Abstract: Oligocene Calcareous Nannofossil Distribution in Deep Sea

Gretchen Blechschmidt, Thomas R. Worsley

The Oligocene epoch is characterized by an increase in provincialism as well as a drop in diversity of many marine microfossil groups. Therefore, it is of particular interest to examine the geographic and stratigraphic distribution of the groups. Calcareous nannofossils, which are among the most common microfossil groups in deep-sea sediments, have been used in this study.

To determine the geographic distribution of these nannofossil species, it is necessary to determine a synchronous datum surface. Almost all paleontologic datum surfaces (highest and lowest occurrences) are to some degree nonsynchronous. To improve this situation, a statistical method based on the stratigraphic distribution of 60 preselected Oligocene nannofossil species at 72 globally distributed Deep Sea Drilling Project sites has been used to generate approximately synchronous data (called compound data) through the Oligocene. The compound data then are correlated with absolute-age estimates. When these compound data are plotted at each site at the appropriate subbottom depth, a compound datum surface is generated. The interval between two compound datum surfaces is a "time slice."< P>

Using a time-slice interval of about 2 m.y., which is approximately equal to the present limits of biostratigraphic resolution of the Oligocene, it is possible to plot the geographic distribution of nannofossil species or groups. Comparison of species distributions through several time slices yields information about radiation and restriction of the nannofossil taxa. This is clearly a powerful tool in paleoenvironmental reconstructions. It also is useful in evaluation of present-day calcareous nannofossil zonations which are limited by provincialism and selective dissolution of taxa.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC