The anomalous magmatism in the southern part of the Santos basin, and the non-continuous salt layer over Abimael Ridge

Abstract

The magmatism in the Santos Basin is related to the following main events: (1) the pre-rift volcanic episode; (2) the syn-rift to late syn-rift episode; (3) the late Cretaceous episode; and (4) the Paleogene episode. This works focuses on the characterization of these events using seismic data and borehole interpretation in the southern part of the basin. The late Cretaceous magmatic episode is recorded in the shelf break region by 3D seismic data. A mini-basin with possible turbidite reservoirs was tested by an exploratory borehole that revealed several igneous features, including volcaniclastic sequences, sills, and volcanic plugs. Basinward, the evaporite layers are thicker and form salt walls above a detachment at the base of the salt reflector, which climbs up oceanward. The analysis of strong reflectors within the distal salt wall suggested three alternative interpretations, including igneous sills within the salt mass, anhydrite lenses, carbonate rafts, carbonate stringers, pre-salt microbialite buildups, and pre-salt volcanic rocks. The interpretation of the seismic and potential field data, constrained by the borehole results, indicates that the strong reflector corresponds to the top of the volcanic layers below the pre-salt microbialites.The anomalous magmatism and salt tectonics styles in the southern part of the Santos Basin are related to a swath of embryonic oceanic crust that was emplaced during the late Aptian to early Albian. The interpretation of a regional profile extending from the continental shelf to the oceanic crust provides evidence for the separation of the salt masses by a failed oceanic spreading center, which is corroborated by the gravity and magnetic models.