Gulf of Cádiz and West Iberian margin: geological and oceanographic setting

Tectonic framework

The southwestern margin of the Iberian Peninsula, at the eastern end of the Azores-Gibraltar zone, is the location of the diffuse plate boundary between Eurasia and Africa. The present plate convergence between the African and Eurasian plates in the Gulf of Cádiz area is ~4 mm/y with a northwest–southeast trend and is accommodated over that broad diffuse deformation zone (Olivet, 1996; Argus et al., 1989). Distinct periods of crustal deformation, fault reactivation, and halokinesis related to the movement between Eurasia and Africa (Malod and Mauffret, 1990; Srivastava et al., 1990; Maldonado et al., 1999; Gutscher et al., 2002; Alves et al., 2003; Medialdea et al., 2004, 2009; Lopes et al., 2006; Terrinha et al., 2009; Zitellini et al., 2009) are known to have controlled the tectonostratigraphic evolution of this part of the Iberian Peninsula. The tectonic structure of this area is a consequence of the distinct phases of rifting since the late Triassic to the Early Cretaceous caused by the opening of the central and North Atlantic basin (Murillas et al., 1990; Pinheiro et al., 1996; Wilson et al., 1996; Srivastava et al., 2000; Borges et al., 2001) and its later deformation during the Cenozoic, especially in the Miocene (Ribeiro et al., 1990; Zittellini et al., 2009). The Gulf of Cádiz Cenozoic evolution was controlled by the Alpine tectonic phases that affected the southern part of Europe. During the Pliocene and Quaternary, the effect of glacio-eustatic variations have rather overprinted structural effects on the margin and resulted in erosion, sedimentary progradation, and incision of major submarine canyons (Mougenot, 1988; Alves et al., 2003; Terrinha et al., 2003, 2009).

The Gulf of Cádiz straddles this oblique-compressive zone between the Eurasian and African plates, extending from the Gloria transform fault zone to the Gibraltar arc, which marks the western front of the Betic-Rif collisional orogen. Since the late Miocene, the northwest–southeast compressional regime has been developed simultaneously with the extensional collapse of the Betic-Rif orogenic front, by westward emplacement of a giant "olistostrome," the Cádiz Allochthonous Unit (CAU), and by very high rates of basin subsidence coupled with strong diapiric activity. At the end of the Messinian, a transtensional regime caused the reopening of the connection between the Atlantic and the Mediterranean through the Strait of Gibraltar (Maldonado et al., 1999). By the end of the early Pliocene, subsidence decreased and the margin evolved toward its present, more stable, condition (Maldonado et al., 1999; Maestro et al., 2003; Medialdea et al., 2004), although the CAU provides an unstable substratum for late Miocene, Pliocene, and Quaternary sedimentation (Medialdea et al., 2004; Zitellini et al., 2009). Some neotectonic reactivation is also evident, as expressed by the occurrence of mud volcanoes, diapiric ridges (Diaz-del-Río et al., 2003; Somoza et al., 2003; Fernández-Puga, et al., 2007; Zitellini et al., 2009), and fault reactivation (Maestro et al., 1998; Gràcia et al., 2003a, 2003b; Lobo et al., 2003; Terrinha et al., 2009). Tectonics have represented a key long-term factor in affecting seafloor morphology, which has exerted strong control on the pathways of MOW and, therefore, on the architecture of the CDS.

Oceanographic setting

The present-day circulation pattern is dominated by exchange of water masses through the Strait of Gibraltar (Fig. F3). This exchange is driven by the highly saline and warm MOW near the bottom and the turbulent, less saline, cool-water mass of Atlantic water at the surface. MOW forms a strong bottom current flowing toward the west and northwest above North Atlantic Deep Water (NADW) (Madelain, 1970; Melières, 1974; Zenk, 1975; Thorpe, 1976; Ambar and Howe, 1979).

After it exits through the Gibraltar Gateway, MOW represents an intermediate water mass, which is warm and very saline, that flows to the northwest along the middle slope (Fig. F3) under Atlantic Inflow and above NADW (Zenk, 1975; Thorpe, 1976; Gardner and Kidd, 1983; Ochoa and Bray, 1991; Baringer and Price, 1999). MOW also represents a flux of ~1.78 Sv through the Gibraltar Gateway, composed of both Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW) (Bryden and Stommel, 1984; Bryden et al., 1994; Millot, 1999, 2009), and generates important alongslope sedimentary processes along the Atlantic margin (Serra et al., 2010a, 2010b). In the Gulf of Cádiz, MOW flows between 500 and 1400 meters below sea level (mbsl) with a velocity close to 300 cm/s at the Strait of Gibraltar (Ambar and Howe, 1979) and ~80–100 cm/s at the latitude of Cape San Vicente (Cherubin et al., 2000). Its distribution is conditioned by the complex morphology of the continental slope, which generates two main cores, between 500 and 700 mbsl (upper core or Mediterranean upper water [MUW]) and 800 and 1400 mbsl (lower core or Mediterranean lower water [MLW]). MLW is further divided into three branches (Fig. F3) (Madelain, 1970; Zenk, 1975; Ambar and Howe, 1979; Johnson and Stevens, 2000; Borenäs et al., 2002). In the western sectors, the interaction of these branches with the seafloor generates big meddies (Richardson et al., 2000).

After exiting the Gulf of Cádiz, MOW has three principal branches (Fig. F4): the main branch flows to the north, the second to the west, and the third to the south, reaching the Canary Islands and then veering toward the west (Iorga and Lozier, 1999; Slater, 2003). The northern branch flows along the middle slope of the Portuguese margin and is further divided into two branches by the influence of the Galicia Bank (Fig. F4). These two branches return to converge and subsequently circulate to the east in the Gulf of Biscay following the continental slope contour (Fig. F4). MOW reaches Porcupine Bank and partly circulates to the north along Rockall Trough until reaching the Norwegian Sea (Iorga and Lozier, 1999; Slater, 2003).

Between ~500 and 1500 mbsl, the water column along the West Iberian margin is dominated by the warm, salty MOW, the two cores of which are centered at ~800 and 1200 mbsl (Ambar and Howe, 1979), and flow as undercurrents northward along the margin and also spread westward. Below MOW at ~1600 mbsl, Labrador Sea Water (LSW) can be found on the margin north of 40.5°N (Fiuza et al., 1998). Below 2000 mbsl, recirculated Northeast Atlantic Deep Water (NEADW) prevails, which represents a mixture of LSW, Iceland Scotland Overflow Water (ISOW), Denmark Strait Overflow Water (DSOW), and to a lesser extent MOW and Lower Deep Water (LDW) (van Aken, 2000). Abyssal water in the region consists of a lower fringe of NEADW that obtains an increasing component of southern-sourced LDW with depth until ~4000 mbsl, where LDW dominates.

Morphosedimentary and stratigraphic framework

The major part of the Gulf of Cádiz comprises a giant outward bulge sloping to the west with irregular surface relief (Fig. F5). The principal physiographic features of this broad slope are

  • A shelf-break located between 100 and 140 mbsl,

  • A steeper (2°–3°) upper slope between 150 and 400 mbsl,

  • Two gently dipping (>1°) wide terraces at 500–750 and 800–1200 mbsl on the middle slope crossed by channels and ridges that trend northeast, and

  • A smooth lower slope (0.5°–1°).

For the most part, this slope lacks submarine canyons, except in the western area of the Algarve margin (Hernández-Molina et al., 2006; Mulder et al., 2006; Marchès et al., 2007). The West Iberian margin is characterized by a steeper slope (~4.5°), which is crossed by several major submarine canyons, but there is also a middle slope terrace (Alves et al., 2003; Terrinha et al., 2003; Llave et al., 2007).

The interaction of MOW with the Gulf of Cádiz margin has resulted in the development of one of the most extensive and complex CDSs ever described. Many authors have highlighted this interaction and have characterized its erosive and depositional features along the middle slope (e.g., Madelain, 1970; Gonthier et al., 1984; Nelson et al., 1993, 1999; Llave et al., 2001, 2006, 2007, 2010, submitted; Habgood et al., 2003; Hernández-Molina et al., 2003, 2006, submitted; Mulder et al., 2003, 2006; Hanquiez et al., 2007; Marchès et al., 2007). This CDS has both large depositional and erosional features (Fig. F2), conditioned by a strong current with speeds reaching nearly 300 cm/s close to the Strait of Gibraltar, slowing to ~80 cm/s at Cape St. Vincent (Kenyon and Belderson, 1973; Ambar and Howe, 1979; Cherubin et al., 2000). The main depositional features are sedimentary wave fields, sedimentary lobes, mixed drifts, plastered drifts, elongated mounded and separated drifts, and sheeted drifts. The main erosional features are contourite channels, furrows, marginal valleys, and moats. All of these features have a specific location along the margin, and their distribution defines five morphosedimentary sectors within the CDS (details can be found in Hernández-Molina et al. [2003, 2006] and Llave et al. [2007]). The development of each of these five sectors at any time is related to a systematic deceleration of MOW as it flows westward from the Strait of Gibraltar, caused by its interaction with margin bathymetry and the effects of Coriolis force. In general, the drifts are composed mainly of muddy, silty, and sandy sediments, with a mixed terrigenous (dominant component) and biogenic composition (Gonthier et al., 1984; Stow et al., 1986, 2002). In contrast, sand and gravel are found in the large contourite channels (Nelson et al., 1993, 1999), as are many erosional features (Hernández-Molina et al., 2006; García et al., 2009). In the proximal sector close to the Strait of Gibraltar, an exceptionally thick (~815 m) sandy-sheeted drift occurs, with sand layers averaging 12–15 m thick (minimum thickness = 1.5 m; maximum thickness = 40 m) (Buitrago et al., 2001).

Four major depositional sequences have been recognized in large contourite systems of the Pliocene and Quaternary sedimentary record, which must be related to MOW paleoceanographic changes. The depositional sequences are separated by four major regional discontinuities (Fig. F6), which from bottom to top are late Miocene (M), intra-lower Pliocene (LPR), base quaternary discontinuity (BQD), and mid-Pleistocene revolution (MPR) (Llave et al., 2001, 2007, 2010, submitted; Hernández-Molina et al., 2002, 2009; Stow et al., 2002). Together, these discontinuities constitute an impressive record of changes in water mass circulation and of the tectonic and/or environmental changes that have affected the evolution of the margin.

Portuguese margin sediments, by contrast, are dominated by hemipelagic muds, with variable admixtures of terrigenous silt and sand, and biogenic components (Baas et al., 1997). Pelagic sedimentation prevailed during interglacial periods. The input of terrigenous material is enhanced during glacial periods because of lowered sea level and input from local sources such as density-driven slope lateral advection and low-concentration turbidity currents, as well as minor influence from bottom currents and ice rafting. High sedimentation rates are typical in this region because of elevated fluxes in both glacial (clay) and interglacial (silt) material; however, distinct climate control results in enhanced sedimentation during colder periods at both orbital and millennial timescales. This pattern is observed in water depths between 2500 and 4600 m (Hall and McCave, 2000; Lebreiro et al., 2009). Detrital input from rivers (Tagus) channeled by turbidity currents is limited to submarine canyon systems (Lebreiro et al., 2009) and abyssal plains (Lebreiro et al., 1997) and does not affect open slope deposition. Ice-rafted detritus occasionally reached the Iberian margin during the last glacial period, especially during Heinrich events when sea-surface temperatures were very low (Lebreiro et al., 1996; Baas et al., 1997; Cayre et al., 1999; Thouveny et al., 2000; de Abreu et al., 2003).

Neotectonic implications

Tectonism during the Pliocene and Quaternary caused by the broad northwest–southeast compressional regime has determined in the short term the local thickness, geometry, and present position of various depositional bodies and in the long term has also contributed to new paleoceanographic changes. Several features of the CDS in the Gulf of Cádiz and west of Portugal can be related to this recent tectonic activity, which has involved the reactivation of faults and diapiric structures related to local movements (Maldonado et al., 1999; Fernández-Puga, 2004; Medialdea et al., 2004, 2009; Fernández-Puga et al., 2007; Neves et al., 2009; Terrinha et al., 2009; Zitellini et al., 2009). The most recent tectonic activity has directly conditioned several features (Llave et al., 2001, 2003; Hernández-Molina et al., 2003; García et al., 2009), including

  • The recent configuration of the channels and ridges sector,

  • The inactivity of several fossil mounded and sheeted drifts identified in Sectors 3 and 4,

  • The recent genesis of the Diego Cão Channel, and

  • The recent overexcavation and northward migration of the Cádiz Channel.

On the West Iberian margin, recent tectonic activity is variable from north to south. The Lisbon margin in the north has undergone significant subsidence through the Pliocene and Quaternary. The Alentejo margin further south has undergone moderate subsidence during the early Pliocene but evolved to a region of mixed transpression-transtension during the late Pliocene and Quaternary, in association with uplift of the Gorringe Bank. Neotectonic activity is recognized in this area at the present time (Alves et al., 2003; Gràcia et al., 2003a, 2003b; Terrinha et al., 2003; Zitellini et al., 2009).

Seismic studies/Site survey data

A broad database of Gulf of Cádiz and the West Iberian margin information collected over the past 40 y by many different nations and cruises provides a superb template for drilling. This data is composed of

  • Bathymetric data, including swath bathymetry of the middle slope using the Simrad EM12S-120 and EM300 multibeam echo-sounder systems;

  • Side-scan sonar image data from the Seamap, OKEAN, Gloria, and TOBI systems;

  • An extensive seismic data grid, including low-resolution multichannel seismic profiles from oil companies (mainly REPSOL and TGS-NOPEC), medium-resolution seismic profiles from Sparker, air gun, Geopulse, and Uniboom systems, high-resolution seismic profiles using a 3.5 kHz system, and ultrahigh-resolution seismic profiles using TOPAS;

  • A variety of core data, ranging from box cores and short gravity cores to giant piston cores;

  • Over 3000 submarine photographs taken with a BENTHOS-372 camera; and

  • Physical oceanographic information.