Late Bajocian ammonites from the Cook Inlet region, Alaska

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Jurassic ammonites of late Bajocian age occur in the Cook Inlet region, Alaska, along the west side of Cook Inlet, and in the southeastern part of the Talkeetna Mountains. The dominant genera are Liroxyites, a subgenus of Oppelia, M egasphaeroceras, n.nd Dettermanites. These have not been recorded outside the Cook Inlet region. The ammonites also include a fair number of specimens of M acrophylloceras, Calliphylloceras, and Lissoceras, but only n, few specimens of Leptosphinctes. Lytoceras, Spiroceras?, and Sphaeroceras are represented by single specimens. These ammonites along the west side of Cook Inlet occur only in the lower part of the Bowser member of the Tuxedni formation. This lower part is overlain unconformably by the upper part, which is characterized by Cranocephalites of probable Bathonian age. The lower part is underlain abruptly but conformably by the Cynthia Falls sandstone member of the Tuxedni formation, which has furnished the middle Bajocian ammonites Chondroceras and Normannites from its lower and middle parts. Beneath the Cynthia Falls sandstone member follows an unnamed siltstone member that has furnished many ammonite genera characteristic of the European zone of Stephanoceras hurnphriesianum. The ammonites from the lower part of the Bowser member are dated as late Bajocian mainly because they include Sphaeroceras; Spiroceras?, and Leptosphinctes that are typically late Bajocian, becu.use they do not include any genera that are typical of the Bathonian or middle Bajocian of Eurasia and north Africa; and because they occur above beds that contain many middle Bajocian ammonites such as Chondroceras, Norrnannites, Telocert;ts, Stemmatoceras, Zemistephanus, Stephanoceras, and Witchellia. The presence of Leptosphinctes itself is good evidence for a late Bajocian age, but its association with Sphaeroceras and an uncoiled n.mmonite that is probably Spiroceras is excellent confirmatory evidence. Furthermore, the resemblance of the Alaskan Leroxyites u.nd Dettermanites to the European Oxycerites and Polyplectites, respectively, indicates an age not older than late Bajocian. Such an age for the lower part of the Bowser member is in line with the presence of Cranocephalites in the overlying beds, as that genus appears to represent much of the Bathonian, according to recent studies in Alaska, Montana, and Greenland. The dominance of the ammonites Liroxyites, ll1 egasphaeroceras, and Dettermanites in the upper Bajocian rocks of Alaska contrasts with their absence in the Tethyan region. Similarly the upper Bajocian rocks of the Tethyan region have furnished many ammonite genera that have 110t yet been found in Alaska. These differences indicate that faunal developments were somewhat different in the two regions during Bajocian time.