Abstract:
Campanian-Maastrichtian sediments recovered from the Demerara Rise off Suriname in the equatorial western Atlantic (ODP Leg 207) consist primarily of nannofossil chalks, clays and claystones and show highly diversified assemblages of calcareous nannofossils. Well-preserved nannofossils are abundant, and their distribution patterns provide a number of biostratigraphical events that allow a needed revision of the calcareous nannofossil zonation for equatorial regions such as this. The major biostratigraphic events consist of the following first and last occurrences (FO's & LO's): FO's and LO's of Broinsonia parca constricta and Uniplanarius trifidus; FO's of Arkhangelskiella cymbiformis, Broinsonia parca parca, Micula prinsii, Lithraphidites quadratus, and the LO of Eiffellithus eximius. Equally important, however, Secondary Index Species have been delineated via quantitative and qualitative nannofossil data collections from Holes 1258A, 1259A & 1260A, and these can be correlated throughout the study area. These events are: the acmes of Lithraphidites quadratus, Bukryaster hayi and Lithastrinus quadricuspis; LO's of the acmes of Kamptnerius magnificus, Gartnerago segmentatum, Ahmuellerella octoradiata, Zeugrhabdotus bicrescenticus; FO's of the acmes of Pseudomicula quadrata, Prediscosphaera incohatus, Ahmuellerella regularis, Lithraphidites preaquadratus, Micula praemurus and K. magnificus; LO's of Staurolithites mielnicensis, Quadrum gartneri, Zeugrhabdotus diplogrammus; FO of Ceratolithoides amplector; and the increase upsection of C. amplector. The study has shown that none of the previously proposed stratigraphic schemes can be applied per se to our study area. The research also demonstrates that a judicious mix of quantitative and qualitative methods gives us broad new opportunities for developing a more robust low-latitude Cretaceous nannofossil biostratigraphy.