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The Gulf of Mexico and South Atlantic Conjugate-Margin Salt Basins: How Similar Are They?


The Gulf of Mexico (GoM) and South Atlantic contain salt basins on conjugate margins: the US and Mexico sides in the GoM; and the Brazil and Angola-Gabon sides in the South Atlantic. Presalt hydrocarbons have been discovered in the South Atlantic but not yet tested in the GoM. Here we compare the crustal architecture of the two sets of conjugate margins using SuperCache data (very long-offset, depth-migrated 2-D seismic) in the northern GoM and published data in other areas in order to highlight the presalt potential in the northern and southern GoM. There are striking similarities in crustal architecture between the GoM and South Atlantic. The base salt (or equivalent unconformity) and Moho define wedges of continental crust that thin from ∼30 km in the proximal domain to >10 km and maybe even >5 km in the distal domain. In the northern GoM, the distal continental crust is hyperextended on faults that dip 7-15 degrees basinward; such faults are more difficult to image in the South Atlantic. A more distal domain of exhumed mantle has been interpreted in the South Atlantic; in the northern GoM, the Moho interpreted from both the seismic data and gravity inversion are compatible with a similar domain. Whether exhumed mantle is present or not, there is a basinward stepup onto normal oceanic crust on each margin. Although the presence of adequate presalt source rocks and reservoirs in the GoM is unknown, the patterns of sedimentary fill during extension are also remarkably similar to those in the South Atlantic. Wedges of synrift strata in proximal areas record the earliest phases of rifting. Synrift wedges are known to young basinward in the Angolan basin, but any similar timing in the GoM is unknown. More distal provinces in Brazil and Angola-Gabon have the well known ‘sag’ sequence above synrift growth strata, with the evaporites representing the upper portion of this sequence. Thus, the base salt is largely unfaulted. The same patterns are seen in the GoM: in the northeast and northwest, a thin unfaulted sequence is observed beneath the salt; in Mexican waters, the sag sequence is up to 5 km thick. It comprises basinward-shifting, unfaulted depocenters, with the youngest (salt) mostly isolated on the US side after oceanic spreading. The evaporites thin and onlap laterally onto SDRs (seaward-dipping reflectors probably representing volcanic highlands) at the southern edges of the South Atlantic salt basins and at the eastern edges of the GoM.