Luciano P. Magnavita

Episodic burial and exhumation in NE Brazil after opening of the South Atlantic

It is a common assumption that elevated passive continental margins have remained high since rifting and breakup. Here, we show that the Atlantic margin of NE Brazil has undergone a more complex history. Our synthesis of geological data, landscape analysis, and paleothermal and paleoburial data reveals a four-stage history: (1) After Early Cretaceous breakup, the margin under went burial beneath a thick sedimentary cover; (2) uplift episodes in the Campanian and Eocene led to almost complete removal of these deposits; (3) the resulting large-scale, low-relief erosion surface (peneplain) was deeply weathered and finally reburied at the Oligocene-Miocene transition; and (4) Miocene uplift and erosion produced a new, lower-level peneplain by incision of the uplifted and re-exposed Paleogene peneplain. Previous studies have identified aspects of this interpretation, but we have defined the absolute timing and magnitude of discrete events of burial and exhumation that followed Early Cretaceous rifting and Eocene–Oligocene peneplanation. We suggest that a late sedimentary cover protected Paleogene weathering profiles until the present day. The uplift phases in Brazil are synchronous with uplift phases in Africa and the Andes. The Andean phases coincided with rapid convergence on the western margin of South America, and the Campanian uplift coincided with a decline in spreading rate at the Mid-Atlantic Ridge. Consequently, we suggest that both vertical movements and lateral changes in the motion of the plates have a common cause, which is lateral resistance to plate motion.

Rifting, erosion, and uplift history of the Recôncavo-Tucano-Jatobá Rift, northeast Brazil

The Reconcavo-Tucano-Jatoba (RTJ) Rift and many other smaller sedimentary basins in northeast Brazil formed during South Atlantic rifting and were subsequently uplifted and exhumed so that Albian marine sediments are now located up to 800 m above sea level some 400 km inland from the Atlantic margin. Local erosion caused by footwall uplift and regional erosion, probably resulting from magmatic underplating and uplift, has removed a large part of the thermal sag phase sediments from the RTJ Rift. The flexural cantilever model, incorporating the flexural isostatic response to simple-shear faulting in the upper crust and pure-shear necking of the lower crust and upper mantle, can explain the main geometrical features observed in the RTJ Rift without resorting to lithosphere scale detachments. The model has also been used to estimate the amount of uplift and erosion of the faulted rift flanks. Rift flank erosion can produce uplift and erosion across the whole RTJ Rift, thus explaining the postrift unconformity which preceded deposition of the Aptian age Marizal Formation. The largest uplift is predicted to occur at the edges of the basin and may explain the anomalously shallow depth to onset of oil generation at the basin margins deduced from vitrinite reflectance data. The model also predicts that maximum burial of much of the basin fill in the RTJ Rift occurred at the end of rifting and not during the postrift infill, as is usually the case. The amount of observed coarse conglomeratic detritus in the Reconcavo subbasin suggests that about 25% of the eroded footwall detritus was deposited and preserved in the adjacent hanging wall half-graben, with the rest being transported more distally as finer material.

Release faults, associated structures, and their control on petroleum trends in the Recôncavo rift, northeast Brazil

Release faults are rift cross faults, which develop to accommodate the variable displacements of the hanging-wall block along the strike of normal faults. Release faults are nearly perpendicular or obliquely oriented to the strike of the normal fault they are related to. They have maximum throws adjacent to the parent normal fault and die out in the hanging wall away from it. They form to release the bending stresses in the hanging wall and do not reflect the orientation of the regional stress field in a basin. Commonly, they show normal-oblique displacements and are preferentially located along the strike ramps. Release faults may also act at the scale of an entire basin, reaching displacements of thousands of meters. Joints, shale, and salt diapirs may develop in association with release faults. Because all these structures represent domains of stress release, they may work as conduits for oil migration and oil traps in extensional basins. This is the case of the Reconcavo basin in northeastern Brazil, a Cretaceous failed rift, connected to the eastern Brazilian continental margin basins. In the Reconcavo basin, two large-scale release faults, with displacements in the order of 3 km, developed in the hanging wall of the rift border faults and control the location of the main oil fields.

The Recôncavo basin

The Reconcavo basin forms the southern end of the inland Reconcavo-Tucano-Jatoba rift system, whose propagation to the north was arrested by the transverse Pernambuco shear zone, causing final rifting and continental break-up to propagate along an alternative path that gave rise to the present continental margin. Between the rift system and the Sergipe-Alagoas basin located along the Atlantic margin lies the triangular Sergipe Microplate that rotated counterclockwise during the early stages of rifting between South America and Africa. Both rift zones, and the microplate between them, are cut in two by the NW-SE trending Vaza-Barris transfer zone along which the border fault of the Reconcavo−Tucano-Jatoba rift flips its position. Thus, to the northeast of the transfer fault the North Tucano and Jatoba basins master faults are placed on the western rift margin; conversely, to the southwest of the Vaza-Barris transfer zone, the Central Tucano, South Tucano, and Reconcavo basins have the main boundary faults on the eastern border of the rift system. Other important cross faults also occur, and among them, the NW-SE Mata and Catu faults, which control the most prolific hydrocarbon trend of the rift, have been interpreted as formed separately as release faults in the hanging wall block of major normal faults.

Rift Border System: The Interplay Between Tectonics and Sedimentation in the Reconcavo Basin, Northeastern Brazil

A geometric and depositional model is proposed to explain the tectonic and sedimentary evolution of the main border of the Reconcavo basin. The architecture of the rift margin is characterized by a rift border system constituted by (1) a master fault, (2) a step, and (3) a clastic wedge. This footwall-derived clastic wedge is interpreted as alluvial fans and fan deltas composed of conglomerates that interfinger with hanging-wall strata. The analysis of the vertical distribution of coarse-grained components of this wedge suggests that its composition is geographically controlled, and no regular inverted stratigraphy is commonly described for this type of succession. During an initial lacustrine phase, turbidites accumulated farther from and parallel to the rift margin. The mapping of marker beds that bound these lacustrine turbidite deposits may be used to infer major periods of clastic influx and, therefore, to correlate with periods of fault-related subsidence or climatic fluctuations in the depositional basin and erosion of the sediment source area. Periods of limited back-faulting and basin expansion toward the main border are distinguished through patterns of progradation and aggradation indicating progressive retreat of the rift border and younging in the footwall direction. The overall evolution of the rift border seems to be related to extension, block rotation, hanging-wall subsidence, and footwall uplift associated with the initial master fault, with limited propagation of faults away from the basin into the footwall.