Late Bajocian Ammonites From the Cook Inlet Region, Alaska By RALPH W. IMLAY JURASSIC AMMONITES FROM SOUTHERN ALASKA GEOLOGICAL SURVEY PROFESSIONAL PAPER 418-A Descr@tion and illivstrations of cephalopods of Middle Jurassic (late Bajocian) 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 Paae Abstract-------------------------------- A-1 Comparisons with other faunas ------------ ----------- A-3 North America, exclusive of Alaska- -------------- 3 Introduction--------.------------------------------- 1 Arcticregion___-_------------------------------ 3 Biologic analysis ............................... 1 Eurasia and north Africa-- ...................... 4 Australasia and Indonesia ---_-------------------- 4 Stratigraphic summary ----- ---- -- -------- -- -- -- - -- -- 2 Geographic distribution -_---- - - - - - - - - - - - -- - - --- - - - - - - 4 North side of Cook inlet ...................... 2 Systematic descriptions..-- - - - - - - -- - - - - - - - - - - - - -- -- -- - 5 Talkeetna Mountains ........................ 2 References----------------------------------------- 13 Age ofthe fauna--------------------------------- 2 Index--_--_-___----------------------------------- 15 ILLUSTRATIONS [Plates follow index] PLATE 1. Lissoceras, Spiroceras?, and Calliphylloceras. 2. Oppelia (Lizozyites) . 3. Megasphaeroceras. 4. Dettermanites. 5. Sphaeroceras, Leptosphinetes, and Macrophylloceras. Page FIGURE1. Index map showing the principal areas of upper Bajocian (Jurassic) rocks in the Cook Inlet region, Alaska-. ---- A-7 2. Index map showing occurrences of upper Bajocian (Jurassic) fossils in the Talkeetna Mountains, Alaska-- - - - _ - - 8 3. Index map showing occurrences of upper Bajocian (Jurassic) fossil in the Tuxedni Bay area, Alaska___- _ - - - - - - - 9 4. Index map showing occurrences of upper Bajocian (Jurassic) fossils in the northern part of the Iniskin Peninsula, TABLES Page TABLE1. Ammonite genera and subgenera from beds of late Bajocian age in the Cook Inlet region, Alaska, showing biolog- ical relationships and relative numbers available for study- _ _ - - - --- --- ------- - - -- - --- - - -- --- - ------- --- - 2. Localities where megafossils were collected from upper Bajocian strata in the Cook Inlet region, Alaska- - - - - - - - 3. Geographic distribution of the upper Bajocian ammonites from the Cook Inlet region, Alaska- - - - - - - - .. - - - - - - - - III JURASSIC AMMONITES FROM SOUTHERN ALASKA LATE BAJOCIAN AMMONITES FROM THE COOK INLET REGION, ALASKA ABSTRACT Bajocian ammonites typical of the Tethyan region and Jurassic ammonites of late Bajocian age occur in the Cook below beds characterized by t,he ammonite Cranocepha- Inlet region, Alaska, along the west side of Cook Inlet, and in the lites, which is known elsewhere only from the Arctic southeastern part of the Talkeetna Mountains. The dominant region and the western interior of the United States. genera are Liroxyites, a subgenus of Oppelia, Jfegasphaeroceras, and Dettermanites. These have not been recorded outside the The evidence sets a lower limit to the age of the Crano- Cook Inlet region. The ammonites also include a fair number of cephalites beds. The late Bajocian ammonite faunule specimens of Macrophylloceras, Calliphylloceras, and Lissoceras, is also unusually interesting because the dominant am- but only a few specimens of Leptosphinctes. Lytoceras, Spiro- monite genera and subgenera present have not been ceras?, and Sphaeroceras are represented by single specimens. found in other parts of the world. These ammonites along the west side of Cook Inlet occur only in the lower part of the Bowser member of the Tuxedni formation. The fossils from the upper Bajocian rocks of the Cook This lower part is overlain unconformably by the upper part, Inlet region, Alaska, were collected by G. S. Martin in which is characterized by Cranocephalites of probable Bathonian 1913, A. A. Baker in 1921, L. B. Kellum and Helmuth age. The lower part is underlain abruptly but conformably by Wedow, Jr. in 1944, R. W. Imlay and D. J. Miller in the Cynthia Falls sandstone member of the Tuxedni formation, 1948, Arthur Grantz in 1951 and 1953, L. F. Fay in which has furnished the middle Bajocian ammonites Chondro- ceras and Normannites from its lower and middle parts. Beneath 1953, and R. L. Detterrnan in 1957 and 1958. the Cynthia Falls sandstone member follows an unnamed silt- Many thanks are due Arthur Grantz and R. L. Det- stone member that has furnished many ammonite genera charac- terman for preparing locality maps and descriptions teristic of the European zone of Stephanoceras humphriesianum. and for furnishing and checking all the stratigraphic The ammonites from the lower part of the Bowser member are information. The position of the lower boundary of dated as late Bajocian mainly because they include Sphaeroceras; Spiroceras?, and Leptosphinctes that are typically late Bajocian, the Bowser member of the Tuxedni formation shown because they do not include any genera that are typical of the on the index maps (figs. 3, 4) is based on an unpub- Bathonian or middle Bajocian of Eurasia and north Africa; and lished map prepared by R. L. Detterman. because they occur above beds that contain many middle Bajo- cian ammonites such as Chondroceras, Normannites, Teloceras, BIOLOGIC ANALYSIS Stemmatoceras, Zemistephanus, Stephanoceras, and Witchellia. The presence of Leptosphinctes itself is good evidence for a late The Alaskan Jurassic arnmonites of late Bajocian age Bajocian age, but its association with Sphaeroceras and an un- described herein include 154 specimens. Their distri- coiled ammonite that is probably Spiroceras is excellent confirma- bution by genera, subgenera, subfamilies, and families is tory evidence. Furthermore, the resemblance of the Alaskan shown in table 1. The table shows that the Sphaerocera- Lerozyites and Dettermanites to the European Oxycerites and Polyplectites, respectively, indicates an age not older than late tidae and the Oppeliidae are the dominant families; Bajocian. Such an age for the lower part of the Bowser member is in line with the presence of Cranocephalites in the overlying TABLE1.-Ammonite genera and subgenera from beds of late beds, as that genus appears to represent much of the Bathonian, Bajocian age in the Cook Inlet region, Alaska, showing biological according to recent studies in Alaska, Montana, and Greenland. relationships and relative numbers available for study The dominance of the ammonites Liroxyites, Megasphaero- Number ceras, and Dettermanites in the upper Bajocian rocks of Alaska Family Subfamily Genus and subgenus of speci- contrasts with their absence in the Tethyan region. Similarly 1 1 mens the upper Bajocian rocks of the Tethyan region have furnished Phylloceratidae ..... Phylloceratinae ....... Macrophylloceras. ......... 10 many ammonite genera that have riot yet been found in Alaska. Calliphyl1oceratinae.- Calliphylloceras ............ Lytoceratidae ...... Lytoceratinae ......... Lytoceras.. ................ These differences indicate that faunal developments were some- Spiroceratidae ............................. Swiroceraa?. ............... what different in the two regions during Bajocian time. IIaploceratidae ............................. Lissoceras.. ..-........... 7 Oppeliidae .......... Oppeliinae............ Oppelia (Liroxyites)........ 50 Stephanoceratidae.. ........................ Dettermanites .............. INTRODUCTION Sphaeroceratidae-.. ........................ Megasphaeroceras.. ........1 I Sphaeloceras ............... 1 The arnmonites described herein have been studied Perisphinctidae ..... Leptosphinctinae ..... Leptosphinctes .............I primarily to demonstrate that racks of late Bajocian L. (Prorsisphinetes?)....... 1 L. ? ....................... age are present in Alaska above beds containing middle I I A-2 JURASSIC AMMONITES FROM SOUTHERN ALASKA the Phylloceratidae, Haploceratidae, and Stephano- matrix, on the internal mold of the pelecypod Lima, ceratidae are much less common; and the Lytoceratidae, and also in close association with belemnite fragments Spiroceratidae, and Perisphinctidae are of minor im- that are corroded and rounded. portance. The most common and characteristic am- Stratigraphically, Mesozoic locality 24821 is near the monites are Liroxyifes, Megasphaeroceras, and Detter- top of the Tuxedni formation (Arthur Grantz, writ- manites. These have not been recorded outside of the ten communication, Nov. 14, 1960). Mesozoic locality Cook Inlet region, Alaska, although tbey resemble the 8572 is near Mesozoic locality 8573 (Martin, 1926, p. ammonites Oxycerites, Sphaeroceras, and Polyplectites 225, 228) which has furnished the ammonite Crano- respectively of the Tethyan region. Their presence cephalites. This ammonite elsewhere in the Talkeetna supports the idea of Arkell (1956, p. 609, 614) that the Mountains has been found in a sandstone a few hun- Pacific region developed as a faunal province, distinct dred feet below the Chinitna formation (Arthur from the Tethyan and Boreal faunal provinces, during Grantz, written communication, Apr. 8, 1959). It Bajocian time. The other genera and subgenera listed occurs aldng the west side of Cook Inlet in the upper are common in the Middle Jurassicof the Tethyari region. part of the Bowser member