Central and South Atlantic Conjugate Margins Pre- and Post-Salt Successions — Recognition, Definition and Implications to Rift Models and Petroleum Systems

David E. Brown¹, Webster U. Mohriak², Haddou Jabour³, and Gabor C. Tari^4
1Canada-Nova Scotia Offshore Petroleum Board, Halifax, NS, Canada
²Petrobras, Rio de Janeiro, Brazil
³OHNYM, Rabat, Morocco
⁴OMV, Vienna, Austria

Basins along the Nova Scotian-Moroccan (non-volcanic/transitional) and Brazilian-West Africa (volcanic) conjugate margins reveal stratigraphic sequences beneath late synrift salt lying above the unconformities defining rift onset, and below the breakup unconformity. They are proximal to basin margin hinge lines in shallow water; in deep water are at the distal basin margins out into the abyssal plain. Recent deep crustal and regional seismic lines have improved resolution of these pre-salt features as well as post-salt troughs controlled by basement-involved faults. In shallow water, they are seen as isolated salt evacuation synclines and half-grabens containing fluvial and playa redbed sediments. Unusual deep water structures, seen as highly rotated fault blocks with growth geometries, have alternative interpretations of continental or oceanic, synrift or post-rift features. Once interpreted as structural relief on magnetically-quiet oceanic crust, internal geometries and presence below the assumed breakup unconformity suggest the basement may not be oceanic. Some evidence supports the presence of attenuated, serpentinized mantle in these areas. The thick pre-salt sedimentary sequences are probably related to rift shoulder uplift at the continental border and future rift spreading axis just prior to rifting. Rapid sedimentation was directed towards the main salt depocentres on both the South and Central Atlantic conjugate margins. Sediments underlying salt in deep waters are not rotated by synrift faults. Their depocenter lies above a deep seismic reflector which may correspond to the Moho or to lower crust detachments. Recognition of these features, identification of basement type, definition and age of breakup events and their interpreted temporal and tectonic association, has implications to different styles of rifting, salt tectonics and basin petroleum systems.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery