ABSTRACT

Calcareous nannoplankton organisms evolved rapidly during the Cenozoic, and are of great value as stratigraphic indicators. A large number of species exhibit world wide distribution, so that this group of fossils is very useful in transoceanic correlation. Calcareous nannofossils may often be recovered from relatively shallow water sediments, permitting the zonation established in pelagic sediments to be used in shelf deposits.

The event which ended the Cretaceous almost annihilated Mesozoic calcareous nannoplankton floras. Highly diversified assemblages of Maastrichtian coccoliths were replaced by early Danian assemblages having only a few genera and species. Species diversity increased rapidly during the Paleocene, and by the middle Paleocene had reached a level comparable to that in the late Mesozoic. Discoasters, which due to their large size are very easy to observe and use stratigraphically, first appeared in the middle Paleocene. Late Paleocene, early and middle Eocene pelagic sediments contain a remarkable succession of rapidly evolving calcareous nannoplankton assemblages, culminating in a diversity maximum in the early Lutetian. Later in the Lutetian, a sudden decline in the number of species occurred, and the late Eocene is characterized by relatively monotonous, slowly evolving calcareous nannoplankton floras. Many species characteristic of the late Eocene became extinct in the latest Eocene or early Oligocene. Large calcareous nannofossils, particularly discoasters, are rare in the Oligocene, but forms bearing smaller coccoliths evolved rapidly. Early Miocene calcareous nannofloral assemblages were dominated by an abundance of stout asteroliths belonging to the Discoaster deflandrei plexus. In the middle Miocene, these were replaced by rich asterolith assemblages with many delicate forms having long, thin arms. The number of species of asteroliths and coccoliths again reached a maximum in the early Pliocene. During the Pliocene a number of species, particularly discoasters, declined and became extinct. The last asterolith, Discoaster brouweri, became extinct at the Nebraskan-Aftonian boundary in the Gulf Coast region. During the Pleistocene, species bearing small coccoliths evolved rapidly, and modern assemblages are dominated by a species which first appeared in the Wisconsinan.

The zonation based on calcareous nannoplankton fossils, proposed here, approximates the stratigraphic resolution which can presently be achieved using planktonic foraminifera. However, monographic studies have been completed only for the Paleocene-lower Eocene, and uppermost Pliocene-Pleistocene-Recent intervals. Stratigraphic resolution should be considerably improved when detailed studies of the middle Eocene-upper Pliocene interval, now in progress, are completed.