The Palaeo-Bathymetry of Base Aptian Salt Deposition on the Angolan Rifted Margin From Flexural Backstripping and Reverse Thermal Subsidence Modelling

Abstract

The bathymetric datum with respect to global sea-level for Aptian salt deposition in the S. Atlantic is hotly debated. Some models propose that salt was deposited in an isolated ocean basin in which local sea-level was 2–3 km below the global level. In this study we determine the palaeo-bathymetry of base Aptian salt deposition on the Angolan rifted continental margin using 2D reverse post-breakup thermal subsidence modelling. The reverse post-breakup thermal subsidence modelling process consists of sequential flexural isostatic backstripping of the post-breakup sedimentary sequences, decompaction of remaining sedimentary units and reverse modelling of post-breakup lithosphere thermal subsidence. The reverse post-breakup lithosphere thermal subsidence requires 2D knowledge of the rifted continental margin lithosphere beta stretching factor which is determined from gravity inversion. The analysis has been applied to the ION-GXT CS1–2400 deep long-offset seismic reflection profile and the P3 and P7+11 seismic cross-sections of Contrucci et al. (2004) offshore N Angola. Reverse post-breakup subsidence modelling restores the proximal autochthonous base salt to near sea-level at breakup time but not the most distal base salt. In contrast the predicted bathymetries for the base distal salt are much greater ranging between 1.0 and 3.0 km. The predicted bathymetries of the first unequivocal oceanic crust are approximately 2.5 km as expected for newly formed oceanic crust of normal thickness. The preferred interpretation is that all Aptian salt in the Angola margin study areas was deposited at or near global sea-level but that while proximal base salt subsided by thermal subsidence alone, the distal salt experienced both late syn-rift and thermal subsidence. This is consistent with seismic evidence which shows that the base distal salt is extensionally faulted indicating that the crust under the distal salt was being actively thinned during salt deposition, while in contrast the proximal salts were formed in a region where crustal thinning had already taken place and had ceased. An alternative interpretation is that the distal salt moved down-slope during breakup to its present position in much deeper water (and is para-autochthonous). The difference between restored base salt palaeo-bathymetries for proximal and distal salt mitigates against, but does not exclude, a depositional origin in a deep isolated ocean basin.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015