Abstract: South Atlantic Paleoceanographic Events Recorded in the Neogene Deep Water Section of the Campos Basin, Brazil

Souza Cruz, C.E. - Petrobras/Cenpes

The Campos Basin, situated offshore of the easternmost projection of the South American shoreline south of 20°S, is an exceptional region for evaluating the interaction between sedimentation and oceanic circulation. This results from the combination of several present day features, such as the basin location (22°-23° S, 39°-41° W), the presence of stratified water masses and very thick wedge of Neogene deep-water sediments shown as very large sigmoidal wedges in seismic dip sections. They are formed by bioturbated, rhythmic mudstone-marlstone bundles of variable thickness, which overlay a dominantly sandy basal section interpreted as a mixed turbidite-contourite system. The turbidite facies comprise chiefly deposits ranging in texture from conglomeratic to sandy, formed by high- density turbidity flows, and less commonly, low-density flow deposits. Contourite facies are the product of reworking of continental slope and rise deposits by bottom currents, and are mainly represented by fine-grained sediments. Deposition of the hemipelagic facies took place in bathyal environments through the slow planktonic "rainfall" on the sea-floor along with deposition of the terrigenous (silty-clayey) fraction, both subject to bottom current action with variable intensity.

Several third-order stratigraphic sequences have been identified in seismic sections, correlated with well logs and nannofossil biostratigraphic data by means of synthetic seismograms. The Neogene stratigraphic sequences show a cyclicity consistent with interpreted third-order eustatic fluctuations of the global cycle chart.

Despite the correspondence of sequence limits with global eustatic lowerings, the currently accepted sequence stratigraphy model does not explain the sequence pattern observed in the Campos Basin. This holds true especially with regard to the nature of the deep-water wedges and the intensity of some erosional unconformities associated to them. These wedges consist entirely of deep-water facies with bathyal faunas and are bounded by erosional surfaces.

Although eustatic sea level changes play a role in this process, the construction of deep water wedges and major erosional unconformities are best explained by changes in the oceanic circulation pattern modulated by climatic changes. The Neogene deep-water section of the Campos Basin clearly records some important paleoceanographic events of the South Atlantic. For instance, the change from Oligocene to Miocene corresponds to a period of intense global deep-water marine circulation brought about by the Circumpolar Antarctic Current following the opening of the Drake Passage. Coeval events in the Campos Basin include a regional deep-water unconformity and extensive reworking of deep-water sands. Two other great deep-water unconformities of Middle Miocene (~ 16.5 Ma) and Upper Miocene (between 5.5 and 8.2 Ma) age in the Campos Basin correlate respectively with the well known East and West Antarctica glaciations. This demonstrates that the erosional and depositional events in the Neogene of the Campos Basin were controlled by the intensity of bottom current activity, by the sediment supply/dispersal ratio, and by their interaction with the basin morphology.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil