doi: 10.1130/0091-7613(2003)0312.0.CO;2 2003;31;19-22Geology
Fernndez-Salas, J. Toms Vzquez, F Lobo, Joao M. Alveirinho Dias, Jess Rodero and Joan GardnerRicardo Len, Teresa Medialdea, Antonio Barnolas, Margarita Garca, Vctor Daz del Ro, Luis Miguel Javier Hernndez-Molina, Estefana Llave, Luis Somoza, M. Carmen Fernndez-Puga, Adolfo Maestro, contourite depositional systemsLooking for clues to paleoceanographic imprints: A diagnosis of the Gulf of Cadiz
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q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or email@example.com.Geology; January 2003; v. 31; no. 1; p. 1922; 4 figures. 19
Looking for clues to paleoceanographic imprints: A diagnosis ofthe Gulf of Cadiz contourite depositional systemsJavier Hernandez-Molina Facultad de Ciencias del Mar, Universidad de Vigo, 36200 Vigo, SpainEstefana LlaveLuis SomozaM. Carmen Fernandez-PugaAdolfo MaestroRicardo LeonTeresa MedialdeaAntonio Barnolas
Instituto Geologico y Minero de Espana, Ros Rosas, 23, 28003 Madrid, Spain
Margarita GarcaVctor Daz del R oLuis Miguel Fernandez-Salas
Instituto Espanol de Oceanografa, C/ Puerto Pesquero s/n, 29640 Fuengirola, Spain
J. Tomas Vazquez Facultad de Ciencias del Mar, Universidad de Cadiz, 11510 Puerto Real, Cadiz, SpainF8 LoboJoao M. Alveirinho Dias
CIACOMAR, Avenida 16 de Junho s/n, 8700311, Olhao, Portugal
Jesus Rodero Instituto Andaluz de Ciencias de la Tierra, Facultad de Ciencias, 18002 Granada, SpainJoan Gardner Naval Research Laboratory, Code 7420, 455 Overlook Avenue SW, Washington, D.C. 20081, USA
ABSTRACTA new morphosedimentary map of the Gulf of Cadiz is presented, showing the contourite
depositional system on the gulfs middle slope. This map is constructed from a broad da-tabase provided by the Spanish Research Council and the U.S. Naval Research Laboratory.Our map shows that this contourite depositional system comprises five morphosedimentarysectors: (1) proximal scour and sand ribbons; (2) overflow sedimentary lobe; (3) channelsand ridges; (4) contourite deposition; and (5) submarine canyons. The Gulf of Cadiz con-tourite depositional system stems directly from the interaction between Mediterranean Out-flow Water and the seafloor; its morphosedimentary sectors are clearly related to the sys-tematic deceleration of the Mediterranean Outflow Waters westward branches, bathymetricstress on the margin, and the Coriolis force. The slopes depositional system can be consid-ered as a mixed contourite and turbidite system, i.e., a detached combined drift and fan.
Keywords: contourite depositional system, middle slope, drifts, channels, diapirs, Gulf of Cadiz.
INTRODUCTIONSince the 1980s, research has confirmed the
essential role of contourite processes in ma-rine environments, to the point that the currentconventional view is that the continental mar-gins are built up by the combined action ofdownslope and along-slope sedimentary pro-cesses. When the along-slope processes dom-inate, they can generate contourite deposition-al systems. We use a new data set to describe,from a regional morphosedimentary point ofview, the contourite depositional systems onthe middle slope of the Gulf of Cadiz as asingle system (Fig. 1). In this unusual system,along-slope processes are clearly dominantwith regard to interaction between Mediterra-nean Outflow Water and the seafloor. Ourstudy used a broad database collected since1989, obtained during several cruises and pro-jects, comprising bathymetric, side-scan sonarimagery, seismic data, and core data (Fig. 2).
MEDITERRANEAN OUTFLOW WATER:THE OCEANOGRAPHIC SCENARIO
The Gulf of Cadizs present-day circulationpattern is characterized by intense oceano-graphic dynamics controlled by the exchangeof water masses through the Straits of Gibral-tar (Fig. 1). This exchange is determined by
the highly saline and warm MediterraneanOutflow Water (MOW) near the bottom andthe turbulent, less saline cool-water mass ofAtlantic Inflow Water on the surface. Medi-terranean Outflow Water forms a strong con-tour current moving from the southeast to thenorthwest along the middle slope above theNorth Atlantic Deep Water (Zenk, 1975). Af-ter passing the Gibraltar seamount, the Medi-terranean Outflow Water forms a turbulentflux ranging from 150 to 200 m wide, whichmoves along a straight channel in a west-southwest direction at a speed of .200 cm/s(Ambar and Howe, 1979). Thereafter, theMediterranean Outflow Water spreads west-ward into the Gulf of Cadiz and progressivelysinks northwestward, descending the conti-nental slope from a depth of 300 m becauseof the dense waters gravity-driven currents.The outflow water loses contact with the sea-floor at 1000 m depth in the east sector andat 1400 m depth in the west sector.
As the Mediterranean Outflow Water moveswestward, its temperature, salinity, and veloc-ity decrease, and it divides into two mainbranches (Madelain, 1970; Ambar and Howe,1979), the Mediterranean Upper Water andMediterranean Lower Water. The Upper Waterforms the upper core, moving as a warm flux
between depths of 500 and 800 m at the baseof the upper slope until Cape San Vicente isreached. The mean velocity is ;46 cm/s. TheMediterranean Lower Water constitutes themore saline, lower core and the MediterraneanOutflow Waters principal volume, at a depthof ;7501200 m and with a mean velocity of;2030 cm/s. Mediterranean Lower Water isaffected by the slope morphology, and dividesinto three minor branches between the Cadizand Huelva meridians (Madelain, 1970; Me-lieres, 1974): (1) the Intermediate Branch,which moves northwestward through the Di-ego Cao channel; (2) the Principal Branch,which is generated in the southern part of theGuadalquivir Bank that modifies the flux to-ward the southeast; and (3) the SouthernBranch, which follows a steep valley towardthe southwest.
The slopes main physiographic features area shelfbreak located between 100 and 140 mdepth; a steeper (2838) upper slope between150 and 400 m depth; and two gently dipping(.18) wide terraces located at water depths of500750 and 8001200 m on the middleslope, having a central sector with channelsand ridges that trend northeast and a smoothlower slope (180.58). The contourite deposi-tional system is located on the middle slope,and comprises the five major morphosedimen-tary sectors detailed next (Figs. 3 and 4).
Proximal Scour and Sand-Ribbons SectorLocated in the southeast area close to the
Straits of Gibraltar, this sector was first de-fined by Kenyon and Belderson (1973). Ero-sive features, mainly erosive scour alignmentsand an abrasion surface, are dominant, but de-positional features, such as extended sanddunes and sand-ribbon fields, have also beenidentified, especially in the northwest zone.
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Figure 1. A: Location of Gulf of Cadiz. B: Regional bathymetric map indicating general cir-culation patterns of Mediterranean Outflow Water. Contours in meters.
Figure 2. Locations of seismic profiles and multibeam data set used to realize morphose-dimentary map. Contours in meters. IGMEInstituto Geologico y Minero de Espan a. CSICConsejo Superior de Investigaciones Cientificas.
Overflow Sedimentary Lobe SectorThis sector is adjacent and seaward of the
proximal scour and sand-ribbons sector, at awater depth of 7501600 m, and has a fanshape opening up downslope in the south. Itis very complex, owing to the effects of de-positional, gravitational, and erosive process-es, and consists of a megasedimentary lobewith bed forms and erosive scours. The mostimportant sedimentary features are sand andmud sedimentary lobes and sand and mudwave fields. Noteworthy erosive features in-clude several furrows that trend northeast. Thelargest (55 km long, 0.81.7 km wide), the GilEanes channel, was first described by Kenyonand Belderson (1973). Northward, the small-est channel, 16 km long and 0.82.3 km wide,is parallel to it. Gravitational processes, suchas slides, are also frequent. This sector waspartially studied previously (Kenyon andBelderson, 1973; Nelson et al., 1993).
Channels and Ridges SectorThis sector, located in the central area, was
studied partially by Nelson et al. (1993), anddescribed in detail by Garca (2002). Today itis dominated by erosive features on the sea-bottom, and five main contourite channelshave been characterized. They are known asthe Cadiz, Guadalquivir, Huelva, Diego Cao,and Gusano channels. These channels are Sshaped, their along-slope zones trend north-west, and their downslope zones trend north-east. They are marked by some structural re-lief features, e.g., the Cadiz and Guadalquivirdiapiric ridges and the Guadalquivir Bank.The channels are asymmetrical, generally pre-senting a deeper north flank; their lengthsrange from 10 km to .100 km, and they are1.510 km wide with ;10350 m of incision.The Cadiz channel is the biggest and the mostimportant, and within it, two recent steep100200-m-deep incisions have been identi-fied, a consequence of overexcavation pro-cesses by the current. Contourite channels arelocated on the southeast flank of the diapiricridge; however, many marginal valleys, withirregular morphology and a northeast trend,have been detected on the diapiric ridgesnorthwest flank. All of these erosive featureswere established over a relict contourite de-positional sector, composed of relict separatedmounded and sheeted drifts, affected by tec-tonics and diapiric phenomena.
Contourite Deposition SectorThis sector is located in the central and
northwest areas. Sedimentary processes aredominant, and the middle slope is built up bytwo kinds of contourite deposits, elongatedand separated mounded drift and sheeted drift.The Faro-Albufeira contourite system hasbeen studied previously (e.g., Gonthier et al.,1984; Llave et al., 2001, among others), andis the largest morphosedimentary feature,comprising five morphological elements.From the upper to the middle slope these arethe erosional surface on the upper slope; theAlvarez Cabral moat (80 km long and 411km wide); the Faro-Albufeira elongated and
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GEOLOGY, January 2003 21
Figure 3. Morphosedimentary map of contourite depositional system on middle slope of Gulf of Cadiz. Sedimentary deposit types and bedforms are shown for five morphosedimentary sectors.
Figure 4. Three-dimensional model from swath bathymetry of central sector of middle slopeof Gulf of Cadiz contourite depositional system, showing main contourite channels andfurrows and main diapir ridge, with northeast trend. Scale bar is in m.
separated mounded drift (80 km long, 12 kmwide, 600 ms thick); the Faro-Cadiz and Bar-tolome Dias sheeted drifts; and erosional fea-tures over the drift. Toward the east, beforethe Faro-Albufeira system, a detached drift(35 km long, 12 km wide, and 100 ms thick)has been determined in the transition betweenthe upper and middle slopes.
Submarine Canyons SectorLocated in the western area, this sector is
characterized by the Portimao, Lagos, Sagres,and San Vicente submarine canyons, whichseparate, from east to west, the Portimao, La-gos, and Sagres sheeted drifts.
DISCUSSION: LOOKING FOR CLUESTO PALEOCEANOGRAPHICIMPRINTS
The Gulf of Cadiz contourite depositionalsystem is a direct consequence of Mediterra-nean Outflow Water interaction with the sea-bottom along the middle slope. Its developmentis conditioned in part by Pliocene-Quaternarytectonics, resolved by the formation of regionalcurvatures on the margin surface and faults anddiapiric structures related to local movements.Although different specific parts of this depo-sitional system have been studied previously by
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others, it has never been approached compre-hensively as a single unique system. Therefore,the defined morphosedimentary sectors shouldbe related to the margins bathymetric stress,the systematically decreasing speed of the dif-ferent undercurrent branches as they movewestward, and the Coriolis force.
The proximal scour and sand-ribbon sector,as postulated by Kenyon and Belderson(1973), results from the Mediterranean Out-flow Waters strongly turbulent current thatcontours the slope after its entrance into theGulf of Cadiz. In this sector, the Mediterra-nean Outflow Waters high velocity generatedthe abrasion surfaces and erosive scour align-ments, but as current velocity decreases, sandribbons and wave fields can be developed. Wesuggest that when the Mediterranean LowerWater interacts with local diapiric ridge struc-tures, a deflection of the main flux could beproduced, generating a small downslope turnof a filament from the principal flux and lead-ing to the genesis of the northeast-trendingfurrows, and the genesis of the overflow sed-imentary lobe. The overflow sedimentary-lobesector is very complex, and has fan-shapeddeposits. This megalobe could be the result ofthe overflow of the Mediterranean Lower Wa-ters small downslope-turn filaments, but thedeposits could also be due to the occurrenceof many slides triggered by the tectonicstructures.
The Mediterranean Upper Water and Med-iterranean Lower Water fluxes are movingnorthwestward, producing the northwest-trending along-slope contourite channels atthe beginning of the channels and ridges sec-tor (Fig. 3). The Mediterranean Upper Wateris flowing northwestward toward the contourite-deposition sector, as a tabular water massalong the toe of the upper slope on a terraceat a water depth of 500800 m. It generatesthe slopes largest sheeted drifts (Faro, Albu-feira, and Bartolome Dias). When the Medi-terranean Upper Water flux interacts with theconcave shape of the margin, it forms a de-tached drift in the transition between upperand middle slope. It then begins to erode theslope to the east of Faro coast. In this region,Mediterranean Upper Water becomes chan-neled and forms the slightly sinuous AlvarezCabral moat. Because of the Coriolis force,causing the Mediterranean Upper Water cur-rent to shear to the right, the flow tends toerode the channels right flank and to createthe Faro-Albufeira elongate and separatedmounded drift on the left side, where the cur-rent slacks. After the Portimao Canyon, thepresence of the Portimao, Lagos, and Sagressheeted drifts suggests the action of a new tab-ular flux of the Mediterranean Upper Water.
The Mediterranean Lower Water also flowsnorthwestward along a deeper 8001200 mterrace, and when it reaches the linear northeast-trending Cadiz and Guadalquivir diapiric ridges,the contourite channels change abruptly fromtheir northwest trend to the new downslopesouthwest trend (Fig. 4). Consequently, an im-portant increase of the current velocity in thisarea was determined by Nelson et al. (1993).
With our new multibeam data, it is now pos-sible to determine (1) that channels are notonly present on the southern flank of the di-apiric ridge; (2) that these asymmetrical chan-nels have an S shape owing to the northwesttrend in those areas lacking the structural con-trol of the ridge; and (3) that these channelshave a southwest trend when the linear dia-piric ridges are present. Therefore, the occur-rence of the marginal valleys on the northflank of the diapiric ridge is also important.This finding is noteworthy because the dis-covery of a secondary circulatory flux down-slope, after Mediterranean Outflow Waterpasses the ridge, would enable us to better un-derstand the genesis of these erosive features,and consequently the Mediterranean OutflowWaters regional hydrodynamic model wouldhave to be revised. Because of the ridges, theMediterranean Lower Water is divided intoseveral minor branches. The Southern Branchis related to Cadiz channel, the PrincipalBranch could generate the Guadalquivir, Huel-va, and Gusano channels, and the IntermediateBranch could generate the Diego Cao channel.
Several features could be related to themore recent tectonic activity that is responsi-ble for the broad northwest-southeast com-pressional regime, such as the reactivation ofthe diapiric ridges. This recent tectonic activ-ity could have increased and changed the sub-marine bottom relief and, consequently, con-trolled changes in the distribution of thecurrents. Therefore, the more recent tectonicactivity could have conditioned several fea-tures, including (1) the recent origin of thechannels and ridges sector, (2) the inactivityof the relict, elongated and separated moundedand sheeted drifts, (3) the recent genesis of theDiego Cao channel proposed by Llave et al.(2001), and (4) the recent overexcavation ofthe Cadiz channel related to movements of thediapiric ridges. In addition, the principal fea-tures over the middle slope are erosive at pres-ent, including the contourite deposition sector,in which an extant superficial erosional featureover the drift has been described (Llave et al.,2001). Therefore, these data support the pro-posals that one of the Mediterranean OutflowWaters major erosive events is related tohighstand stages, such as the present one, thatthese erosive events represent a winnowingstage, and that the major depositional features,i.e., the drifts, are currently inactive on a re-gional scale. So, from the seismic analysis ofthe Quaternary sedimentary record, a secondmajor erosive event should be considered at theend of the lowstand stage (Llave et al., 2001).
CONCLUSIONSThe contourite depositional system of the
Gulf of Cadiz could be considered as a mixedcontourite and turbidite system, a companiondrift fan associated with the western North At-lantic margin (Faugeres et al., 1999). However,the Gulf of Cadiz contourite depositional sys-tem does not fit into that model, because thecontourite processes are dominant on the mid-dle slope, and downslope processes dominatethe lower slope and continental rise. These pro-
cesses do not interact simultaneously, as in theNorth Atlantic margin model. We could callthis system a detached combined drift fan, con-sidering the margins sedimentary stacking pat-tern, opposite to the pattern on the Hatteras orHebrides margins, where the along-slope pro-cesses developed in a more distal part of themixed system. Conventional wisdom nowholds that the largest drift deposits tend to de-velop in marine basins. However, as indicatedherein, many kinds of deposits could composea contourite depositional system, and much re-search remains to be done to determine in detailthe genesis and relationships of the sedimen-tary facies. A contourite depositional systemcan be formed by drift deposits, but also bysedimentary wave fields, channels, moats, fur-rows, scours, levees, sand ribbons, and sedi-mentary lobes. All of these deposits could, tak-en together, be considered clues to thepaleoceanographic imprints related to the samewater mass, but they could also be individual-ized, mainly due to the local behavior of thecurrent due to seabottom stress, producingbranches of the current, secondary flow, fila-ments, internal waves, local turbulence, over-flows, helicoidal flow, and other features.
ACKNOWLEDGMENTSThis work was supported by the projects Comision In-
terministerial de Ciencia y Tecnologia MAR-98-02-0209(TASYO).
REFERENCES CITEDAmbar, I., and Howe, M.R., 1979, Observations of the
Mediterranean outflowII. The deep circulation inthe vicinity of the Gulf of Cadiz: Deep-Sea Research,v. 26A, p. 555568.
Faugeres, J.C., Stow, D.A.V., Imbert, P., and Viana, A.,1999, Seismic features diagnostic of contourite drifts:Marine Geology, v. 162, p. 138.
Garca, M., 2002, Caracterizacion morfologica del sistemade canales y valles submarinos del talud medio delGolfo de Cadiz (SO de la Pennsula Iberica): Impli-caciones oceanograficas [Tesis de Licenciatura]: Ca-diz, Spain, University of Cadiz, 114 p.
Gonthier, E.G., Faugeres, J.C., and Stow, D.A.V., 1984,Contourite facies of the Faro Drift, Gulf of Cadiz,in Stow, D.A.V., and Piper, D.J.W., eds., Fine-grained sediments, deep-water processes and facies:Geological Society [London] Special Publication15, p. 275292.
Kenyon, N.H., and Belderson, R.H., 1973, Bed forms ofthe Mediterranean undercurrent observed with side-scan sonar: Sedimentary Geology, v. 9, p. 7799.
Llave, E., Hernandez-Molina, F.J., Somoza, L., Daz-del-Ro, V., Stow, D.A.V., Maestro, A., and AlveirinhoDias, J.M., 2001, Seismic stacking pattern of theFaro-Albufeira contourite system (Gulf of Cadiz): AQuaternary record of paleoceanographic and tectonicinfluences: Marine Geophysical Research, v. 22,p. 475496.
Madelain, F., 1970, Influence de la topographie du fond surlecoulement mediterraneen entre le detroit de Gi-braltar et le Cap Saint-Vincent: Cahier Oceanograp-hiques, v. 22, p. 4361.
Melieres, F., 1974, Recherches sur la dynamique sedimentairedu golfe de Cadiz (Espagne) [Ph.D. thesis]: Paris,France, University of Paris, 235 p.
Nelson, C.H., Baraza, J., and Maldonado, A., 1993, Med-iterranean undercurrent sandy contourites, Gulf ofCadiz. Spain: Sedimentary Geology, v. 82,p. 103131.
Zenk, W., 1975, On the Mediterranean outflow west of Gi-braltar: Meteor Forschungen Ergebnisse, v. 16,p. 2334.
Manuscript received June 11, 2002Revised manuscript received September 12, 2002Manuscript accepted September 13, 2002
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