CALCAREOUS NANNOFOSSIL REVISED BIOSTRATIGRAPHY OF THE LATEST ALBIAN-EARLIEST CAMPANIAN TIME INTERVAL (LATE CRETACEOUS)

Abstract Calcareous nannoplankton appeared in the Late Triassic and rapidly evolved during the Jurassic and Cretaceous, reaching a diversity maximum in the Late Cretaceous. The widespread distribution, the high abundance and assemblage diversity of Late Cretaceous nannofloras make calcareous nannofossils a very effective biostratigraphic tool for high-resolution dating and correlations. This study is focused on the revision of nannofossil biostratigraphic events in the Upper Cretaceous. The major objective is to refine the biostratigraphic schemes available for the latest Albian-earliest Campanian time interval by the investigation of eight selected sections. These are located in different sedimentary basins in order to assess nannofossil bioevents in different paleoceanographic regimes and/or latitudes and evaluate the applicability of the three major nannofossil zonations. Sites and sections studied are: DSDP Site 463 (Mid-Pacific Mountains – Pacific Ocean), ODP Site 763 (Exmouth Plateau – Indian Ocean), ODP Site 1138 (Kerguelen Plateau – Indian Ocean), ODP Site 1261 (Demerara Rise – Atlantic Ocean), Monte Petrano (Umbria-Marche Basin – Italy), Clote de Chevalier (Vocontian Basin – France), Rock Canyon, Pueblo (Western Interior Basin – Colorado), Tarfaya Core (Tarfaya Basin – Morocco). Biostratigraphic investigation was performed on a total of 564 smear slides analyzed using standard light microscope techniques under cross polarizers and transmitted light, at 1250X magnification. I produced semi-quantitative range charts of all taxa providing information on their preservation, total and single taxon abundance. Taxonomic revision was applied to cases of potential misidentification and discrepant attribution. Taxonomic notes and remarks based on personal observation are provided. Also, photographic plates illustrate all zonal/subzonal markers and some other common species. The results were compared to those from other sections published in literature to test reliability of biozonations and discriminate global, regional or local events. Nannofossil biohorizons were also examined in the context of stage boundary definition. More specifically, for each stage boundary the data documented for GSSP sections and/or presented for GSSP proposals were analyzed and the result of my investigation were used to provide information about the applicability of nannofossil events. Adopting the three zonations, a total of 17 zonal and 4 subzonal markers were recognized as potentially cosmopolitan biohorizons. The achieved resolution is comparable for the three biozonations and higher subdivision is possible only at regional level and in restricted time intervals. Moreover, nannofossil events were proved to be extremely useful to biostratigraphically constrain paleoceanographic events of the Late Albian-Early Turonian interval. Namely, Oceanic Anoxic Event 1d, the Mid-Cenomanian Event and Oceanic Anoxic Event 2 were successfully characterized in the studied sequences.