Abstract This work addresses the geological and geophysical interpretation of salt structures in selected Brazilian sedimentary basins, from intracratonic Palaeozoic evaporites in the Amazon and Solimões basins to divergent margin evaporite basins formed during the Mesozoic break-up of Gondwana. There is an intriguing correlation between evaporite basins and hydrocarbon accumulations in all the selected basins discussed. The Solimões and Amazonas basins developed evaporite depositing environments as the Middle Carboniferous sea was closing during a plate convergence phase. The salt basin along the eastern Brazilian and western African margins developed along the Mesozoic rifts of the South Atlantic. Regional seismic interpretation and potential field (gravity and magnetic) data over the eastern Brazilian and west African margins suggest a very thick autochthonous salt layer deposited over rifted continental crust and particularly above the thick sag basin sediments over the hyperextended crust that marks the transition from continental to oceanic crust. Most of the hydrocarbon discoveries in the eastern Brazilian and western African margins are in post-salt turbidite and carbonate reservoirs, but recent discoveries in the deepwater salt basins along the southeastern Brazilian margin indicate that pre-salt plays will represent an important contribution to hydrocarbon production in the near future.

Abstract Salt deposited over rift sequences along passive margins flows basinward in response to thermal subsidence and non-uniform sediment loading. The flow results in extensional tectonics in the salt and the overlying sediments where the space is not constrained and in compressional tectonics where it is constrained. Salt waves form as compression propagates inward from the borders of the laterally contracting body. Upslope flow of the salt is particularly effective in creating folds having regular wavelength because of the increasing resistance. Improved seismic resolution permits recognizing the complex internal structure of the folds, whereas isolated 2D sections may be misleading. The resulting salt waves easily can be mistaken for diapirs in the conventional sense; their flanks may appear discordant when, in fact, they are not. Alternating materials of different rheological properties greatly enhance the formation of uniformly spaced waves or folds. Apart from the dominant halite, the alternating material may consist of mostly mono- or polymineralic evaporite rocks such as anhydrite, carnallite, bischoffite, or tachyhydrite, as well as alternating shale and sandstone/siltstone layers interlayered with, or overlying, the evaporites. Syndeformational sedimentation over the evaporites results in the formation of minibasins. The layers overlying the evaporites participate in the folding, with upward decreasing amplitude, so that definition of the upper boundary of the evaporite sequence on seismic sections is not always straightforward. Seismic lines from Brazil’s Atlantic margin are used to illustrate these points.

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Abstract On a predrift reconstruction of their facing margins, almost all of Brazil's and southwest Africa's petroleum reserves occur within a nearly continuous north-south-trending strip, the South Atlantic Megatrend, whose formation was controlled by continental breakup. It took almost 40 m.y, from earliest Valanginian to late Albian time, for Africa and South America to separate, as South America rotated clockwise relative to Africa about a pole located in northeast Brazil. Rifting mechanism and continental deformation were controlled by the distance from the shifting pole of differential rotation. The rift was cut into two approximately equal portions by the Ponta Grossa dike swarm and associated volcanics that formed over the Paraná–Tristan da Cunha hot spot. South of the dike swarm continental, breakup was fast and characterized by intense volcanic activity (seaward dipping reflectors). In this area, a euxinic sea formed by Aptian time over the oceanic crust. To the north of the dike swarm, continental breakup was slower, and a northward-tapering rift valley lying deep below sea level formed over extended continental crust. This depression was flooded catastrophically by sea water in late Aptian time, depositing a 2000-m-thick salt body. The oil fields constituting the South Atlantic Megatrend occur either within the salt basin, both above and below the salt, or near the pole of differential rotation in transtensional inland rifts that contain no salt. The opening of the Atlantic divided this megatrend into segments that now alternate between the Brazilian and African margins: an oil-producing segment on one margin is faced by a relatively sterile segment on the other. This distribution may result from alternating polarity of rifting by simple shear. Oil richness, defined here as the volume of original reserves per unit length of a segment, increases along the megatrend with distance from the pole of relative rotation. Far from the pole, as in the Lower Congo and especially in the Campos Basin where the rift is wide, the rifted continental crust extends far offshore, oil richness is high, and the largest petroleum accumulations occur offshore.

Abstract Thin-skinned gravitational gliding of sediments above a detachment layer of salt or shale is common on passive margins. Changes in surface slope result in a domain of extension upslope and a domain of contraction downslope. Contractional domains tend to occur under present-day deep water and are thus not well understood. In the deep-water Santos Basin, Brazil, a contractional domain contains a suite of salt-cored structures. Angular folds (chevron and box folds), as well as concentric folds, are common in the upper part of the Aptian evaporite sequence, which appears to comprise alternating layers. In general, angular and concentric folds form by flexural slip during shortening of mechanically layered sequences. Their occurrence in the Santos Basin is evidence in favor of horizontal contraction. The lower part of the Aptian evaporite sequence appears to be mostly rock salt. It has been squeezed out from under synclines into spaces created by growing anticlines. In places, the layered evaporite sequence has been thickened or even repeated across thrust faults and ramp anticlines. An overlying sequence of open-marine sediments has been condensed or eroded over anticlines but forms local depocenters. These depocenters are asymmetric (of foreland style) next to isolated thrusts but symmetric in synclines or between thrusts of opposite vergence. The structural styles have been reproduced in physical models, properly scaled for gravitational forces, in which salt is represented by silicone putty and sediments are represented by sand. The models were shortened horizontally by a screw jack. The experiments illustrate the importance of horizontal contraction and syntectonic sedimentation in shaping salt-cored structures. They have been used to establish criteria that may be diagnostic of construction.