Calcareous nannofossil biozonation of the Miocene and revision of the helicoliths and discoasters

Abstract

"Calcareous nannofossils" form a heterogeneous group of minute calcareous objects that range in size from 1 to 30 microns. The majority of the fossils resemble the coccoliths of the exterior calcareous cover (coccosphere) of the Haptophyceae and therefore it is generally accepted that these fossils are remains of such unicellular algae. Reviews of basic information about the calcareous nannofossils have been published by Hay (1977), Gartner (1977), Haq (1978), Tappan (1980) and Lord and Taylor (1982). The recognition of the calcareous nannofossils as a worthwhile tool for biostratigraphic correlations is generally credited to Bramlette and his coworkers Riedel, Sullivan, Martini and Wilcoxon (1954-1967). Following these pioneering efforts, intensive taxonomic and biostratigraphic studies of calcareous nannofossils have been carried out, and these have formed the basis for several biozonal schemes of the Mesozoic and the Cenozoic Systems. The Cenozoic zonations are far more refined than the Mesozoic ones, mainly owing to the great number of Cenozoic sections with well-preserved nannofossil content obtained by the coring of deep ocean sediments. The two comprehensive zonations of the Cenozoic most frequently used are the compilations of Martini (1971) and of Bukry (1971,1975). Although both zonal schemes have so far provided a good working basis for biostratigraphic correlations, they are not applicable everywhere. The Neogene of the Mediterranean area has proved especially hard to calibrate with the standard Zonations of the open oceans. Our biostratigraphic investigation was concentrated on Miocene Mediterranean sections, firstly because numerous land sections and cores of this interval were either available in the Utrecht collections or easily accessible, and secondly because the "standard" Miocene zonations were inadequate for biostratigraphic correlations in the Mediterranean region. As most of the Mediterranean sections were fragmentary, a large number of sections were studied in order to cover the Miocene interval completely and to check the reproducibility of our initial biostratigraphic results. This study led to the composition of a new "Mediterranean Miocene Zonation" (fig. 44). The applicability of our biostratigraphic data from the Mediterranean sections was checked on some DSDP cores and land sections in extra-Mediterranean areas. The use of our new markers in combination with the conventional ones that we found in these sections resulted in a more refined general biozonal scheme: the "Integrated Miocene Zonation" (fig. 45). A large amount of taxonomic information was gathered during the progress of our investigation. of all the taxa encountered we chose to make a thorough revision of the taxonomy of two major groups, namely the helicoliths and the discoasters. As a basis for this revision we used our new findings concerning the ultrastructure of the nannoliths of these groups (chapters 4 and 5). Both groups were chosen because of their rich and diverse representation in our Miocene samples. In order to complete our knowledge about these nannofossil groups and to reconstruct "evolutional patterns" we expanded our study by incorporating data from several pre-Miocene and postMiocene sections and samples (fig. 2).