Santos Drift System: Stratigraphic organization and implications for late Cenozoic palaeocirculation in the Santos Basin, SW Atlantic Ocean
Cláudio S. L. Duarte, Adriano R. Viana, 2007. "Santos Drift System: Stratigraphic organization and implications for late Cenozoic palaeocirculation in the Santos Basin, SW Atlantic Ocean", Economic and Palaeoceanographic Significance of Contourite Deposits, A. R. Viana, M. Rebesco
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High-quality 2D and 3D seismic data were analysed to investigate the stratigraphic organization of the sedimentary deposits and the impact of the palaeocirculation of the SW Atlantic Ocean in the construction of the Santos Basin slope from late Palaeogene to Recent time. Seven seismic sequences were identified based on their external geometry, internal seismic pattern and seismic boundaries. A correlation between these sequences, the glacio-eustatic curves and the major climatic–palaeoceanographic events was attempted. The base of the studied sequences is the c. 28 Ma intra-Oligocene Rupelian–Chatian unconformity. Six other seismic horizons corresponding to the sequence boundaries were mapped ranging from the late Oligocene to the Pliocene. Variations in the bottom-current intensity were characterized by the seismic pattern of each sequence and by the evolution of margin physiography. A 100 km long channel-like gutter, the Santos Channel, was excavated at the foot of an intra-slope escarpment. It was the locus of major flow circulation until the middle Miocene, when the margin physiography reorganization transferred the main axis of deep current action downslope and excavated the São Paulo Channel at the foot of the present continental slope. Two major contourite drifts were accumulated in response to the different combinations of bottom-current axis position and slope physiography. Those drifts constitute the Santos Drift System. Conversely to the present-day circulation pattern, with southward flow above the slope dominated by the western boundary Brazil Current, both at surface and deep waters, the geological record indicates that the palaeocirculation in the Santos Basin was marked by the opposite sense of circulation of surface waters (southward palaeo Brazil Current) and of the intermediate to deep waters (northward Southern Ocean Current). The study indicates that periods of relative sea-level rise to highstands correspond to increase in drift accumulation whereas during lowstands slope drift sedimentation is reduced.
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There has lately been a growth in the number and level of studies of contourite deposits. Most recent studies of contourites have two major lines of interest. One, propelled by the oil industry's continuous move into increasingly deep waters, concerns their economic significance. The other involves the stratigraphic/palaeoceanographic record of ocean circulation changes imprinted on contourite deposits that can be a key to understanding better the climate-ocean connection. The application of many different theoretical, experimental and empirical resources provided by geophysics, sedimentology, geochemistry, petrology, scale modeling and field geology are used in the 16 papers of this volume, proposing answers to those two main aspects. The papers are subdivided into two major categories (economic interest and stratigraphic/palaeoceanographic significance), with case studies ranging from well-documented drifts to new examples of modern and fossil series, involving a large diversity of geographic and physiographic scenarios worldwide.