From: RIES, A. C., BUTLER, R. W. H. & GRAHAM, R. H. (eds) 2007. Deformation of the Continental Crust: TheLegacy of Mike Coward. Geological Society, London, Special Publications, 272, 345–359.0305-8719/07/$15 © The Geological Society of London 2007.

Geology and tectonics of the South Atlantic Brazilian salt basins

IAN DAVISON

Earthmoves Ltd, Chartley House, 38–42 Upper Park Road, Camberley GU15 2EF, UK(e-mail: i.davison@earthmoves.co.uk)

Abstract: This paper first reviews the salt basins and depositional ages in the South Atlanticsalt province. This comprises a series of salt basins separated by basement highs, deepgraben (that never dried up), later volcanic highs and subaerial ocean spreading ridges. Initialhalite and anhydrite deposition occurred first in the Sergipe–Alagoas Basin of NE Brazil atc. 124.8 Ma, and was closely followed by deposition in the Kwanza Basin, Angola between124.5 and 121 Ma. The later potassium–magnesium-rich salts were deposited in the Sergipe–Alagoas and Gabon–Congo basins before 114.5 Ma. The age of the main Santos–Campos saltis not known precisely, but the latest anhydrites deposited on the southern margin of theSantos Basin post-date volcanic rocks dated at 113.2 Ma. The paper then compares the salttectonics of the wide Campos–Santos Basin segment with the narrow South Bahia basinssegment. Sediment loading in the Santos Basin produced a landward-dipping base salt,which led to the development of counter-regional faults, and inhibited downslope sliding, andenhanced later contractional effects caused by either gravity spreading or regional tectoniccompression. Folding occurred in simultaneous pulses across the Santos Basin, suggestingthat regional tectonic compression occurred. The narrow salt basins of South Bahia havea steeply dipping base salt horizon (4°) and pronounced folding, which initiates at theoceanward pinch-out of the salt and propagates back up the slope. The topographic highs,above fold anticlines, are rapidly eroded on narrow margin slopes, which allows the folds togrow more easily to large amplitudes at the top salt horizon.

anhydrite deposits overlie Precambrian base-ment or 113.2 Ma age volcanic rocks in thisarea (Dias et al. 1994). The South Bahia basins(Cumuruxatiba, Jequitinhonha and Camamu)are separated from the Espírito Santo Basin bythe Abrolhos Volcanic High which was eruptedand intruded during Palaeocene times (Sobreira& Szatmari 2000) (Fig. 2). The Sergipe–AlagoasBasin is separated from the Camamu Basin bythe Jacuípe Basin, which is interpreted to havebeen a deep sediment-starved graben duringdeposition of the salt that may never have driedup (Fig. 1). Salt has not been penetrated inthe deeper part of the Sergipe–Alagoas Basin(>1000 m) and seismic data also suggest theabsence of salt structures. This contrasts withthe Gabon salt basin which extends out tonear the ocean–continent crustal boundary. TheSergipe–Alagoas Basin is bounded to the northby the Ascension Fracture Zone, which is ass-umed to have been a basement high during theAptian. Farther north, the Ceará Salt Basin con-tains thin Aptian salt of the Paracuru Formation(Fig. 1). Halite is up to several metres thick inwells CES-42 and CES-46 (Regali 1989). Thisis the only recorded Aptian halite along bothsides of the Equatorial Atlantic margin, althoughAptian anhydrite has also been recognized in theBarreirinhas Basin of Brazil (Azevedo 1991) andin the interior Araripe Basin (Fig. 1).

The South Atlantic salt province comprises aseries of basins that are separated by: (1) deeprifts that never dried up (Jacuipe Basin); (2)basement highs where no salt was deposited(Florianópolis High, Ascension Fracture Zone);or (3) post-salt volcanic highs (Royal Charlotteand Abrolhos) (Fig. 1). These basins containsome of the largest oilfields (Marlim Complex,c. 10x109 barrels) discovered worldwide in thelast 20 years, and salt tectonics controlled theconfiguration of many of the producing reser-voirs. This paper reviews the tectonic and deposi-tional history of the South Atlantic salt basins,and then illustrates the differing structural stylesalong the southern and central Brazilian Atlanticmargin. Salt tectonic styles of a broad margin arecompared with those of a narrow steeper marginswhere less sediment has been deposited.

South Atlantic salt deposition

Distribution of the Brazilian salt basins

The Brazilian salt province is separated intoat least three original salt basins (Ceará, Sergipe–Alagoas, and the South Bahia–Espírito Santo–Campos–Santos Basin). The southern end of theSantos Basin is bounded by the FlorianópolisHigh, which was a basement high duringthe Barremian to early Aptian period. Thin

346 I. DAVISON

Age and nature of Brazilian evaporitesequences

Depositional age estimates of the evaporitesdiffer along the Brazilian margin and aresummarized below (and in Fig. 1).

The salt in the Sergipe–Alagoas Basin of NEBrazil occurs in two separate intervals in theMaceió Formation (Paripueira Member) and theMuribeca Formation (Ibura Member) of Aptianage (Uesugui 1987; Feijó 1994). The Paripueira

Member was deposited during palynologicalzones P-230 (Inaperturopollenites crisopolensis)to P-260 (Inaperturopollenites turbatus). It con-sists of halite beds interbedded with shales andis c. 100 m in thickness. This salt is estimatedto have deposited around 124.8 Ma (R. Wynne-Jones, pers. comm.) The later Ibura Memberof the Sergipe–Alagoas Basin is restricted to theupper part of P-270 zone (Sergipea variverrucata)(Uesugui 1987) onshore and on the shallow shelf.The overlying Riachuelo Formation contains

Fig. 1. Map of the South Atlantic salt province. Salt Basin outlines compiled by author.

347SOUTH ATLANTIC BRAZILIAN SALT BASINS

late Aptian planktonic Foraminifera (Ticinellabejaouaensis) and ammonites dated at 114.5 Ma(Koutsoukos et al. 1993). The Ibura salt in theSergipe–Alagoas Basin was deposited in asabkha environment with coarse clastic fandeposits occurring on the borders of the basin inhalf-grabens, adjacent to major normal faults.Bituminous black shales occur interbedded withthe halite, dolomite and algal limestones, andthese are an important hydrocarbon source rockin the northern basins of Brazil. The Ibura salt isa MgSO4-free potassium mineral deposit formedof stacked cycles of halite, carnallite and sylviteup to 800 m in thickness. Several of these cyclescontain >10 m thick units of primary tachyhy-drite (CaMg2Cl6.12H2O; Meister & Aurich 1972;Wardlaw 1972; Wardlaw & Nicholls 1972;Borchert 1977).

Farther south in the Santos Basin, on theFlorianópolis High, anhydrite and carbonatesof the Ariri Formation lie unconformably above

the Curumim Volcanic series in well 1-SCS-3,which has been dated at 113.2P0.1 Ma (Diaset al. 1994). The age of the main salt of theEspírito Santo, Campos and Santos basins is notknown, but is thought to be coeval as this is asingle continuous salt basin (Fig. 2).

The thin halite of the Mundaú and Icaraísub-basins is an isolated occurrence along theEquatorial margin in Ceará and is also assignedto the P-270 zone of late Aptian age (Regali1989).

African salt basins

The West African Salt Basin is separated intoat least three basins: Douala, Rio Muni and themain Gabon–Congo–Angola Salt Basin (Fig. 1).A structural high may have been created alongthe Ascension Fracture Zone during the Aptian,which separated Rio Muni from the main WestAfrican Salt Basin to the south. The Rio Muni

Fig. 2. Map of the southern Brazilian Atlantic margin salt basins showing location of the cross-sections in thisstudy. Salt structures in Santos and Campos from Jamieson et al. (2002), and in Cumuruxatiba, Jequitinhonhaand Camamu from Cainelli & Mohriak (1998) and the author’s own work.

348 I. DAVISON

salt has lacustrine–continental strata above it(Dailly 2000), whereas the main Gabon–AngolaSalt Basin is immediately followed by fullymarine Albian carbonates (R. Bate pers. comm.).

The salt in Gabon and Congo does notinclude MgSO4 salts and contains thick tachy-hydrite units (de Ruiter 1979). The salt in Gabonis particularly rich in carnallite, with beds up to400 m thick, which produce strong seismic reflec-tions within the evaporite interval. Interbeddedcarnallite, halite and thin black shale layersare also present in the shallow part of the CongoBasin (Belmonte et al. 1965). Bischofite andtachyhydrite occur at the top of some evaporitecycles and are considered to be original sedimen-tary deposits, which are extremely rare world-wide except in the conjugate Sergipe–Alagoas(Wardlaw & Nicholls 1972; Borchert 1977) andGabon–Congo basins (Teisserenc & Villemin1990). The most extensive and thickest potassiumsalt deposits are present onshore in the coastalKouilou region of Congo, where 10 evaporiteintervals are recognized with a cumulative thick-ness of potassium minerals of more than 100 m(Belmonte et al. 1965). These salt deposits wereexploited in the Holle Mine area for severalyears, but this was abandoned as a result offlooding from an aquifer (Warren 1999).

The significance of the MgSO4-free salt isthat it cannot have formed from evaporation ofnormal seawater, yet the deposits appear to be ofprimary sedimentary origin. The chloride miner-als sylvite, carnallite, tachyhydrite and bischofiteare diagnostic of brines enriched in CaCl2, pro-bably by hydrothermal water–rock interaction.The most prolific rock host would be basalt,altered to spilitic greenstone, where albitizationreleases Ca into brine and chloritization absorbsMg from brine (Hardie 1990; Jackson et al.2000), and a mid-ocean ridge spreading centrewas probably developed during the later saltdeposition.

The African salt basins are believed to have bepartially separated from the Brazilian salt basinsby subaerial mid-ocean ridge spreading centres,with the salt onlapping and thinning onto theridge (Jackson et al. 2000). Evaporation draw-down may have taken the local sea level downwell below the present-day level (c. 3 km) duringthe early Aptian to expose the spreading centre.Seaward-dipping reflectors occur sporadicallyalong both margins outboard, and possiblyunderneath, the seaward edge of the salt, andprovide possible evidence for subaerial spreading(see example from Santos Basin, Fig. 4d; Jacksonet al. 2000; Henry et al. 2004). Plate reconstruc-tion work carried out by the author also suggeststhat ocean spreading had already commenced

before the late Aptian salt was deposited, as200 km of separation of the two continents is pre-dicted at that time (http://www.earthmoves.co.uk/products/south_atlantic/index.html).

The salt in the Doula sub-basin, which is themost northern salt basin on the African side,is poorly known (Fig. 1). The current interpreta-tion of the oceanic–continental crust boundary inthis area suggests that the salt is located on oceancrust (Meyers et al. 1996). However, the salt mayhave been allochthonous and originally depos-ited higher on the shelf then flowed basinward, asit appears to have done in southern Rio Muni inBlock K of Equatorial Guinea.

Ages of African salt basins

In Angola, the top of the salt is estimated to havebeen deposited before 121–124.5 Ma, based onthe age range of early Aptian planktonic Fora-minifera Leupoldina cabri found above the saltin a confidential well in one of the shelfal blocksin Angola and in the DSDP-364 well, whichwas suspended above the frontal allochthonoussalt massif (Caron 1978; R. Wynn Jones pers.comm.). (It should be noted that the Aptian stagelasts from 125 Ma to 112 Ma; Gradstein et al.(2004)). The Kwanza salt post-dates the extinc-tion of Inaperturopollenites crisopolensis and istherefore slightly younger than the ParipueiraMember of the Sergipe–Alagoas Basin.

The evaporites in Gabon are thought tobe the equivalent of the later Ibura Member(121–114.5 Ma, Uesugui 1987), and the Gabonand Sergipe basins were probably a single basinat that time. However, no evaporites equivalentto the older Paripueira Member of Sergipe–Alagoas (124.8 Ma) have been identified inGabon (Doyle et al. 1982). It is not clear wherethe change in age of the salt between Angola andGabon occurs and it is not clear whether the saltis diachronous within a single basin.

Salt Tectonics in the GreaterCampos–Santos–Espírito Santo Basin

Faulting controls on salt deposition

Large normal faults are present in both theCampos and Santos basins, which offset the baseAptian salt by up to 2 km (Figs 3 & 5). The pre-salt sag phase strata and salt thickness changedramatically across the fault shown in Figure 5,but there is no evidence of fanning of thesag-phase reflectors into the fault, whichsuggests that a fault scarp already existed beforedeposition of the salt. Variations in salt thickness

349SOUTH ATLANTIC BRAZILIAN SALT BASINS

and facies are controlled by faulting, with thinsalt (<1 km) present over highs and thicker salt(<2) in lows. Large normal faults (>2 kmthrow) offsetting the base and top salt have alsobeen identified in the Sergipe–Alagoas Basin.Localized pods of salt are restricted to the down-thrown side of the fault, suggesting that a largesurface topography was present, where saltinfilled the lows (e.g. Figueiredo 1985), but itis still not clear whether later faulting in theSergipe–Algoas Basin occurred during theAptian after the salt was deposited, or faultscarps existed before the salt was deposited.

Salt basin geometry

The Santos Basin is the widest salt basin in theSouth Atlantic, of up to 500 km width from theSantos Hinge Line to the frontal edge of the salt.The salt basin narrows to 150 km wide in theEspírito Santo Basin (Fig. 2). The Santos–Campos–Espírito Santo Basin (S–C–ES Basin)contains the largest oil and gas fields in the SouthAtlantic and has been the subject of severalimportant papers on salt tectonics (e.g. Cobbold& Szatmari 1991; Demercian et al. 1993;Cobbold et al. 1995; Mohriak et al. 1995; Fiduket al. 2004). Most of the hydrocarbon fields

are combination structural–stratigraphic traps,created by movement of the salt in Cretaceousand Tertiary turbidite sandstone reservoirs.

The large rift flank uplift created the Serrado Mar Mountains, adjacent to the Santos andCampos basins, which rise to over 2.2 km abovesea level (Fig. 3). This topography developedduring the synrift stage and made a contributionto the differential gravitational stress affectingthe salt basin. The flanking mountains and thepalaeotopography of the sea bed during theCretaceous created a differential stress of up toc. 100 MPa, which produced downslope slidingand frontal toe compression in the salt and over-lying strata (Fig. 3). The amount of differentialstress occurring at the leading edge of the salt canbe estimated by using Archimedes’ Principle andmeasuring the maximum height of the freeboardbetween sea bed on oceanic crust and above thefrontal allochthonous salt massif (Fig. 3). Thefreeboard is up to 600 m in the NE Santos Basin(Mohriak 1988) and 1400 m in the Kwanza Basin(Marton et al. 2000), and this is approximatelyequivalent to a differential stress of 6 MPa and14 MPa, respectively (using a density differencebetween seawater and salt or sediment of1000 kg m−3 and assuming zero strength of thesediment; Fig. 3). Hence, the frontal stress is

Fig. 3. Force balance on the Santos continental margin indicating topography, which created a maximumdifferential stress of c. 100 MPa, causing downslope compression on the continental margin. The allochthonoussalt sheet has a freeboard up to 710 m which was produced by a compressional stress of at least 7 MPa. Insettable showing freeboard of frontal salt structures in the South Atlantic.

350 I. DAVISON

much less than that predicted from the margintopography. This is probably due to stress dissi-pation as a result of present-day folding and slipalong the salt décollement.

The sedimentary loading of the Santos Basinhas created a downwarp of the regional basesalt horizon, which has subsided 4.5 km belowthe regional Early Cretaceous position to createa landward dip of c. 1.7° (Fig. 4a). The sedimentloading occurred in Late Cretaceous times, so thelandward base salt dip has created an effectivephysical buttress since that time. The landwarddip of base salt inhibited downslope sliding ofoverlying sedimentary strata, and promotedcounter-regional faulting caused by lateral saltexpulsion in front of the prograding sedimentwedge (see also Ings et al. 2004). Landward dip ofbase salt has also helped to cause a large amountof contractional folding and thrusting from LateCretaceous through to Tertiary times (Cobboldet al. 1995; Cobbold 2004). Analysis of the sedi-mentary stratal growth patterns in the synclinesbetween salt-cored anticlines indicates that atleast one phase of contraction occurred in asimultaneous pulse across the outer half of theSantos salt basin over a 300 km wide zone. Thiscontractional event is tentatively correlated withan end-Maastrichtian erosional unconformity inthe shallow shelf area (Pereira & Macedo 1990).Stresses were so high in the outer Santos Basinsalt sheet that major thrust faulting occurred,which repeated and thickened the evaporitesequence to as much as 4.5 km in thickness(Fig. 4c).

In the Campos and Espírito Santo basins thebase salt dips oceanward or is almost flat, anddownslope sliding has produced mainly regionalseaward-dipping listric faults that sole out inthe Aptian salt. Counter regional faults are lessimportant, and the compressional folding is lessmarked than it is in the Santos Basin.

In the outer central and northern part of theSantos Basin the salt interval is a strongly reflec-tive interbedded sequence which is believed tobe a halite–anhydrite–clastic sediment sequenceof Aptian age strata (Fig. 4b; Mohriak et al.2004). The uppermost seismic reflection of this

layered sequence is particularly strong and hasbeen called the enigmatic reflector (Mohriaket al. 2004). This is probably an anhydrite layer,and most of the salt diapirs appear to be cappedby this reflector (Fig. 4b). Thick halite occursnear the base of the evaporite sequence, andhas been intruded into the overlying mixedevaporite–clastic layered sequence (Fig. 4b).Outside the fold cores the layered sequence isclearly imaged (Fig. 4b). However, these layeredrocks are tightly folded in the cores of theanticlines and the steeply dipping beds arenon-reflective, giving the impression that thefolds have massive halite cores (Fig. 4b).The strongly anisotropic anhydrite–halite–clasticlayers enhance fold amplification and some foldsgrew into ptygmatic shapes.

Frontal edge of the salt

The ocean–continent transition zone lies slightlyinboard of the salt basin, where salt has beendeposited on either normal oceanic crust orseaward-dipping reflectors (SDRs). The salt hasbeen overthrust c. 20 km over oceanic crust in theSantos Basin, but in some areas it is difficult tolocate a sharp ocean–continent transition and itis not clear whether the outer 50–100 km part ofthe Santos Basin salt lies on oceanic or continen-tal crust. The seismic reflection data are incon-clusive, because the crust below the frontal saltmassif is poorly imaged over a 50 km wide zone.The gravity signature is also very transitional inthis area and an abrupt density change does notoccur at the ocean–continent transition.

Salt tectonics in the Jequitinhonha andCamamu basins of South Bahia

The topographic slope is much steeper in theJequitinhonha and Camamu basins than it isfarther south. The maximum width of the saltbasin reaches c. 70 km in the South Bahia basins.In the central portion of the Jequitinhonha Basinthe present-day sea bed has a 3.1° dip from thelandward edge to the seaward edge of the salt,representing a total relief of 3400 m (Fig. 6).

Fig. 4. (a) Regional cross-section through the Santos Basin showing the landward dip of the base salt caused bysedimentary loading.

351SOUTH ATLANTIC BRAZILIAN SALT BASINS

This has created a large gravitational componentof downslope sliding, generating a maximumdifferential stress of 40 MPa (with an assumedsediment density of 2200 kg m−3). A very largeextensional listric fault formed at the landwardedge of the salt basin, which produced a mega-rollover anticline that is c. 10 km wide. Earlyturbidite sandstone reservoirs may be trapped inthe hanging wall of this fold (Fig. 6a and b).

The rest of the salt basin is dominated by fold-ing. The geometry of growth strata around thefolds indicates that they grew as a result ofdownslope gravity spreading with compressionoccurring at the leading edge of the salt becauseof salt pinch-out. The fold-growth strata suggestthat the folds propagated backwards up the slopefrom the frontal salt pinch-out (Fig. 6a), with theearliest initiation of folds in the deepest water.

(b)

Fig. 4. (b) Close-up of the layered evaporite–clastic sequence in the central and northern Santos Basin, showing alower halite section that intrudes through the upper layered part of the evaporite sequence. The evaporites arecapped by the very strong reflector, informally known as the enigmatic reflector (Mohriak et al. 2004), which isbelieved to be an anhydrite layer capping the evaporite sequence. TWT, two-way travel time.

352 I. DAVISON(c

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353SOUTH ATLANTIC BRAZILIAN SALT BASINS

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354 I. DAVISON

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355SOUTH ATLANTIC BRAZILIAN SALT BASINS

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356 I. DAVISON

357SOUTH ATLANTIC BRAZILIAN SALT BASINS

This is the opposite sense to the usual forward-propagation of mountain fold and thrust belts.The folds that are currently the most active arehighest up the continental slope, although all thefolds in the section (Fig. 6b) have created sea-bedtopographic relief and are still active. Erosionhas removed up to 0.75 km of strata from thecrest of the major folds (Fig. 6b). Rapid erosionallows the roof of the fold to uplift more easily,and the salt-cored anticline continued to create alarge fold amplitude defined at top salt horizon.This positive feedback effect makes this a‘fold factory’ with a physical environment idealfor growing folds with large compressive stress,low-viscosity salt layers that fill fold cores, andhigh erosion rates on the fold crests.

Conclusions

The Brazilian and African salt basins developeddiachronously in the Aptian, and individualbasins were separated by basement highs andlows. The oldest salt in the South Atlantic isthought to be the Sergipe–Algoas Paripuiera saltdated at c. 124.8 Ma, followed by the KwanzaBasin salt at 121–124.5 Ma, and then the IburaMember and Gabon salt (114.5–121 Ma). Theage of initiation of the main Santos–Campos

salt basin is not known but the latest evaporitedeposition appears to be latest Aptian (c. 112–113 Ma). It is still not clear whether the salt isdiachronous within basins, or between basins orboth. The original salt thickness and distributionwere partly controlled by normal faulting, withthin salt deposited on pre-salt highs. The outerpart of the salt basin is interpreted to lie onoceanic crust with the mid-ocean ridge spreadingcentre exposed subaerially as a result of eva-poritic drawdown. A limited amount (c. 20 km)of overthrusting of the salt onto oceanic crust hasoccurred.

In the Santos Basin, a large sediment load wasapplied in Cretaceous times, which pushed thebase salt horizon down so that it dipped land-ward over a large part of the basin (Fig. 7). Thisimpeded downslope regional listric faults fromforming, and counter-regional faults and uprightdiapirs and contraction folds are more dominantin the outer part of the basin. Numerical model-ling also indicates a similar relationship withlandward-dipping base salt (Ings et al. 2004).Contractional deformation is more pronouncedin the Santos Basin, as the landward dip of thebase salt caused a buttressing effect duringlater compressive events. One regional com-pressive event has been identified near the endof the Maastrichtian. Sediment loading in the

Fig. 7. Summary of the different styles of salt tectonics on the Brazilian margin. (a) Broad margin with flat andlandward-dipping base salt horizon. Folding and counter-regional fault development are favoured on broadmargins. (b) Narrow basin with a relatively steep oceanward-dipping base salt horizon. The salt pinch-out causescompression to propagate back up the slope and later turbidites may be trapped behind the folds.

358 I. DAVISON

Espírito Santo and Campos basins occurredmainly in the Tertiary and is less important thanin the Santos Basin. Base salt dips gently seawardin these basins and regional down-to-basin listricfaults are better developed, and contractionaldeformation is less important.

The Jequitinhonha and Camamu basinsexhibit pronounced contractional folding,because a larger gravitational component devel-oped as a result of the steep topography on thesemargins (Fig. 6). The first folds formed at theleading edge of the salt and early turbidite reser-voirs may have been trapped in the outer partof the basin on the landward side of these folds(Fig. 7). Later folding developed higher up on theslope so that later turbidites were trapped higheron the slope behind actively growing folds(Fig.7). The early folds in deep water were alsoactive at this time, but they grew less quickly, asthere was less erosion of the fold crests in deepwater. Very large salt-cored folds (2 km ampli-tude) developed on narrow margins as the crestsof the folds were rapidly eroded which facilitatedfolding. These folds developed on the continentalslope, where extension would normally beexpected.

I wish to thank the late Mike Coward for his inspirationin my early research years working on Precambianmobile belts in Africa, which ultimately led to me work-ing on the tectonics of the South Atlantic. I would liketo thank R. Wynn Jones for useful discussions onthe age of the salt in the South Atlantic. G. Tari andJ. Turner made useful comments on this paper.

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