Upper Maastrichtian - Eocene Benthic Foraminiferal Biofacies Of The Brazilian Margin, Western South Atlantic

Benthic foraminiferal biofacies were delimited for the upper Maastichtian through upper Eocene of five Brazilian marginal basins (Sergipe-Alagoas, Mucuri, Campos, Santos and Pelotas) and two DSDP Sites 356 and 20C of the western South Atlantic. The biofacies were determined based on the benthic foraminiferal assemblages and associated parameters, including percentage of planktic foraminifera (% planktics), lithology, and percentage of radiolarians (% rads). The biofacies show basin-to-basin differences, and are primarily distinguished by the agglutinated/calcareous taxon percentage and the dominant three or four species. Biofacies A is composed of 100% calcareous taxa and dominates in the Eocene. Biofacies B has up to 10% agglutinated taxa and occurs from the middle Paleocene through the upper Eocene. Biofacies C has 11% to 25% agglutinated taxa and is present from the upper Maastrichtian through the upper Eocene. Biofacies D contains a balanced percentage of calcareous and agglutinated taxa (similar to 50% each), and is always associated with Biofacies E in the marginal basins. Biofacies E is dominated by agglutinated taxa, especially tubular forms (Bathysiphon, Nothia, Rhizammina, Psammosiphonella). This biofacies correlates with the so- called "flysch-type" biofacies of Berggren and Gradstein (1981), and occurs exclusively in the marginal basins from the Maastrichtian through upper Eocene, although it dominates in the Paleocene. The biofacies distribution reveals distinct environmental settings as the Brazilian margin built outwards in response to tectonic activity and increased terrigenous input. The biofacies record a deep-water setting close to or below the calcite compensation depth (CCD) during the Maastrichtian-Paleocene along the entire eastern Brazilian margin. Progradation of the shelf and shoaling of the slope during the Eocene probably is the principal reason for the abrupt change from agglutinant-rich biofacies (E and D) to calcareous-rich biofacies (A, B, and C) in the early Eocene. The Campos Basin continued to record abyssal to lower bathyal conditions while the Sergipe-Alagoas and Mucuri basins shoaled to neritic palaeodepths by the late Eocene. Changes in relative sea level, including a global sea level fall in the late Paleocene followed by global sea level rise in the early Eocene, as well as changes in the position of the CCD along the Brazilian margin affected the development of foraminiferal biofacies in the marginal basins. The distal DSDP sites were at greater palaeodepths than the sites in the marginal basins, but were not below the CCD during the Maastrichtian Eocene and biofacies were dominantly calcareous. We conclude that the CCD was shallower along the productive Brazilian continental than in the pelagic areas due to the greater flux of organic matter.