--> Temporal and Spatial Evolution of Rifting in the Brazilian Southeastern Passive Margin

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Temporal and Spatial Evolution of Rifting in the Brazilian Southeastern Passive Margin

Abstract

In the dawn of offshore petroleum exploration geologists realized that passive margins had a dual tectono-stratigraphic evolution: a deep rifted package (Pre-Rift and Syn-Rift Sequences) covered by sediments displaying sag geometry (Drift Sequence). The angular unconformity separating them was considered synchronous to the breakup of the continent. In the last two decades scientists understood that rifting is a phased process that evolves through different mechanisms, in different times and positions in the margin. Rifting tends to shift in time from the proximal to distal margin, focusing towards the final locus of breakup. Post-rift unconformities throughout the margin are not synchronous. Post-rift strata may be coeval with syn-rift strata in more distal positions. Only an entire vision of the margin out to the oceanic crust allows the correct identification of the age and the stratigraphic level of the breakup unconformity. Detailed mapping of the rifted sequences in the petroleum provinces of the Southeastern Margin of Brazil (Santos, Campos and Espírito Santo Basins) confirmed this overall trend; but, also pointed out a much more complex history. Rifting started concentrated in the present proximal margin in the Hauterivian-Barremian. During Barremian-Early Aptian rifting spread practically all over the basins, out to the present COB. The exception was a central axial area where fault-controlled sedimentation ceased and localized thermal sag deposition started. In this area, the post-rift unconformity is the oldest. During this rift climax, continental crust broke up and lithospheric mantle exhumed in a continuous belt surrounding the 3 basins; suffering significant normal faulting. By Mid- to Late Aptian rifting practically came to a halt and the region started to develop a larger continuous sag basin. Few spots display fault-controlled sedimentation during this time. In the Lattermost Aptian-Earlymost Albian rifting re-started with great intensity but only in the distal margin, atop hyper-extending crust. The proximal margin continued to down warp under thermal subsidence. As salt was deposited, the proximal half of it was laid down in sub-horizontal giant sag lakes or gulfs, while the distal half was deposited in actively subsiding grabens. Lithosphere breakup occurred right after salt deposition (111 Ma). The real breakup unconformity is nowadays hidden in the contorted beds immediately overlying the salt bodies developed by halokinesis.