Age: late Bajocian.

[Note by RBB: USGS loc. number given as Mesozoic loc. GC3-1116 - obviously an error]

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 north of Togiak Bay. This dating is based mostly only the association of Normannites, Stephanoceras, and Stemmatoceras, 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 Spiroceras, 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 as 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 probably 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 inthe Talkeetna Mountains as well as west of Cook Inlet.
Most of the ammonite genera and subgenera present in the Twist Creek Silstone and in equivalent beds in southern Alaska are characteristic of the Tethyan Realm. The exceptions include