Foraminiferal characterisation and taxonomy of Oligocene-Miocene Congo Fan deep sea sub-environments, offshore Angola.
AbstractThe Congo Fan has been accumulating sediment since the mid-Oligocene, and is of particular interest to the oil industry due to abundant large reservoirs contained within the meandering sandy palaeochannels from largely Miocene and Oligocene deposits. The high sedimentation rates and rich benthic foraminiferal faunas also provide an interesting record of Miocene and Oligocene palaeoceanographic change in the southeast Atlantic, a stratigraphic time interval that has had no recovery from proximal ODP and DSDP Sites. Three oil wells spanning the Upper Oligocene to Middle Miocene from the distal part of the Congo Fan (Block 31, approximately 2000 m water depth) are studied for both benthic and planktonic foraminifera using ditch cutting samples at 10 m spacing. In addition to assemblage data, 8,80 and 813C measurements have been obtained from Cibicidoides spp. for the Miocene which, along with several planktonic foraminiferal datums, provides a relatively well-constrained age model. The Oligocene sections are dominated by agglutinated benthic foraminifera and the age model is less accurate. A full taxonomy has been carried out on all foraminifera encountered, with over 150 agglutinated, 80 calcareous benthic, and 27 planktonic species described and pictured. A number of sedimentological environments contain characteristic faunas. Channel deposits are either barren or contain current-sorted calcareous specimens, levee deposits have higher abundances of transported calcareous specimens and are affected by hydrodynamic sorting, overbank deposits are dominated by high diversity in situ agglutinated faunas. Oligocene sediments from the Congo Fan consist almost entirely of agglutinated foraminifera due to deposition below a locally raised CCD. A level of decreased diversity and increased abundance (the 'Scherochorellcf event) in the intra-Upper Oligocene records a probable expansion of the oxygen minimum zone associated with polar cooling. During the Early Miocene a gradual and persistent increase in the percentage of calcareous foraminifera is mirrored by increasing benthic 813C, suggesting reduced bottom-water acidification and a lowering of the CCD. A dramatic shift in the shallow infaunal morphogroup (-16 Ma) indicates lower oxygen and a second episode of oxygen minimum zone expansion, which is coincident with cooling in this location and adds evidence for raised colder bottom waters. Global cooling at this time was probably responsible for increasing the strength of the polar front, and in turn strengthening offshore winds affecting an increase in upwelling and surface water productivity