Jurassic (Bathonian or Early Callovian) Ammonites From Alaska and Montana By RALPH W. IMLAY SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 374-C Descr$tions and illustrations of ctphalopods of possible late Middle Jurasric (Bathonian) age UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1962 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. CONTENTS Page Page C- 1 Age of the faunas-Continued C- 1 Callovian versus Bathonian in Greenland- - - - _ - - _ - - C-2 Callovian versus Bathonian in Alaska and Montana- -- - Stratigraphic summary- __ --______ _ - - - -- - ---.- -- -.- - - C-2 Paleogeographic considerations- - -_-- -- ---- ---- Cook Inlet region, Alaska -______--------.-.--..--c-2 Summation of the evidence- - - _._ _ - _ _ - - - - - - - - - - - - Iniskin Peninsula-_-_______----.--------~.--C-2 Comparisons with other faunas---------___----------- Peninsula north of Chinitna Bay----- __._ _ _._ - C-3 \Vestern interior of Canada- - - -- -- -____------- --- Talkeetna Mountains ----___-_ - - -- ---- - - -- - -- C-3 Arctic region-_-_---___-_----------------------- Western Montana- - -----__-----------------.---C-5 other regions--__-__-____----------------------- Rocky Mountain front north of the Sun River- (2-5 Geographic distribution ___-___ --- - ---------- ------ -- - Drummond area--- ---_____ _--- -- -.-- ---- -- - C-10 Summary of results- --_-____-_----_---_-_----------- Age ofthe faunas-----------_----------------------- GI0 Systematic descriptions--_ _ _ - _ - - - - - - - - - - - - - - - - - - - - - - - Evidence from Alaska---____________--------------C-10 Literature cited _-_-_---______----------------------- Evidence from Montana --_-_____ --- - - -- .--- --- - - C-12 Index---__--___-_-_------------------------------- ILLUSTRATIONS [Plates 1-3 follow index] PLATE 1. Holcophylloceras, Oecotraustes (Paroecotraustes) ?, and Arctocephalites (Cranocephalites). 2. Arctocephalites?, Siemiradzkia?, and Arctocephalites (Cranocephalites). 3, 4. Arctocephalites (Cranocephalites). 5. Arctocephalites (Cranocephalites) and Arcticoceras?. 6. Arctocephalites?, Cobbanites, and Xenocephalites?. 7. Parareineckeia and Cobbanites. 8. Cobbanites. Page FIGURE1. Index map of the principal areas of Jurassic rocks in the Cook Inlet region, Alaska- _ _ _ _-___-_-_-___-_-___-__c-4 2. Index map showing occurrences of fossils in the Arctocephalites (Cranocephalites) beds in the Talkeetna Mountains, Alaska---------_----------------------------.---------------------------------------------------- 3. Index map showing occurrences of fossils in the Arctocephalites (Cranocephalites) beds in the peninsula south of Tuxedni Bay, Alaska--_____-____-----------~-----------~------------------------------------------- 4. Index map showing occurrences of fossils in the Arctocephalites (Cranocephalites) beds in the Iniskin Peninsula, Alaska--_----_____-------.---------------.----.-----------.-------------------------------------- 5. Index map showing occurrences of fossils in the Arctocephalites (Cranocephalites) beds in northwestern Montana fromthe Sun River area northward__________-------------------------------------------------------- 6. Index map showing occurrences of fossils in the Arctocephalites (Cranocephalites) beds in the Drummond area in west,ern Montana-_-___________.________-_____________________-__________-___________-------------- 7. Correlation of some Middle and Late Jurassic formations in the Cook Inlet region, Alaska, and in western Montana- TABLES Page TABLE1. Ammonite genera in the Arctocephalites (Cranocephalites) beds in Alaska and Montana showing biological relation- ships and relative numbers available for study- _ _ ___________._____________---_--______________-_------_C-2 2. Occurrences and stratigraphic positions of the Callovian ammonites from the upper part of the Bowser member of the Tuxedni formation, above the beds containing Arctocephalites (Cranocephalites)_ _ _ --_________-----------C-3 3. Localities at which ammonites of Bathonian or early Callovian age have been collected in the Arctocephalites (Crano- cephalites) beds in the Cook Inlet region, Alaska -__-__________________._________________________-_-_--_C-16 4. Localities at which ammonites of Bathonian or early Callovian age have been collected in the upper member of the Sawtooth formation in western hlontana- -- -- -.__-_ ___ _ _.- - -_ ___ _ __ _ _ __ ___ _ __ _ _ __---_--_ ___---- ------- C-17 5. Geographic distribution of fossils from the Arctocephalites (Cranocephalites) beds in the Cook Inlet region, Alaska- C-20 6. Geographic distributicn of fossils from the Sawtooth formation in northwestern Montana ----_-___.------------ C-21 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY JURASSIC (BATHONIAN OR EARLY CALLOVIAN) AMMONITES FROM ALASKA AND MONTANA ABSTRACT Cranocephalites beds are biologically close to ammonites in the Jurassic ammonites of possible late Middle Jurassic (Bathonian) Callovian and are not likely to be older than late Bathonian. age occur in western Montana in the upper member of the Saw- Considering both stratigraphic and faunal evidence, the Cra- tooth formation and in the Cook Inlet area, Alaska, in the nocephalites beds are tentatively correlated with the Bathonian middle part of the Bowser member of the Tuxedni formation. rather than the Callovian stage and with the late rather than They are characterized by Cranocephalites, a subgenus of Arc- the early Bathonian. If this correlation is valid, the ammonites tocepi~alites. They include, also, Holcophylloceras, Parareineck- of Bathonian time occupied two distinct realms of which one eia, a new genus of the Reineckeiidae, Cobbanites, a new genus of included central and southern Europe and the Tethyan region the Perisphinctidae, and some specimens assigned questionably from 3lexico to Indonesia, and the other included the Arctic to Oecotraustes, Xenocephalites, Arcticoceras, Siemiradzkia, and region and part of the North Pacific Ocean. Arctocephalites. Stratigraphically the beds containing these ammonites occupy INTRODUCTION the position of the Bathonian stage or the basal part of the The ammonites described herein have been studied Callovian stage of Europe. In the Iniskin Peninsula, Alaska, primarily to determine whether the middle Jurassic they lie unconformably on beds containing late Bajocian am- monites and grade upward into beds containing early Callovian (Bathonian) stage is represented by sedimentary rocks ammonites. In the Talkeetna Mountains, Alaska, they lie on in Alaska and the western interior of the United States. beds of Early Jurassic age and are overlain by beds containing The study was prompted at this time in order to test typical early Callovian ammonites, but are more restricted in recent inferences and statements by W. J. Arkell distribuLion than the Callovian beds. The fact that in many (1956, p. 609) that a retreat of the Arctic Ocean from places the Callovian beds rest directly on Bajocian beds could mean either that the Cranocephalites beds were never dzposited bordering lands began after early Bajocian time, in those places or that they were eroded before the deposition that the seas retreated from most of the known land of the Callovian beds. areas of the world about middle Bathonian time, In Montana the Cranocephalites beds seem to grade donn- that the Arctic Ocean was probably isolated from the ward into beds containing middle Bajocian ammonites and are other oceans during the Bajocian-Bathonian regression, overlain sharply with possible disconformity by beds character- ized by the early Callovian ammonite Arcticoceras. As the and that Bathonian rocks are absent in North America Arcticoceras beds are equivalent to at least part of the European north of southern Mexico with the possible exception zone of Macrocephalus macrocephalites at the base of the Callo- of one place in northern Alberta. Evaluation of the vian, the Cranocephalites beds cannot be younger than the fossil evidence bearing on the presence or absence of basal part of that zone. A somewhat older age for the Crano- the Bathonian in the United States and Canada should cephalites beds in Montana would be favored if a disconformity exists between the Cranocephalites and Arcticoceras beds. Con- be interesting to many geologists. ceivably the Cranocephalites beds could span the entire Batho- The fossils from the Cook Inlet region, Alaska were nian as well as the basal Callovian, but such a long time span collected by G. C. Martin in 1913, A. A. Baker in 1921, for a single ammonite faunule seems unlikely. Helmuth Wedow and L. B. Kellum in 1944, C. E. Faunally the beds characterized by Cranocephalites cannot be Kirschner in 1946, D. J. Miller and R. W. Imlay in dated exactly. They do not contain any genera typical of the 1948, Arthur Grantz in 1951-53, R. D. Hoare in 1952, Callovian of Europe and only two ammonites that probably L. F. Fay in 1953, and R. 1,. Detterman in 1958. belong to the Bathonian genus Siemiradzkia. A Callovian age is slightly favored by the presence of the family Reineckeiidae, The fossils frorn western Montana were collected by by the fact that the new genera Parareineckeia and Cobbanites C. F. Deisa in 1940 and 1941, R. M. Garrels in 1940, range upward into beds containing typical Callovian ammonites, Josiah Bridge in 1941, R. W.Imlay in 1944-58, J. B. and by the probability that Cranocephalites