Abstract

The bathymetric datum with respect to global sea level for Aptian salt deposition in the South Atlantic is hotly debated. Some models propose that the salt was deposited in an isolated ocean basin in which local sea level was between 2 and 3 km below the global level. In this study, we use reverse post-break-up subsidence modelling to determine the palaeo-bathymetry of base Aptian salt deposition on the Angolan rifted continental margin. The reverse post-break-up subsidence modelling consists of the sequential flexural isostatic back-stripping of the post-break-up sedimentary sequences, decompaction of remaining sedimentary units and reverse modelling of post-break-up lithosphere thermal subsidence. The reverse modelling of post-break-up lithosphere thermal subsidence is carried out in 2D and requires knowledge of the continental lithosphere stretching factor (β), which is determined from gravity anomaly inversion. The analysis has been applied to the ION-GXT CS1-2400 deep long-offset seismic reflection profile, and two seismic cross-sections (P3 and P7+11) from offshore northern Angola. Reverse post-break-up subsidence modelling restores the proximal autochthonous base salt to between 0.2 and 0.6 km below global sea level at the time of break-up. In contrast, the predicted water-loaded bathymetries of the more distal base salt restored to break-up time are much greater between 2 and 3 km. The predicted bathymetries of the first unequivocal oceanic crust at break-up are approximately 2.5 km, as expected for newly formed oceanic crust of ‘normal’ thickness. Several interpretations of these results are possible. Our preferred interpretation is that all Aptian salt on the northern Angola rifted continental margin was deposited between 0.2 and 0.6 km beneath global sea level, and that the proximal salt subsided by post-rift (post-tectonic) thermal subsidence alone; while the distal salt formed during late syn-rift, when the underlying crust was actively thinning, resulting in additional tectonic subsidence (followed by post-rift thermal subsidence). An alternative interpretation is that the distal salt is para-autochthonous and moved downslope into much deeper water during and just after break-up. We do not believe that a deep isolated ocean basin, with a local sea level 2–3 km beneath that of the global sea level, as has been proposed, is required to explain the Aptian salt deposition on the northern Angolan rifted continental margin.

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