Abstract Only Several wells drilled during the 70s and 80s in shallow waters of Pará-Maranhão offshore basin, Brazilian Equatorial Atlantic Margin, discovered Albian to Cenomanian source rocks, good Cenomanian to Santonian reservoirs, and, most importantly, recovered oil. Petrophysical reassessment of those wells using new techniques and knowledge combined with new 3D seismic acquisition methods and interpretation models guided by new theories on the breakup of continental margins, has shed light and improved understanding of the tectono-stratigraphic evolution of the Pará-Maranhão offshore basin and its petroleum system. The breakup of the Brazilian Equatorial Atlantic Margin at the Pará-Maranhão offshore basin occurred during the late Albian (102 Ma). The immediate post-breakup marine transgression over ancient continental areas coupled with the irregular paleotopography inherited from the rift phase created localized ponds and conditions for the source rock deposition. In the same period, the global geological record registers two anoxic global events, OAE1d (Breistroffer) and OAE2 (Bonarelli). Those two events enhanced the potential for source rock deposition during the early drifting phase in the Pará-Maranhão offshore basin. The new 3D seismic data show features suggesting that after the initial transgressive phase, when the source rock was deposited, recurrent deltaic progradation occurred and delivered sandy deposits on the continental shelf. Seismic imaging (geometry and three-dimensional distribution of the depositional bodies), well data (logs, lithology, biostratigraphy), and paleoecologic interpretation strongly suggest that the sandy bodies deposited on the shelf are turbidities likely fed by the same fluvial system ascribed for the deltaic building. The subsequent overburden of the drift succession pushed the source rocks into the oil window. The close proximity of source rock to the reservoirs, helped by faults active just after the breakup, represent an exceptional condition for migration and focalization of the hydrocarbon into the reservoirs. Some wells drilled in the analyzed area had oil shows and even have produced oil. However, all of them were classified as noncommercial. The posteriori assessment led to two conclusions: (1) the 2D seismic data available at that time precluded good imaging and hence compromised the interpretation of the tectono-sedimentary evolution of the basin and its petroleum system; and (2) the drilling and resource assessment at that time also precluded a conclusive result on the potential of those wells. Nevertheless, new data, techniques, and knowledge available nowadays have enhanced the confidence considering this area as a potential oil province.

Abstract Two gravitational fold and thrust belts (GFTBs) from the deep waters of the Pará-Maranhão and Barreirinhas basins were interpreted in two-dimensional seismic sections and analyzed using the concepts of fault-related folding and taper-wedge mechanics. These basins lie in the equatorial Atlantic Ocean margin of Brazil, a classic example of transform to oblique (transtensional) continental margin. There, deep-water anoxic shales of probably Turonian age served as decollement zones to Late Cretaceous and Paleogene predominantly siliciclastic wedges to slide down from upper slope (extension) to lower slope (contraction) realms. The structural style of both fold belts is typical thin-skinned tectonics with imbricate thrust faults branching upward from the detachment level with associated fault-related folding. The direction of tectonic transport is from the coast to offshore (southwest to northeast). The similarities and differences between the GFTBs were highlighted. The most striking difference regards the distribution of contractional strain throughout the fold belt. In the Pará-Maranhão GFTB, a single major contraction event was achieved via eight regularly spaced imbricate thrust faults. Intervening slices present all types of classic fault-related folding (fault-bend, fault-propagation, and detachment folds). In the Barreirinhas GFTB, an earlier minor contraction was also spread over regularly spaced faults; however, a major late contraction was achieved by one main thrust fault with one major fault-related fold associated to it. This fold is predominantly a shear fault-bend fold whose geometry varies slightly along strike. Their main similarities regard the discrete nature of the detachment level, their structural coherence and narrowness, their low to moderate wedge taper, and their noncritical nature of the taper wedge. Both present syntectonic growth strata that record variations in the balance between the rates of sedimentation and structural uplift of the fold, and folding by limb rotation. The fault-related folding thus determined and the parameters established for wedge taper are slightly different from those presented by active submarine fold and thrust belts at convergent margins and passive margins throughout the world. The decollement dip of the GFTBs is significantly lower, and the bathymetric slope is somewhat higher than elsewhere around the globe.

Abstract The Pará-Maranhão and Barreirinhas basins in the equatorial margin of Brazil contain gravitational gliding systems composed of three structural domains: a proximal extensional, a distal contractional, and a transitional (or translational) domain between the two others. The main faults of these domains detach on a decollement surface of shales and marls, presumably overpressured. Several methods were applied to investigate these thin-skinned tectonics systems, including interpretation of seismic sections, physical modeling, numerical modeling, restoration of cross sections, and integration with field data. These methods indicated that thrusts developed in a classical backstepping sequence with younger thrusts developing in the hanging wall and with landward migration of depocenters through geologic time. Out-of-sequence thrusts were observed locally. The results of cross section restorations suggested that the total amount of shortening exceeded the total amount of stretching in the basal layers, close to the detachment surface, whereas stretching exceeded shortening in the upper layers. Our conclusions point out that gravitational gliding was caused by the combined effect of sedimentary loading, slope gradient, and probably, pore fluid overpressure, with gliding events being triggered by episodic reactivations of the intervening Romanche and Saint Paul fracture zones.

Abstract The central equatorial Brazilian margin is divided into the Amazon and Barreirinhas divergent segments separated by the Pará-Maranhão transform segment. Analysis of regional 2D seismic lines allowed the definition of the crustal architecture of the margin. In the study area, the Barreirinhas segment has a proximal domain with a 30–35 km-thick continental crust, a 20–40 km-wide necked domain where the crust thins to 10 km, and an outboard domain with hyperextended continental crust. The Pará-Maranhão and Amazon segments consist of exhumation domains and their transition to ocean crust. Their structural styles indicate that this is a magma-poor passive margin with oceanic crust formed in a slow spreading centre. The Pará-Maranhão segment is bounded by two branches of the Saint Paul Fracture Zone that displace crustal domains with structures that document the transition from the distal part of a transform margin to an oceanic fracture zone. Two groups of post-rift volcanic complexes have been identified in the exhumation and oceanic domains, and whose distribution is controlled by the fracture zones. Late Cretaceous–Recent gravitationally-driven slide systems and mass-transport deposits indicate long-lived margin collapse and sediment redistribution fundamentally controlled by the underlying crustal structure of this part of the northeastern Brasilian passive margin.