This report is based upon five collections of fossils containing cephalopods and gastropods, from the type section of the Calico Bluff Formation at Calico Bluff, Yukon River, eastern Alaska. Earlier work on USGS collections from Calico Bluff published by Gordon (1957, USGS Prof. Paper 283) had established the presence of late Visean (early Chester) goniatites in the lower part of the formation. Determination of plant spores from some of the same levels in the upper part of the formation have shown types not known to occur elsewhere below rocks of Late Pennsylvanian age.
Despite the age determination from the spores, we feel that the megafossils -- at least the cephalopods and gastropods -- indicate a possibility of Late Mississippian (Chester) or Early Pennsylvanian (Morrow) age. The ammonoids do not appear to be as late as Middle Pennsylvanian (Atoka) age and they most certainly are not Late Pennsylvanian in age.
The problem, then, lies between the possibilities of Late Mississippian and Early Pennsylvanian age. Gordon feels that all of the cephalopods found could occur in rocks of Late Mississippian age, but some of them, such as those identified as Glaphyrites? and Stenopronorites? resemble species of Early Pennsylvanian age. One of the nautiloids, Tylonautilus sp. is of a genus not at present known in rocks of Pennsylvanian age, but fairly common in rocks of Late Mississippian age. But it is only a fragment and could be reworked. The poor preservation of so much of the material is what causes most of the problem. One wonders how much of the supposed ornament is caused by distortion after deposition of the material and how much of it is original. Yochelson's opinion, derived from the gastropods is that they are more like Early Pennsylvanian types, but not enough is known of the Late Mississippian forms and much that we are now learning indicates they are much like the Pennsylvanian species. So for the present, pending the study of other fossils from this section, such as the corals, bryozoans, brachiopods, and ostracodes, we feel that the age of these beds might be considered to be Late Mississippian (Chester) or Early Pennsylvanian (Morrow).