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The Jurassic beds in the McCarthy area of the Chitina Valley represents at least the lower part of the Lower Jurassic, the Bathonian or basal part of the Callovian stages, and the upper Oxfordian and Kimmeridgian stages. The Lower Jurassic fossils are in general rather poorly preserved. Many of the identifications are querried and hence have little value in determining the exact age within the Lower Jurassic. Thus collections F-3A and F-45 contain ammonites resembling Wahneroceras which, if correctly identified, are good evidence for a Hettangian age. However, the specimens do not show certain critical features necessary for the identication of the genus. In contrast the Sinemurian stage of the Lower Jurassic is represented by Arnioceras (locs. F-4 and F-35) which in northwest Europe ranges through the lower and midle parts of the stage. The ammonites Crucilobiceras (Loc. F-1) and Microderoceras (Mes. loc. 14472) could be either uppermost Sinemurian or lower Pliensbachian. In addition the pelecypod "Entolium" semiplicatum (Hyatt) is probably indicative of a Sinemurian age as it occurs in abundance with ammonites of that age in eastern Oregon and locally in the Sierra Nevada (Sailors Canyon Formation). The Weyla-bearing limestone has furnished no fossils of age value other than Weyla itself, which genus is known only from the Lower Jurassic. I understand that Si Muller has examined the Weylas from the McCarthy area and identified some of them with a species that occurs in Nevada and Oregon in beds of late Pliensbachian to Toarcian age. Most of the collections from the Jurassic shale above the Weyla-bearing limestone contain species of Buchia of late Oxfordian and Kimmeridgian age. These include Buchia concentrica (Sowerby) (locs. F-17, F-19, F-34, F-50, 61-ABC-26, Mes. loc. 14495 and 14496), B. rugosa (Fischer) at many localities, and B. mosquensis (von Buch) at a few localities. Of these species, B. concentrica (Sowerby) is of late Oxfordian to early Kimmeridgian age (see Prof. Paper 314-G, p. 159, 165). Near the top of its range elsewhere in Alaska it is associated with B. rugosa (Fischer) and less commonly with B. mosquensis (von Buch), both of which range higher into beds of probably Portlandian age. The ammonite Prionodoceras (locs. 61-ABC-27, 61-ABC-36, 14495, 14496 and 14497) in the Boreal region is of late Oxfordian to early Kimmeridgian age and is commonly associated with Buchia concentrica (Sowerby). I suspect that it was collected lower than B. rugosa (Fischer), but I would be much interested in knowing your conclusions concerning the stratigraphic positions and ranges of these fossils in the McCarthy area. Three of the collections (locs. 61-60, F-28, 61-ABC19) from the shale above the Weyla-bearing limestone are of considerable interest because they contain genera of ammonites that have been found in the Cook Inlet area in the middle part of the Bowser member of the Tuxedni formation and in the lower part of the Chinitna formation. In my paper dealing with the Callovian ammonites from Alaska (Prof. Paper 249-B) I referred the ammonites in question to Procerites? (pl. 54) and Reineckeia (pl. 55). As your collections do not contain any of the ammonites, such as Cadoceras and Kepplerites, that are common in the Chinitna formation, I suspect that the collections represent older beds equivalent to some part of the Bowser member. In any case their age is either Bathonian, or early Callovian and they are distinctly older than the beds containing Buchia. We still lack descriptions of localities F-6, F-50 and 61-19. When these arrive, Mesozoic numbers will be assigned to the collections and a list sent to you. Also, sometime we would like to have a quadrangle map, or maps, showing the exact occurrences of the fossil localities. Any stratigraphic information that you can furnish relating to the localities to the tops or bottoms of formation, or to marker beds, will be nice to have in our record books. |