Salt tectonics in the western Gulf of Cadiz, southwest Iberia

AUTHORS

Hugo Matias � REPSOL, Paseo de la Castel-lana, 280, 4th Floor, 28035 Madrid, Spain;

Salt tectonics in the western Gulfof Cadiz, southwest Iberia

present address: IDL-Instituto D. Luis, Universityof Lisbon, Lisbon, Portugal;
Hugo Matias, Pedro Kress, Pedro Terrinha, [email protected]
Hugo Matias is an exploration geophysicist at
Webster Mohriak, Paulo T. L. Menezes, Luis Matias,Fernando Santos, and Frode Sandnes
REPSOL, where he has been involved in eval-uation projects in North Atlantic margins, BlackSea, and northwest Africa. He received his B.Sc.and M.Sc. degrees and Ph.D. in geophysics fromthe University of Lisbon, where he is also asenior researcher. He was formerly with PartexOil and Gas. His research interests focus onbasin analysis, with particular emphasis on salttectonics and deep-water petroleum systems.

Pedro Kress � Yacimientos PetroliferosFiscales SA, Macacha Güemes 515, Piso 23,C1106BKK Buenos Aires, Argentina;[email protected]

Pedro Kress obtained his degree in geologyfrom the Universidad de Buenos Aires-Argentina(1989) and his Ph.D. from FU-Berlin (Germany)in 1994. He joined YPF as an undergraduate in1985, worked in the Austral and Neuquina basins(Argentina), Espirito Santo, Campos, and Santosbasins (Brazil), Spain and Global Regional Ge-ology. He has been with YPF International Explo-ration Group, Buenos Aires, Argentina since2010.

Pedro Terrinha � Laboratorio Nacional deEnergia e Geologia-Unit of Marine Geology,Estrada da Portela, 2720-866 Amadora, Portugal;[email protected]

Pedro Terrinha obtained his degree in geologyfrom the University of Lisbon, M.Sc. degree

ABSTRACT

This study presents the results from the interpretation of anextensive and recent regional two-dimensional seismic surveyfocused on the understanding of the salt tectonics in the west-ern Gulf of Cadiz (GoC). Two different salt units were iden-tified: an autochthonous salt unit of the Late Triassic or theEarly Jurassic (Hettangian) and an allochthonous unit thatoriginated from the Hettangian salt. Interpretation of the pat-tern of distribution of the salt in the basin allowed subdivision ofthe area of study into three distinct salt domains: the easterndomain characterized by the presence of a conspicuous al-lochthonous salt nappe (Esperança Salt), the central domaindominated by salt diapirs with mild deformation of Miocenestrata and wide salt-withdrawal minibasins, and the southwest-ern domain where present-day tectonics induces impressivesalt deformation affecting the sea floor. This complex patternis mainly the result of the interaction of inherited basementstructure, complex tectonic history, and stress regime of thebasin. The intense halokinesis observed has created severalsalt-related trap geometries and fluid migration pathways. Asthe focus of worldwide exploration along passive margins isgradually shifting to deep-water regions, the western GoC hasthe potential to become a deep-water petroleum province inthe near future.

and Ph.D. in 1998 at Imperial College, UnitedKingdom. He has worked in rift and inversiontectonics of the Portuguese Mesozoic-Cenozoicmarginal basins and neotectonics of the south-west Iberian Margin. He taught at the Universitiesof Evora and Lisbon and currently works as aresearcher at the LNEG, Portugal.

INTRODUCTION

Recent developments in hydrocarbon exploration offshorePortugal, in particular, in the Algarve Basin (Figure 1), haveincreased the interest in salt tectonic studies in this area. The

Webster Mohriak � Petroleo Brasileiro S.A.-Petrobras/Universidade Estadual do Rio deJanerio-Faculdade de Geologia, Rio de Janeiro,Brazil; [email protected]

Webster Mohriak graduated in geology fromthe Universidade de São Paulo (Brazil) in 1977

Copyright ©2011. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received March 15, 2010; provisional acceptance April 26, 2010; revised manuscript receivedDecember 13, 2010; final acceptance January 27, 2011.DOI:10.1306/01271110032

AAPG Bulletin, v. 95, no. 10 (October 2011), pp. 1667– 1698 1667

and received his Ph.D. in geology at OxfordUniversity in 1988. Senior explorationist atPetrobras, he has been conducting regionalbasin analysis projects with special emphasis onpetroleum geology and tectonic evolution ofthe South Atlantic sedimentary basins, withseveral publications on divergent conjugatemargins.

Paulo T. L. Menezes � UniversidadeEstadual do Rio de Janerio/Petrobras, Rio deJaneiro, Brazil; [email protected]

Paulo T. L. Menezes received his B.Sc. degree(1986) in geology from Universidade Estadual doRio de Janeiro (UERJ), an M.Sc. degree (1990)in geophysics from Universidade Federal doPará (UFPA), and a Ph.D. (1996) in geophysicsfrom the National Observatory (RJ). He has beena professor at UERJ since 1997. In 2006, hejoined Petrobras. His research interest is in in-terpretation of potential field and electro-magnetic data.

Luis Matias � Centro de Geofísica da Uni-versidade de Lisboa, Edificio C8, Piso 6, CampoGrande, 1749-016 Lisboa, Portugal;[email protected]

Luis Matias, born in Portugal received his Ph.D.in geophysics (seismology) from the Universityof Lisbon. He is a researcher at the InstitutoD. Luiz and professor at the Sciences Faculty ofthe University of Lisbon. His main research in-terests include seismology, marine geophysics,deep geology, seismotectonics, and naturalhazards. Since 2000, he has authored and co-authored more than 25 refereed articles.

Fernando Santos � Centro de Geofísica daUniversidade de Lisboa, Edificio C8, Piso 6,Campo Grande, 1749-016 Lisboa, Portugal;[email protected]

Fernando Monteiro Santos is an assistant pro-fessor at the University of Lisbon. He receivedhis Ph.D. (1994) on geophysics from the Uni-versity of Lisbon. His research interests are fo-cused mainly on applied geophysics usingelectromagnetic methods and also in inverseproblems.

Frode Sandnes � Fortis Petroleum NorwayAS, Solbraveien 20, N-1383 Asker, Norway;[email protected]

Frode Sandnes is vice-president for businessdevelopment at Fortis Petroleum. He received

1668 Salt Tectonics in the Western Gulf of Cad

development of a comprehensive study of the petroleum sys-tems of this basin is inevitably connected to the analysis of thesalt system and related tectonics. The Triassic–Hettangian saltidentified in the onshore Algarve Basin has been studied in thefield, at outcrops, and in an undergroundmine (Terrinha et al.,1990; Terrinha, 1998). However, the knowledge of the cor-relative extensive offshore unit has, so far, been limited by thescarcity of coverage and poor resolution of available seismicdata. Although some diapiric structures were identified in thepast (Terrinha, 1998; Lopes, 2002; Lopes et al., 2006), a de-tailed description of the salt units, together with the fault-related families, needed to be readdressed based on goodquality multichannel seismic (MCS) profiles. Salt tectonicsis an important factor in subbasin development and thin-skinned mechanisms of rifting and tectonic inversion in boththe onshore and offshore basins along the west and south-west Iberian continental margins (Rasmussen et al., 1998;Terrinha, 1998; Kullberg, 2000;Alves et al., 2006; Lopes et al.,2006; Matias, 2007) and, therefore, salt kinematics and ge-ometry are important factors in the evaluation of hydrocarbongeneration and accumulation in the Algarve Basin, offshorePortugal.

Applied petroleum exploration research on the AlgarveBasin has increased during the last few years mostly becauseof the release of commercial seismic data from the hydrocar-bon industry. Until 1998, most of the published studies fo-cused on stratigraphy and tectonics in the onshore part of thebasin (Palain, 1976; Rocha, 1976; Mougenot, 1989; Terrinhaet al., 1990; Kullberg et al., 1992; Cabral, 1995; Terrinha andRibeiro, 1995; Terrinha, 1998). In recent years, as a conse-quence of the success of international research projects, sev-eral regional models for the geodynamic evolution of the Gulfof Cadiz (GoC) have been proposed. These provided a sig-nificant contribution to the knowledge and understandingof the region (Tortella et al., 1997; Maldonado et al., 1999;Gutscher et al., 2002; Gracia et al., 2003a, b; Terrinha et al.,2003; Medialdea et al., 2004; Zitellini et al., 2004, 2009;Gutscher et al., 2009; Rosas et al., 2009; Terrinha et al., 2009;Zitellini et al., 2009).

Regional studies conducted by several oil companies in the1970s have shown that extensional and compressional struc-tures mapped in the offshore Algarve Basin can be related tosalt tectonics. A series of anticlinal structures associated withmoderate salt movement are located within the shelf area andalong the shelf edge (Terrinha, 1998; Lopes, 2002; Lopes et al.,2006). Despite these observations, only a few studies havedirectly addressed the offshore salt distribution and associated

iz, Southwest Iberia

his M.Sc. degree in petroleum geosciences fromthe Norwegian Institute of Technology. Sandneshas 32 yr of experience in the internationaloil industry from positions including businessdevelopment advisor (Bayerngas Norge AS),vice president business development (GenesisPetroleum Corporation Plc), chief geologist(TGS-Nopec Europe), and general manager(Saga Petroleum, Jakarta).

ACKNOWLEDGEMENTS

We thank Teresa Medialdea and Claudia Bertonifor revising the document and for their con-structive criticisms, and also Elena Hernaiz forproducing some of the artwork herein pre-sented. We also thank Martin Jackson for hisconstructive criticisms. We are especially gratefulto TGS for allowing publication of their multi-client seismic data set. We also thank Cabrita daSilva and Teresinha Abecassis for continuousand stimulating support from the Divisão para aPesquisa e Exploração de Petróleo (DPEP).Within the industry, Seismic MicrotechnologyInc., donated the software. During 2003–2005,Hugo Matias was funded by Fundação para aCiência e Tecnologia through a Ph.D. scholarship.The AAPG Editor thanks the following reviewersfor their work on this paper: Arthur E. Berman,Stuart D. Harker, and Martin P. A. Jackson.

tectonics. Terrinha (1998) discussed some prominent saltstructures in the onshore area, such as the Albufeira and theLoulé salt walls (Figure 1). Studies on the offshore part of theAlgarve Basin have also been conducted by Lopes (2002) andLopes et al. (2006), focused on the Mesozoic and Cenozoicsequences, respectively. Lopes (2002) used reprocessed seis-mic data, together with two-dimensional (2-D) gravity for-ward modeling, to interpret the diapiric salt structures. Bothauthors addressed the evaporite deposition and its implica-tions on the basin evolution. However, because of limitationsin seismic data quality, both studies only presented schematicmaps with outlines of the salt diapirs in a restricted area. Inaddition to the recognition of the Hettangian salt unit, alsofound onshore, Terrinha (1998) proposed the existence ofan allochthonous salt sheet emplaced during the Jurassic–Cretaceous transition, which controlled subbasin developmentin the easternmost area of the basin.

This study shows the results of the regional interpretationof a new high-resolution high-quality 2-D seismic data set ac-quired in the offshore Algarve Basin by TGS in 2000–2001.The objective of this study is to contribute to the knowledge ofsalt tectonics in the offshore Algarve Basin through (1) iden-tification and description of the salt units present in the basinwithin the whole western GoC; (2) presentation of schematicand detailed maps of the distribution of the different saltunits; (3) geometric characterization of the salt-related struc-tures and fault families; (4) definition of major salt tectonicdomains; and (5) preliminary discussion on the implications ofsalt tectonics on the conceptual petroleum systems for thisexploratory frontier.

GEOLOGIC FRAMEWORK OF THE GULF OF CADIZ

The evolution of the southern Iberian margin is more com-plex than most other North Atlantic margins. It developed as arift basin adjacent to a transform plate boundary from the EarlyJurassic through the Late Cretaceous. It was subjected to in-version tectonic episodes between the rifting phases (Terrinhaet al., 2002), and during the Late Cretaceous through the Ho-locene went through frontal and oblique convergence of north-west Africa and Iberia with the formation of the Pyrenees andBetics orogens (Figure 2). The major structural trends of themargin, during the Mesozoic, were dominated by half-grabenstructures, particularly in the Triassic–Jurassic–Early Creta-ceous time interval (Terrinha, 1998; Maldonado et al., 1999;Matias et al., 2005; Mohriak, 2005; Mohriak et al., 2008). The

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1670 Salt Tectonics in the Western Gulf of Cadiz, Southwest Iberia

main faults were reactivated during the Late Cre-taceous to the Neogene compressional episodesthat resulted in tectonic inversion.

Structure and Physiography

The study area (Figure 1) comprises much of theoffshore Algarve Basin, which is part of a 165-km(103-mi) long, 100-km (62-mi)-wide northeast-southwest–striking trough developed west of theGoC and part of the Sagres Basin. Water depthsrange from 200 m (656 ft) on the shelf to 3000 m(9843 ft) in the southwestern area (ultradeep wa-ters). To the north, the Algarve Basin is boundedby a basal unconformity of the Early Triassic thatoverlies metamorphosed graywackes and shalesof Carboniferous age folded and thrust duringthe Variscan orogeny. The southern boundarywas composed of a series of basement highs: SaoVicente Highs, Albufeira High, and GuadalquivirBank. The Guadalquivir Bank (Figure 1), a horstduring Mesozoic rifting, is characterized by a con-spicuous high-gravity anomaly (Gràcia et al., 2003a)and separates the Algarve Basin from the Alloch-thonous Unit of the GoC (AUGC) (Figure 1)(Medialdea et al., 2004, 2009).

The AUGC refers to a large complex of tec-tonic and gravitational imbricate bodies that ex-tend from theGibraltar Arc across theGoC throughmost of the Horseshoe Abyssal Plain that yield achaotic seismic facies in MCS profiles. These bod-ies started to form in association with the firststages of southwest Iberia–northwest Morocconorth-south–oriented collision, that is, initial stagesof inversion of the rift basins, and continued to formand deform during Neogene times through a pro-cess of thrust stacking, large-scale mass transportdeposits, and gravitational instability mostly asso-ciated with the westward-directed overthrust ofthe Betic-Rif orogen and formation of theGorringebank andCoral Patch ridge (Maldonado et al., 1999;Medialdea et al., 2004; Iribarren et al., 2007;Gutscheret al., 2002; Terrinha et al., 2009).

The main extensional faults strike north-southparallel to the Atlantic Ocean in the west andnortheast-southwest to east-west in the centraland eastern parts where the Tethys rifting had astronger influence. The north-south and northwest-southeast–striking faults subdivided the basin invarious compartments. To the west, the onshoreAlgarve Basin is bound by a Paleozoic basementhigh with a northeast-southwest orientation thatextends from the onshore across the shelf, where itis covered by Mesozoic strata, that are in turn un-conformably overlain byCenozoic sediments. Thisfeature links the south and west Iberia margins andconnected the Boreal and Tethyan oceanic realms(Terrinha et al., 2002) in theMesozoic. The centraland western shelf and slope of the Algarve Basinincised by canyons that transport sediments ontothe Horseshoe Abyssal Plain. The eastern sector ofthe basin is bounded to the west by the offshoreprolongation of the Sao-Marcos Quarteira Fault(SMQF) Zone (Figure 1) (Terrinha, 1998; Lopes,2002; Lopes et al., 2006). The central sector isbounded to the west by the Portimão Fault Zone(PMFZ), which also corresponds to the PortimãoCanyon.

Basin Evolution

The Algarve Basin consists of two basins: one ofMesozoic age and another of Cenozoic age.

TheMesozoicAlgarve Basin is a rift basinwhosehistory began with the deposition of the first syn-rift terrigenous sediments of the Early Triassic andendedwith the deposition of themarine limestonesof the Cenomanian (Figure 2). This sucession iswell exposed onshore; offshore, it is imaged onseismic reflection profiles and it was penetrated byfour oil industry wells (Figure 1). Four main riftingphases can be established based on the correlationof sedimentary facies, sediment thickness variations,and age of faulting (Palain, 1976; Rocha, 1976; Rey,1983; Manupella, 1988; Lopes, 2002; Terrinhaet al., 2002). The earliest rifting started with the

Figure 1. (A) Map of the Iberian continental margin showing the western Gulf of Cadiz (Algarve and Sagres basins), major tectonicfeatures, and adjacent offshore basins. (B) Map of the western Gulf of Cadiz illustrating the interpreted seismic grid and main structuralelements. Map coordinates refer to WGS84/UTM zone 29.

Matias et al. 1671

deposition of the continental clastics in the EarlyTriassic that graded into finer clastics, carbonates,and evaporites interbedded with tholeiitic basaltsof the Hettangian (Verati et al., 2007; Martinset al., 2008), followed by shelf carbonates of theSinemurian, and limestones and marls with ammo-

1672 Salt Tectonics in the Western Gulf of Cadiz, Southwest Ib

noids of the Pliensbachian and the early Toarcian(Figure 2). The second stage of rifting started inthe Aalenian(?)–Bajocian, with shelf carbonatesthat progressively graded into limestones and marlswith ammonoids until the late Callovian. The thirdepisode started in the mid-Oxfordian that lasted

Figure 2. Simplified stratigraphy with main tectonic events in the Algarve Basin and main seismic units interpreted in this article. AUGC =allochthonous units of the Gulf of Cadiz; GB = Guadalquivir Bank.

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until the Tithonian. Sedimentation initiated withdeposition of shelf carbonates that graded intomarlswith ammonoids. The fourth period of rifting is onlywell dated from the earliest Cretaceous throughthe Albian. Sedimentation initiated with Purbeckfacies in the Berriasian, followed by deposition oflagoon and marine limestones with an event ofsiliciclastic sedimentation in the Barremian. TheUpper Cretaceous is represented by marine shelflimestones of the Cenomanian. The four phases ofrifting each culminate with basinwide facies ho-mogenization and are separated by clear erosiveunconformities. Three episodes of compressionoccurred between the rifting phases (Terrinha et al.,2002). The first one occurred in the Pliensbachianand the other two at the end of the Callovian andthe Tithonian. The main extensional fault ori-entations that accommodated the rifting werenorth-south and northeast-southwest in the west-ernmost parts of the basin and east-northeast-west-southwest to east-west in the central and easternparts. Northwest-southeast to north-south–strikingfaults bound the main grabens and half grabens.The SMQF is a northwest-southeast–striking faultthat resulted from inversion of a Paleozoic thrustthat separates the central from the eastern part ofthe basin in which the major subsidence occurred.The PMFZ is a north-south–striking fault that re-sulted from reactivation of a lateVariscan fault thatseparates the western and central domains of thebasin (Figure 1).

The Cenozoic Algarve Basin contains stratafrom Paleocene through the Holocene (Figure 2).The Paleogene is only recorded offshore, drilled bythree out of five oil industry wells. It consists oflimestones, silts, and marls (Figure 1). The oldestdeposits range from Paleocene to Oligocene, lieunconformably on folded Lower Cretaceous rocks,and they are folded and thrusted (Terrinha, 1998;Lopes et al., 2006; Roque, 2007). The Neogeneand the Quaternary crop out offshore and onshoreand are well dated as Burdigalian through theHolocene. The Neogene unconformably overliesthe previous folded and thrusted units, suggestingthat most of the tectonic inversion of the rift basinis constrained from post-Cenomanian to the lateOligocene–Aquitanian.Nevertheless, evidence from

outcrops and seismic profiles suggests that somethrusting affected the lowest Miocene strata in thebasin. Tectonic inversion was mainly accommo-dated by southward-directed thrusting. Pliocenethrough present compression is evident from milddeformation of the upper Miocene, Pliocene, andQuaternary, fromuplift of Pliocene littoral segmentsabove interglacial sea levels, and from historicalandmeasured seismicity. Present-day compression,deduced from earthquake focal mechanisms, isnorthwest-southeast oriented (Ribeiro et al., 1996).The main cause of present-day active tectonics isthe oblique collision between northwest Africaand southwest Eurasia in the study area, which iscausing thrusting and dextral wrenching.

Paleogeography and Relation of OffshoreIberia and Central Atlantic Salt Basins

The Hettangian salt unit in the Algarve Basin ispart of amuch largerUpper Triassic–Lower Jurassicsalt depositional system that extended throughoutthe rift system that preceded opening of the cen-tral Atlantic (Ziegler, 1986;Mohriak et al., 2008),forming conjugate salt basins in the Grand Banks–Lusitanian basins and Nova Scotia–Moroccan mar-gins (Figure 3).

During the middle Permian to Early Triassic,extension occurred on the former Appalaches andMauritanides fold belt, between Laurentia and theSahara platform. As a result, a series of rifts and pull-apart basins developed, which successively linkedinto a broader rift system, following the structuralgrain of the older Variscan compressive defor-mation in the North Atlantic. During the Triassic,extensional deformation acted on the western partof the Fennosarmatian craton, splitting off Green-land fromNorway and propagating southward intoLaurentia (Ziegler, 1989). At the same time, in theTethys, spreading propagated from east to west,across the Variscan fold belt, separating the Iberiacraton from the Sahara platform. The three pro-cesses interacted to create a patchy distribution ofgrabens and half grabens between the presentCanada and Iberia (Figure 3).

In the central Atlantic, crustal extension contin-ued between the Sahara platform and Laurentia

Matias et al. 1673

during the Triassic to Early Jurassic until breakupin theAalenian.Northward propagation of oceanicspreading was limited by the Newfoundland-Gibraltar Transform Fault zone (NGTF), whichtransferred extension into the Tethys spreading axis(Schettino and Scotese, 2002). North of the NGTFalong the West Iberia Margin, rifting and subse-quent thermal subsidence prevailed until the lateEarly Cretaceous, as oceanic breakup propagatedfrom south to north in the Barremian (Srivastavaet al., 2000). Figure 3 shows a schematic distribu-tion of the rift pattern and the salt basins devel-oped between Laurentia (North American plate),the Sahara platform (Gondwana), the Iberia cra-


ton, and the Armorican platform (Eurasian plate)during the Early Triassic–Late Jurassic. Basin re-construction along the Saharan platform (Gond-wana) and the Variscan Nova Scotia fold belt (aspart of the North American plate) follow Davison(2005), whereas basin geometries between Laur-entia, Iberia, and Eurasia have been taken fromseveral sources (Lefort, 1984; Tankard andBalkwill,1989; Wilson et al., 1989; Srivastava and Verhoef,1992).

Because the Algarve Basin is located north ofthe NGTF, it is worthwhile comparing its initialrifting with that of the Lusitanian, Jeanne d’Arc,Carson, and Bonniton basins that were adjacent

Figure 3. Schematic Late Triassic–Early Jurassic paleogeography of the central Atlantic. Based on Enachescu (1987), Tankard et al.(1989), Ziegler (1989), Srivastava and Verhoef (1992), Davison (2005), and Wielens et al. (2006).

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before the oceanic breakup. These basins show asimilar stratigraphic evolution, although their depo-centers behaved independently. In the Jeanne d’ArcBasin, Middle Triassic clastic sequences (EurydiceFormation) were followed by Rhaetian–Sinemuriansalt sequences (Argo Formation), and subsequentlycovered by a carbonate platform (Iroquois Forma-tion of the Pliensbachian), indicative of a slowdownof rifting (Sinclair et al., 1992).

The Lusitanian Basin was separated from theJeanne d’Arc Basin by the Springdale horst beforethe Barremian oceanic breakup (Enachescu, 1987,1992; Wielens et al., 2006), and its stratigraphicrecord for the initial rifting phase is similar (Wilsonet al., 1989; Kullberg, 2000; Uphoff, 2005; Alveset al., 2006), with Triassic clastic sequences (SilvesFormation), followed by the evaporites of the Da-gorda Formation (uppermost Triassic–Lower Ju-rassic) overlain by a Sinemurian shallow dolomiticplatform (Coimbra Formation). The initial riftingof theAlgarve Basinwas coevalwith thementionedbasins, and the stratigraphic record is similar, withthe exception of the occurrence of a tholeiitic basaltevent that occurred in the Sinemurian (Mirandaet al., 2008).

DATABASE AND METHODOLOGY

Database

A 2-D regional seismic reflection survey, consist-ing of a set of 58 lines of 2-Dmulticlient reflectionseismic data, was acquired by TGS in 2001 duringthe PDT00 and PD000 cruises (TGS, 2001). TheTGS survey was the main database for this study(Figure 1). The total coverage of the 2-D seismicdata set (5820 km [3616 mi]) is 17,890 km2

(6907mi2), extending from the shelf to the basinalpart of the GoC (Figure 1). The TGS seismic sur-vey was acquired with a 6000-m (19,685-ft)-lengthstreamer, and the line spacing was about 8 × 4 km(5 × 2.5 mi) in a north-northwest–south-southeastand east-northeast–west-southwest direction, re-spectively, with a shot point interval of 25 to 30 m(82 to 98 ft). Recording lengthwas 12,000ms two-way traveltime (TWT).

These 2-D seismic data were acquired with asampling rate of 2 ms and later resampled, duringprocessing, at 4 ms. Data quality is good after ap-plication of multiple removal during processing.Seismic data are displayed according to Society ofExploration Geophysicists inverse polarity, mean-ing that an increase in acoustic impedance across aninterface produces a negative-amplitude excursionon the seismic trace (trough).

Well data used in this study were provided bytheDirecçãoGeral deGeologia e Energia (DGGE),through the library Divisão para a Pesquisa e Ex-ploração de Petróleo (DPEP). These data consist ofdrilling reports, well-log geologic evaluation, checkshots, and regional exploration reports conductedby several oil companies. Data from wells in theeastern GoC were taken from Lanaja et al. (1987).

All the available data were loaded into a King-dom Suite project to ensure the proper integrationof all interpretation elements. The project datum isat mean sea level (0 m). The projection coordinatesystem used is the UTM29 (Universal TransverseMercator), and the datum ellipsoid is WGS84. Allthe maps have been plotted using this coordinatesystem. As for the well data and associated depths,they have all been referenced to true vertical depthsubsea (TVDSS), that is, corrected for kelly bushingelevation and drilling deviations. Throughout thisarticle, seismic sections and maps are displayed inTWT in milliseconds. Depths and thickness areexpressed in meters, where time-depth conversionwas conducted.

Methodology

The methodology used in this article is based pri-marily on the interpretation of 2-D seismic dataconstrained by well-log calibration. The interpre-tation of morphological features associated withsalt deformation and halokinesis on 2-D seismicreflection is generally complex (geometry and ver-tical extent of the diapirs) because of their highlyirregular shapes and deterioration of the imaging ofthe subsalt formations. Iteration between interpre-tation of seismic lines and gravity maps (i.e., verticalderivative and analytical signal) and 2-D gravitymodeling was successfully used to alleviate this

Matias et al. 1675

Figure 4. (A) Lithostratigraphic chart of the Triassic–Hettangian units defined onshore by Rocha (1976); (B) Offshore Triassic rocks at the bottom section of Ruivo-1, 6-Y-1 BIS, and C1wells (see location of wells in Figure 1).

1676SaltTectonics

inthe

Western

GulfofCadiz,SouthwestIberia

problem (Matias et al., 2005; Matias, 2007). Cali-bration of seismic lines and the age of horizons and/or unconformities was done using check-shot datafrom well Ruivo-1 and Algarve 1, drilled on theshelf and slope, respectively. Depth conversion ofTWT isochron maps was conducted using a layer-cake model based on the constant velocities takenfrom Ruivo-1 and Algarve 1 wells. The Ruivo-1well penetrated the lowermost Jurassic evaporitesand stopped drilling when Hettangian basaltswere penetrated (Figure 4).

The nomenclature adopted to classify the mainsalt structures and associated tectonicswas based onRowan et al. (1999) andHudec and Jackson (2007).Additional data from unpublished stratigraphic re-ports and previous studies provided useful infor-mation on stratigraphy, tectonics, and halokinesis inthe Iberian southern margin. This integrated ap-proach of interpretation allowed identification and

correlation of the main salt tectonic morphologicalelements of interest.

INTERPRETATION OFSEISMIC MEGASEQUENCES

The seismic-stratigraphic framework of the studyarea on the offshore Algarve Basin was simplifiedand divided into four seismic-stratigraphic units(Matias et al., 2005): unit 0, unit 1, unit 2, andunit 3, which were defined based on their seismiccharacter and well calibration, corresponding tobasement, salt, early post–salt, and late post–saltstratigraphic sequences (Figures 2, 5). The seismicdata quality is much better in the eastern and cen-tral sectors of the basin than in the western sector.Toward the southern part of the basin, the pres-ence of the AUGC that has a seismic chaotic facies

Figure 5. Salt diapirs exhibiting salt tongues, primary welds, and crestal faults in the shelf, slope, and deep basin. TWT = two-waytraveltime; K/J. Unc. = Unconformity Separating Cretaceous from Jurassic; gp = growth patterns; SMB = salt minibasin; PW = primaryweld; Unit 0 = Paleozoic–Triassic basement; Unit 1 = Late Triassic–Hettangian Salt; Unit 2 = Jurassic through Paleogene; Unit 3 = Mioceneto Holocene. For location of this seismic line, see Figure 1. Data courtesy of TGS.

Matias et al. 1677

significantly obscures the imaging of deeper reflec-tors and limits any interpretation for the pre–saltsequences. Three seismic profiles (Figures 5–7) ori-ented parallel and perpendicular to the continentalmargin in the shelf, slope, and basinal areas illus-trate the main salt features and the seismic strati-graphic units. Thickness maps of unit 1 and unit 2(Figure 8A, B, respectively) also aid in the interpre-tation of the salt tectonics and illustrate the depo-sitional trends of the salt and lower post–salt units.

Unit 0 (Paleozoic–Triassic Basement)

In the study area, the base of the Mesozoic–UpperTriassic unit corresponds to the acoustic basement(herein defined has the deepest coherent reflection)in the offshore Algarve Basin (Figures 5–7). The topof unit 0 corresponds to a discontinuous and poorlydefined reflector, which is associated with the base


of the uppermost Triassic–lowermost Jurassic evap-oritic unit. As a consequence, the top of the acousticbasement does not exactly match the top of thePaleozoic basin basement, which is expected to lie100 to 300 m (328–984 ft) below the base of theevaporitic series. In some areas, the basement im-age is similar to the horizon corresponding to thetop of the Triassic salt diapirs. To overcome thisproblem, mapping of the top of the basement wasaided by the results from gravity inversion, whichallowed a much better discrimination betweenbasement highs and salt diapirs (Matias, 2007).

Unit 1 (Upper Triassic–Hettangian Salt)

This unit is the focus of this study. Figure 4 illus-trates the onshore and offshore lithofacies of theTriassic–Hettangian unit. In the western part ofthe GoC, the autochthonous Triassic–Hettangian

Figure 6. Composite regional seismic line with main salt tectonic domains. TWT = two-way traveltime; K/J. Unc. = UnconformitySeparating Cretaceous from Jurassic; gp = growth patterns; SMB = salt minibasin; PW = primary weld; Unit 0 = Paleozoic–Triassicbasement; Unit 1 = Late Triassic–Hettangian Salt; Unit 2 = Jurassic through Paleogene; Unit 3 = Miocene to Holocene. For location of thisseismic line, see Figure 1. Data courtesy of TGS.

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salt was penetrated by well Ruivo-1 (Figure 4),which drilled 224 m (735 ft) of Upper Triassic–Hettangian rocks composed primarily of anhydritewith minor interbeds of claystone, limestone, andsalt. To the east, the correlative sequence in the ex-ploratory well 6-Y-1 BIS consists of 200 m (656 ft)of interbedded anhydrite, limestone, and claystone,whereas the well GC C-1 drilled a lower section ofsalt deposits, composed of thick layers of halite,anhydrite, potassium salt, and red clays (Figure 4).

In most cases, the top of this evaporite-bearingunit corresponds to a high-amplitude reflection,derived from the high impedance contrast gener-ated at the boundary with the overlying sediments(Figures 5, 6). Poor seismic imaging seems to berelated more with steep flanks of the diapirs ratherthan to this impedance contrast. Internally, theevaporite unit is characterized by a chaotic seismic

character. The salt base is an extremely elusivereflector and is not easily identified on the seismicprofiles. Thus, to avoid major errors in the map-ping of the salt base, the present interpretationwas hinged on the best images below the salt dia-pirs and their lateral correlation with salt welds,that is, places where the base of the salt was clearer(Figure 9). This evaporite unit is overlain by Neo-gene, Cretaceous, or Jurassic sediments (Figures 5–7), depending on its degree of deformation.

Three salt stratigraphic units were identified.First, an autochthonous unit of Hettangian age,defined as a salt horizon at their original Hettan-gian stratigraphic position, that is spread acrossthe entire area (Figure 8C); second, a para-autoch-thonous unit corresponding to those diapirs that,despite lateral and vertical salt migration, are stillconnected with the Hettangian mother salt horizon

Figure 7. Regional seismic line with salt structures in the shelf. TWT = two-way traveltime; gp = growth patterns; PW = primary weld; st =salt tongue; Unit 0 = Paleozoic–Triassic basement; Unit 1 = Late Triassic–Hettangian Salt; Unit 2 = Jurassic through Paleogene; Unit 3 =Miocene to Holocene. For location of this seismic line, see Figure 1. Data courtesy of TGS.

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Figure 8. (A) Thickness map (in meters) of unit 1, (B) thickness (in meters) of Mesozoic–Lower Cenozoic (unit 2), and (C) synoptic mapwith main salt geomorphological features. Map coordinates refer to WGS84/UTM zone 29.

1680 Salt Tectonics in the Western Gulf of Cadiz, Southwest Iberia

or were only severed at their roots; and third, anallochthonous unit observed in the eastern area only(Figure 8).

The currently available seismic data set wasacquired and processed with no special emphasison the subsalt imaging. Still, the top and base ofthe allochthonous salt tongues and nappes areimaged quite well on most of the lines. Seismicresolution and line spacing are good enough forthe interpretation of major nappes and salt tonguestructures, but lateral and areal correlation ofstructures (e.g., faults) is unreliable and can onlybe resolved using three-dimensional seismic data.The TWT isopach map of unit 1 was depth con-verted using a constant velocity of 4500 m/s, asshown in panels A and C of Figure 8. The synopticmap in Figure 8C was based on the thickness mapof unit 1 (Figure 8A).

Unit 2 (Jurassic–Paleogene)

The unit 2 is bounded at the base by the Top Salthorizon and at its top by a regional erosional sur-face, interpreted as the Miocene unconformity,which is associated with an important regional up-lift and basinwide erosion (Figures 5–7). This unitwas drilled in several wells in the offshore Algarve

(e.g., Ruivo-1, Algarve 1, Corvina 1, and Impe-rador 1) and is mainly composed of limestones andmarls of the Jurassic, siliciclastics and limestonesof the Cretaceous (Berriasian–Cenomanian), andshelf limestones of the Paleogene.

Locally within unit 2, another important sur-face has been interpreted corresponding to thePaleogene unconformity (Figures 5–7). On theshelf, this unit is characterized by high-amplitudeand low-frequency seismic reflectors (Figure 5). Inthe deep basin, these reflectors are continuous atthe base and become slightly discontinuous to-ward the top of the section. The TWT isopachmap of unit 2 was depth converted using a con-stant velocity of 3500 m/s for the early post–saltlayers (Figure 8B), based on the well Algarve 1.

TheMesozoic horizonswithin unit 2 show clearexamples of wedging toward extensional faultsand salt diapirs, attesting to the synsedimentarygrowth of the latter and their control ofminibasins(Figures 5, 6, 9), whereas these geometric featuresare not observed so clearly in the Paleogene.

Unit 3 (Miocene–Holocene)

The base of this unit is characterized by a high-amplitude and continuous reflector identified as

Figure 9. Salt minibasin SMB1(see Figures 5, 6, 8 for location).D1 to D3 and S1 to S4 corre-spond to main discontinuitiesand sequences in this example.Depicted in black are the shapesof the salt diapirs. Divergent re-flections in opposite directionsof S1 and S2 indicate growth ofboth diapirs. Consistent diver-gence toward the south aboveD3 indicates growth of southerndiapir only. D1 corresponds toa salt weld caused by completeexhaustion of salt between thetwo diapirs. D4 marks the onsetof compression of the diapir andpossibly basin inversion. Datacourtesy of TGS. TWT = two-waytraveltime.

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the Miocene unconformity. The top of the unit isthe present sea floor. This unit includes the Mio-cene turbidite system (Ledesma, 2000; Martinezdel Olmo et al., 2006), the contouritic units(Llave et al., 2006), and the AUGC (Medialdeaet al., 2004). The Neogene unit is characterizedby parallel to subparallel reflectors and, in somecases, it exhibits transparent seismic character as-sociated with marine shales (Figures 5, 6). In theshelf and base of slope area, it is highly channelized(Figures 5, 7), indicating that an active system of


feeder channels provided the depocenter of thebasin with extensive clastic sedimentation.

SALT-RELATED STRUCTURAL STYLES:SEISMIC EXAMPLES AND INTERPRETATION

Whereas relatively simple salt diapirs were de-scribed onshore (Terrinha et al., 1990; Terrinha,1998), the salt tectonic geometries become grad-ually more varied going offshore. Figures 5 and 6

Figure 10. Thrust faultsassociated with salt coredfold. Two onlap surfaceshave a particularly impor-tant meaning: (A) this on-lap surface clearly recordsa pulse of major short-ening in the early Mioceneand (B) onlap above thearched crest of the talldiapir suggests that thediapir was compressionallyrejuvenated in the Plio-cene–Pleistocene but is notbeing squeezed today. Datacourtesy of TGS. TWT =two-way traveltime; gp =growth patterns; PW =primary weld; SW = sec-ondary weld; Unit 0 =Paleozoic–Triassic Base-ment; Unit 1 = LateTriassic–Hettangian Salt;Unit 2 = Jurassic throughPaleogene; Unit 3 =Miocene to Recent.

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depict a variety of structural types of salt struc-tures, bell-shaped bodies, salt walls, salt apophasis,and an allochthonous salt nappe (Esperança salt).Also, a series of tectonic structures formed in as-sociationwith the emplacement of these salt bodies,such as, crest faults, welds, toe thrusts, rim syn-clines, and so on. The Triassic–Hettangian autoch-thonous salt is widespread across the basin, indi-cating original deposition of evaporites from theGuadalquivir Bank in the south to the northern-most onshore extensional fault line in the north(Figure 8).

Salt Pillows, Diapirs, and Minibasins

Salt pillows spread across all of the central andeastern parts of the basin as shown in Figure 8.They consist of structures with a maximum of

1500 m (4921 ft) height, can be more than 10 km(6.2 mi) across (Figures 5, 6, 8) and are the oldeststructures in the basin because they affect only thelower packages of the Mesozoic, unit 1 (Figures 5,6). Several salt piercement features occur through-out the shelf, slope, and deep basin (Figure 8C).Most of the salt diapirs are located along diapiricridges that strike (1) northeast-southwest in theeastern sector, (2) east-west in the central sector;and (3) west-northwest–east-southeast in the west-ern sector. Salt walls and diapirs have very differentshapes in cross sections, as evident in Figures 5–7,10, 11, showing development of tongues and bulbsat their tops. However, in their lower parts, they areof two types only: those that developed from pre-viously formed pillows or those that consist of nar-row stems formed on top of basement extensionalfaults. The salt walls are very steep and range from

Figure 11. Present-daydiapirs in the southwestarea with crestal collapsefaults affecting the over-lying sediments. For loca-tion of this seismic line,see Figure 6. Data courtesyof TGS. TWT = two-waytraveltime; gp = growthpatterns; PW = primaryweld; Unit 0 = Paleozoic–Triassic Basement; Unit 2 =Jurassic through Paleo-gene; Unit 3 = Miocene toRecent.

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11 km (7 mi) length × 2.5 km (2 mi) wide in theeastern domain (SW1; Figure 8) to 24 km (15 mi)long and 5.6 km (4 mi) wide (SW2; Figure 8) in thesouthwestern domain. Isolated subcircular diapirshave radii from 1.5 to 7 km (0.9–4mi). The thickestdiapirs found in the basin are located in the south-west area, forming salt walls that may reach 8 km(5 mi) in thickness and that deform the present-day sea floor (SD1, Figures 8, 10, 11).

The isopach map of seismic unit 2 (Figure 8B)shows a clear control of the thickness of the Me-sozoic by the trend of the salt structures. Note inFigure 8 the development of a west-to-east lineardepocenter adjacent to a diapiric ridge in the cen-tral part of the basin, as well as the location of themini basin SMB1 in close association to the salt walllocated to the south. The same thing happens in thesouthwest area where the thickest subbasins andsalt welds are confined by the development of di-apirs and the southern basement high (Sagres Pla-teau; Figure 1). On seismic sections, these thick


syndeformational depocenters commonly have di-vergent reflection pattern, a wedge external shape,and are characterized by thickening toward thecenter of the minibasin (e.g., SMB1; Figures 5, 6,9). The most conspicuous feature is an elongatedminibasin (SMB1; Figure 8C) with a length ofabout 37 km (23 mi) and a width of about 10 km(6 mi), with Mesozoic–Lower Cenozoic sedimentsreaching a maximum thickness of about 5.5 km(3 mi). In general, average thickness in these mini-basins is about 4.5 km (3 mi) (Figure 8B).

Salt Tongues

The central area of the offshore Algarve Basin ischaracterized by some conspicuous salt structuressuch as tongues associated with diapirs rising fromthe para-authothonous salt diapirs (Figures 5, 7).The tongues are connected to the top parts of thediapirs, indicating that these tongues developed inthe later stage of the halokinesis. All the observed

Figure 12. Deformed salt nappe (Esperança nappe) with toe-thrust faults and crestal faults affecting Pliocene–Quaternary sediments.For location of this seismic line, see Figure 6. Data courtesy of TGS. TWT = two-way traveltime; SMB = salt minibasin; Unit 0 = Paleozoic–Triassic Basement; Unit 1 = Late Triassic–Hettangian Salt; Unit 2 = Jurassic through Paleogene; Unit 3 = Miocene to Recent.

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tongues developed within the sedimentary units ofthe Late Jurassic and the Cretaceous but have alsoaffected the Miocene sediments.

Primary and Secondary Welds

The Hettangian salt was deformed into salt pil-lows (most likely during the Early Jurassic, almostimmediately after salt deposition and post–salt sed-imentation started), forming a series of topographichighs and lows. The depressions were filled withLower toMiddle Jurassic sediments. In several cases,the formation of salt pillows and evolving salt di-apirs locally depleted the basal salt layer, resultingin salt welds (Figures 5–12), which were mainlyformed during the Late Mesozoic. Salt evacuationand deformation has had an important function inMesozoic sedimentation, as can be seen by the de-velopment of several withdrawal minibasins thatshow marked growth patterns of diverging reflec-tors in different ages (Figures 5, 9).

Thrust or secondary salt welds are related tocompressional tectonic regimes, associated folds,thrust faults, and consequent compression of di-apirs (Jackson and Cramez, 1989; Rowan, 1995;Vendeville and Nilsen, 1995; Rowan et al., 1999),resulting in isolated salt bodies almost detachedfrom their feeders (Figure 10). The southwestdomain (Figures 8, 10) exhibits intensive foldingand isolated salt diapirs that intruded along thefold axis. Also close to the northern boundary ofthe Guadalquivir Bank, several “squeezed” diapirscan be seen in seismic sections (Figure 5). De-formation of the present-day sea floor (Figure 6,southwest domain II) and tilting of Miocene throughQuaternary sediments (Figure 5) indicates thatthese diapirs were recently compressed as a resultof the collision of the African and Eurasian plates.Similar secondary salt welds have been described inthe Perdido and Mississippi Fan fold belts (Rowanet al., 1993, 1999; Rowan, 1995) associated withdeformation of autochthonous salt.

Crestal Faults

Some halokinetic structures display importantstructural deformation of the overburden asso-

ciated with crestal faults. These are generally high-angle normal faults on the crests of diapirs (Rowanet al., 1999). The faults dip inward toward thedownsagged part, forming symmetric grabens and/or depressions. They are caused by extension ofthe overburden and/or dissolution processes withinthe salt, creating instability and the collapse of theoverlying layers (Rowan et al., 1999; Bertoni andCartwright, 2005). Examples of crestal faults ontop of salt walls and circular diapirs are shown inFigures 5, 7, 11, and 12. The faults and associateddepressions may reach a diameter of approximately4 to 6 km (3 to 4 mi), as measured on cross sectionbetween its opposite external rims.

Toe-Thrust Faults

Toe-thrust faults are landward-dipping reverse faultsnear the downdip toes of salt bodies or their evac-uated equivalents, and are generally arcuate inmapview (Rowan et al., 1999). In the southwestern areaof theAlgarveBasin, toe thrusts are interpreted closeto the Hettangian salt hinge line (southernmost di-apir in Figure 6), when the salt climbs stratigraph-ically. Analog processes occur in the Campos andEspírito Santo basins offshore Brazil, Angola mar-gin, and offshore the Canadian and Moroccan mar-gins (Tari et al., 2000; Mohriak, 2005; Haddou andTari, 2007; Mohriak et al., 2008).

SALT TECTONIC DOMAINS

Interpretation of the pattern of distribution of thesalt in the basin allowed subdivision of the basininto three sectors, the eastern domain, the centraldomain and the southwest domain (Figure 8).

The Eastern Domain: The EsperançaSalt Nappe

The eastern domain is mainly characterized by thepresence of a conspicuous allochtonous nappe, oc-cupying mostly the eastern part of the basin, con-fined to the area located between the GuadalquivirRidge and the present-day lower slope (Figures 8,12). It occupies an area of approximately 1400 km2

Matias et al. 1685

and has an average thickness of about 600 m(1969 ft) (Figures 6, 8, 12–15). The presence ofthis allochthonous salt unit was first proposed byTerrinha (1998), and its detailed geometry and spa-tial distribution in structural maps were first de-scribed byMatias (2007), who named it “Esperançasalt.”

The western and eastern limits of the Esperançasalt could not be defined with precision. The top ofthe Esperança salt is highly irregular, slightly dia-piric at places. Fewwithdrawal basins are above theEsperança salt. In these, the salt has been displacedlaterally, forming salt pillows, minibasins, and ter-tiary welds bounded by roller faults (Figures 6, 13,


14) mostly during the Paleogene. At present, mi-nor reactivation of the Esperança salt structures isexpressed by faulting and/or tilting of recent sub-surface strata overlying salt anticlines (Figures 13,14). TheAlgarve-2well located in this area reachesonly the Paleocene and does not cut across any salthorizons.

Genetically, the Esperança salt is interpretedas a large salt nappe (ramping up from the autoch-thonous salt to the Upper Mesozoic section. Thissalt body originated in the easternmost part of thebasin, forming a nappe that spread southwestward(Figures 12, 15). It probably resulted from coales-cence of extruding Triassic–Hettangian salt walls

Figure 13. Northwest-southeast line illustrating the southwestward movement of the Esperança salt. TWT = two-way traveltime; gp =growth patterns; SMB = salt minibasin; TW = Tertiary weld; Unit 0 = Paleozoic–Triassic Basement; Unit 1 = Late Triassic–Hettangian Salt;Unit 2 = Jurassic through Paleogene; Unit 3 = Miocene to Holocene. For location of this seismic line, see Figure 1. Data courtesy of TGS.

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and diapirs, located in the vicinity of the Gualdal-quivir bank (east-northeast–west-southwest strik-ing salt walls SW1a, SW1b; Figures 7, 8), a processdescribed by Hudec and Jackson (2006) and anal-ogous to halokinetic processes that operate in theGulf of Mexico, Lower Congo Basin, and offshoreMorocco (Wu et al., 1990a, b; Tari et al., 2000;Wuand Bally, 2000; Mohriak, 2005; Hudec and Jackson,2006; Mohriak et al., 2008).

The age of emplacement was determined basedon the tie to wells. The touchdown of post–saltlayers on the continuous reflector at the base ofthe salt relatively dates them as emplaced at theJurassic–Cretaceous boundary (Matias, 2007), in-dicating that this body was most likely extrusion-ally emplaced during the Upper Jurassic compres-sional pulses (Terrinha et al., 2002). Also, the layersat the base of the salt layer seem to be conformable

Figure 14. Roller faults and tertiary welds affecting Mesozoic–Cenozoic sediments, related with the Esperança salt unit. TWT = two-waytraveltime; gp = growth patterns; TW = Tertiary weld; Unit 1 = Late Triassic–Hettangian Salt; Unit 2 = Jurassic through Paleogene; Unit 3 =Miocene to Holocene. For location of this seismic line, see Figure 1. Data courtesy of TGS.

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with the base of the salt sheet (Figure 12), providingsupport for an extrusional origin instead of intrud-ing the nearby sediments.

The presence of roller faults with steep tiltedfootwalls and the evidence of tertiary welds sup-port the interpretation that this salt nappe ac-commodated extension and acted as a decollementfor the Cretaceous through Cenozoic cover, mainlyin the lower slope area (Figures 13, 14, 15A).


Cenozoic compression has deformed the salt nappe,as expressed by toe-thrust faults in its westernlimits, inducing small anticlines with faults at theircrests and toe thrusts in the front edge (Figure 12).Extension seems to have been more intense duringthe pre-Miocene, considering the small throw of theroller faults observed at the Miocene unconformitylevel (Figure 13). Compression occurred for muchlonger periods and affected Pliocene–Quaternary

Figure 15. (A) Simplified dia-grams illustrating the Esperançasalt showing southwestwardcompression and basinward ex-tension. Bathymetry is also over-laid to show salt movementperpendicular to sea floor con-tours. See Figure 8 for location.(B) Roho system (Rowan et al.,1999, used with permission ofAAPG).

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strata. This outstanding salt structure appears tobe a Roho system (sensu Rowan et al., 1999)(Figure 15B). Roho systems are defined by seaward-dipping typically listric normal faults accommo-dating a significant amount of extension in the roofsequence of a salt sheet or nappe (Rowan et al.,1999). The deformation is driven by gravity slidingon top of allochthonous salt. Analogies betweenthe Esperança salt and the Sigsbee Salt Canopy inthe Gulf of Mexico are inevitable (Hall et al., 2000;Mohriak, 2005; Mohriak et al., 2008). The cur-rently available 2-D seismic coverage is not suf-ficient to map the details of the Esperança saltnappe and will be soon enriched with additionaldata that hopefully will allow further and moredetailed studies.

The Central Domain

The central domain extends from the upper slopeto the deep basin and is characterized on the shelfand slope by isolated diapirs, whereas the centraldeep-water basin is dominated by elongated saltwalls (Figure 8). These salt walls are sinuous elon-gated ridges that strike east-west in the map view.They are associated with the main extensional faultstrike of the basement and separate deepminibasinsformed by salt evacuation, hence influencing Me-sozoic deposition. In the central area, most of thediapirs seem to have affected sediments up to thebase of the Neogene, indicating that halokinesisended before the Late Cenozoic.

Some diapirs developed crestal faults causedby extension (Figure 5). Throughout the area, sig-nificant salt evacuation occurred, associated withthe rise of salt from the autochthonous Hettangianlayer. This resulted in several salt welds, with sed-imentation occurring contemporaneously with saltmovement, as evidenced by growth patterns fromthe Lower Jurassic to the Cretaceous, and the de-velopment of salt minibasins with thick Mesozoic–Early Cenozoic–aged depocenters (Figures 5, 6, 8).Salt tongues are commonly associated with saltpiercements or diapirs (Figure 5). Close to theGuadalquivir Bank, the salt layer could not deformbasinward and was squeezed against the basementwalls, creating landward salt tongues and pierce-

ments of the upper layers along the GuadalquivirBank (Figure 5).

Southwestern Domain

The southwestern area is dominated by intensivepresent-day compression as a result of the dex-tral transpression that results from oblique col-lision between northwest Africa and southwestIberia (Zitellini et al., 2004; Rosas et al., 2009;Terrinha et al., 2009; Zitellini et al., 2009). SeveralUpper Cenozoic diapirs in the southwestern area(Figures 6, 10, 11) are associated with highly foldedsediments in a thrust zone (Figures 8, 10), affectingthe salt and creating thrust welds. These diapirs arethe most prominent salt diapirs in the entire area,affecting Upper Cenozoic sediments and deformingthe sea floor (Figures 6, 11). This domain also con-tains several dormant toe-thrust anticlines at theoceanward edge of the salt basin (Figure 6). It in-cludes the southern limit of the Sagres Plateau andGualdalquivir Bank, where salt diapirs have beeninterpreted, showing that salt deposition occurredin a vast area beyond the offshore Algarve Basinsensu strictu (Figure 8), which was probably con-nected with the northwestern Morocco salt basins(Figure 3).

STRUCTURAL CONTROLS ONSALT DEFORMATION

Understanding the controls on the observed ar-chitecture of the Triassic–Hettangian salt in theoffshore Algarve Basin is fundamental for devel-oping a depositional and tectonic model for thisimportant salt system. Themorphological featuresthat characterize salt units in the offshore AlgarveBasin (Figures 8, 15A) are the result of the inter-action of several factors associated with the basinevolution (Figure 16).

Depositional Limits and Original Thickness

The presence of three outcropping salt diapirs on-shore, the Loulé halite diapir, the Albufeira gyp-sum diapir, and the Faro diapir (Terrinha, 1998),

Matias et al. 1689

confirms that salt was also deposited northward ofthe shelf area. In most salt basins worldwide, theoriginal pinch-out of the mother salt layer is diffi-cult to define on seismic data. In the Algarve Basin,initially, it was thought that salt originally onlappedand pinched out to the south and west, along theflanks of the outer basement highs (GuadalquivirBank, Albufeira High, and SaoVicente Highs). Thepresence of several diapirs located on the southernflank of these basement highs (southwestern do-main; Figure 8), strongly suggests that the limitsof the salt deposition extended south of the studyarea. To thewest, seismic quality is poor and it is noteasy to determine its limits. The proposed bound-aries are based on identification of several toe-thrustfaults, interpreted to be the ramping up of autoch-thonous and allochthonous salt onto oceanic ortransitional crust.


The average thickness of the autochthonoussalt (calculated from the TWT thickness map ob-tained from the top and bottom authocthonous salthorizons and using an interval velocity of 4500 m/s)is about 1500 m (4921 ft), which could be con-sidered as a fair approximation for the initial salt layerthickness. An estimate of the original mother saltthickness would also have to consider the alloch-thonous salt tongues and the Esperança salt, whichwould increase initial thickness. However, it canalso be less because of inconclusive seismic imagingand poor lateral definition of salt walls, which com-monly result in overestimation of salt thickness.

Basement Structure and Tectonics

The Triassic rifting events and the resulting base-ment structural grain are dominated by north-south

Figure 16. Simplified regional maps illustrating different tectonic controls on salt deformation. TWT = two-way traveltime.

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to northwest-southeast– and east-west–strikingfaults, which bisect the study area in three mainsubbasins bounded by pronounced basement highs(Figure 16A). This structural pattern has playedthree main factors in the observed salt tectonics:

1. Paleorelief and salt thickness. Seismic sectionsfrom the study area illustrate possible relation-ships between basement horst and grabens andthe location of salt diapirs (Figures 5–7). Twoalternative possibilities, suggested by the appar-ent relation of salt walls/diapirs and basementfault, are (1) its distribution was controlled bythe geometry of active rifting, that is, salt wasdeposited during the synrift stage, which is inagreement with important lateral variations ofthe thickness of the Triassic–Hettangian unitsas shown in the early work of Palain (1976);and (2) the salt was deposited early in the postriftstage and its distribution was controlled by rift-generated topography that had not yet beenburied. Insufficient data exist to determinewhich model is correct, but either way, lateralvariations in original salt thickness might haveinfluenced the halokinetic structural styles.Also, the basement-related northwest-southeast–striking faults seem to have exerted some con-trol on the trends of salt walls and/or diapirsbecause they acted as strike-slip faults offsettingsome salt walls and basement highs (Matias,2007).

2. Physical barriers to salt and sediment flow(Figure 16A). The presence of basement highsbounding the basin limits is observed since theearly stage of basin evolution. Some major struc-tures (Guadalquivir Bank, Albufeira High, SãoVicente Highs; Figure 1) were important fac-tors limiting the southward salt movement andinhibiting the displacement of salt for long dis-tances. This type of basement control on saltbasins is also seen in the Campos and LowerCongo basins (Mohriak, 2005) because the base-ment highs promote an upward movement ofthe salt bodies, which are preferentially locatedin regions of lesser geopressures. This also partlyexplains why, offshore Algarve, the salt does notform a smooth basinwide extensional decolle-

ment surface for large-scale gravity sliding and/or spreading, such as is observed in many di-vergent margin basins with salients in the basinconfiguration (e.g., northwest Morocco, LowerCongo, Kwanza, and Campos basins [Tari et al.,2000; Mohriak et al., 2004; Mohriak, 2005;Mohriak et al., 2008]).

3. Thermal contraction (Figure 16B). A regionalseaward tilt produced by thermal contractionof the rifted margin, after the deposition of theTriassic–Hettangian salt, has created the gentleslope that enhances and accelerates the seawardflow and the initial stages of salt deformation.The downslope gravitational component was re-strained by the southern basement highs andenhanced the diapirism andMesozoicminibasinformation.

Overburden Thickness andCenozoic Compression

The development and continuous rise of salt diapirsafter compressive pulses in the Late Mesozoic–Early Cenozoic was controlled by overburden thick-ness and the capacity of the salt to breach throughthe overlying strata (Figure 16C). This only oc-curred in areas where the overburden was thin,whereas areas with thick overburden remained rel-atively undeformed. As a result of north-south Ce-nozoic compression, the halokinesis in the AlgarveBasin is still active as seen in the southwest area(Figures 6, 10, 16D).

Salt-Sediment Interaction

The interpretation of salt-related structures andthe previous discussions highlight the fact that saltdeformation has significantly controlled basin sedi-mentation in three ways: (1) salt-withdrawal mini-basins, (2) acted as a detachment for folds andthrusts, and (3) formation of topographic highs.Differential sedimentary loading drives salt mobi-lization and affects the evolution of salt bodies. Inturn, salt deformation affected the sediment path-ways, the stratigraphic architecture, and the dis-tribution of Miocene turbidite systems (Matias,2007). Active salt withdrawal divided the area into

Matias et al. 1691

at least five minibasins (Figure 8). Seismic inter-pretation of SMB1 indicates that these salt bodieswere deformed extensively during the Mesozoic.The salt bodies formed bathymetric highs, focusingsedimentation into the central parts of the mini-basins, which resulted in differential loading ofthe underlying salt, enhancing salt withdrawal.This positive feedback of the salt-sediment systemworked only after the initial minibasins were formed(Varnai and Weimer, 1998; Hudec et al., 2009).The basin highs also controlled the erosional pat-terns, as shown by important erosional truncationsby the Miocene unconformities in areas surround-ing the salt diapirs. It probably influenced the litho-facies distribution within overlying sediments, al-though not enough well data exist in the basinalarea to support this suggestion.

Varying Stress Regimes

The Algarve Basin was located in an area wheredifferent stress regimes interacted with each other,in particular, (1) Triassic–Early Jurassic extensionpredating the opening of the Atlantic Ocean, fol-lowed by (2) Middle Jurassic–Early Cretaceousstrike slip faulting at varying strikes and rates,transferring slip from the Atlantic into the Tethysdomains, along the Gibraltar Transform Fault;(3) Late Cretaceous compression, as a consequenceof anticlockwise rotation of Iberia during opening ofthe Biscay Bay against Africa followed by (4) Ce-nozoic compression between Africa and Eurasia(Figure 16D).

IMPLICATIONS TOHYDROCARBON EXPLORATION

Salt-Related Traps and Plays

Several play types offshore the Algarve Basin weredefined by the extensive 2-D seismic data set usedand, to our knowledge, almost all of these remainuntested to date. In the offshore part of the AlgarveBasin, almost all the play types are salt related. Thesalt-cored toe-thrust anticlines provide spectacularstructures in the southwestern domain. Several traps


are associated with salt diapirs, defining the classi-cal salt flank play (truncated upward flexures anddragged sediments) in the central domain. Towardthe eastern part of the basin, the allochthonous Es-perança salt and subsalt traps may include anticlinalstructures beneath the salt layer, with reservoirswithin the Jurassic strata; however, these targets arenot imaged convincingly on the presently available2-D seismic data. To date, this subsalt play type hasnot been tested in theAlgarve Basin, but it has beensparsely drilled in the Canadian margin (Mohriaket al., 2008). The deformation undergone by theEsperança salt and the overburden sediments is alsointeresting in terms of petroleum exploration be-cause it puts in positive structural position the gas-prone Miocene turbidites that are present in theGoC (Garcia-Mojonero et al., 2003) and also cre-ates rollover structures associated with extensionalfaults.

The pre–salt targets (below the authocthonoussalt) are also an undrilled play in the Algarve Basinand have also been suggested as a potential play inthe Lusitanian Basin (Uphoff et al., 2002; Uphoff,2005). It is presently a very popular and successfulplay in the Santos Basin offshore Brazil (Berman,2008; Carminatti et al., 2008;Mohriak et al., 2008),and it has been also tested in other passive marginbasins such as the Gulf of Mexico and west Africa.However, it involves a greater risk, higher drillingcosts, and geologic interpretation uncertainty be-cause of imaging limitations of the top and base salthorizons, which may mask the subsalt structures,and a more difficult prediction of the subsalt sedi-mentary facies and reservoir characteristics.

Hydrocarbon Migration

Themigration of fluids associated with halokinesisis supported by two arguments: (1) presence of sev-eral fluid escape structures and sea floor directhydrocarbon indicators such as gas chimneys andpockmark features; (2) seismic amplitude anoma-lies in the overburden.

In a couple of locations, seismic amplitudeanomalies were observed on Pliocene–Quaternarysediments overlying salt diapirs (Matias et al., 2005),associated with crestal faulting. At various points,

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where the faults intersect porous layers, such as thePliocene–Pleistocene sands known to bear gas else-where in the area (eastern GoC), small brightspots are clearly visible on the seismic (Figure 17A).These seismic disturbances (dimming and/or bright-ening of reflectors) are typically associated withthe presence of fluids or gas in shallow sediments(O’Brien et al., 1998; Teige and Hermanrud, 2004;Schroot et al., 2005). This provides additional evi-dence for fluid migration focused along salt diapirs.The proposed migration process in the offshoreAlgarve Basin can be summarized as:

1. In several locations, the formation of salt pillowsand diapirs depleted the entire basal salt layer,resulting in salt welds placing Mesozoic sedi-ments in direct contact with Triassic sediments,with the development of minibasins. This couldfavor themigration of hydrocarbons generated intheMesozoic and/or subsalt Triassic or Paleozoicsource rocks intoMesozoic–Cenozoic sediments.

2. During the Late Mesozoic–Early Cenozoic, thebasin has undergone important compression,which triggered the upward salt and fluids flowthat was highly focused along diapir flanks.

3. Salt halokinesis and fluid migration processes oc-curred during the Cenozoic and led to accumu-lation of hydrocarbons in near-surface sediments(shallow gas) or, in some cases, even reachingthe surface as chimneys or mud volcanoes.

The presence of the allochthonous Esperançasalt layer and the proposedmigration of fluids alongsalt flanks might have an important drawback interms of exploration attractiveness, for example,the presence of overpressured sediments belowsalt. Overpressure is commonly found underneathallochthonous salt layers as a result of undercom-paction of these subsalt sediments. This phenom-enon is well known in the Gulf of Mexico (Houseand Pritchett, 1995).

CONCLUSIONS

Two different salt units were identified in the off-shore Algarve Basin: a lower autochthonous unit ofthe Late Triassic/Early Jurassic (Hettangian) and anallochthonous unit. The Hettangian unit is mainlycharacterized by an autochthonous salt layer of amaximum of 1500 m (4921 ft) of thickness, saltdiapirs, salt walls and pillows, and occurs through-out the basin. The allochthonous unit originatedfrom theHettangian salt andwas emplaced as awidenappe during the Late Jurassic–Early Cretaceousand is confined to the eastern part of the basin.

Most salt movement was initiated by extension,which was probably triggered during the Early toMiddle Jurassic clastic progradation episodes. Al-though the time of initiation of salt movementcannot be precisely determined, its final movementoccurred during the Pliocene–Quaternary and, lo-cally, it is observed to affect the present-day ba-thymetry. Growth on Mesozoic structures duringthe Cretaceous and the Jurassic has been widelyrecognized in the basin, suggesting that salt move-ment and displacement were contemporaneouswith deposition along salt structures.

Figure 17. Salt-related faults and seismic anomalies (dim andbright spots) in Pliocene–Quaternary sediments associated withmigrated hydrocarbons. Data courtesy of TGS.

Matias et al. 1693

Interpretation of the pattern of distribution ofthe salt in the basin allowed subdivision of the areaof study into three distinct salt domains: (1) theeastern domain characterized by the presence of aconspicuous allochthonous salt nappe (EsperançaSalt), (2) the central domain dominated by salt di-apirs with mild deformation of Miocene strata andwide salt-withdrawal minibasins, and (3) the south-western domain where present-day tectonics in-duces impressive salt deformation affecting the seafloor. This complexpattern ismainly the result of theinteraction of inherited basement structure, com-plex tectonic history, and stress regime of the basin.

As a consequence of halokinesis, several salt-related traps and plays were created, together withwidespread salt chimneys that might have acted asmigration conduits along salt diapir flanks for hy-drocarbons generated in, and expelled from, Juras-sic and Cretaceous source rocks and later remi-grated using the faults as pathways into Tertiaryturbiditic reservoirs. The progressive compres-sional deformation of vast amounts of salt duringthe Late Cretaceous–Early Tertiary period createda world-class frontier salt basin from an explora-tion standpoint.

As the focus of worldwide exploration alongpassive margins is gradually shifting to deep-waterregions, the offshore Algarve Basin has the po-tential to become a deep-water petroleum prov-ince in the near future.

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Salt tectonics in the western Gulf of Cadiz, southwest Iberia