area of the Talkeetna Mountains could be explained by folding, faulting, slumping, or ~erosion. 

The differences, however, are most readily explained 

by erosion in latest Bajocian time, like that which 

took place west of Cook Inlet (Detterman and Hartsock (1966, p. 35)). Erosion during that time in the 

area of the Talkeetna Mountains is supported. by two 

facts: (1) upper Bajocian beds have been found 

there to date only in two places; and (2) these beds 

are dated by the ammonites present as not younger 

than early late Bajocian. 

WEST SIDE OF COOK INLET 

The Twist Creek Siltstone was defined by Detterman (1963, p. C33; Detterman and Hartsock, 1966, 
STRATIGRAPHIC SUMMARY 5 

R. 7 E.' 

30 29 28 27 26 25 

31 32 33 36 

T. 22 N. 

(-------+------+-------'1--t-------t-~~----r------~ T. 21 N. 

0 

I 

0 

I 

2 MILES 

I 

14 

23 

3 KILOMETERS 

12 

13 

24 

FIGURE 3.-Upper Bajocian ammonite localities in the Boulder Creek area of the Talkeetna Mountains. Numbers 

on map refer to listing in tables 2 and 3. 

p. 35, fig. 2, tables 1 and 2) for a sequence of beds 

that was previously included in the lower part of the 

Bowser Member of the Tuxedni Formation (Kirschner and Minard, 1948; Imlay, 1953, table 5; 1962, 

p. A2). That sequence was considered by Detterman 

to be a separate formation because it consists mostly 

of a distinctive, soft, dark-gray thin-bedded to massive siltstone that weathers brownish-gray, contains 

many fossiliferous limestone concretions and many 

thin beds of volcanic ash, is overlain unconformably 
6 LATE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA 

by the Bowser Formation, and is underlain conformably by thick-bedded to massive graywacke and conglomerate composing the Cynthia Falls Sandstone. 

On the Iniskin Peninsula, the Twist Creek Siltstone 

thins southward from 128 m to a featheredge. On the 

peninsula between Chinitna Bay and Tuxedni Bay, it 

thins northward from 125 to 91 m (Detterman and 

Hartsock, 1966, p. 35). 

The Twist Creek Siltstone on those two peninsulas 

(figs. 4 and 5) was dated as early late Bajocian by 

Imlay (1962, p. A-6) on the basis of its ammonite 

faunule and its conformable position above two formations that contain ammonites of late middle Bajocian Age (Imlay, 1964, p. B7, B14). 

LOWER KUSKOKWIM-BRISTOL BAY AREA 

The late Bajocian ammonite Leptosphinctes was 

collected about 54 km north-northeast of the head of 

Togiak Bay (fig. 6) from beds consisting of argillite 

0 

I 

0 

and thin bands of fine-grained tuff. This occurrence 

is within a sequence consisting of "massive coarsegrained volcanic conglomerate at least 152 1neters 

thick overlying fine and medium-grained tuffs at least 

30 meters thick" (J. M. Hoare, written cornmun., 

:March 1979) . 

These beds are considered to be in the younger 

part of the Gemuk Group and are probably at least 

305 m thick, but reliable estimates of their thiekness 

cannot be made because of tight folding and much 

faulting (Hoare, 1961, p. 599). 

AGE AND CORRELATION 

The Twist Creek Siltstone on the west side o:f Cook 

Inlet and the lithologically and faunally equivalent 

beds in the Talkeetna Mountains were dated by 

Imlay (1962, p. A2, A3) as early late Bajocian 

(Strenoceras sub furcatum zone) on the basis of the 

2 

I 

i 

TUXEDNI 

I 

3 MILES 

I 

4 KILOMETERS 

FIGURE 4.-Upper Bajocian ammonite localities in the Tuxedni Bay area, west of Cook Inlet. Numbers on map refE:r to 

· listing in tables 2 and 3. 
AGE AND CORRELATION 7 

Camp Pt. 

/· 

/.·· 

/. 

/ .· 

I 

I 

Gulf Island V 

/ .·· I .·· Mt. Chinitna 

X 

/ .· 

/ .·· / .· 

/ .·· I .· / .·· 

0 3 MILES 

0 3 4 KILOMETERS 

FIGURE 5.-Upper Bajocian ammonite localities in the northern part of the Iniskin Peninsula, west of Cook Inlet. 

Numbers on map refer to listing in tables 2 and 3. Top of Twist Creek Siltstone is indicated by dotted lines; its 

base is indicated by dashed lines. 

association of the ammonites Sphaeroceras, Spiroceras ?, and Leptosphinctes. That dating is herein upheld by the presence within the same beds of Norma.nnites, Ste,mntatoceras, and Stephanocer-as, which 

are rare or unknown elsewhere in the world later 

than early late Bajocian (see fig. 7) ; by the presence 

of Ca.domites, which in Europe ranges from the highest part of the middle Bajocian through the Bathonian (Arkell, 1952, p. 70; Kopik, 1974, p. 9) ; and by 

the absence of Chondroceras, which is common nearly worldwide in the SteJJhanoceras humphriesianum 

zone of the middle Bajocian (Imlay, 1973, p. 19) but 

is unknown in beds of late Bajocian Age. 

The only evidence to date that the Twist Creek 

Siltstone and equivalent beds in Alaska could be at 

least in part as old as latest middle Bajocian 

consists of the presence of Cadomites and Leptosphinctes, which in southeast France occur as low 

stratigraphically as the uppermost middle Bajocian 

(Teloceras blagdeni subzone of Stephanoce1·as humphriesianum. zone) (Pavia, 1971, p. 83, 100, 101, 

tables 2 and 3). Similarly, Leptosphinctes has also 

been found in the Boulder Creek area of the Talkeetna Mountains (Imlay, 1964, p. B18, B54, B55, 

pl. 28, figs. 4-6) associated with species of Stemmatoceras, Chondroceras (abundant), and NoTmannites (U.S. Geol. Survey Mesozoic loc. 8572) that are 

identical with species that occur west of Cook Inlet in 

the Fitz Creek Siltstone and in the Cynthia Falls 

Sandstone of middle Bajocian Age but that do not 

occur in the overlying Twist Creek Siltstone. This 

occurrence of Leptosphinctes with many specimens of 
8 

61°00' 

LATE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA 

RIVER 

r-------------~--------~------------~N~y~a~c----~~----------~--+------------~ 

50 

I I 

I I 

I 

50 

~ 

<Q"' 

c1~ ~ 

~ 

BRISTOL 

I 

I I 

0 50 

BAY 

I 

100 

50 

I 

I 

100 MILES 

I 

150 KILOMETERS 

FIGURE 6.-Approximate occurrence of an upper Bajocian ammonite locality in the lower Kuskokwim-Bristol Ba3r area 

in southwestern Alaska. Number on map refers to listing on tables 2 and 3. 
AGE AND CORRELATION 

AGE MIDDLE BAJOCIAN LATE BAJOCIAN 

E: 

~ 

c::: 

·~ Cl) 

·~ E: ...c:: 

Q.. ~ ·c:: 

Genus E: ~ 0 .... ~ r.;;, ~ Cl) 

::::... ...c:: -2 c::: .!::; 

-e ~ ·~ i: 

~ 'Q) Cl) ~ c::: ~ 

~ Cl) 

~ Q.. 

0 ~ Ill Cl) ~ Cl) ·~ ~ t.,) ~ I:)) c::: 

·~ Cl) 0 Ill 

·~ 0 c::: t.,) Cl) .!::; ~ ~ 0 .!::; c::: ...c:: c::: c::: ·'t:::: c::: 0 ~ Ill ~ i: 

0 ....... ....... ~ ~ 

Cl) a Cl) Cl) c_, ~ 

Spiroceras --------- ..... 

~ 

Lissoceras ---------- ........ 

.... 

Normannites --------

Stephanoceras --------

Stemmatoceras -------

Cadomites ---------- ..... 

;' 

Sphaeroceras --------

Leptosphinctes -------

FIGURJ~ 7.-Diagram showing European ranges of certain ammonite genera present in beds of late Bajocian Age in 

southern Alaska. 

9 
10 LATE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA 

TABLE 2.-Geographic distribution of late Bajocian 

[Numbers 1 and 2 are keyed to locality numbers in figs. 2 and 3. Numbers 3-11 are keyed to locality numbers in fig8. 4 and 

places as the localities listed under those individual numbers. Higher numbers are USGS Mesozoic locality numbers.] 

Talkeetna Mountains 

Sheep 

Creek Boulder Creek 

area area 

Unnamed beds equivalent to 

Genus and species Twist Creek Siltstone 

1 2 

.-1 C'l .-1 0 C'l:) 0) 0 .-1 CN C'l r:-. 0) 00 00 00 0) 0) 0) 

00 I.Q C'l I.Q I.Q I.Q I.Q I.Q I.Q -.::j< 00 0 0 0 0 0 0 0 CN C'l:) C'l:) C'l:) C'l:) C'l:) C'l:) CQ 

Phylloceras (Macrophylloceras) grossicostatum Imlay__________ X ----- __________ ----- X _______________ _ 

(M.) cf. P. (M.) grossicostatum Imlay ______________ ------- ----- ----- ----·- ___________________________________ _ 

Calliphylloceras freibrocki Imlay _ --------------------------- ----- ----- ----- ----- ----- ----- ________________ -----

sp ------------------------------------------------------- X ----- ----- ----- ----- ----- ----- ------ -----

Lytoceras sp ------------------------------------------------ ----- ----- ----- __________ ----- __________ ------ -----

Spiroceras? sp ---------------------------------------------- ----- ----- ----- ----- _____ ----- __________ ------ -----

Lissoceras bakeri Imlay ------------------------------------- ----- ----- ----- ------ _____ ----- __________ ------ ____ _ 

Oppelia (Liroxyites) kellumi Imlay ------------------·-------- ----- ----- ----- ----- ----- X X ----- ------ -----

Normannites vigoTosus (Imlay) ---------------------------- X X __________ ----- X _____ ---·-- -----

bouldeTensis Imlay, n. sp -------------------------------- ----- ----- ----- ----- ----- ----- X _____ ------ ----- Stephanoceras cf. S. caamanoi McLeaTn -----------------------· __________________________________________________ _ 

StemmatoceTas cf. S. arcicostatum Imlay --------------------- X ----- ----- ----- ----- X ----- ---·-- -----

sp ------------------------------------------------------ ----- ----- ----- ----- ----- ----- ----- ----- X 

Cadomites magnus Imlay, n. sp ------------------------------ ----- ----- ----- ------ ----- ----- ----- X ---·-- -----

Megasphaeroceras rotundum Imlay--------------------------- X X X X ----- ------ X 

cf. M. rotundum Imlay -----------------------------------· __________________________________________________ _ 

sp ------------------------------------------------------ ----- ----- ----- ----- ----- ----- ----- ----- ---·-- -----

Sphae?·oceras talkeetnanum Imlay ---------------------------- ----- ----- X ----- ----- ----- ----- ----- ---·-- -----

Leptosphinctes cliffensis Imlay --·- ---------------------------- ----- ----- ----- ----- ----- ----- ----- ----- ---·-- ----- cf. L. leptus Buckman ------------------------------------ ----- _____ ----- ___________________________________ _ 

(Prorsisphinctes?) delicatus Imlay ------------------------- ----- ___________________________________ ------ ____ _ 

(P. ?) sp ------------------------------------------------- ----- ----- ----- ----- ------ ----- ----- ----- ---·-- -----

L.? sp -------------------------------------------------------- ----- ----- ----- ----- ----- ----- ----- ----- ---·-- -----

Chondrocerras at Mesozoic loc. 8572 contrasts with 

the absence of C hondroce1·as in the Twist Creek Siltstone; it shows that the lowest part of the range of 

Leptosphinctes in Alaska is the same as it is in 

France and definitely favors an early late Bajocian 

Age assignment for the Twist Creek Siltstone and 

equivalent beds in Alaska. 

Similar early late Bajocian ammonites have been 

found elsewhere in North America in east-central 

Oregon near Seneca and Huntington and in the western interior region in southeastern Idaho and adjoining parts of Wyoming and Utah. In east-central Oregon, the ammonite faunule of that age is nearly the 

same as the faunule in the Twist Creek Siltstone in 

southern Alaska (Imlay, 1973, p. 28-31); the faunule 

in east-central Oregon differs by containing more 

specimens of Sp1~roceTas and by the presence of the 

genus LupheTites. In the western interior region, the 

upper part of the Sliderock Member of the 'Twin 

Creek Limestone has furnished a much less varied 

ammonite faunule, consisting only of Spirroceras, 

M egasphaeroceTas, Stemmatoceras, and StephanoceJ·as (Imlay, 1967, p. 26-29, 59). Nonetheless, the 

species of those genera are identical with or very 

similar to species in eastern Oregon and in Alaska. 

Elsewhere in North America, the early lab~ Bajocian is possibly represented in the Smithers area of 

British Columbia by specimens that have been assigned questionably to M egasphaeroceras by Frebold 

and Tipper (1973, p. 1130, pl. 3, figs. 2, 3). Likewise, 

the early late Bajocian is represented near Mixtepec, 

Oaxaca, in south-central Mexico by LeJJtosphinctes 

(Burckhardt, 1927, p. 27, pl. 10, figs. 11, 1:2) and 

Parastrenoceras (Burckhardt, 1927, p. 90, pl. 16, 

figs. 10, 11, 16; Ochoterena F., 1963, p. 1-15, pis. 

1-4). 
GEOGRAPHIC DISTRIBUTION 11 

ammonites in southeTn Alaska by a1·eas and localities 

5 and in table 3. The columns numbered 6-11 represent two ammonite collections that are probably from some of the same 

West side of Cook Inlet Lower 

'l~uxedni KuskokwimBay Iniskin Peninsula Bristol Bay 

area area 

Twist Creek Siltstone Gemuk Group 

(upper area) 

3 4 5 6 7 8 9 10 11 6-11 12 

C'l 0) 0 "'d' -.::14 -.::14 tO Cf:) M 1.0 M 1.0 ...-4 00 0) M "'d' C\l 00 0 ...-4 M M .-4 M -.::14 .-4 ...-4 0) M 0 ...-4 Ci) ...-4 ...-4 C\l C\1 ~ ~ 0 Ci) M 0 0) M M 1.0 0 0 M 0 ~ ~ ...-4 ...-4 C\l C\1 ...-4 Ci) .-4 ....... Ci) .-4 ...-4 tO ...-4 0 ...-4 ~ ...-4 ...-4 ...-4 

C'l C\1 C\1 ....... ....... C\1 ....... ....... C\1 C\1 C\1 ...-4 C\1 C\1 C\1 M M M 

----- ----- ----- ----- ----- X ----- ----- X X X X 

----- ----- ----- ----- ----- ----- ----- ----- ----- ----- X _____________________________________________ ----- ----- ----- X ----- ----- ----- X ---------------------

----- ----- ----- ----- ----- X ----- ----- ----- X ----- ----- ----- X X ----- ---------------------

X ----- ----- ----- ----- ------ ---- ----- ----- ----- ----- ----- -----· ----- ----- ----- ---------------------

X ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ---------------------

X X X X X X ----- ----- ----- X ----- ----- ----- ----- ----- ----- X ---------------------

X ----- ----- ----- ----- X X X X X X X X X X X ------------------·---

----- ----- ----- X X X X ----- -----· ----- ----- X X X ----- ---------------------

X X ----- ----- ----- X X X 

----- ----- ----- ----- X 

X 

AMMONITE FAUNAL SETTING 

X X 

X 

X 

The early late Bajocian ammonite genera found in 

southern Alaska are mostly of Tethyan origin. They 

are distinguished, however, from assemblages of the 

same age in southern Eurasia by the absence of any 

genus of the Parkinsoniidae and by the presence of 

M ega.sphaeroce1·as and Oppelia (Liroxyites). Megasphaeroceras has been found elsewhere in eastern 

Oregon (Imlay, 1973, p. 8, 54, 82, pl. 40, figs. 4-7) 

and in the western interior of the United States 

(Imlay, 1967, p. 96, pl. 14, figs. 1-6, 9-16, pl. 16, figs. 

1-4) and is possibly represented at Tensas Creek in 

the S:mithers area of central British Columbia (Frebold and Tipper, 1973, p. 1113, 1127, 1128, pl. 3, figs. 

2, 3). The subgenus Liroxyites has been found elsewhere in northern Alaska associated with ammonites 

of middle Bathonian age (Imlay, 1976, p. 14, pl. 1, 

X X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

fig. 15). The fact that no ammonites of early late 

Bajocian Age have been found in arctic Alaska or in 

arctic Canada is good evidence that the ammonites of 

that age found in southern Alaska entered from the 

Pacific Ocean. Furthermore, such ammonites apparently lived considerably south of their present site in 

southern Alaska, as indicated by their close resemblances to species in Oregon and in the western region and by paleomagnetic data (Stone and Packer, 

1979,p. 558-560,fig. 6). 

GEOGRAPHIC DISTRIBUTION 

Figure 1 shows the principal areas in southern 

Alaska in which late Bajocian ammonites have been 

found. Figure 2 shows the major areas in which such 

ammonites have been found in the Talkeetna Mountains, and figure 3 is a detailed index map of late 
12 LATE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA 

Bajocian ammonite occurrences in the Boulder Creek 

area of those mountains. Figures 4 and 5 are fairly 

detailed index maps of late Bajocian ammonite occurrences west of Cook Inlet. A single ammonite occurrence of that age in the lower Kuskokwim-Bristol 

Bay region is shown in figure 6. The geographic occurrences of the late Bajocian ammonite genera and 

species by areas and localities are shown herein in 

table 2. Detailed descriptions of those localities are 

given in table 3. 

TABLE 3.-Description of upper Bajocian ammonite localities 

in southern Alaska 

Locality No. USGS 

(figs. Mesozoic 

2-6) loc. No. 

Collector's 

field No. 

Collector, year of 

collection, description. of 

locality, and stratigraphic 

assignment 

L__ 2482L__ 53AGz144 ___ Arthur Grantz and L. F. 

Fay, 1953. Talkeetna 

Mountains near head of 

Sheep Creek, lat 61 o 59' 

17" N., long 147°38'35" 

W., Anchorage (D-2) 

quadrangle. Tuxedni 

Group, near top in beds 

equivalent to the Twist 

Creek Siltstone. 

2___ 8572____ 13Am21 _____ G. C. Martin, 1913. Talkeetna Mountains at 

altitude of 4,200 ft 

(1,280 m) on ridge west 

of creek that enters 

main Boulder Creek 

from north, 3 miles ( 4.8 

km) above its junction 

with the East Fork. 

Probably from westcentral part of sec. 2, T. 

21 N., R. 17 E., Anchorage ( D-4) quadrangle. 

Tuxedni Group in beds 

equivalent to the Twist 

Creek Siltstone west of 

Cook Inlet. 

2___ 3029L__ 72ADf17 ____ Don Hartman and R. W. 

Imlay, 1972. Talkeetna 

Mountains, on west side 

of a tributary of Boulder Creek, at same place as USGS Mesozoic loc. 

30592. Tuxedni Group, 

from beds equivalent to 

the Twist Creek Siltstone west of Cook Inlet. 

2___ 30580___ 74AI8 ______ R. W. Imlay and Jordan 

Pflaker, 197 4. Talkeetna 

Mountains. From 9 m 

of reddish-brown siltstone about 21 m below 

beds of Bathonian Age 

and 21 m above beds of 

late middle Bajocian 

Age. The reddish-brown 

siltstone occurs at the 

west base of a bluff and 

at the east end of a fairly flat area and is 

underlain by 44 m of 

TABLE 3.-Description of upper Bajocian ammonite localities 

in southern Alaska-Continued 

Locality No. USGS 

(figs. Mesozoic 

2-6) Joe. No. 

2 __ _ 30583 __ _ 

2 __ _ 30589 __ _ 

2 __ _ 30590 __ _ 

2 __ _ 3059L __ 

Collector's 

field No. 

Collector, year of 

collection, description of 

locality, and stratigraphic 

assignment 

brown to gray 8iltstone. 

NW 1,4 sec. 26, T. 22 N., 

R. 7 E., Anchorage ( D3) quadrangle. Tuxedni 

Group in beds equivalent to the Twist Creek 

Siltstone west of Cook 

Inlet. 

74ADt 97 ___ R. L. Detterman, 1974. 

Talkeetna Mountains. 

From brown-weathering 

siltstone and shale con- taining limestone con- cretions. On ridge 2.22 

miles (3.52 km) N. 83° 

E. of V ABM Suicide at 

altitude of 4,460 ft (1,341 

m), SW 1,4 SE: 1,4 sec. 34, T. 22 N., R. 7 E., 

Anchorage (D-4) quadrangle. Tuxedni Group, 

30-46 m below beds at 

USGS Mesozoic loc. 

30589 in beds equivalent 

to the Twist Creek Siltstone west of Cook Inlet. 

74ADt106 ___ R. L. Detterman and R. 

W. Imlay, 1974. Talkeetna Mountains. From 

siltstone and gray limestone containing limestone concretions. On 

ridge top 0.35 mile (0.56 

km) south of USGS 

Mesozoic loc. :W583 at 

altitude of 4,510 ft 

(1,356 m), N'\V % NE 1

4 sec. 3, T. 21 N., R. 7 

E., Anchorage (D-4) 

quadrangle. Tuxedni 

Group, in beds equivalent to the Twist Creek 

Siltstone west of Cook 

Inlet. 

74ADT 107 __ R. L. Detterman and R. 

W. Imlay, 1974. Talkeetna Mountains, on east slope of ridge about 

midway between USGS 

Mesozoic locs. 30589 and 

30591 at altitude of 

4,000 ft (1,219 m), NE 1

4 NE 14 sec. 3, T. 21 

N., R. 7 E., Anchorage 

D-4) quadrangle. Tuxedni Group, float from 

beds equivalent to the 

Twist Creek Siltstone 

west of Cook Inlet. 

74ADt108 ___ R. L. Detterman and R. 

W. Imlay, 1974. Talkeetna Mountains. 

Brown-weathering siltstone containing limestone concretions. Downslope 0.2 mile (0.32 km) 

from USGS Mesozoic 

loc. 30590 at altitude of 

3,670 ft (1,119 m), NW 
GEOGRAPHIC DISTRIBUTION 13 

TABLE 3.-Desc?·iption of upper Bajocian mnmonite localities TABLE 3.-Desc?·iption of uppe1· Bajocian ammonite localities 

in southen~ Alaska-Continued in southern Alaska-Continued 

Locality No. USGS 

(figs. Mesozoic 

2-6) loc. No. 

2 __ _ 30592 __ _ 

g __ _ 21282 __ _ 

4 __ _ 22709 __ _ 

5 __ _ 22710 __ _ 

s __ _ 11034 __ _ 

6 __ _ 19934 __ _ 

Collector's 

field No. 

74ADt109 __ _ 

48AI81 ____ _ 

51AGz140 __ _ 

51AGz141 __ _ 

21ABF-42 __ 

44AWWF-3 _ 

Collector, year of 

collection, description of 

locality, and stratigraphic 

assignment 

cor. sec. 2, T. 21 N., R. 

7 E., Anchorage· (D-4) 

quadrangle. Tuxedni 

Group, float from beds 

equivalent to the Twist 

Creek Siltstone west of 

Cook Inlet. 

R. L. Detterman and R. 

W. Imlay, 1974. Talkeetna Mountains. 

Brown to gray siltstone 

containing limestone 

concretions. Downslope 

0.2 mile (0.32 km) N. 

85 o E. from USGS 

Mesozoic loc. 30591 at 

altitude of 3,440 ft 

(1,036 m). NW lit NW 

% sec. 2, T. 21 N., R. 

7 E., Anchorage (D-4) 

quadrangle. Tuxedni 

Group, from beds equivalent to the Twist Creek 

Siltstone west of Cook 

Inlet. 

R. W. Imlay and D. J. 

Miller, 1948. Tuxedni 

Bay area, on tributary 

entering Bear Creek 

from the southeast, 4.75 

miles (7.6 km) S. 22o 

W. of Fossil Point. 

Twist Creek Siltstone, 

300 ft (91 m) above 

base. 

Arthur Grantz, 1951. Tuxedni Bay area, about 

0.3 mile (0.5 km) above 

mouth of tributary en- tering Bear Creek from 

southeast at a point 2.53 

miles ( 4 km) from Tuxedni channel. Twist 

Creek Siltstone, about 

100 ft (30.5 m) above 

base. 

Arthur Grantz, 1951. Tuxedni Bay area, about 0.5 

mile ( 0.8 km) above 

mouth of same tributary 

described under Mesozoic loc. 22709. Twist 

Creek Siltstone, about 

260 ft (79.6 m) above 

base. 

A. A. Baker, 1921. Iniskin 

Peninsula, on right fork 

of Cliff Creek about 

2,700 ft (823 m) above 

junction with left fork 

and 2.46 miles ( 4 km) 

S. 15 o W. of mouth of 

Fitz Creek. Twist Creek 

Siltstone, about 100 ft 

(30.5 m) above base. 

Helmuth Wedow, Jr., 1944. 

Iniskin Peninsula, small 

tributary on right side 

of Cliff Creek about 

5,100 ft (1,554 m) above 

junction with Fitz Creek 

Locality No. USGS 

(figs. Mesozoic 

2-6) loc. No. 

6 __ _ 21314 __ _ 

11036 __ _ 

7 __ _ 19943 __ _ 

7 __ _ 21313 __ _ 

7 __ _ 21315 __ _ 

g __ _ 26593 __ _ 

9 __ _ 11035 __ _ 

Collector's 

field No. 

48AI30 ____ _ 

21ABF-43 __ 

44AWWF-12 

48AI29 ____ _ 

48AI31 ____ _ 

57 ADt5 ____ _ 

21ABF-44 __ 

Collector, year of 

collection, description of 

locality, and stratigraphic 

assignment 

and 2.42 miles ( 3.9 km) 

S. 14 o W. of mouth of 

Fitz Creek. Twist Creek 

Siltstone, about 85 ft 

(26 m) above base. 

R. W. Imlay and D. J. 

Miller, 1948. Iniskin 

Peninsula, on Cliff 

Creek 3.66 miles ( 5.8 

km) N. 87° E. of Tonnie 

Peak. Same location as USGS Mesozoic locs. 

11034 and 19934. Twist 

Creek Siltstone, 10-100 

ft ( 3-30 m) above base. 

A. A. Baker, 1921. Iniskin 

Peninsula, on right 

fork of Cliff Creek 

about 8,000 ft (2,438 m) 

above junction with Fitz 

Creek and 2.58 miles ( 4 

km) S. 15° W. of mouth 

of Fitz Creek. Twist 

Creek Siltstone, 125-

175 ft. ( 38-53 m) above 

base. 

Helmuth Wedow, Jr., 1944. 

Iniskin Peninsula, on Cliff Creek about 200 ft 

( 67 m) downstream 

from a prominent cas- cade, 114 miles (2 km) 

above junction with Fitz 

Creek and 3.35 miles 

( 5.3 km) east of Tonnie 

Peak. Twist Creek Siltstone, 125-175 ft (38-

53 m) above base. 

R. W. Imlay and D. J. 

Miller, 1948. Iniskin 

Peninsula, on Cliff Creek 

just below a cascade. 

Same location and stratigraphic position as USGS Mesozoic locs. 

11036 and 19943. 

R. W. Imlay and D. J. 

Miller, 1948. Iniskin 

Peninsula, on Cliff Creek near USGS Mesozoic 

locs. 21313 and 19943. 

Twist Creek Siltstone, 

float from lower 175 ft 

(53 m). 

R. L. Detterman, 1957. 

[niskin Peninsula, on Cliff Creek near USGS 

Mesozoic loc. 21313, 

lat 59°49'20" N., long 

153°11'00" W. Twist 

Creek Siltstone, 200 ft 

( 61 m) above base. 

A. A. Baker, 1921. Iniskin 

Peninsula, on right fork 

of right fork of Cliff 

Creek about 9,000 ft 

(2,743 m) above junction with Fitz Creek 

and 2.68 miles ( 4.2 km) 

S. 16° W. of mouth of 

Fitz Creek. Twist Creek 

Siltstone, lower part. 
14 LATE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA 

TABLE 3.-Desc'ription of upper Bajocian am'monite localities 

in southern Alaska-Continued 

Locality No. 

(figs. 

2-6) 

10 __ 

1L_ 

1L_ 

6-11 

6-11 

12 __ 

USGS 

Mesozoic 

loc. No. 

20001 __ _ 

21318 __ _ 

27099 __ _ 

31713 __ _ 

31714 __ -

31122 __ _ 

Collector's 

field No. 

Collector, year of 

collection, description of 

locality, and stratigraphic 

assignment 

44AWWF-68 Helmuth Wedow, Jr., and 

L. B. Kellum, 1944. Ini- . ·. ·skin Peninsula; on north.: 

east side of Tonnie 

Creek about 200 ft (61 

m) downstream from 

top of a cascade, which 

is 0.9 mile (1.4 km) S. 

48° E. of Tonnie Peak. 

Twist Creek Siltstone, 

about 100 ft (30.5 m) 

above base. 

48AI35 _____ R. W. Imlay and D. J. 

Miller, 1948. Iniskin 

Peninsula, on Tonnie 

Creek 1 to 1.05 miles 

(1.6 km) S. 51° E-. of 

Tonnie Peak. Same location as USGS Mesozoic loc. 27099. Twist 

Creek Siltstone, 50-125 

ft (15-38 m) above 

base. 

58ADt2 ______ R. L. Detterman, 1958. IniB37 (29471 on specimen) 

No field num- ber (29466 on specimen) 

skin Peninsula, on Tonnie Creek 0.81 mile N. 

55o W. of Iniskin Bay 

Association No. 1 well. 

Lat 59°45'30"N., long. 

153°15'05"W. Same location as USGS Mesozoic loc. 21318. Twist 

Creek Siltstone, 75-100 

ft ( (23-30 m) above 

base. 

Carleton Beal, 1937. Iniskin Peninsula. Probably in Cliff Creek at or near USGS Mesozoic 

loc. 21314 (No. 6 on fig. 

5), judging by the large 

number of ammonite 

genera present. Probably in NW % SE % sec. 3, T. 5 S., R. 23 W., 

Iliamna ( D-1) quadrangle. Twist Creek 

Siltstone, probably in 

lower 100 ft (30 m). 

Carleton Beal?, 1937? Iniskin Peninsula. Probably on Tonnie Creek at or near USGS Mesozoic 

Joe. 20001 (No. 10 on fig. 5), judging by the 

small number and the 

kind of ammonite gen- era present. Probably in 

SW % NE % NE 14 sec. 7, T. 5 S., R. 23 W., 

Iliamna (D-1) quadrangle. Twist Creek Siltstone. 

GC3-1116 ___ W. L. Coonrad and J. M. 

Hoare, 1975. Lower 

Kuskokwin-Bristol Bay 

region. From north-facing scree slope consisting of argillite and thin 

bands of · fine-grained 

tuff. At altitude of 

about 1,400 ft (427 m) 

TABLE 3.-Description of upper Bajocian ammonite localih'es 

in southern Alaska-Continued 

Locality No. 

(figs. 

2-6) 

USGS 

Mesozoic 

loc. No. 

Collector's 

field No. 

Collector, yea:r of 

collection, description of 

locality, and stratigraphic 

assignment 

in SE cor. NW lA, sec. 3, T. 8 S., R. 63 W., 

lat 59°30'51" N., long 

159°59'36" W., Good- news ( C-3) quadrangle, 

Gemuk Group, upper 

part. 

SYSTEM A TIC DESCRIPTIONS 

Family PHYLLOCERATIDAE Zittel, 1884 

Subfamily PHYLLOCERATINAE Zittel, 1884 

Genus PHYLLOCERAS Suess, 1865 

Subgenus MACROPHYLLOCERAS Spath, 192'/ 

Phylloceras (Macrophylloceras) grossicostatum Imlay 

Plate 1, figures 1-3 

Phylloceras (Macrophylloceras) grossicostatum Imlay, n. sp., 

1953, U.S. Geological Survey Professional Paper 249-B, 

p. 74, pl. 25, figs. 11-13, 15, 16. 

Macrophylloce1·as cf. M. grossicostatum Imlay, 1962, U.S. 

Geological Survey Professional Paper 418-A, A5.. pl. 5, 

figs. 6, 7. 

Phylloceras (Macrophylloceras) grossicostatum Imlay, 1980, 

U.S. Geological Survey Professional Paper 1091, p. 16, pl. 1, 

figs. 12, 13. 

This species originally was based on six specimens 

from the lower part of the Chinitna Formation west 

of Cook Inlet (Imlay, 1953, p. 74), but more recently 

it was collected from the underlying Bowser Formation and Twist Creek Siltstone in the same area and 

from equivalent beds in the Talkeetna Mountains. To 

date, 13 septate specimens have been collected from 

the Bowser Formation and equivalent beds and 14 

specimens from the Twist Creek Siltstone and equivalent beds. These collections show that the .innermost whorls are smooth and that very fine .riblets and 

striae appear at a diameter of about 26 mm. These 

riblets pass into striae near the umbilicus, become 

stronger ventrally and adorally, and persist to diameters of 60-70 mm. In addition, at a diameter of 

about 30 mm, some broad, low, gently flexuous ribs 

appear on the upper two-thirds of the flanks but tend 

to fade out on the venter at diameters less than 60 

mm. Adorally, these broad ribs become prominent or 

fairly prominent within about one-fourth of a whorl, 

or at diameters of 60-70 mm, and become separated 

by broad flat interspaces. The coarseness of these 

ribs varies somewhat/from ·one specimen to another ' / 

and is intermediate on the holotype. 
SYSTEMATIC DESCRIPTIONS 15 

TY1Jes.-Holotype, USNM 108014; para types, 

USNM 10815-10817; hypotypes, USNM 273501-

273503. 

Occurrences.-Twist Creek Siltstone west of Cook 

Inlet at USGS Mesozoic loc. 11035, 21313, 21314, 

31713 and 31714. Equivalent unnamed beds in the 

Talkeetna Mountains at USGS Mesozoic locs. 24821 

and 30589. 

Subfamily CALLIPHYLLOCERATIN AE, Spath, 1929 

Genus CALLIPHYLLOCERAS Spath, 1927 

Calliphylloceras freibrocki (Imlay) 

Plate 1, figures 4-7 

Phylloceras (Calliphylloceras) freibrocki Imlay n. sp., 1953, 

U.S .. Geological Survey Professional Paper 249-B, p. 73, 

pl. 26, figs. 7-11. 

Calliphylloceras freibrocki (Imlay), 1980, U.S. Geological 

Survey Professional Paper 1091, p. 16, pl. 1, figs. 7-9, 11. 

This species is represented in the Twist Creek Siltstone by three specimens that closely resemble the 

type specimens from the Chinitna Formation in 

shape, in fineness of ribbing (lirae), and in the 

gently sigmoidal pattern of both ribs and constrictions on the flanks. 

Types.-Holotype, USNM 108006; paratypes, 

USNlVI 108007, 108008; hypotypes, 273504, 273505. 

Occurrence.-Twist Creek Siltstone west of Cook 

Inlet at USGS Mesozoic locs. 20001 and 31714. 

Family HAPLOCERATIDAE Zittel, 1884 

Genus LISSOCERAS Bayle, 1879 

Lissoceras bakeri Imlay 

Plate 2, figure 8 

Lissoceras bakeri Imlay, 1962, U.S. Geological Survey Professional Paper 418-A, p. A6, pl. 1, figs. 1-6, 9-12. 

The nearly complete adult specimen illustrated 

herein shows that the adult body chamber occupies 

nearly half a whorl and retracts a little from the 

outermost septate whorl. At a diameter of 135 mm, 

the whorl height is about 60 mm and the umbilical 

width is 30 mm. 

Types.-Holotype, USNM 130886; para types, 

USNM 130887a, b, 130888, 130889a-c, 130890; hypotype, 273506. 

Occurrence.-Twist Creek Siltstone west of Cook 

Inlet at USGS Mesozoic locs. 11034, 19934, 21282, 

21314, 21315, 22709, 22710, and 31713. 

Family OPPELIIDAE Bonarelli, 1894 

Subfamily OPPELIIN AE Bonarelli, 1894 

Genus OPPELIA Waagen, 1869 

Subgenus LIROXYITES Imlay, 1962 

Oppelia (Liroxyites) kellumi Imlay 

Plate 2, figures 1, 2, 6, 7 

Oppelia (Liroxyites) kellumi Imlay, n. sp., 1961, Journal of 

Paleontology, v. 35, no. 3, p. 470, pl. 63, figs. 5, 7-9. 

Oppelia (Li·roxyites) kellumi Imlay, 1962, U.S. Geological 

Survey Professional Paper 418-A, p. AS, pl. 2, figs. 1-12. 

This species, now represented by 68 specimens, is 

the second most abundant species in the upper Bajocian beds of southern Alaska. It is characterized by 

its discoidal shape, highly evolute coiling, sharp venter, broad falcoid ribs and falcoid striae that persist 

onto the body chamber, and by the presence of faint 

broad spiral bands. Its ornamentation varies considerably in strength from one specimen to another. 

Three specimens in which the ribs and bands are 

fairly conspicuous are illustrated herein. 

Types.-Holotype, USNM 130886; paratypes, 

USNM 130887 a, b, 130888, 130889a-c, 130890 ; hypotypes, USNM 273507-273509. 

Occurrences.-Twist Creek Siltstone west of Cook 

Inlet at USGS Mesozoic locs. 11035, 11036, 19943, 

20001, 21282, 21313, 21314, 21315, 21318, 27099, 

31713 and 31714. Equivalent beds in the Talkeetna 

Mountains at USGS Mesozoic locs. 30583 and 30589. 

Family STEPHANOCERATIDAE Neumayr, 1875 

Genus NORMANNITES Munier-Chalmas, 1892 

Normannites vigorosus (Imlay) 

Plate 4, figures 1-4 

Dettermanites vigorosus Imlay, n. sp., 1961, Journal of Paleontology, v. 35, no. 3, p. 472, pl. 64, figs. 1-3. 

Dettermanites vigorosus Imlay, 1962, U.S. Geological Survey 

Professional Paper 418-A, p. A12, pl. 4, figs. 1-9. 

This species is represented by 17 specimens. It 

differs ·from most species of Normannites by attaining a larger size, by having more secondary ribs per 

primary rib, and by the secondary ribs being much 

weaker than the primary ribs. Among the 17 specimens available, the strength of ribbing varies appreciably, but the number of secondary ribs per primary 

rib varies from three to four. Generally, two secondary ribs arise from tubercles near the middle of the 

flanks, and one or two secondary ribs arise freely between the tubercles. 

Normannites vigorosus (Imlay) greatly resembles 

N. formosus Buckman ( 1920, pl. 151, figs. 1-3; Westermann, 1954, p. 259, pl. 22, fig. 4, pl. 23, figs. la-c; 

Imlay, 1973, p. 84, pl. 41, fig. 19) but differs by having stronger primary ribs and much weaker secondary ribs. 

Type.-Holotype, USNM 130895; para types, 

USNM 130896, 130897a, b; hypotypes, USNM 

273510, 273511. 

Occurrences.-Twist Creek Siltstone on the Iniskin 

Peninsula at USGS Mesozoic locs. 11034-11036, 
16 LATE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA 

19934, 21314, 21318, and 31713. Equivalent beds in 

the Talkeetna Mountains at USGS Mesozoic locs. 

8572, 24821, and 30589. 

Normannites boulderensis Imlay, n. sp. 

Plate 3, figures 2-8 

This species is represented by three internal molds 

preserved in concretions. The whorls are coronate in 

section, much wider than high, and each embraces 

nearly half of the preceding whorl. The body chamber 

is incomplete but occupies three-fourths of a whorl 

and is not contracted from the preceding septate 

whorl. The umbilicus is moderately wide. Its wall is 

steeply inclined and rounds evenly into the flanks. 

On septate whorls, the primary ribs are sharp, 

high, widely spaced, incline slightly forward, and 

terminate near the middle of the flanks in acute 

prominent tubercles. From these pass two, or rarely 

one, somewhat weaker secondary ribs that incline 

forward on the flanks and cross the venter transversely. A few ribs arise freely along the zone of 

tuberculation. All ribs gradually become stronger and 

sparser during growth. Adorally on the body chamber, the secondary ribs become even stronger than 

the primary ribs. 

The holotype at a diameter of 95 mm has a whorl 

height of 30 mm and an umbilical width of 45 mm. 

The paratype at a diameter of 80 mm has a whorl 

height of 22 mm, a whorl thickness of 35 mm, and an 

umbilical width of 42 mm. 

The suture line is similar to that on N ormannites 

vigorosus (Imlay) (1962, p. Al2, pl. 4, fig. 7) but 

differs by its ventral lobe being of about the same 

length as the first lateral lobe. 

The three specimens herein assigned to Normannites boulderensis Imlay, n. sp., bear some resemblance to the coarsely ribbed variant of N. vigo'rosus 

with which they are associated. They differ by having much stronger and fewer secondary ribs that 

adorally on the body chamber become as strong or 

stronger than the primary ribs. 

These three specimens bear more resemblance to 

Normannites orbignyi Buckman (1927, pl. 734, figs. 

1-3; Westermann, 1954, p. 138, pl. 5, figs. 3, 4, pl. 6, 

fig. 1; Imlay, 1973, p. 82, pl. 41, figs. 9, 10, 18, 20) but 

differ by being less evolute, by bearing more prominent tubercles, and by the largest specimen attaining 

a much larger size. 

Types.-Holotype, USNM 273512, paratypes 

273-513, 273514. 

Occurrences.-Twist Creek Siltstone on the Iniskin 

Peninsula at USGS Mesozoic loc. 31713. Unnamed 

beds in the Talkeetna Mountains at USGS Mesozoic 

loc. 30589. 

Genus STEPHANOCERAS Waagen, 1869 

Stephanoceras cf. S. caamanoi McLearn 

Plate 3, figure 1 

One small septate Alaskan ammonite bea:rs ribs 

and tubercles that are similar in strength and density to those on the inner whorls of Stephanoceras 

caamanoi McLearn (1930, p. 5, pl. 2, fig. 2; 1932a, 

p. 55, pl. 2, fig. 2, pl. 3, fig. 7, pl. 4, fig. 8; Frebold, 

1964, pl. 15, figs. la, b). This ammonite differs from 

Stephanoceras (Skirroceras) kirschneri Imlay 

(1964, p. B47, pl. 18, figs. 1-4, pl. 19; 1973, p. 87, 

pl. 30, fig. 13; pl. 42, figs. 1-10) at a comparable size 

by having fewer and finer primary ribs, much 

weaker tubercles, and fewer secondary ribs per 

primary rib. 

Figured specimen.-USNM 273515. 

Occurrences.-Twist Creek Siltstone at USGS 

Mesozoic loc. 31713. 

Genus STEMM A TOCERAS Mascke, 1907 

Stemmatoceras cf. S. arcicostatum Imlay 

Plate 5, figures 1-10 

Six somewhat crushed septate specimens ar1e characterized by fairly evolute coiling; by a depressed 

whorl section; by fairly strong, widely spaced pri- · 

mary ribs that curve slightly forward and ter:minate 

in acute tubercles a little above the middle of the· 

flank; and by weak secondary ribs that arise from 

the primary ribs by threes and fours and then arch 

gently forward on the venter. 

These specimens differ from Stemmatoceras albertense McLearn (1928, p. 20, pis. 5-7; Warren, 1947, 

p. 67, pl. 5, fig. 1; Frebold, 1957, p. 50, 51, pl. 21, 

figs. 2a, b, pl. 23, figs. la-c) by having finer and more 

numerous secondary ribs. The specimens show more 

resemblance to S. aTcicostatum Imlay (1967, p. 90, 

pl. 8; figs. 1, 2; pl. 9, figs. 1-11) but apparently have 

a more depressed whorl section and sharper primary 

ribs. 

Figured specimens.-USNM 273516-273521. 

Occurrences.-Twist Creek Siltstone on Iniskin 

Peninsula, USGS Mesozoic loc. 31713. Equivalent unnamed beds in the Talkeetna Mountains at USGS 

Mesozoic locs. 24821 and 30589. 

Stemmatoceras sp. 

Plate 6, figures 1-5; plate 7, figure 2 

One fairly large laterally compressed am1nonite 

consists of parts of two septate whorls and half of a 

body whorl. The septate whorls bear strong primary I 
SYSTEMATIC DESCRIPTIONS 17 

ribs that incline gently forward and terminate in 

strong tubercles near the middle of the flanks. From 

these tubercles pass three or four rather weak secondary ribs that incline forward on the flanks but 

cross the venter nearly transversely. Other secondary 

ribs arise freely along the zone of tuberculation, resulting in nearly five secondary ribs for each primary rib. 

On the body chamber, the primary ribs are low, 

very broad, become less distinct adorally, and terminate in acute tubercles near the middle of the flanks. 

The 8econdary ribs are very broad and low at the 

adapical end of the body chamber and become faint 

adorally within one-fourth of a whorl. 

This ammonite differs from some ammonites described previously as Stemmatoce1·as n. sp. indet. 

(Imlay, 1964, p. B48, pl. 20, figs. 1-4) by having 

slightly weaker primary ribs and a higher whorl section. Its septate whorls greatly resemble those of the 

holotype of S. palliseri McLearn (1932b, p. 114, pl. 2, 

pl. 5, fig. 1). It likewise resembles the septate whorls 

of an Alaskan specimen that was compared with S. 

palliseri McLearn by Imlay (1964, p. B48, pl. 20, figs. 

5, 6, pl. 21, figs. 2, 4), but this ammonite differs by 

having weak instead of strong ribbing on its body 

chamber. 

Figured specimen.-USNM 273522. 

Occurrence.-Unnamed beds in the Boulder Creek 

area of the Talkeetna Mountains at USGS Mesozoic 

loc. 30591. 

Genus CADOMITES Munier-Chalmas, 1892 

Cadomites magnus Imlay, n. sp. 

Plate 4, figure 10; plate 7, figures 1, 3, 4 

One fairly large specimen is septate except for the 

outermost one-fourth whorl, which is crushed ventrally. The outer two septate whorls are fairly well 

exposed, are depressed coronate in section, and become more depressed during growth. 

On the next to outermost septate whorl, the primary ribs are moderate in height and spacing, become 

broader, lower, and more widely spaced adorally, 

curve gently forward on the umbilical wall, and terminate in weak nodes a little below the middle of the 

flanks. From these nodes pass three to four very fine, 

closely spaced secondary ribs. In addition, one or two 

very fine secondary ribs arise freely between the 

nodes. 

Adorally on the outermost septate whorl, the primary ribs become much lower, broader, and more 

widely spaced, and the secondary ribs are replaced 

by densely spaced striations, or lirae. 

On the preserved adapical part of the nonseptate 

whorl, the primary ribs are even lower and broader, 

and the upper parts of the flanks as well as the venter are smooth. 

On the septate whorls at a diameter of 84 mm the 

whorl height is 33 mm and the whorl thickness is 44 

mm. At a diameter of 132 mm, the whorl height is 

44 mm and the whorl thickness is 62 mm. 

The secondary ribs on the Alaskan specimen are 

comparable in fineness and density with the secondary ribs on C ad01nites exstinctus ( Quenstedt) 

(1887, p. 630, pl. 74, figs. 30, 32-34; Hahn, 1971, 

p. 110, pl. 9, figs. 10-13) and on some specimens that 

have been assigned to Cadomites rectelobatus 

(Hauer) by Sturani (1964, p. 22, pl. 2, figs. 6, 8). 

The Alaskan specimen differs, however, by its primary ribs becoming weaker, broader, and sparser 

during growth, by its secondary ribs becoming much 

finer on the outermost septate whorl, by the ventral 

part of the adapical fourth of the body chamber becoming smooth, and by attaining a much larger size. 

The ribbing on the next to the outer septate whorl 

of the Alaskan specimen is also comparable in fineness with that on two ammonites that were illustrated under the new generic names Polystephanus 

and Stegeostephanus by Buckman (1922, pl. 311 and 

312) but which were considered to be synonyms of 

Cadomites deslongchampsi (d'Orbigny) by Arkell 

(1952, p. 80). Nonetheless the lectotype of that species (Arkell, 1952, v. 3, p. 79, text-fig. 21) as well as 

a specimen assigned to that species by Hahn (1971, 

p. 111, pl. 9, fig. 16) differ from the Alaskan specimen by having coarser ribbing that persists onto the 

body chamber. 

In summation, the Alaskan specimen bears fine 

ribbing as in C. exstinctus (Quenstedt) and on some 

specimens of C. deslongchampsi (d'Orbigny) and C. 

rectelobatus (Hauer). The Alaskan specimen differs 

by its secondary ribs becoming much finer on its outermost septate whorl, by its primary ribs becoming 

much broader and more widely spaced adorally, and 

by its venter becoming smooth on at least the adapical part of the body chamber. 

Type.-Holotype, USNM 273523. 

Occurrence.-Unnamed beds in the Talkeetna 

Mountains at USGS Mesozoic loc. 30590. 

Family SPHAEROCERATIDAE Buckman, 1920 

Genus MEGASPHAEROCERAS Imlay, 1961 

Megasphaeroceras rotundum Imlay 

Plate 4, figures 8, 11 

Megasphaeroceras rotundum Imlay, n. sp., 1961, Journal of 

Paleontology, v. 35, no. 3, p. 471, pl. 63, figs. 1-4, 6. 
18 LATE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA 

Megasphaeroceras rotundum Imlay, 1962, U.S. Geological 

Survey Professional Paper 418-A, p. A10, pl. 3, figs. 1-4, 6. 

This species, represented by 106 specimens in 

available collections, is the most common species in 

the upper Bajocian beds of southern Alaska. It is 

characterized by stout to moderately stout whorls, a 

depressed whorl section; a tiny umbilicus; sharp, 

forked gently flexuous, forwardly inclined ribs on the 

septate part of the shell, and much weaker and 

broader ribs and some striae on the body chamber. 

The ribbing is much weaker on the internal molds of 

the septate whorls than on the shell and becomes 

weak to indistinct on internal molds of the adult body 

chamber. The specimens illustrated herein bear more 

shelly material on their body chambers than do most 

specimens of the species and hence bear more distinct ribbing. 

Types.-Holotype, USNM 130898; para types, 

USNM 130899a-d, 130900 ; hypotypes, USNM 

273524 and 275775. 

Occurrences.-Twist Creek Siltstone west of Cook 

Inlet at USGS Mesozoic locs. 11035, 11036, 19943, 

21282, 21313-21315, 21318, 22709, 26593, 27099, 

31713, and 31714. Equivalent unnamed beds in the 

Talkeetna Mountains at USGS Mesozoic locs. 24821, 

30291, 30580, 30589, and 30592. 

Genus SPHAEROCERAS Bayle,, 1878 

Sphaeroceras talkeetnanum Imlay 

Plate 2, figures 3-5 

Sphaeroceras talkeetnanum Imlay, 1962, U.S. Geological 

Survey Professional Paper 418-A, p. All, pl. 5, figs. 1-4. 

This species, originally based on a single specimen, 

is now represented by another specimen that differs 

only by being a little more globose. 

Types.-Holotype, USNM 130902; hypotype, 

USNM 273525. 

Occurrences.-Unnamed beds in the Talkeetna 

Mountains at USGS Mesozoic loc. 8572; Twist Creek 

Siltstone on the Iniskin Peninsula at USGS Mesozoic 

loc. 31713. 

Family PERISPHINCTIDAE Steinmann, 1890 

Subfamily LEPTOSPHINCTIN AE Arkell, 1950 

Genus LEPTOSPHINCTES Buckman, 1920 

Leptosphinctes cf. L. leptus Buckman 

Plate 4, figures 6, 7, 9 

Six laterally crushed ammonite fragments ·bear 

sharp, moderately spaced primary ribs that incline 

slightly forward on the flanks. Most of the primary 

ribs divide at about three-fifths of the height of the 

flanks into pairs of slightly weaker ribs that trend 

forward only slightly. A few primary ribs :remain 

simple, and some secondary ribs are indistinctly connected with the primary ribs. A constriction appears 

to be present near the adoral end of the large~,t fragment. 

The rib pattern on these specimens is sirnilar to 

that on Leptosphinctes leptus Buckman (1~120, pl. 

160) from England and to that on L. cf. leptus Buckman (Imlay, 1973, p. 91, pl. 47, fig. 25), from eastern 

Oregon. 

Figured specimens.-USNM 273526. 

Occurrence.-Unnamed beds in the lower Kuskokwim-Bristol Bay area of southwestern Alaska at 

USGS Mesozoic loc. 31122. 

Subgenus PRORSISPHINCTES Buckman, 1921 

Leptosphinctes (Prorsisphinctes?) sp. 

Plate 4, figure 5 

One specimen is characterized by high, sharp, 

widely spaced primary ribs that incline slightly forward on the flanks. They divide at about three-fifths 

of the height of the flanks into pairs of weaker secondary ribs that incline considerably forward. 

The rib pattern is similar to that on the inner 

whorls of L. (Prorsisphinctes) pseudomartinsi (Siemiradzki) (Arkell and others, 1957, p. L314, fig. 

395) from Europe and to that on the inner whorls of 

L. sp. from eastern Oregon (Imlay, 1973, p. 91, pl. 

47, fig. 24). Its ribs are a little higher and :3harper 

than those on L. cli.fjensis Imlay from the Iniskin 

Peninsula west of Cook Inlet (Imlay, 1962, p. A12, 

pl. 5, figs. 10, 11). Its ribbing is much stronger than 

that on L. (Prorsisphinctes ?) delicatus Imlay (1962, 

p. A13, pl. 5, figs. 8, 9). 

Figured specimens.-USNM 273527. 

Occurrence.-Unnamed beds in the lower Kuskokwim-Bristol Bay area of southwestern Alaska at 

USGS Mesozoic loc. 31122. 

REFERENCES CITED 

Arkell, W. J., 1950-1958, Monograph of the English Bathonian 

ammonites: London, Palaeontographical Society, 264 p. 

(In 8 pts, pub. separately. See especially pt. 8 (1952), 

p. 73-102, pls. 9-11.) 

Arkell, W. J., and others, 1957, Mesozoic Ammonoidea, in 

Moore, R. C., ed., Treatise on invertebrate paleontology, 

Part L, Mollusca 4, Cephalopoda, Ammonoidea: New 

York and Lawrence, Kans., Geological Society of America 

and University of Kansas Press, p. L80-L437. 

Buckman, S. S., 1909-1930, Type ammonites: London, privately published, 7 v. 
REFERENCES CITED 19 

Burckhardt, Carlos, 1927, Cefalopodes del Jurasico de 

Oaxaca y Guerrero: Mexico, Instituto Geologico Boletin 

47, 108 p., 34 pls. 

Detterman, R. L., 1963, Revised stratigraphic nomenclature 

and age of the Tuxedni Group in the Cook Inlet region, 

Alaska: U.S. Geological Survey Professional Paper 

475-C, p. C30-C34, figs. 68.1, 68.2. 

Detterman, R. L. and Hartsock, J: K., 19.66, Geology of the 

Iniskin-Tuxedni region, Alaska: U.S. Geological Survey 

Professional Paper 512, 78 p., 6 pls., 7 figs. 

Frebold, Hans, 1957, The Jurassic Fernie group in the 

Canadian Mountains and foothills: Canada Geological 

Survey Memoir 287, 197 p., 44 pls., 5 figs. 

--- 1964, Illustrations of Canadian fossils-Jurassic of 

western and arctic Canada: Canada Geological Survey 

Paper 63-4, 107 p., 51 pis. 

Frebold, Hans, and Tipper, H. W., 1973, Upper Bajocianlower Bathonian ammonite fauna and stratigraphy of 

Smithers area, British Columbia: Canadian Journal of 

Earth Sciences, v. 10, no. 7, p. 1109-1131, 8 pls., 3 figs. 

Hahn, Wolfgang, 1971, Die Tulitidae S. Buckman, Sphaeroceratidae S. Buckman und Clydoniceratidae S. Buckman 

(Ammonoidea) des Bathoniums (Brauner Jura e) 

im slidwestdeutschen Jura: Baden-Wiirttemberg Geologisches Landesamt J ahreshefte, v. 13, p. 55-122, 9 pls., 

13 figs. 

Hoare, J. M., 1961, Geology and tectonic setting of lower 

Kuskokwim-Bristol Bay region, Alaska: American Association of Petroleum Geologists Bulletin, v. 45, no. 5, p. 

594-611, 3 figs. 

Imlay, R. W., 1953, Callovian (Jurassic) ammonites from the 

United States and Alaska-Part 2. Alaska Peninsula and 

Cook Inlet regions: U.S. Geological Survey Professional 

Paper 249-B, p. 41-108, pis. 25-55, figs. 3-9. 

--- 1961, New genera and subgenera of Jurassic (Bajocian) ammonites from Alaska: Journal of Paleontology, 

v. 35, no. 3, p. 467-474, pis. 63, 64. 

--- 1962, Late Bajocian ammonites from the Cook Inlet 

region, Alaska: U.S. Geological Survey Professional 

Paper 418-A, 15 p., 5 pls., 4 figs. 

--- 1964, Middle Bajocian ammonites from the Cook Inlet 

region, Alaska: U.S. Geological Survey Professional 

Paper 418-B, 61 p., 29 pis., 5 figs. 

--- 1967, Twin Creek Limestone (Jurassic) in the western interior of the United States: U.S. Geological Survey Professional Paper 540, 105 p., 16 pls., 18 figs. 

--- 1973, Middle Jurassic (Bajocian) ammonites from 

eastern Oregon: U.S. Geological Survey Professional 

Paper 756, 100 p., 47 pls, 8 figs. 

--- 1976, Middle Jurassic (Bajocian and Bathonian) 

ammonites from northern Alaska: U.S. Geological Survey Professional Paper 854, 22 p., 4 pls., 5 figs. 

--- 1980, Middle Jurassic (Bathonian) ammonites from 

southern Alaska: U.S. Geological Survey Professional 

Paper 1091, 42 p., 12 pls., 11 figs. 

Kirschner, C. E. and Minard, D. L., 1948, Geology of the 

Iniskin Peninsula, Alaska: U.S. Geological Survey Oil 

and Gas Investigations Preliminary Map 95, scale 1 inch 

to 4,000 feet. 

Kopik, Janusz, 1974, Genus Cadomites Munier-Chalmas, 1892 

(Ammonitina) in the upper Bajocian and Bathonian of 

the Cracow-Wielun Jurassic Range and the Gory 

Swietokrzyskie Mountains (southern Poland), in Z 

badan stratygraficzno-paleontologicznych w Polsce, v. 

VII: Poland Instytut Geologiczny Biuletyn 276, p. 7-53, 

pls. 1-11, 2 figs. (p. 1-43 are in English). 

McLearn, F. H., 1928, New Jurassic Ammonoidea from the 

Fernie Formation, Alberta: Canada Geological Survey 

Bulletin 49, p. 19-22, pls. 4-8. 

--- 1930, Notes on some Canadian Mesozoic faunas: 

Royal Society of Canada Proceedings and Transactions, 

3d ser., v. 24, p. 1-8, 2 pis. 

--- 1932a, Contributions to the stratigraphy and palaeontology of Skidegate Inlet, Queen Charlotte Islands, 

British Columbia [continued]: Royal Society of Canada 

Proceedings and Transactions, 3d ser., v. 26, sec. 4, p. 

51-84, 10 pis. 

--- 1932b, Three Fernie Jurassic ammonoids: Royal 

Society of Canada Proceedings and Transactions, 3d 

ser., v. 26, sec. 4, p. 111-115, 5 pls. 

Ochoterena F., Hector, 1963, Amonitas del Jurasico Medio y 

del Calloviano de Mexico-[Pt.] 1. Parast'renoce1·as gen. 

nov.: Mexico Universidad Nacional Aut6noma, Instituto 

de Geologia Paleontologia Mexicana, no. 16, 26 p., 5 pls. 

Pavia, Giulio, 1971, Ammoniti del Baiociano superiore di 

Digne (Francia SE, dip. Basses-Alpes): Societa Paleontologica ltaliana Bollettino, v. 10, no. 2, p. 75-142, 

pis. 13-29, 8 figs. 

Quenstedt, F. A. von, 1883-88, Die Ammoniten des Schwabischen Jura: Stuttgart, 3 v., 1,140 p., 126 pls. 

Stone, D. B., and Packer, 1979, Paleomagnetic data from 

the Alaska Peninsula: Geological Society of America 

Bulletin, pt. 1, v. 90, no. 6, p. 545-560, 6 figs. 

Sturani, Carlo, 1964, Ammoniti mediogiurassiche del Veneto; 

Faune del Baiociano terminale (zone a garantiana e a 

pwrkinsoni) : Padua Universita Istituti di Geologia e 

Mineralogia Memorie, v. 24 (1963-1964), 43 p., 4 pls., 30 

figs. 

Warren, P. S., 1947, Description of Jurassic ammonites from 

the Fernie formation *** Alberta: Alberta Research 

Council Report 49, p. 67-76, 7 pis. 

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p., 33 pls. 

*U.S. GOVERNMENT PRINTING OFFICE: 1982-361-594/112 

A 

Age and correlation of ammonites with 

standard Bajocian zones of western 

Page 

Europe -------------------------- 6, 7 

alberteme, Stemmatoceras -------------------- 16 

Ammonite faunal setting--------------------- 10, 11 

arcicostatum, Stemmatoceras ------------- 10, 16; pl. 5 

B 

bakeri, Lissoceras ---------------------- 10, 15; pl. 2 

Biologic analysis ---------------------------- 2 

blagdeni, Teloceras -------------------------- 7 

Boulder Creek, ammonite collection 

localities ------------------------ 5, 12 

boulderensis, Normannites --------------- 10, 16; pl. 3 

c 

caamanm:, Stephanoceras ---------------- 10, 16; pl. 3 

Cadomites ------------------------- 2, 7, 17; pl. 4, 7 

deslongchampsi -------------------------- 17 

exstinctus ------------------------------- 17 

magnus ------------------------- 10, 17; pl. 4, 7 

rectelobatus ----------------------------- 17 

CaUiphyUoceras ------------------------- 2, 15; pl. 1 

freibrocki -------------------------- 10, 15; pl. 1 

sp. ------------------------------------ 10 

(CaUiphylloceras) freibrocki, PhyUoceras -------- 15 

Calliphylloceratinae ------------------------- 2, 15 

Chondroceras ----------------------------- 3, 7, 10 

cliffensis, Leptosphinctes --------------------- 10, 18 

Cobbanites --------------------------------- 2 

costidensus, Cranocephalites ------------------

Cranocephalites --------------------------~--

costidensus ----------------------------- 2 

Cynthia F'alls Sandstone---------------------- 6 

D 

delicatus, Leptosphinctes (Prorsisphinctes) ------ 10, 18 

deslongchampsi, Cadomites ------------------- 17 

Dettermanites vigorosus ---------------------- 15 

E 

exstinctUf, Cadomites ------------------------ 17 

F 

formosus, Normannites ----------------------- 15 

freibrocki, CaUiphylloceras --------------- 10, 15; pl. 1 

Phylloceras (CaUiphylloceras) -------------- 15 

G 

Geographic distribution----------------------- 11 

grossicostatum, Macrophylloceras -------------- 14 

Phylloceras (MacrophyUoceras) -------- 10, 14; pl. 1 

H 

Haploceratidae ----------------------------- 15 

humphriesianum, Stephanoceras ---------------

INDEX 

(Italic page numbers indicate descriptions and principal references] 

Page 

Iniskin Peninsula, ammonite collection 

localities --------------~-------- 1, 7, 12 

K 

kellumi, Oppelia (Liroxyites) ------------- 10, 15; pl. 2 

kirschneri, Stephanoceras (Skirroceras) _________ 16 

Kuskokwim-Bristol Bay areas, ammonite 

collection localities--------------- 1, 6, 12 

L 

Leptosphinctes ------------------ 2, 6, 7, 10, 18; pl. 4 

cliffensis ------------------------------- 10, 18 

leptus ----------------------------- 10, 18; pl. 4 

sp. ------------------------------------ 10 

(Prorsisphinctes) -------------------------

delicatus ---------------------------- 10, 18 

pseudomartinsi ---------------------- 18 sp. _____________________________ 10, 18; pl. 4 

Leptosphinctinae --------------------------- 2, 18 

leptus, Leptosphinctes ------------------- 10, 18; pl. 4 

Liroxyites ------------------------------- 2, 11, 15 

(Liroxyites), Oppelia ------------------------- 11, 15 

kellumi, Oppelia -------------------- 10, 15; pl. 2 

Lissoceras ----------------------------- 2, 15; pl. 2 

bakeri ----------------------------- 10, 15; pl. 2 

Localities. See Geographic distribution and see Occurrences heading in text under Systematic 

descriptions. 

Lupherites --------------------------------- 10 

Lytoceras ----------------------------------

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

Lytoceratidae ------------------------------

Lytoceratinae ------------------------------

M 

2 

10 

Macrophylloceras grossicostatum --------------- 14 

(MacrophyUoceras), Phylloceras --------------- 2, 14 

grossicostatum, Phylloceras ----------- 10, 14; pl. 1 

magnus, Cadomites ------------------- 10, 17; pl. 4, 7 

Megasphaeroceras -------------- 2, 3, 10, 11, 17; pl. 4 

rotundum __________________________ 10, 17; pl. 4 

sp. ------------------------------------ 10 

N 

Normannites ----------------------- 2, 7, 15; pl. 3, 4 

boulderensis ------------------------ 10, 16; pl. 3 

formosus ------------------------------- 15 

orbignyi -------------------------------- 16 

vigorosus -------------------------- 10, 15; pl. 4 

0 

Oppelia ------------------------------------ 15 

(Liroxyites) ----------------------------- 11, 15 

kellumi -------------------------- 10, 15; pl. 2 

Oppeliidae --------------------------------- 2, 15 

Oppeliinae --------------------------------- 2, 15 

orbignyi, Normannites ----------------------- 16 

Page 

p 

paUiseri, Stemmatoceras --------------------- 17 

Parastrenoceras ---------------------------- 10 

Parkinsoniidae ------------------------------ 11 

Perisphinctidae ----------------------------- 2, 18 

Phylloceras (Calliphylloceras) freibrocki --------- 15 

(Macrophylloceras) ----------------------- 2, 14 

grossicostatum ------------------ 10, 14; pl. 1 

Phylloceratidae ----------------------------- 2, 14 

Phylloceratinae ------------------------------ 2, 14 

Polystephanus ------------------------------ 17 

(Prorsisphinctes), Leptosphinctes --------------- 18 

delicatus, Leptosphinctes ------------------ 10, 18 

pseudomartinsi, Leptosphinctes ------------ 18 

sp., Leptosphinctes ------------------ 10, 18; pl. 4 

pseudomartinsi, Leptosphinctes (Prorsisphinctes) _ 18 

R 

rectelobatus, Cadomites ----------------------- 17 

rotundum, Megasphaeroceras ------------- 10, 17; pl. 4 

s 

Sheep Creek, beds nearly identical with Twist 

Creek Siltstone ------------------- 2 

Sphaeroceras ------------------------- 2, 7, 18; pl. 2 

talkeetnanum ----------------------- 10, 18; pl. 2 

Sphaeroceratidae --------------------------- 2, 17 

Spiroceras -------------------------------- 2, 7, 10 

sp. ------------------------------------ 10 

Spiroceratidae ------------------------------ 2 

Stegeostephanus ----------------------------- 17 

Stemmatoceras ---------------- 2, 7, 10, 16; pl. 5, 6, 7 

alberteme ------------------------------- 16 

arcicostatum ----------------------- 10, 16; pl. 5 

paUiseri -------------------------------- 17 

sp. ----------------------------- 10, 16; pl. 6, 7 

Stephanoceras ----------------------- 2, 10, 16; pl. 3 

caamanoi -------------------------- 10, 16; pl. 3 

humphriesianum zone -------------------- 7 

(Skirroceras) kirschneri ------------------- 16 

Stephanoceratidae -------------------------- 2, 15 Stratigraphic summary, lower KuskokwimBristol Bay area ------------------- 6 

Talkeetna Mountains --------------------- 2 

west side of Cook Inlet-------------------- 4 

Strenoceras subjurcatum zone----------------- 6 

Systematic descriptions ---------------------- 14 

T 

Talkeetna Mountains -------------------------

ammonite collection localities·------------- 1, 4, 11 

lithologically and faunally equivalent beds ____ 6 

talkeetnanum, Sphaeroceras -------------- 10, 18; pl. 2 

Teloceras blagdeni subzone -------------------- 7 

Tuxedni Bay area, ammonite collection 

localities------------------------- 6, 8 Twin Creek Limestone, Sliderock Member, 

ammonite faunule ----------------- 10 

Twist Creek Siltstone 

age and correlation--------------------- 6, 7, 10 

stratigraphic summary -------------------- 4 

identical beds---------------------------- 2 

v 

vigorosus, Dettermamtes --------------------- 15 

vigorosus, Normannites ----------------- 10, 15; pl. 4 

PLATES 1-7 

Contact photographs of the plates in this report are available, at cost, 

from U.S. Geological Survey Library, Federal Center, 

Denver, Colorado 80225 
PLATE 1 

[All figures are natural size] 

FIGURES 1-3. Phylloceras (Macrophylloceras) grossicostatum Imlay (p. 14). 

1. Hypotype, USNM 273501, from USGS Mesozoic loc. 31713. 

2. Hypotype of immature specimen, USNM 273503, from USGS Mesozoic loc. 248.21. 

3. Hypotype, USNM 273502, from USGS Mesozoic loc. 21314. 

4-7. Calliphylloceras freibrocki (Imlay) (p. 15). 

4. Hypotype, USNM 273505, from USGS Mesozoic loc. 31714. 

6-7. Hypotype, USNM 273504, from USGS Mesozoic loc. 31714. 
CEOLOGJCAL ~W H\TY 

5 

4 

6 

MACROPHYLLOCERAS AND CALLIPHYLLOCERAS 

/ 

7 
PLATE 2 

[All figures are natural size] 

FIGURES 1, 2, 6, 7. Oppelia (Liroxyites) kellumi Imlay (p. 15). 

1, 2. Ventral and lateral views of hypotype, USNM 273509, from USGS Mesozoic loc. 20001. 

6. Hypotype, USNM 273508, from USGS Mesozoic loc. 20001. 

7. Hypotype, USNM 273507, from USGS Mesozoic loc. 31713. Shows broad falcoid ribs, falcoid striae, 

and faint spiral bands and striae. 

3-5. Sphaerocwras talkeetnanum Imlay (p. 18). 

Lateral and ventral views of hypotype, USNM 273525, from USGS Mesozoic loc. 31713. 

8. Lissocerras bakerri Imlay ( p. 15) . Adult specimen showing complete body chamber. Hypotype, USNM 273506, from USGS Mesozoic loc. 

31713. 
8 

f,JRO.\T!TES, SPHAEROCERAS, AND LISSOCERAS 
PLATE 3 

[All figures are natural size] 

FIGURE 1. StephanoceTas cf. S. caamanoi McLearn (p. 16). 

Figure specimen, USNM 273515, from USGS Mesozoic loc. 31713. 

2-8. Nonnannites bouldwrensis Imlay, n. sp. (p. 16). 

2, 3. Lateral and ventral views of paratype, USNM 273513, from USGS Mesozoic loc. 30589. 

4. Lateral view of paratype, USNM 273514, at USGS Mesozoic loc. 30589. Shows characteristie ribbing on a septate whorl. 

5-8. Suture line and ventral, lateral, and apertural views of holotype, USNM 273512, from USGS Mesozoic 

loc. 31713. 
CEOLO<:J('AL :-;t' lt\'EY 

3 

7 8 

STEPHANOCERAS AND NORMANNJTES 
FIGURES 1-4. 

5. 

6, 7, 9. 

8, 11. 

10. 

PLATE 4 

[All figures are natural size] 

Normannites vigo?'osus (Imlay) (p. 15). 

1-3. Apertural, lateral, and ventral views of hypotype, USNM 273511, from USGS Mesozoic lot!. 11036. 

4. Hypotype, USNM 273510, from USGS Mesozoic loc. 19934. 

Leptosphinctes (Pro?'sisphinctes?) sp. (p. 18). 

Figured specimen, USNM 273527, from USGS Mesozoic loc. 31122. 

Leptosphinctes cf. L. leptus Buckman (p. 18). 

Laterally crushed fragments, USNM 273526, from USGS Mesozoic loc. 31122. 

M egasphaeroce1·as 'rotundum Imlay ( p. 17). 

8. Small septate hypotype, USNM 275775, from USGS Mesozoic loc. 30291. 

11. Hypotype, USNM 273524, from USGS Mesozoic loc. 11035. Shows distinct ribbing on adult body 

chamber. 

Cadomites magnus Imlay n. sp. (p. 17). 

Holotype, USNM 273523, from USGS Mesozoic loc. 30590. Shows about 80 mm of body chamber. 
(;EOLO(;I('AL St'n\'EY PROFE~~IONAL PAPER 111-.n 1'1..\TE I 

10 11 

NORMANNITES, LEPTOSPHINCTES, MEGASPHAEROCERAS, AND CADOMITES 
PLATE 5 

[All figures are natural size] 

FIGURES 1-10. Stemmatoceras cf. S. arcicostatum Imlay (p. 16). 

1, 2. Ventral and lateral views of septate whorls, USNM 273519, from USGS Mesozoic loc. 24821. 

3, 4. Ventral and lateral views of a fragment of a septate whorl, USNM 273520, from USGS Mesozoic loc. 

30589. 

5. Lateral view of some septate whorls, USNM 273518, from USGS Mesozoic loc. 30589. 

6, 7. Lateral and apertural views of a specimen on which the adoral one-third of the outermost whorl is 

nonseptate, USNM 273517, from USGS Mesozoic loc. 30589. 

8. Lateral view of a worn specimen, USNM 273516, from USGS Mesozoic loc. 31713. 

9, 10. Lateral and ventral views of a ventrally crushed fragment, USNM 273521, from USGS Mesozoic 

loc. 30589. 
CEOLOGJCAL SUHn.:Y 

1 

8 9 

STEJJMATOCERAS 

3 

PllOF'ESSIONAL l'AP J.;r{ lli--9 PLATE 5 

4 

7 

10 
PLATE 6 

[All figures are natural size] 

FIGURES 1-5. Stemmatoceras sp. (p. 16). 

1, 2. Lateral and ventral views of a fragment belonging to the same innermost whorl shown in figures 4 and 5. 

3, 4. Ventral and lateral views of the two septate inner whorls shown in figure 5 plus a small part of the body chamber. 

5. Lateral view of body chamber and of two septate whorls of hypotype, USNM 273522, from USGS Mesozoic loc. 30591. 

PLATE 7 

[All figures are natural size] 

FIGURES 1, 3, 4. Cadornites rnagnus Imlay, n. sp. (p. 17). 

Ventral, apertural, and lateral views of holotype, USNM 273523, from USGS Mesozoic loc. 30590. Note 

that the incomplete body chamber, represented by one-fourth of a whorl, becomes smooth adorally ex- cept for low, broad, widely spaced ribs on the lower parts of the flanks. These ribs become faint adorally. Compare with view on pl. 4, fig. 10. 

2. Sternrnatocems sp. (p. 16). 

Ventral view of ada pi cal part of body chamber of specimen shown on plate 6. Note low to faint ribs. 
<;EOLO(;!C:\L SUH\'1-:Y 

CADOMITES AND STEMMATOCERAS