Santos Drift System: stratigraphic organization and implications for late Cenozoic palaeocirculation in the Santos Basin, SW Atlantic Ocean

Abstract

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 S~o 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. the coastal ranges (Serra do Mar and Serra da Mantiqueira). Fig. 4. (a) Sea surface temperature satellite image (Landast NOAA-12) illustrating the meandering pattern of the warm Brazil Current and the giant eddy developed at the Cabo Frio salient and penetrating onto the shelf. (b) Numerical model plots of Brazil Current behaviour in the Santos Basin based on temperature (~ and intensity represented by the length of the arrows (m s -l) (from Souza 2000).

The economic importance of contourites

“…However, their regional extent and abrasive aspect, with along-slope trending rather than downslope scouring characteristics, suggest a contour current-related erosion. The South Atlantic Brazilian margin is prone to such unconformities, such as the examples described by Viana (2001), Gomes & Viana (2002), Lima (2003) and Duarte & Viana (2007). The physical coincidence between some of these unconformities and sequence boundaries is also probable, and has been demonstrated for the Santos Drift example (Duarte & Viana 2007), where deep-water drift boundaries were correlated to regional sequence boundaries tied with exploratory wells.…”

“…The South Atlantic Brazilian margin is prone to such unconformities, such as the examples described by Viana (2001), Gomes & Viana (2002), Lima (2003) and Duarte & Viana (2007). The physical coincidence between some of these unconformities and sequence boundaries is also probable, and has been demonstrated for the Santos Drift example (Duarte & Viana 2007), where deep-water drift boundaries were correlated to regional sequence boundaries tied with exploratory wells. The upslope-migrating package, a frequent feature on separated or plastered drifts (Faug~res et al 1999), is occasionally misinterpreted as lowstand prograding wedges in the sense of their sequence stratigraphic position.…”

“…In such a case, vigorous SW-flowing slope surface currents, in a pattern similar to that of the presentday Brazil Current (see discussion by Duarte & Viana 2007), would erode and induce instability at the tip of the shelf edge clinoforms prograding from the shelf edge, where the gradient increases. Such erosion induces the introduction of sediments into the basin via successive mass flows, whose heads are aligned for several tens of kilometres along the isobaths.…”

“…Recent papers by Llave et al (2001), Reeder et al (2002), Viana et al (2002b), Carter et al (2004 and Lu & Fulthorpe (2004), and others presented in this volume (Carter 2007;Duarte & Viana 2007) have dealt with palaeoceanographic aspects of contourites. Despite the occurrence of gas hydrates in some contourite systems around the world (e.g.…”

Seismic expression of depositional elements associated with a strongly progradational shelf margin: northern Santos Basin, southeastern Brazil

“…In addition to that, tectonic adjustments in the Serra do Mar range caused a reorganization of the drainage system, leading Paraíba do Sul River to migrate north and to start delivering its load to Campos Basin (Karner & Driscoll, 1999). As a result, a dramatic reduction in sedimentation rate took place in northern Santos Basin, leading to transgression and reworking by bottom currents (Duarte & Viana, 2007).…”