Structural Modeling Based on Sequential Restoration of Gravitational Salt Deformation in the Santos Basin (Brazil)

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Marine and Petroleum Geology xxx (2012) 1e17

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Structural modeling based on sequential restoration of gravitational salt deformation in the Santos Basin (Brazil)

Sávio Francis de Melo Garcia a,*, Jean Letouzey b, Jean-Luc Rudkiewicz b, André Danderfer Filho c, Dominique Frizon de Lamotte d a Petrobras E&P-EXP, Rio de Janeiro, Brazil b IFP Energies Nouvelles, France c Universidade Federal de Ouro Preto, Ouro Preto/MG, Brazil d Université de Cergy-Pontoise, France article info abstract

Article history: The structural restoration of two parallel cross-sections in the central portion of the Santos Basin enables Received 8 December 2010 a first understanding of existent 3D geological complexities. Santos Basin is one of the most proliferous Received in revised form basins along the South Atlantic Brazilian margin. Due to the halokinesis, geological structures present 22 November 2011 significant horizontal tectonic transport. The two geological cross-sections extend from the continental shelf Accepted 2 February 2012 to deep waters, in areas where salt tectonics is simple enough to be solved by 2D restoration. Such cross- Available online xxx sections display both extensional and compressional deformation. Paleobathymetry, isostatic regional compensation, salt volume control and overall aspects related to structural style were used to constrain basic Keywords: fl Salt tectonics boundary conditions. Several restoration algorithms, such as simple shear, exural slip and free methods, section restoration were used to restore the sedimentary deformation, including salt gravity gliding. The results of the 2D Santos Basin restoration are consistent with five major sequences of sedimentary evolution: (1) the brittle pre-salt passive margins deformation, (2) the significant and fast salt deposition, (3) the initial post-salt deformation with predom- South Atlantic inant rafting tectonics, (4) the Late Cretaceous progradational deposition and coeval development of deep water compressional minibasins, and (5) the Cenozoic sedimentary deposition, with less intense salt tectonics. A 1D subsidence analysis based on the 2D restored results is shown as a useful restoration control tool. The 1D results indicate that an initially proximal infill evolves towards distal regions under salt tectonics control. The 1D diagrams also record the history of the overburden movements through lateral depocenter migration in minibasins areas, submitted to large horizontal salt spreading. The results highlight an important isostatic movement during salt deposition, large but not enough to eliminate a needed depression to accommodate the thick evaporites. By quantifying the halokinetic lateral deformation through time, the results suggest less intensity of the phenomenon throughout the Paleogene, with minor impacts on the petroleum system in this period. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction restoration methods. Numerical tools are needed to control both decompaction and flexural isostasy in cases in which there are major Salt tectonics is one of the most complex deformation processes changes in properties, such as density and compressibility, between operating in sedimentary basins. Most current methods of sequential evaporites and other sediments. restorations imposed to geological events, however, are based on The study area in the deep water of the Santos Basin is intensely simplified geometrical approaches. Several poorly controlled deformed by salt tectonics. This passive margin basin is situated simplifications are generally required to simulate such mode of offshore Southeastern Brazil, the most prolific petroleum province of deformation, and are not really integrated with geological processes the South Atlantic (Fig.1). The deep offshore region, close to the study and properties, limiting applicability and effectiveness of the area, constitutes a new frontier for petroleum exploration. The basin was developed upon stretched and thinned continental crust of a ruptured Gondwanian margin (Zalán et. al., 2011). The wide * fl Corresponding author. Present address: Petrobras E&P-EXP,Av. Chile, 330,13th oor, expression of the Santos Basin bathymetry gives rise to the São Paulo Rio de Janeiro, ZIP 20031-170, Brazil. Tel.: þ55 21 2144 0613; fax: þ55 21 2144 1818. E-mail addresses: [email protected], [email protected] Plateau, which was developed over a thick salt layer, a fundamental (S.F.deM. Garcia). element of the basin evolution.

In this work, a comprehensive restoration technique integrating 1D burial geohistories, salt amount monitoring and regional setting constraints as supplementary calibration tools is applied in order to minimize the strong impact of simpliﬁcations on basin evolution results. Paleobathymetry models, ﬂexural isostasy and the monitoring of the variation of thicknesses within a consistent structural style are used to guide and constrain all restoration steps. There- fore, the structural modeling character of this work is not just a technique application but also a geological methodology aimed to integrate concepts and data.

2. Geological setting

The Santos Basin constitutes a good example of a passive margin basin, ﬁlled by sediments deposited during rift and drift phases. It results from an asymmetrical partition of the Gondwanian “super- continent” whereby the Brazilian rifted margin remained wider than the conjugate African margin. The Santos Basin spreads over 3.52 Figure 1. Regional location map outlining the study area (red polygon) including two x105 km2 down to 3000 m water depth. The current knowledge of its restored cross-sections (black lines), six available wells (yellow circles) and four architecture comes from more than one hundred wells and studied pseudo-wells (red circles). Contours represent water depth in meters, rein- forced by the blue shading sea ﬂoor topography. Notice the aborted spreading center in numerous 2D and 3D seismic surveys. It is bounded to the Northeast the southern region, and its northward propagation along a pre-salt fault trend. Its by the Cabo Frio High and to the Southwest by the Florianópolis possible continuity within the study area could account for a local crustal thinning Platform (Fig. 1). The studied area covers around 6000 km2. anomaly. 2.1. Stratigraphy

Several authors, such as Guerra (2008), Rouby et al. (1993) and The stratigraphic framework of the Santos Basin presented Szatmari and Demercian (1993), discussed the evolution of the herein (Fig. 2) is in agreement with the general proposal of Moreira Santos Basin using restoration methods. Cobbold et al. (2001), et al. (2007). The maximum sedimentary thickness of the Santos Meisling et al. (2001) and other authors have also interpreted the Basin is approximately 12 km (Pereira and Macedo, 1990). The complex thin-skinned deformation above the Aptian salt in this crystalline basement outcropping onshore is characterized by basin using structural analyses and regional interpretation. More granites and gneisses of Precambrian age (Neoproterozoic Ribeira recently, a new framework including a failed sea ﬂoor spreading Belt). The Guaratiba Group represents the carbonate, siliciclastic center in the southern limit of the basin as well as an outer high and evaporitic sediments of the Camboriu, Piçarras, Itapema, Barra located on the São Paulo Plateau have been described in several Velha and Ariri Formations, deposited before the drift phase, during papers and presentations (Braga et al., 2003, Carminatti et al., 2008, periods of major and minor fault activations (rift phase). The Scotchman et al., 2006), highlighting the diverse nature of the signiﬁcant basaltic volcanism of the Camboriu Formation that underlying basement. underlays these formations and unconformably covers the pre-

Figure 2. Simplified stratigraphic chart of the Santos Basin outlining the fiveteen horizons interpreted in the seismic profiles during the study. The major Early Cretaceous progradation event is highlighted between the horizons 04 and 09.

Cambrian basement is regarded as the economic basement for basin regions. Sand-rich gravity flows are described in distal petroleum exploration. These Neocomian basalts were followed by structural lows controled by salt tectonics. This sedimentary continental sedimentation of the Piçarras and Itapema Formations, package represents the onset of a retrogradational pattern that in half-grabens formed by faulting and block rotation during the accounts for the largest marine transgression in the Santos Basin initial rift phase (Barremian). and which culminated in the Turonian oceanic anoxic event (OAE-2 An unconformity separates the Barremian sequences from the event, Arai, 1988). overlying Barra Velha Formation, which was deposited during the Most of the Late Cretaceous siliciclastic progradational sequences Aptian in a relatively less active fault system. This Aptian package is were deposited in continental paleoenvironments, reflecting a shift composed of carbonates and shales, typical of a transitional envi- of the continental shelf limits towards the offshore. After a strong ronment, from continental to shallow marine. The Late Aptian salt erosional episode around the Cretaceous-Cenozoic limit, a basin- sequence, recorded by the Ariri Formation, is about 2500 m-thick on wide regressive episode gradually shifted the coastal line about two average. Such salt formation was unconformably deposited above hundred kilometers eastward, allowing the development of a wide Late Aptian syn-rift limestones, in a very short period of time, during prograding depositional wedge. The Cenozoic sedimentary the transition from continental to oceanic conditions. The salt layer sequences of the Itamambuca Group include sediments deposited extends across the West African and Brazilian continental margins from proximal alluvial fans (Ponta Aguda Formation) to distal (Karner and Gamboa, 2007), and was deposited under high sedi- marine shales and sandstones (Marambaia Formation), with mentation rates, in the order of 1 km over a period of 0.5 Ma occurrences of carbonate platforms (Iguape Formation), overlain by (Dias, 1998). the most recent shelf sedimentation (Sepetiba Formation). The There are three major sequences overlying the Guaratiba Group, greatest geographic distribution of the Ponta Aguda Formation is composing the drift phase: the Camburi, Frade and Itamambuca recorded from the Early to Middle Paleogene, reaching a maximum groups (Fig. 2). These groups comprise Albian to Cenomanian thickness of 2200 m. The Iguape Formation is recorded from sequences, an intervening Late Cretaceous progradational episode, Oligocene to present day, reaching a maximum thickness of 2700 m. and the Cenozoic sequence, respectively. The post-salt sequences The pelites of the Marambaia Group are interlayered with sandy were deformed by gravity-driven tectonics with variable displace- bodies of the Maresia Member which are considered as meandering ments along the basin (Assine et al., 2008), mainly due to heteroge- channels or fans in less confined batial areas. Finally, the coarse- to neities of the salt thickness, preexistent relief and progressive fine-grained sandstones and coquinas of the Sepetiba Formation basement tilt. were deposited on the continental shelf during the last 4.2 Ma. The Albian to Cenomanian sequences of the Camburi Group Fifteen horizons were interpreted on the 2D seismic sections include the first deposits clearly related to the drift evolution after and were tied to the stratigraphic chart of Moreira et al. (2007) the deposition of the evaporites. Proximal siliciclastic sediments, (Fig. 2). These horizons were used to build two structural cross- shallow-water limestones in the continental shelf, and marls and sections (Fig. 3), which are around 120 km long and less than shales in the distal basin were deposited during Albian time. The 20 km apart. These cross-sections were subsequently used as input overlying Cenomanian sequence records deltaic and alluvial silici- data for the 2D structural restoration analysis, were special atten- clastic proximal fans, with shales and marls from the shelf to distal tion was given to the salt tectonics during the drift phase.

Figure 3. The two geological cross-sections A-A’ and B-B’ (see location in Fig. 1) are based on ﬁfteen interpreted horizons. The ten layers above the salt are intensely deformed. The Cabo Frio Fault separates extensional and compressive domains. A proximal low and an intermediate high constitute inherited structures, developed before the onset of salt tectonics. Classical minibasins occur in the distal compressional domain. Indicators A to G help to correlate similar compartments in both cross-sections.

2.2. Regional structural framework this feature northwards (Fig. 1). Towards the study area, a set of normal faults forms a less significant SW-NE-trending structural The structural inheritance presented herein is based mainly on low. The continuity of all these regional structures outlines a region previous studies of several authors in the literature. Rifting occurred of fragile basement from the southern failed rift basin up to the in the Santos Basin from Late Jurassic to Early Cretaceous along pre- proximal part of the study area (Fig. 1). The depression thus formed, existing structures (Pereira and Macedo, 1990). The oldest sedi- which predated salt deposition, later acted as a controlling depo- mentary record of the rift phase in the Santos Basin dates about 135 center for the main Late Cretaceous progradational wedge. This Ma ago (Moreira et al., 2007). Basement heterogeneities have structural low is parallel to the Cabo Frio Fault suggesting controlled the main rift architecture through time (horst and a common structural control for both features (Fig. 1). Although grabens bounded by normal faults) (Zalán et al., 2011). Long NE- available or published data is limited in the Santos Basin, we trending lineaments are identified among the most significant assume that the regional crustal thinning, including the failed rift heterogeneities. Those structures have been consistently mapped branch, is similar to Lavier and Manatschal’s model (2006), onshore and can also be observed offshore (Braga et al., 2003), at involving an H block in front of a V-shaped basin (Fig. 4). According least as far as the Santos hinge line (Fig. 1). Reactivations of pre- Lavier and Manatshal (2006) and Péron-Pinvidic and Manatschal existing lineaments and coeval shear zones controlled the evolu- (2010), the H-Block (Hanging wall) represents a relatively under- tion of the coastal mountain ranges (Serra do Mar and Mantiqueira) formed upper crust piece, thinner than 20 km, bounded by a con- and intervening Cenozoic onshore grabens (e.g., Almeida, 1976; tinentwards thinning fault and an oceanwards exhumation fault, Zalán and Oliveira, 2005); as well as the compartmentalization of with preserved pre-rift sediments cover. The authors highlight the the basin offshore. H-block resembles a remarkable retardation of subsidence whose Rifting developed an extensively stretched and thinned conti- origin is not yet well understood. Péron-Pinvidic and Manatschal nental crust within the Santos Basin still attached to the Brazilian (2010) defined the ‘V’-shaped basins as representing schemati- margin known as the São Paulo Plateau, a major deepwater phys- cally the temporal and spatial evolution of different and consecu- iographic feature (Mascle and Renard, 1976; Kumar et al., 1977; tive phases of rifting, with associated crustal blocks formation. The Cande and Rabinowitz, 1978; Guimarães et al., 1982; Demercian, crustal thinning, broadly distributed in the upper crust, is also 1996; Cobbold et al., 2001; Gomes et al., 2002; Scotchman et al., probably reflected in the basin compartmentalization by shear 2006, Carminatti et al., 2008). An aborted sea-floor spreading zones (Zalán et al. 2009). centre or failed breakup basin inherited from the initial rifting Asignificant structural high known as the “Outer High of the complexity has been described in the southern part of the São Paulo Santos Basin” (Gomes et al., 2002)orthe“Santos External High” Plateau, between the Florianópolis Platform and the São Paulo (Carminatti et al., 2008) is located slightly southeastward of the Ridge (Fig. 1)(Carminatti et al., 2008; Gomes et al., 2002; Gomes studied area. It corresponds to an important basement structure in et al., 2009; Mohriak, 2001; Meisling et al., 2001; Scotchman the central part of the São Paulo Plateau where the marine sedi- et al., 2006). An aligned sequence of en échelon grabens prolongs mentary infill is relatively thin, around 1000 meters, when compared

Figure 4. Schematic block diagram of the Santos Basin adapted from the model of Lavier and Manatshal (2006) and regional transect from Zalán et al. (2011). Notice in the block diagram the aborted southern spreading center where a hypothetical H-Block can be interpreted. In the regional transect is interpreted the same H-Block (indicated by H) and a local Moho exhumation (indicated by a black arrow). This scenario suggests a strongly thinned crustal thickness beneath the São Paulo Plateau.

Please cite this article in press as: Garcia, S.F.M., et al., Structural modeling based on sequential restoration of gravitational salt deformation in the Santos Basin (Brazil), Marine and Petroleum Geology (2012), doi:10.1016/j.marpetgeo.2012.02.009 S.F.M. Garcia et al. / Marine and Petroleum Geology xxx (2012) 1e17 5 to more proximal regions. As demonstrated by recent wells, Aptian Within the São Paulo Plateau, an impressive stratified salt column carbonates were deposited on this wide basement high under is located on the Santos External High (Freitas, 2006). This package is shallow-water marine environment (Carminatti et al., 2008). The not autochthonous, as it has been laterally displaced above the basal complexity of depositional paleoenvironments, shifting from detachment, controled by irregularities of the underlying basement restricted and shallow water to deep water beyond the Santos relief. More rugged features observed on cross-section A-A’ can be External High, made the understanding of this carbonate succession related to the preexisting structural framework (Fig. 3). a tectonic and stratigraphic challenge. There is not much available data (paleo water depth, relief, among others) to constrain or cali- 3. Methodology brate the paleoenvironmental restoration of these post-rift sequences. The restoration procedure presented here comprises the following backwards successive steps: (1) sediment removal and decompaction 2.3. Structural implications of the salt tectonics with isostatic flexural compensation; (2) fault and salt movement restoration fitting the paleogeometry to a reference target; (3) final The Santos Basin is the southernmost Atlantic basin where Late paleobathymetric adjustments. Compared with other restoration Aptian evaporites were deposited (Mohriak et al. 2008). Climatic methods (e.g. Rowan, 1993), this one is a simplified approach, which conditions were favorable for salt deposition on the São Paulo simultaneously considers flexural isostatic compensation and vertical Plateau and in other basins to the North. Those conditions were decompaction. In addition, it does not consider eustasy and litho- enhanced by the structural alignment of paleohighs such as the sphere cooling. The effects of these processes are compensated in the Florianópolis Platform and the São Paulo Ridge (Fig. 1). The struc- final paleobathymetric calibration. Fault-related and salt movement tural highs acted as efficient barriers at the southern limits of the deformations are restored with traditional algorithms such as simple basin blocking the marine water circulation derived from the proto- or inclined shear and move-on-fault, taking into account a conserva- Atlantic Ocean (Leyden et al. 1976). A fast and short (ca. 0.5 Ma) tive cross-sectional area of the sedimentary layers, including the salt. regional subsidence pulse of over 1000 m occurred during the Late The assumption of salt cross-sectional area preservation is limited Aptian probably as a consequence of the isostasy induced by the only below its overburden and has no intention to be realistic. It is thick salt layer deposition (prompted by the calculations from Van proposed to constrain the uncoupled restoration fitting (both pre-salt Den Belt and De Boer 2007). Nevertheless, the existing tectonic by faulting during rift phases and post-salt solely by salt tectonics), barriers still blocked the entrance of the surrounding oceanic waters through the structural coherence of the salt deformation within and kept the basin sufficiently closed to allow for salt deposition. a controlled amount (area) of salt. The model is constrained by Evaporites can flow similarly to a viscous material geologically adefined paleogeometry (shelf-slope-rise), subsequently adjusted speaking when they are differentially loaded by overlying sedi- using the constraints of the well biostratigraphic data. The restoration ments since the earliest depositional stages (Demercian, 1996). was performed using RECON-MS (Petrobras in-house developed The salt motion occurred westward towards the Southeast, software). between the Cretaceous Santos hinge line and the salt pinch-out, To better understand the structural restoration carried out here, close to the São Paulo Ridge (Fig. 1). The high accumulation rates the restoration cycles followed major sequences based on the since post-middle Cretaceous deformed the thick Aptian salt stratigraphic groups proposed by Moreira et al. (2007) (Fig. 2). An sequence resulting in a relatively shallow bathymetry over the São exception is made for the Guaratiba Group, subdivided in two parts, Paulo Plateau (Mohriak et al. 2008). During the drift phase, the salt considering the salt layer as an individual major sequence on and the overburden were intensely deformed in a convergent account of its relevant tectonic role. radial transport (Cobbold and Szatmari, 1991), and the deformation increasingly varies from distension near the coast to 3.1. Backstripping with flexural isostatic compensation compression in deep waters region (Fig. 3). The most active tectonic time was the Late Cretaceous, as a response to the load of The backstripping approach shows that a significant part of the progradational wedges over the thick salt layer (in the compart- basin subsidence is due to sedimentary loading and the layering ments C D E of Fig. 3). needs to be corrected throughout the time for the compaction The Cabo Frio Fault (Fig. 1 and 3) marks the major transition effects. The decompaction solution applied here is based on between the extensional and compressional salt tectonic domains Sclater and Christie (1980), similarly discussed in Bender et al. (Mohriak and Szatmari, 2001). Beyond this fault, the classical min- (1989). The decompaction of the underlying layers related to the ibasins behave passively within the compressional domain. The removal of the uppermost sedimentary layer is calculated in evolution of the Cabo Frio Fault is related to huge volumes of sedi- vertical lines along the cross-sections. More regular and greater ments deposited since Albian times over the thick salt layer which distance of discretization is applied to the flexural isostatic provides the accommodation space. The progradation evolving correction. Thus, the cross-sections are prepared in view of from the shallow shelf towards the basin inflated even more the a detailed facies definition along the layering to improve these original salt thickness in deep waters creating a relative restriction calculations. The parameters used in this procedure are listed in for sedimentation (Ge et. al., 1997). The salt layer reacts to its Table 1. The remaining decompacted units are referenced with proximal squeezing pushed by the overburden deposition, thus respect to the base of the removed unit whose overall geometry is becoming locally exposed in reactive diapirs, inhibiting distal based on the isostatic compensation, calculated considering depocenters. Under such constraints, the shelf / slope / rise system aconstantlithosphericflexural rigidity for the entire cross- evolves with proximal thick layering and pushes laterally the thin sections and during the entire geologic time. distal set in the footwall of the Cabo Frio Fault (Mohriak et al. 2008). The flexural isostasy can play a significant tectonic-sedimentary The minibasins were filled by minor sedimentary flows bypassing control, affecting several aspects of the basin evolution, such as the slope. These conditions changed at the end of the Late Creta- bathymetry, fault geometry, thickness variation, uplift rate and, ceous progradational event, when the tectonic salt flow strongly erosional events. The unloading approach considering local Airy- decelerated and virtually ceased during the Cenozoic. Therefore, the type isostasy cannot approximate differential effects of the minibasins became more significant, developping more extensive regional subsidence. The magnitude of the isostatic compensation sandy bodies in less confined depocenters (Moreira et al. 2007). depends on the dimension of the applied sedimentary load, as well

Table 1 Erosional features are noticed in both cross-sections: in the Facies parameters applied to decompaction and flexural isostatic compensation. Salt is proximal and slope regions, linked to sea level changes and close to considered non-compressible, without variations during the decompaction salt diapirs. Although unconformities are crucial for the regional calculations. stratigraphy (Fig. 2), the erosions have no great magnitude and are 3 Facies Density (g/cm ) Porosity (initial) Decay (1/km) not taken into consideration in the restoration. The impact that Shale 2.68 0.46 0.46 these erosions would cause is certainly smaller than the uncer- Siltstone 2.67 0.48 0.47 tainties involved in the salt movement restoration. Sandstone 2.66 0.43 0.35 Mudstone 2.69 0.39 0.49 Wackestone 2.7 0.42 0.47 Grainstone 2.71 0.37 0.47 3.2. Structural block restoration Salt 2.16 ee Water 1.03 ee Crust 2.85 ee Several algorithms were applied to restore the deformed Mantle 3.33 eegeometry of each cross-section. Length, cross-sectional area, or Asthenosphere 3.18 eeboth, can be retained for the restored layers, depending on each algorithm. Therefore, neither gain nor loss of material is significant to the resulting geological features over time. We have sought the as on the flexural rigidity of the lithosphere. The assumption of simplest methods to obtain restoration solutions. Considering a constant elastic thickness through time and space is an acceptable dominant ductile deformation for the salt layer and brittle defor- simplification even though the lithospheric rigidity varies due to mation for other sedimentary layers, each cross-section has been either its physical integrity or with the thermal regime (Watts, subdivided into minor individual blocks with similar rheology, 2001). A lithospheric elastic thickness of 5 km is assumed (for whenever necessary at each restoration cycle. These blocks are a discussion on this value see, e.g., Roberts et al., 1998; Tiley et al., limited by well defined geological elements, such as stratigraphic 2003) and a crustal density of 2.78 g/cm3. contacts, salt diapir boundaries, sea floor or normal faults. Each The loads of the sedimentary sequences along the Santos Basin block is restored by geometrical transformations to a target (which extends over more than 400 km beyond the study area) are geometry, constrained by the paleobathymetry and by lateral block not entirely represented in the cross-sections A-A’ and B-B’, each boundaries (faults, diapirs or cross-section borders). around 120 km long. Moreover, the post-salt sedimentary Simple shear transformation is able to preserve the blocks’ sequences become progressively shortened in the study area cross-sectional area in 2D restoration. These shear transformations during the sequential uncoupled restoration, suggesting even have been used to restore brittle blocks in the extensional defor- shorter load lengths. The consequent underestimated unloading mation domain and also in less deformed situations of the distal can produce inaccuracies in the isostatic calculations. In order to domain. Flexural unfolding transformations preserving length on simulate a sedimentary load more similar to the total flexural load the layering shortening direction have been rarely used to restore and also to restore the missing sedimentary segments which have the folding of deep layering in some minibasins blocks. Consid- been pushed outside of the study area by halokinesis, the cross- ering major deformation concentrated close to the faults and sections are laterally extrapolated (Fig. 5). The isostatic approach diapirs edges, sometimes partial restoration steps were super- provides a paleobathymetric estimation which requires additional imposed, increasing shear restoration amount to better restore calibration at the end to fit the real equilibrium. some intensely deformed salt-sediment boundaries. Sometimes

Figure 5. The cross-section B-B’ is laterally extrapolated based on the regional transect X-Y-Z from Carminatti et al. (2008). The thick evaporites in the southeastern extrapolation are very important to the study area restoration once their deformation provided the accommodation space for the prograding sedimentary pile.

Figure 6. Paleo-bathymetry model through time based on well data for the platform and slope regions (no data available to the continental rise region). The gray shading polygons represent the summarized ranges of water depth described in the biostratigraphic wells reports in each area. The red dashed lines are regional bathymetry models for each region. Colored circles (blue, green and red) show the bathymetry expected for each restored age. Notice the pre-existing interpreted depression predating the deposition of salt.

Figure 7. The shelf break (white points) and foot of the slope (black points) are shown here for the cross-section B-B’. These critical points have been interpreted for each restored horizon above the salt layer, including the Albian carbonates top (H9). Notice the Cabo Frio Fault which limits the thicker proximal domain from the post-salt minibasins that developed in the more distal domain. proximal parts of the cross-sections, i.e., on the shelf and upper slope destruction of salt welds, as well as random increase and decrease in domains. Compressional gravity-driven structures and minibasins salt thickness. In all these situations, the local subsidence rates are restricted beyond the slope edge, where relatively thinner post- applied to brittle layers and deformation rates of the underlying salt layers are deposited. During gravity spreading, the thick salt ductile salt layer should be consistent with the paleobathymetry layer gradually becomes more deformed. Once this layer is locally profile. Figure 8 shows a detailed restoration of the applied meth- depleted or welded, either in the footwall of the growth faults or in odology. This example of the structural restoration steps includes: the compressional domain below minibasins, both the overburden and salt thicknesses must be consistent with the paleobathymetry. I) The initial (present day) condition of cross-section A-A’; This must be checked in every restored cycle. II) The decompaction of the youngest layer and flexural isostatic Erosive features are observed on the slope domain. Such features compensation; represent restriction for accommodation space or sediment bypass III) First paleobathymetric approximation, calculated according to along the slope. The slope domain in these situations is positioned the decompaction procedure. The blue line represents the over a regional basement high and shows a shortened width. regional bathymetry used as target. The salt layer is suppressed Restrictions to the salt flow and also to the salt thickness variation to allow the best block restoration adjustment to the decou- are expected. On the other hand, where a thicker layer is deposited pled post-salt package. Small circles show two salt welds that after salt withdrawal, either due to local collapse or gravity slide, this should be preserved, whereas a small cross indicates a salt shift must also be consistent with the paleobathymetry in the salt thickness to be maintained; layer restoration. IV) Post-salt package adjusted to the top target line, seeking the If salt thickness variation is required through time, certain best replacement of the initial amount of salt below the features should be avoided, i.e. the successive random formation and package. This step leads to a complete restoration cycle.

Figure 8. Example of the ﬁrst restoration steps showing sequential sketches for the partial results of the cross-section A-A’. Notice two expected salt welds highlighted by circles in the step 3, as well as a salt thickness highlighted by cross, under a minibasin, to remain ﬁxed during the restoration.

Figure 9. Restoration outcomes of pre-salt stages applied without paleobathymetric data to cross-sections A-A’ and B-B’. Notice the extension restored represent less than 10% from the deformed cross-section. The resulting topography reaching up 3000 meters of depression in the cross-section A-A’ is consistent with the accommodation space needed for the thick salt layer deposition.

4. Restoration and discussion post-salt sequences can be explained by the major factors controlling salt tectonics: the preexisting relief at the time of salt Outcomes of fourteen different restoration restoration cycles are deposition and the sedimentation rate. These factors provide presented. These results represent the geological evolution for different conditions for the gravitational deformation as, for fifteen horizons interpreted in cross-sections A-A’ and B-B’. The instance, changing in the thickness relations between the ductile pre-salt sequence is the first of five major sequences considered in salt layer and the brittle overburden through time. These various the structural evolution. The last three restoration outcomes (Fig. 9) aspects of salt tectonics have been already well discussed by several are related to deformation of this sequence when brittle tectonics authors (e.g., Cobbold and Szatmari, 1991; Weijermars et al., 1993; was prevalent. Strongly different, the following eleven restoration Lerche and Petersen, 1995; Garcia, 1999; Mohriak and Szatmari, outcomes (Figs. 10, 11, 13 and 14) record the effects of thermal 2001; Hudec et al., 2009). subsidence and ductile salt deformation. The subdivision of the The five sequences are described below.

Figure 10. Cross-sections A-A’ and B-B’ restored at the salt deposition stage. The obtained scenario shows an original salt layer up to 4000 meters thick in the cross-section A-A’. The faint pink color represents the amount of salt added in order to restore the salt motion towards distal portions.

Figure 11. Restored proﬁles of cross-sections A-A’ and B-B’ for the Albian to Cenomanian stages that shows a climax of the rafting tectonics in the Santos Basin. The fast extension of the thin Albo-Cenomanian layers is obtained by the underlying salt layer movement, pushed to distal portions by the thick post-salt sediments deposition in the proximal area.

4.1. Pre-salt sequence 5). This feature is consistent with the hypothesis of a northward lateral prolongation of the brittle normal structures farther south The quality of the subsalt seismic imaging is not good enough to into the failed spreading centre (Figs. 1, 4 and 5). This structural low solve uncertainties related to the pre-salt layers. In general, the tilted had presumably already been formed before the time of salt depo- blocks involving the basement and syn-rift strata beneath the base of sition (Fig. 9). The restored geometry at the pre-salt sequence top the salt were defined only by a few faults without good definition of suggests that significant topography existed in the basin prior to salt the dip of the pre-salt series (Fig. 9). The unconformities subdividing deposition. However, the lack of paleobathymetric data prevents any this sequence are not easily mapped and, moreover, no data about accurate calibration and therefore, this geometry remains quite the target relief of these sequences is available. The restoration of speculative, resulting only from the isostatic and structural adjust- these sequences characterized by a graben- and horst-dominated ments applied during the restoration. The present-day architecture fault system was marked by block rotation and significant exten- derived from the interpretation of seismic images is the most sion is observed in the proximal basement low domain (Figs. 3 and deformed situation of the pre-salt layers. Thus, the pre-Aptian basement architecture is assumed here to have been less deformed in the past.

4.2. The Aptian salt sequence

The salt layer is progressively pushed towards the distal regions of the basin due to gravitational instabilities (Fig. 10). The light pink segment of the salt layer represents the recovered amount resulting from its restored movement from present day to deposition time. The dark pink segment has a cross-sectional area equal to the present day cross-section. In the study area, the salt layer presents different average thicknesses in each cross-section: it is around 1600 m-thick in cross-section A-A’ and about only 700 m-thick in cross-section B-B’ (Fig. 3). The restored salt layer is more homogeneous and is on average around 2800 m-thick in cross-section A-A’ and about 1200 m-thick in cross-section B-B’ (Fig. 10). The thickness differences consider both segments of the salt layer. During the restoration of the halokinesis shortening, virtual gaps rest over the position from where large amount of salt was pushed outside the study area. When we regard this salt amount as replacing the lateral gaps (light pink segment), the isostatic Figure 12. Restoration model for the Albian carbonates. The seismic image shows the response of the pushed salt prevents minor deformation due to the present day conﬁguration of the Albian growth structures. The restoration model requires a formerly thicker salt basin (thicker than 2500 m) to accommodate the salt loading absence. The loading replacement applied throughout observed thickness growing of the carbonate layers. the entire restoration workﬂow leads to a better shape for the

Figure 13. Restored outcomes of the cross-sections A-A’ and B-B’ presenting the progradation of the Late Cretaceous. The lateral movement during the development of the Cabo Frio Fault (black arrows) is showned in relationship to a vertical fixed position (dashed line). The total extension in the study area (red arrows) is also showned according the light pink segment of the salt layer is pushed out of the study area. The large lateral deformation during this period provides the formation of first salt welding in many compartments. initial basement geometries in both cross-sections, with cumula- 4.3. Albian to Cenomanian sequences tive isostatic effects through time. No attempt was made to restore the top of the restored salt layer at its deposition time, and a sub- The first post-salt sedimentary sequences include the Albian horizontal bathymetric gradient (<1%) was assumed. The restora- carbonates and part of the Late Cretaceous marine siliciclastics. In tion was not applied to the region of the added segments (light pink both cross-sections, several blocks separated by gaps of several salt). A thick regular salt layer was added to provide the loading kilometers compose these sequences. There is some uncertainty effects, without any bathymetric adjustment. It is impossible to about their restored spacing, especially for the blocks from the interpret any structural feature at salt deposition time, before the middle to the distal part of the cross-sections (Fig. 11). The sedimentation of the overburden units, based exclusively on low deformed blocks document the large magnitude of raft tectonics resolution data. operating at the onset of halokinesis. In the continental rise

Figure 14. Restored outcomes of the Cenozoic layers for cross-sections A-A’ and B-B’ showing that salt tectonics is no longer active in the proximal domain, and remains relatively quiescent within the more distal domain, developing the minibasins. domain, a large salt volume covered by a thin overburden favors (Fig. 13). In early evolution of these sequences, the proximal area is a fast gravitational gliding. Different sedimentation domains and characterized by a thicker post-salt package, already compartmented paleoenvironments were linked with the shelf / slope / rise by diapirs. Structural and thickness differences are observed among geometries. these Late Cretaceous restored geometries in both cross-sections. The thickness variations interpreted in the seismic data for the Such differences are important for the progradation evolution in Albian and Cenomanian layers show a thick proximal portion the hanging wall of the Cabo Frio Fault. There was a general tendency developed in continental shelf and slope domains. An underlying to develop antithetic faults close to the slope edge, while the slope thick salt layer provides the initial minimum basin amplitude to advanced as a consequence of progradation. This tendency seems to accommodate the carbonate shelf development. Thin and folded be related to differences between the thick continental shelf and the distal Albian carbonate layer interpreted as continental rise deposits thin continental rise overburden domains, controling the Cabo Frio are clearly restored in their initial unfolded stage and reconnected at Fault since its onset. A signiﬁcant basinwards retreat of the Cabo Frio their correct paleobathymetric position (Fig. 11 and 12). In the footwall is restored and little or no fault gap will remain to be restored following sequences, before the development of the classical mini- in subsequent phases. With the progradation development, the basins, these carbonates were translated basinward by raft tectonics. largest depocenters are formed beyond the continental shelf. This Thickness variations in the Albian sequence are observed on seismic shift in sedimentation pattern is described by Assine et al. (2008).The lines (Fig. 3) and allow to interpret a sedimentary growing that sedimentation was probably controled by a bypass through the thick represents the beginning of the halokinesis process. Restoration continental shelf overburden, which was relatively blocked against results corroborate the relationship between progradation and the the structural high, causing adjustments in the Cabo Frio Fault squeezing of the salt basinwards. The development of the Cabo Frio geometry. The ﬁrst salt welds are formed beneath thick post-salt fault during the Albian to Cenomanian sequences (Fig. 11)issug- depocenters. gested in the transition from the thicker proximal overburden to the thinner distal one. 4.5. Cenozoic sequences

4.4. Late Cretaceous progradation sequences The topmost and youngest layers are the least deformed (Fig.14). The Cenozoic aggradational sequences drape uniformly over the During the massive progradation of the Late Cretaceous sedi- nearly-emerging diapirs. The overburden became very thick, pre- ments, the salt movement accommodated the development of senting many welding points in the salt layer which block the multiple overburden blocks limited by faults or salt structures previously intense salt tectonics. The reactive diapirism under rapid

Figure 15. First restoration steps showing the decompacted Miocene situation just after the removal of the uppermost layer. The present day in details images is compared with the decompacted scenario of the central image. The decompaction outcome, dependent of the isostatic parameterization, is refered to the present day referential bathymetry (difference in the striped grey area). The detail 1 highlights an isostatic result locally higher than the referential bathymetry (potential erosion) while the detail 2 highlights a relatively lower isostatic position in the minibasins domain (potential aggradation).

Please cite this article in press as: Garcia, S.F.M., et al., Structural modeling based on sequential restoration of gravitational salt deformation in the Santos Basin (Brazil), Marine and Petroleum Geology (2012), doi:10.1016/j.marpetgeo.2012.02.009 14 S.F.M. Garcia et al. / Marine and Petroleum Geology xxx (2012) 1e17 geometry model for salt deposition is discussed by Montaron and between 92 and 70 Ma (Figs. 9 and 10). A geohistorical graph Tapponier (2010). The isostatic effect is obvious in the 1D subsi- replacing all components through time illustrates how the over- dence geohistory graphics discussed later. burden, spreading to downstream situations, is progressively replaced at each age by the moving upstream overburden. For 4.7. Structural restoration style instance, the first overburden deposited above a fictional point in Albian times has moved away, being now replaced by another Albian The analysis of the variations in salt thickness and amount helps segment which was deposited farther north-northwest. This can be understand the structural coherency and different style of the salt easily observed, for example, in a fictional point in the middle of the layer deformation as well the overall restoration results in the study cross-sections at the initial and final restored geometries of the Late area. Intense salt deformation begins in a salt layer with more Cretaceous progradation (arrows and dashed lines in the Fig. 13). homogeneous thickness. The salt flowing out in each cross-section is Built from restored cross-sections results, the 1D subsidence not proportional to the salt existing at present day. According to the and overburden historical graphs (Fig. 16) replace the traditional restoration results, the total amount of salt that occurs in both cross- image of layers only under increasing compaction by layers with sections is uneven, 1.6 times greater in the cross-section A-A’ than in tectonic movement. The added value of this approach for the the cross-section B-B’ at the beginning, for the Aptian restored stage analysis of thermal effects depends on the magnitude and timing of (Fig. 10) and 2.4 times greater at the present day (Fig. 3). Comparing the lateral movement that the system has suffered. The difference the same ages, the average thickness changes more than 4 times in may be significant for situations with greater lateral movements if, the B-B’ cross-section, as opposed to less than 3 times in the cross- for instance, the thermal data from deeper burial (less thick salt section A-A’. In other words, wherever there is less salt at the layer) is moved to replace a pushed overburden that is relatively present day (cross-section B-B ’), not only there was relatively more colder (thicker salt layer), overlying a petroleum source rock in the past but also the salt movement was greater through time. kitchen (with a structurally modified geothermal history), similar These hypotheses suggest that the distribution of salt was spatially to the geohistory of the pseudo-well 3B (Fig. 16). Apart from this more homogeneous in the past. The salt layer was more efficiently context, the halokinesis deformation should not significantly affect pushed away in the B B’ cross-section, which is also consistent with the petroleum system, since the source rocks (underlying the salt the less fragmented deformation style observed in the orverburden layer) and salt welds (i.e. potential migration windows) remain of this cross-section. relatively static through time. The results suggest variable strain The decoupled restoration solution assumes diachronism rates for salt tectonics in the Santos Basin, supporting the assess- between the salt gravity-driven deformation and the older tectonism ment of Araújo et al. (2005) that the major part of the salt defor- affecting layers below the salt. The salt layer acts as a detachment mation occurs until the end of Cretaceous, with minor movements layer for the gravity-driven deformation above this rheological limit. afterwards. Even if local kitchens might be mature early, these For the package deformed by salt tectonics, the cumulative extension authors consider the peak of oil expulsion at the Late Campanian, represents an increase of nearly 60% in the study area. The cumulative with significant migration pulse from 44 Ma to the present day. In extension for the brittle pre-salt domain represents an increase of other words, during the entire Paleogene several elements of the only approximately 6%. The fault extension indicating negligible petroleum systems remain almost in the same vertical position as upper crustal stretching while large beta stretching factors do not they are today. restore completely the paleobathymetric indicators (Scotchman et. The 1D historical graphs are also useful as consistency control al., 2006). Depth-dependent lithosphere thinning, in which stretch- tool for several restoration parameters such as bathymetry, isostasy ing of the lower crust and lithosphere mantle greatly exceeds that of and salt thickness variation. Some abrupt changes in isostasy and the upper crust, has been observed at many non-volcanic and paleobathymetry can be observed through few 1D restored schemes volcanic rifted continental margins including conjugate margin pairs and seem to stand out as boundaries for the five major sequences (Kusznir and Karner, 2007). The restoration results for the brittle (red arrows in Fig. 16). The general bathymetric model (Fig. 6) deformation domain below the salt layer are consistent with the illustrates the critical isostatic adjustment at salt deposition time assumption of Moreira et al. (2007). For these authors, the main faults passing from a major, deep and dry continental depression to of the rift phase ceased their activities or underwent rare reactivation a shallower environment. The prograding wedge in the Late Creta- after the deposition of Barra Velha Formation (Fig. 2)andthis ceous reverses the deepening bathymetric evolution, advancing the deformation, almost totally produced before the salt deposition, was shelf break basinwards. The tectonic-sedimentary control is fully restored during the restoration cycles of the rifting phase. imposed on the consistent 1D graphs. Most of the accommodation space created during salt tectonics 4.8. Geohistory based on 1D modeling is due to the lateral expulsion of salt under the continuous burial of the Santos Basin. The 1D geohistory graphs provide a good The 1D subsidence histories allow simple and useful evolutionary summary of this progradation history (Fig. 16). Pseudo-well 1B analysis. It is possible to give new uses to the 1D geohistory inte- shows significant proximal sedimentation easily accommodated in grating them with the 2D restoration results. The classical 2D and 3D Albian to Cenomanian times; whereas pseudo-well 2A indicates basin modeling simulations are, usually, constrained under struc- Late Cretaceous progradation and pseudo-well 4A shows Cenozoic tured meshes. In such simulations and many previous 1D studies, representative thickness. The clastic progradation from Late phenomena related to salt deformation are solved by the simple salt Cretaceous imposes greater deformation on the salt layer. Pseudo- thickness variation. The spatial deformation of the structures can be wells 1B, 2A and 4A in Figure 16 are located in post-salt depocenters explored. A pseudo-well can be extracted from the intersection from where thick salt layers had been gradually squeezed out at points of a vertical line with all horizons at a fixed position in the different times (Turonian for pseudo-well 1B, top Cretaceous for present day cross-sections. A historical component of each repre- pseudo-well 2A and Miocene for pseudo-well 4A). In contrast, sentative age can be extracted in the same vertical position from all pseudo-well 3B is located above a thick salt wall near the Cabo Frio remaining restored outcomes. In terms of the traditional basin Fault. This pseudo-well reveals the Late Cretaceous evolution of modeling, the lateral tectonic transport due to the halokinesis affects a minibasin, coeval with a progradation event. Pseudo-well 3B also most significantly the petroleum system mainly under minibasins, shows the salt layer undergoing continuous thickening, related to moving at the period of greatest activity of the Cabo Frio Fault, lateral spreading.

Figure 16. Eight graphs based on the restored cross-sections A-A’ and B-B’ show subsidence and overburden 1D geohistories. The location map provides the pseudo-wells 1B, 2A, 3B and 4A positions. A pseudo cross-section composed by projected parts shows a continuous progradation from proximal regions to the distal ones from the Albian up to present day.

Such graphs represent a quality control for the restoration process available wells and seismic data; and (3) adjustments in the salt layer by crossing the one-dimensional evolution against the cross-sections under volume conservation, integrating the paleobathymetric and constraints imposed by the workflow. This simultaneous check helps isostatic results. The 2D restoration is revisited through 1D comple- identify critical points, hence improving the restoration results. mentary analysis, which can also be used as a calibration tool. The calibration workflow intends to make all restoration stages more 5. Conclusions integrated through time and space. The workflow could also be suitably applied in other basins and/or complex situations, for The approach of integrating structural restoration tools with instance, where erosive features are important. isostatic control and paleobathymetric model is crucial to obtain The restored geometry of the pre-salt sequence (predating the reliable results. A relatively simple 2D workflow based on classical salt deposition) results from the decoupled backwards restoration. backwards cyclic steps was carried out in this work: (1) isostatic Although not conclusive, the obtained geometry suggests that the flexural response to unloading and decompaction; (2) fault-related structural high has subdivided the post-rift environment into two block restoration to fit a paleobathymetric model to the few sub-basins, filled and unified during the salt deposition.

Afterwards, the structural high worked as a bulkhead during the Bacia de Santos: importância na exploração de hidrocarbonetos)). Revista e salt deformation. Brasileira de Geociências 28 (suppl. 2), 111 127. Bender, A.A., Mello, U.T., Chang, H.K., 1989. Bidimensional reconstitution of the For the post-salt layers deformed by halokinesis, the brittle geologic history of sedimentary basins: theory and its application in the clastic package developed early in the proximal domains, in Campos Basin. ((Reconstituição bidimensional da história geológica de bacias contrast with the thin sequences of the distal regions, shaped by sedimentares: teoria e uma aplicação na bacia de Campos)). Boletim de Geo- ciências da Petrobras 3 (1/2), 67e85. the ductile rheology of the thick salt layer. These thin sequences Braga, L.F.S., Costa, C.M., Gama, F., Fontoura, C., Cunha, A.S., Dourado, F., Correa, F.S., either behaved as small rafts during early extension or developed 2003. Magneto-Structural Imaging (MSI) and regional basement of the Santos a subsequent and continuous shortening in the minibasins area. A Basin, Brazil. In: SBGf. International Congress of The Brazilian Geophysical Society, 8. CD-ROM, Rio de Janeiro, p. 4. constant salt cross-sectional area assumption through time is not Cande, S.C., Rabinowitz, P.D., 1978. Mesozoic seafloor spreading bordering conjugate a realistic constraint in the study area. A first approximation is continental margins of Angola and Brazil. In: Proceedings of Offshore Tech- related to the 2D nature of the restoration while salt movements nology Conference, 10, pp. 1869e1871. Houston, Texas, OTC3268. Carminatti, M., Wolff, B., Gamboa, L.A.P., 2008. New exploratory frontiers in Brazil. occur in a complex 3D space. Moreover, segments with additional In: World Petroleum Congress, 19. Madrid, Spain, WPC proceedings, 11 f. cross-sectional salt area are brought from distal areas, consistent Cobbold, P.R., Meisling, K.E., Mount, V.S., 2001. Reativation of an obliquely rifted with the large volume of salt, known beyond the study area. This margin, Campos and Santos Basin, Southeastern Brazil. American Association of e approach provides a good scenario of restoration for the two Petroleum Geologists BulIetin 85 (11), 1925 1944. Cobbold, P.R., Szatmari, P., 1991. Radial gravitational gliding on passive margins. geological cross-sections presented in this paper. Geometric pale- Tectonophysics 188, 249e289. oenvironmental model, paleobathymetric data and flexural isos- Demercian, L.S., 1996. Halokinesis in evolution of the south of Santos Basin from the tasy calculations provide guidelines to restore independent Aptian to Upper Cretaceous (A halocinese na evolução do sul da Bacia de Santos do Aptiano ao Cretáceo superior). Master Dissertation, Universidade Federal do packages above and below the ductile salt layer. Constant, low Rio Grande do Sul, Porto Alegre, Brazil. elastic thickness assumed in the restoration processes does not Dias, J.L., 1998. Stratigraphic and sedimentological analysis of Aptian stage in part of the eastern margin of Brazil and Malvinas Plateau: considerations about the cause major problems to the methodology approach. On the other fi fl rst marine incursions and ingressions of the South Atlantic Ocean (Análise hand, the uncertainties in the restoring transformations re ect the sedimentológica e estratigráfica do andar aptiano em parte da margem leste do lower resolution data and established constraints. The results are Brasil e no Platô das Malvinas: considerações sobre as primeiras incursões e robust and offer opportunities for corrections and improvements ingressões marinhas do Oceano Atlântico Sul Meridional). PhD thesis, fl Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil. both in the restoration tool and the work ow. Freitas, J.T.R., 2006. Evaporite depositional cycles of the Santos Basin: an analysis of Geohistory 1D analysis based on 2D cross-section restoration cyclostratigraphy data from two wells and seismic traces (Ciclos deposicionais results are complementary tools which improve the whole 2D pal- evaporíticos da Bacia de Santos: uma análise cicloestratigráfica a partir de dados de 2 poços e de traços de sísmica). PhD thesis, Universidade Federal do inspastic process. Inconsistencies produced during restoration steps Rio Grande do Sul, Porto Alegre, Brazil. can be easily observed in these graphs and used again as a new input Garcia, S.F.M., 1999. Three-dimensional study of effects of halokinesis in passive for further optimization loops. The 1D graphs allow for a consistent margins (Estudo tridimensional de efeitos da halocinese em margens passivas). evaluation of the local bathymetric variation through time. The 1D Master Dissertation, Universidade Federal de Ouro Preto, Brazil. Ge, H., Jackson, M.P.A., Vendeville, B.C., 1997. Kinematics and dynamics of salt subsidence graphs confirm that the sedimentary evolution in the tectonics driven by progradation. American Association of Petroleum Geologists study area results from a continuous sedimentary progradation. They Bulletin 81 (3), 398e423. illustrate salt thinning first in the proximal portions and, then, Gomes, P.O., Kilsdonk, B., Minken, J., Grow, T., Barragan, R., 2009. The Outer High of the Santos Basin, southern São Paulo Plateau, Brazil: pre-salt exploration outbreak, gradually occurring in the most distal areas. They also provide paleogeographic setting, and evolution of the syn-rift structures. 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