CALCAREOUS NANNOFOSSILS AND THE SANTONIAN CAMPANIAN AND CAMPANIAN/MAASTRICHTIAN BOUNDARIES ON THE BRAZILIAN CONTINENTAL-MARGIN - HISTORICALOVERVIEW AND STATE-OF-THE-ART/

Recognition of the Campanian stage on the Brazilian Continental Margin , using calcareous nannofossils, has been historically problematic. Th is paper constitutes an overview of earlier works, showing how nannofo ssil biostratigraphic ideas have evolved since Troelsen & Quadros prov ided the first biozonation of this region in 1971. Recent studies have provided data which have helped to clarify these apparent biostratigr aphic problems, and allows this region to be placed in a global biostr atigraphic context. The earliest researchers identified the Santonian/ Campanian boundary by the last occurrences of 'Lithastrinus grillii' a nd Petrobrasiella venata. P. venata was later abandoned as an index sp ecies due to its rarity and, instead, the last occurrences of Marthast erites furcatus and 'Lithastrinus grillii' became the most-used marker s. However, the stratigraphic age of these biohorizons diverged from t hose quoted in the literature. In the Brazilian basins, these extincti ons, rather than having occurred in the Campanian as was recorded else where,were considered to mark the top of the Santonian, as suggested b y correlations with other microfossil groups (primarily foraminifera a nd palynomorphs). To explain this phenomenon, the existence of a conde nsed sequence was postulated for most of the Brazilian marginal basins , where the uppermost Santonian deposits were apparently indistinguish able from those of the lowermost Campanian. In line with current corre lations presented in the nannofossil literature, and with new informat ion obtained from core and side-wall samples, it is now believed that the extinction of these species did occur in the Campanian in the Braz ilian basins, whilst the last occurrence of Lithastrinus moratus (prev iously misidentified as Lithastrinus grillii) has become a useful Sant onian marker. Thus the Santonian/Campanian boundary in Brazil lies in a stratigraphic position similar to elsewhere in nannofossil terms, th at is below CC18. The Campanian/Maastrichtian boundary was initially c haracterised in nannofossil terms in Brazil by the last occurrence of Broinsonia parca constricta, and later by the last occurrence of Eiffe llithus eximius. Recently acquired data has shown that the sequence of events in the Brazilian marginal basins is similar to that of the Sis singh/Perch-Nielsen standard biozonation scheme through this interval. Again, correlations in the literature with the recently defined bound ary (in macrofossil terms) thus allow the boundary to be determined be tween the last occurrences of Broinsonia parca constricta and Uniplana rius trifidus, that is, in CC23b. (C) 1997 Academic Press Limited.