Computer-processed, probability-based statistical analysis of the published stratigraphic information on 25 common calcareous nannofossil species present in 51 long sections of Oligocene strata indicates that the 5 Oligocene zones of the standard Paleogene zonation of Martini (1970) are very reliable. However, the available number of subdivisions is far greater than previously supposed and is likely to increase as more sections are studied. It also is evident from this study that the sequence of biostratigraphic datum surfaces for an area is dependent on both evolution and the environment in which it is found, so that sequences differ significantly among biogeographic provinces.

In this analysis, water depth and latitudinal position were used to separate 51 sections from the equatorial region and Northern Hemisphere into 4 overlapping groups. The sections first were separated into high and low-latitude groups, neritic and oceanic groups, and into the combinations of these, making a total of 8 groups. A most probable biostratigraphic sequence of datum surfaces was found for each of these groups and compared with each other, and with the composite based on all by means of cross sections and a fence diagram.

It is suggested that maximum gain in biostratigraphic resolution will result from intensive study of new and previously described sections with the aim of separating as many datum surfaces as possible from each other. The description of new species and detailed taxonomic classification, although important to local problems, will add little to global biostratigraphic resolution.

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