Calcareous nannofossils and the Santonian/Campanian and Campanian/Maastrichtian boundaries on the Brazilian Continental Margin: historical overview and state of the art

Abstract

Recognition of the Campanian stage on the Brazilian Continental Margin, using calcareous nannofossils, has been historically problematic. This paper constitutes an overview of earlier works, showing how nannofossil biostratigraphic ideas have evolved since Troelsen & Quadros provided the first biozonation of this region in 1971. Recent studied have provided data which have helped to clarify these apparent biostratigraphic problems, and allows this region to be placed in a global biostratigraphic context. The earliest researchers identified the Santonian/Campanian boundary by the last occurrences of ‘ Lithastrinus grillii’ and Petrobrasiella venata. P. venatawas later abandoned as an index species due to its rarity and, instead, the last occurrences of Marthasterites furcatusand ‘ Lithastrinus grillii’ became the most-used markers. However, the stratigraphic age of these biohorizons diverged from those quoted in the literature. In the Brazilian basins, these extinctions, rather than having occurred in the Campanian as was recorded elsewhere, were considered to mark the top of the Santonian, as suggested by correlations with other microfossil groups (primarily foraminifera and palynomorphs). To explain this phenomenon, the existence of a condensed sequence was postulated for most of the Brazilian marginal basins, where the uppermost Santonian deposits were apparently indistinguishable from those of the lowermost Campanian. In line with current correlations presented in the nannofossil literature, and with new information obtained from core and side-wall samples, it is now believed that the extinction of these species didoccur in the Campanian in the Brazilian basins, whilst the last occurrence of Lithastrinus moratus(previously misidentified as Lithastrinus grillii) has become a useful Santonian marker. Thus the Santonian/Campanian boundary in Brazil lies in a stratigraphic position similar to elsewhere in nannofossil terms, that is below CC18. The Campanian/Maastrichtian boundary was initially characterised in nannofossil terms in Brazil by the last occurrence of Broinsonia parca constricta, and later by the last occurrence of Eiffellithus eximius. Recently acquired data has shown that the sequence of events in the Brazilian marginal basins is similar to that of the Sissingh/Perch-Nielsen standard biozonation scheme through this interval. Again, correlations in the literature with the recently defined boundary (in macrofossil terms) thus allow the boundary to be determined between the last occurrences of Broinsonia parca constrictaand Uniplanarius trifidus, that is, in CC23b.