ABSTRACT

Calcareous nannoplankton fossils of the Tertiary are generally well known, and their importance in stratigraphy has been amply demonstrated. Cretaceous calcareous nannofossils have been described in a number of papers, and it is evident that many species can be recognized. Workable stratigraphic schemes for subdivision of the Upper Cretaceous have been slow to evolve because of (1) the great diversity of Upper Cretaceous assemblages, (2) the similarity of many coccolith species in this interval, and (3) the lack of continuous sections with adequate, well-preserved nannofossil floras.

Twelve zones based on calcareous nannofossils are now recognized within the interval Cenomanian-Maastrichtian, approximating the degree of subdivision readily attainable with planktonic foraminifers. Four of the zones lie with the Cenomanian-Turonian interval, another four probably within the Coniacian-Santonian, and four within the Campanian-Maastrichtian. Many of the genera and species characteristic of the Upper Cretaceous evolved during the Cenomanian. The increase in diversity is a marked feature of Cenomanian and Turonian assemblages. Diversity remained more or less constant until the Maastrichtian, but evolution proceeded rapidly in some groups, such as Arkhangelskiella, Kamptnerius, and some other coccolith genera during the Coniacian-Santonian. The Campanian was a period of relative evolutionary quiescence, and is generally difficult to subdivide. The Maastrichtian is characterized by successive elimination of many species with a resulting decrease in diversity of the assemblages. The end of the Maastrichtian coincides with an abundance-diversity minimum marking a level of great change in calcareous nannoplankton fossil assemblages.

Reference sections for the zones which are recognized are in Kansas and Alabama. The section along the Alabama River between Selma and Millers Ferry is particularly valuable as one of the best exposed series of outcrops available anywhere. It is also unique in providing rich, diverse, well-preserved calcareous nannofloras at all levels.