W.U. Mohriak

South Atlantic divergent margin evolution: rift-border uplift and salt tectonics in the basins of SE Brazil

Abstract The South Atlantic Ocean evolved after rupture of the São Francisco–Congo–Rio de la Plata–Kalahari cratonic landmass and the Late Proterozoic fold belts. Break-up in the South Atlantic realm developed diachronously: rifting started in the south (Argentina) during the Jurassic and progressed towards the equatorial segment. The central portion was controlled by a rift-resistant cratonic nucleus (the São Francisco–Congo craton) and as a result underwent development of narrow basins; parts controlled by Neoproterozoic fold belts developed wide basins. The final break-up of western Gondwana and the onset of plate divergence were marked by thick wedges of seaward-dipping reflectors, located near the incipient ocean-ridge spreading centre that had already been formed by the time Aptian evaporites were deposited. Subsequently, a few episodes of intraplate tectonic and magmatic activity affected the Santos, Campos and Espírito Santo basins. Post-break up development of the offshore basins was affected by gravity gliding over the Aptian evaporites. Continental uplift may be invoked as the main cause of salt mobilization, generating prograding clastic wedges that thickened basin-wards and produced a loading effect on the salt basin. Coupled with onshore erosional unloading and the effects of the gravity gliding, this probably resulted in further flexural uplift of the continental margin.

Jaibaras trough: an important element in the early tectonic evolution of the Parnaíba interior sag basin, Northern Brazil

Abstract The Jaibaras Trough (JT) is a late Precambrian to Early Paleozoic basin that is related to the subsequent formation of the Paleozoic–Mesozoic Parnaiba intracratonic basin in northern Brazil. Based on available geological and geophysical data, we propose that the JT is a prototypical rift basin formed by brittle reactivation processes along ancient shear zones associated with the NE–SW Transbrasiliano Lineament of the Brazilian–Panafrican Orogeny. The JT evolution is partially constrained by the stratigraphic sequences outcropping at the border of the basin or drilled by exploratory boreholes. The preserved sedimentary column records, in a broad sense, the sedimentary and tectonic controls on the early subsidence in the Parnaiba Basin. These sequences are constituted by basal fault-scarp-related paraconglomerates followed laterally and vertically by fluvial–lacustrine sandstones interbedded with reddish shale. The JT geological evolution is also constrained by petrology of four magmatic events (Coreau Dike Swarm, Mucambo and Meruoca plutons and mafic Parapui Suite) related to continental rifting. The Late Precambrian–Early Paleozoic age of JT is perfectly compatible with continental fragmentation during the processes of lithospheric stretching that affected western Gondwana at that time. The contemporaneous development of a magmatic-sedimentary system, dated as Cambrian to Ordovician and confined to the rift trend, raised the thermal gradient and produced a mantle softening discontinuity, giving rise to a broad uplifted region. This might explain the close temporal and spatial association between the Transbrasiliano Lineament and the Early Paleozoic depositional axis of the Parnaiba Basin. An integrated interpretation of the JT and the early tectonostratigraphic behaviour of the Parnaiba Basin indicates that the JT might constitute a precursory stage that was slightly inverted before the initiation of sag deposition in the intracratonic basin.

Petroleum geology of the Campos Basin, offshore Brazil

Abstract The Campos Basin, offshore Brazil, is the most prolific basin in the western South Atlantic, with more than thirty hydrocarbon accumulations currently accounting for about 60% of Brazilian oil production. Intensive drilling and seismic, gravity and magnetic data have contributed to the recognition of four tectono-stratigraphic units related to the rifting and break-up of Pangea. The lowest sequence consists of Neocomian clastics deposited on basalt dated at 120–130 Ma, and reflects the fault-controlled subsidence associated with the stretching that preceded the emplacement of oceanic crust. The Aptian proto-oceanic stage is characterized by a sequence of evaporitic rocks that have undergone intense diapiric activity in deep water. An open-marine environment begins with a thick sequence of Albian/Cenomanian limestones, locally with clastic input, which grades upwards and basinwards into deep water marls and shales. This section is structurally associated with detached listric normal faults that sole out on the Aptian evaporites. Finally, the marine Upper Cretaceous to Recent clastic section is characterized by a more quiescent phase of thermal subsidence, with some residual halokinetic activity that increases in intensity towards deeper waters. The hydrocarbon accumulations are distributed throughout the stratigraphic column of the basin from Neocomian to Miocene. The reservoirs range from fractured basalts and porous bioclastic limestone (coquinas) in the Lagoa Feia Formation, to limestones and sandstones in the Macaé Formation, and sandstones in the Campos Formation. Detailed geochemical analyses undertaken on cutting, core and oil samples show that almost all the hydrocarbon accumulations discovered to date are sourced mainly from lacustrine calcareous black shales deposited in a closed Upper Neocomian lake system, having saline to hypersaline waters of alkaline affinities. The extreme anoxic conditions in this lacustrine environment resulted in the deposition of fine, well laminated organic-rich (TOC up to 9%) calcareous black shales, with high-quality organic matter composed almost entirely of low-sulphur type-I kerogen, originating from lipid-rich algal and bacterially-derived material. The excellent hydrocarbon source potential of these sediments, combined with the appropriate thermal history, produced the necessary conditions to yield low-density oil (around 30° API) characterized by a low sulphur content (around 0.30%), and significant quantities of alkanes (up to 70%). Diagnostic features in the biological markers from this depositional environment include: low concentration of steranes, presence of β-carotane, gammacerane and 28,30-bisnorhopane, very high concentrations of hopanes and high relative abundances of tricyclic terpanes up to C34.

Basin-forming processes and the deep structure of the Campos basin, offshore Brazil

Abstract This paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins. The events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors: shear zones, thin layers of intrusive rocks, underplated material, gneissic banding, fluids and heterogeneities in the crust and mantle. Several types of sedimentary basins have been recognized with the aid of deep seismic reflection data: basins formed along active master faults; basins formed along inactive faults; basins with no fault control; basins formed by low angle detachment faults; and basins formed by pervasive pure shear, or an approximately pure shear, where the lower crust has been locally extended by a different amount than the upper crust. The mechanisms of basin forming processes recognized in deep seismic profiles are: (a) simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle; (b) thinning of the lower crust and mantle by ductile processes, convective mechanisms or flow in the mantle and the upper crust by faulting, resulting in an overall pure shear; and (c) thinning of the lower crust associated with phase changes in the mantle or non-conservation of mass. Two end-member models of basin formation involving lithospheric stretching may be recognized in the South Atlantic: simple shear stretching, where only the upper crust is involved in the process and mantle uplift is offset relative to the basin; and pure shear stretching, where uplift of the mantle vertically balances the basin fill. The first mechanism has been suggested for the onshore Tucano Basin. The data obtained for the offshore Campos Basin suggests regional lithospheric stretching and crustal thinning with Moho uplift compensating for sediment accumulation in the basin depocentre. Complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the Campos basin.

Deep seismic reflection profiling of sedimentary basins offshore Brazil: Geological objectives and preliminary results in the Sergipe Basin

Abstract The first deep seismic reflection profiles offshore Brazil were acquired in Campos Basin and processed to 10 s TWT in 1984. Starting in 1989, Petrobras acquired an extensive data set of deep seismic profiles using special acquisition equipment capable of effectively penetrating through the sedimentary layers and imaging the whole crustal architecture. These deep (18 s TWT) seismic reflection profiles extend across the Atlantic-type marginal basins, from the platform to the deepwater province, presently considered frontier regions for petroleum exploration. This work addresses the geological objectives of a deep seismic profile in the Sergipe Basin and discusses the results obtained by integrating regional seismic, gravity and magnetic data. When combined, these data provide evidence that deep seismic reflectors observed in the Sergipe Basin are related to intracrustal-upper mantle structures rather than sedimentary features. The deep seismic reflection profile in the Sergipe Basin also suggests that, rather than a non-volcanic passive margin, the deepwater extension of this basin is marked by several magmatic structures, including thick wedges of seaward-dipping reflectors and volcanic plugs. These magmatic features are associated with basinforming processes resulting from lithospheric extension during the breakup of Gondwana in the Early Cretaceous and subsequent emplacement of oceanic crust. These results are compared to the crustal scale structures observed in the Campos Basin, in the southeastern margin of Brazil. The interpretation of the deep structure of these basins indicates that final separation between the South American and African plates formed passive margins characterized by different patterns of crustal attenuation underlying the rift blocks.

Crustal architecture of South Atlantic volcanic margins

Integration of seismic, potential field, and borehole data from the conjugate margins of eastern Brazil and West Africa indicates that the rift architecture varied along strike, and that volcanic episodes (or lack thereof) may substantially affect petroleum exploration of the deep-water provinces. In this chapter we discuss various pairs of conjugate sedimentary basins, from Pelotas and Namibia in the south to the SergipeAlagoas and Rio Muni counterparts in the north. The following aspects are emphasized: (1) rift depocenters are controlled by border faults subparallel to the margin and by transverse faults that may continue as transform fractures in the oceanic crust; (2) in many basins along the southernmost segment of the South Atlantic, Early Cretaceous volcanics underlie continental lacustrine synrift sediments of Neocomian age; (3) in the northern segment of the salt basin, prerift sediments with no volcanic material underlie the synrift sediments; (4) in some segments of the margin, the transition from outer rift blocks to oceanic crust is characterized by wedges of seaward-dipping reflectors with a possible origin associated with the initial phases of oceanic crust emplacement; (5) locally, the outermost rift blocks seem to be highly eroded by postrift uplift caused by shearing or by magmatic underplating; (6) subsequent to the rift sequence, a quiescent period marked by a sag basin above a regional unconformity predated the deposition of Aptian evaporites; (7) the South Atlantic salt basin along both margins was controlled by tectonic and volcanic elements, and locally, salt was deposited directly on the volcanic substratum; (8) volcanic ridges formed before and after salt deposition, and separated portions of the rift and salt basins before the final breakup installed a divergent regime with pure oceanic crust; (9) basement-involved extensional faults, volcanic activity, and enhanced continental margin uplift and denudation are indicative of reactivation of rift-phase faults after salt deposition in some segments of the margin; and (10) tectono-magmatic episodes climaxed in the later Cretaceous-early Tertiary, forming large volcanic complexes along the conjugate margins. Three main episodes of magmatic activity are observed in the South Atlantic salt basins: the Late Jurassic-Early Cretaceous event is related to the Parana-Etendeka flood basalts and the volcanic rocks that occur in the offshore basins; it is followed by thick wedges of volcanic rocks interpreted as seaward-dipping reflectors; and the Late Cretaceous-early Tertiary event is related to hotspot and leaking fracture zones. Fitting together of deep-imaging, multichannel seismic profiles at approximate conjugate margin positions south of the Maceio-Ascension-Kribi Fracture Zone system shows a high degree of asymmetry for the crustal architecture and the rift sequences, suggestive of a simple shear mechanism of lithospheric stretching. Although there is no evidence for synrift volcanism at this segment of the margin, the development of seaward-dipping reflector packages observed in the Brazilian side suggests that magmatic activity in this region may also be asymmetric.

Salt tectonics in the western Gulf of Cadiz, southwest Iberia

This study presents the results from the interpretation of an extensive and recent regional two-dimensional seismic survey focused on the understanding of the salt tectonics in the western Gulf of Cadiz (GoC). Two different salt units were identified: an autochthonous salt unit of the Late Triassic or the Early Jurassic (Hettangian) and an allochthonous unit that originated from the Hettangian salt. Interpretation of the pattern of distribution of the salt in the basin allowed subdivision of the area of study into three distinct salt domains: the eastern domain characterized by the presence of a conspicuous allochthonous salt nappe (Esperanca Salt), the central domain dominated by salt diapirs with mild deformation of Miocene strata and wide salt-withdrawal minibasins, and the southwestern domain where present-day tectonics induces impressive salt deformation affecting the sea floor. This complex pattern is mainly the result of the interaction of inherited basement structure, complex tectonic history, and stress regime of the basin. The intense halokinesis observed has created several salt-related trap geometries and fluid migration pathways. As the focus of worldwide exploration along passive margins is gradually shifting to deep-water regions, the western GoC has the potential to become a deep-water petroleum province in the near future.

Crustal architecture and tectonic evolution of the Sergipe-Alagoas and Jacuípe basins, offshore northeastern Brazil

Abstract The Sergipe-Alagoas and the Jacuipe basins are divergent margin basins located in the South Atlantic Ocean, at the northeastern Brazilian margin. High-quality 18 s (two-way travel time) seismic reflection profiles, which extend from the coastline towards the boundary with the oceanic crust, were integrated with potential field data and calibrated with the results of several exploratory boreholes. When combined, these data provide evidence that the deep seismic reflectors in the deep-water region are related to the crustal architecture of the basin, rather than to sedimentary features. Arrays of high-impedance reflectors, previously interpreted as top of basement, probably correspond to detachment planes near the base of the crust, or to underplated igneous rocks above the Moho. The extensional processes that were active during rifting resulted in normal crustal faults that apparently detach near the seismic Moho in the deep-water region. These crustal faults control and rotate syn-rift sedimentary units along the margin. The seaward portion of the rift seems to have been uplifted and highly eroded during a post-break-up tectonic event. The deep seismic reflection profiles also indicate that the deep-water province is characterized by wedges of reflectors that dip seawards. These features probably correspond to seaward-dipping reflectors (SDR), which are composed of volcanic rocks marking the transition to oceanic crust. The SDR wedges are locally associated with volcanic intrusions. Several volcanic plugs in deep-water region are aligned with oceanic fracture zones that apparently penetrate through the whole crust and reach the upper mantle. On the other hand, there are some diapiric structures located near the boundary between continental and oceanic crust that bear evidence for deep-water salt tectonics.

Transform zones in the South Atlantic rifted continental margins

Abstract Integration of seismic, potential field, and borehole data from conjugate basins along the South Atlantic continental margin, particularly the northeastern Brazilian and northwestern African segments, indicates that the rift architecture is controlled by fracture zones that extend from the oceanic crust and penetrate through the continental crust, locally corresponding to Precambrian structures in cratonic regions. The fracture zones may divide the continental margin into several compartments with independent sedimentary depocentres, separate crustal domains along oceanic transforms, and affect the rift architecture by shearing. Oceanic transform zones may leak igneous rocks originated from the mantle. This work discusses conjugate sedimentary basins in the South Atlantic salt basins, particularly from Jacuípe to Sergipe-Alagoas on the Brazilian side, and from Gabon to Rio Muni on the African side. The following aspects are emphasized: (1) rift depocentres are controlled by border faults subparallel to the margin and by transverse faults that may continue as transform fractures in the oceanic crust; (2) the southernmost segment of the South Atlantic continental margins is characterized by Early Cretaceous volcanic rocks that underlie continental lacustrine Neocomian to Barremian syn-rift sediments; (3) the pre-rift sequences (Mesozoic and Palaeozoic sediments) that underlie the syn-rift depocentres in Gabon and Sergipe/Alagoas are mainly devoid of volcanics; (4) there is seismic evidence of magmatic underplating in the deeper portions of the continental crust, which are expressed by antiformal features locally aligned with transform fractures; (5) basement-involved extensional faults and volcanic activity along leaking transform faults are imaged along several conjugate segments of the margin, particularly along the equatorial margin (Romanche fracture zone); (6) in some segments of the divergent margin, the transition from outer rift blocks to oceanic crust is characterized by wedges of seaward-dipping reflectors with a possible origin associated with emplacement of oceanic ridges; (7) locally, the outermost rift blocks near the continental-oceanic crust boundary seem to be highly eroded by post-rift uplift caused by transform fault shearing or by magmatic underplating; (8) tectonomagmatic episodes climaxed in the Late Cretaceous/Early Tertiary in northeastern Brazil and extended to the Late Tertiary on the West African margin, forming large volcanic complexes along transverse lineaments that affect both oceanic and continental crust.

Phanerozoic regional geology of the eastern Brazilian margin

The South American divergent continental margin extends from East Brazil towards the continental margin off Argentina. It is limited to the north by the Atlantic equatorial segment marked by transcurrent movements associated with oceanic fracture zones, and towards the south, by the fracture zones in the Malvinas–Falklands Plateau and the subduction zone north of Antarctica. Plate tectonics concepts relate the diverse tectono-magmatic phases identified in the South Atlantic continental margins to Mesozoic–Cenozoic tectonic reactivation following the breakup of Gondwana. In the extensional segment of the eastern Brazilian margin, the region encompassing the Espirito Santo, Campos and Santos Basins correspond to a major petroleum province.