Cretaceous stage boundaries in the southern Interior Plains of Canada

Marine Cretaceous rocks of Berriasian to Aptian age are restricted to northwestern Alberta and north­eastern British Columbia where many stage and substage boundaries may be drawn only tentatively on the basis of limited molluscan faunas. A complete marine Albian succession is similarly restricted, although the middle and upper divisions of the stage are much more widespread. The complete Albian succession of the Peace River district contains a refined and well-documented sequence of ammonite and foraminiferal zones and would be a most suitable continental standard. The Albian-Cenomanian (Lower-Upper Cretaceous) boundary has been reliably established in the continuous foraminifer- and mollusc-bearing sequences of the northwestern plains, but eastward it becomes enclosed in a hiatus. The same is true of successive stage boundaries from the Cenomanian­Turonian to the Santonian-Campanian. The bases for establishment of these boundaries, therefore, can be fully considered only on the western flank of the basin. The Campanian-Maastrichtian boundary may be precisely drawn in western Saskatchewan and traced westward to the Rocky Mountain front. Zones based on ammonites and inoceramid bivalves form· the cornerstone of the biostratigraphy and chronostratigraphy of the southern Interior Plains and hold the key to the stage boundaries. Zones based on assemblages of benthonic foraminifers, rarely of planktonic foraminifers, supplement the molluscan zones. The foraminiferal zones are less reliable and less useful, however, because some benthonic assemblages are weakly diachronous, most foraminiferal zones span several molluscan zones, and many stage boundaries fall within individual foraminifer zones.