Influence of the Atlantic inflow and Mediterranean outflow currents on Late Quaternary sedimentary facies of the Gulf of Cadiz continental margin

ELSEVIER Marine Geology 155 (1999) 99–129

Influence of the Atlantic inflow and Mediterranean outflow currentson Late Quaternary sedimentary facies of the Gulf of Cadiz

continental margin

C. Hans Nelson a,Ł, Jesus Baraza b, Andres Maldonado c, Jesus Rodero c, Carlota Escutia a,John H. Barber, Jr. a

a US Geological Survey, 345 Middlefield Rd. MS999, Menlo Park, CA 94025, USAb Instituto de Ciencias del Mar, CSIC, Paseo Juan de Borbon s=n. 08039, Barcelona, Spain

c Instituto Andaluz de Ciencias de la Tierra, CSIC=Universidad Granada, 18071, Granada, Spain

Received 28 February 1998; accepted 21 September 1998

Abstract

The late Quaternary pattern of sedimentary facies on the Spanish Gulf of Cadiz continental shelf results froman interaction between a number of controlling factors that are dominated by the Atlantic inflow currents flowingsoutheastward across the Cadiz shelf toward the Strait of Gibraltar. An inner shelf shoreface sand facies formed by shoalingwaves is modified by the inflow currents to form a belt of sand dunes at 10–20 m that extends deeper and obliquelydown paleo-valleys as a result of southward down-valley flow. A mid-shelf Holocene mud facies progrades offshore fromriver mouth sources, but Atlantic inflow currents cause extensive progradation along shelf toward the southeast. Increasedinflow current speeds near the Strait of Gibraltar and the strong Mediterranean outflow currents there result in lack of muddeposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf. At the outershelf edge and underlying the mid-shelf mud and inner shelf sand facies is a late Pleistocene to Holocene transgressivesand sheet formed by the eustatic shoreline advance. The late Quaternary pattern of contourite deposits on the SpanishGulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contoursand the Mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys betweenthe ridges. Coincident with the northwestward decrease in outflow current speeds from the Strait there is the followingnorthwestward gradation of contourite sediment facies: (1) upper slope sand to silt bed facies, (2) sand dune facies on theupstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against thediapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar containsmedium–fine sand beds interbedded with mud. The adjacent valley floor facies is composed of gravelly, shelly coarse tomedium sand lags and large sand dunes on the valley margins. By comparison, the northwestern drift contains coarsesilt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Because of the complexpattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean outflow current, the larger-scalebedforms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grainedcontourite deposits of adjacent sediment drift facies. Radiocarbon ages verify that the inner shelf shoreface sand facies(sedimentation rate 7.1 cm=kyr), mid-shelf mud facies (maximum rate 234 cm=kyr) and surface sandy contourite layer of0.2–1.2 m thickness on the Cadiz slope (1–12 cm=kyr) have deposited during Holocene time when high sea level results

Ł Corresponding author. Fax: C1 650 329 5299; E-mail: [email protected]

0025-3227/99/$ – see front matter 1999 Published by Elsevier Science B.V. All rights reserved.PII: S 0 0 2 5 - 3 2 2 7 ( 9 8 ) 0 0 1 4 3 - 1

100 C.H. Nelson et al. / Marine Geology 155 (1999) 99–129

in maximum water depth over the Gibraltar sill and full development of the Atlantic inflow and Mediterranean outflowcurrents. The transgressive sand sheet of the shelf, and the mud layer underlying the surface contourite sand sheet ofthe slope, correlate, respectively, with the late Pleistocene sea level lowstand and apparent weak Mediterranean outflowcurrent. 1999 Published by Elsevier Science B.V. All rights reserved.

Keywords: Gulf of Cadiz; Strait of Gibraltar; Mediterranean outflow currents; Atlantic inflow currents; shelf and slopefacies; sedimentation rates; sediment ages; contourite facies

1. Introduction

The Gulf of Cadiz is located northwest of Gibral-tar Strait in the eastern Atlantic Ocean along a re-entrant of the Spanish coast (Fig. 1). Above 300 mwater depth there is a strong southeastward inflow ofNorth Atlantic Surficial Water (NASW — hereaftercalled Atlantic inflow) over the Gulf of Cadiz shelfthat intensifies toward the Strait of Gibraltar (Figs. 2and 3) (Ambar and Howe, 1979a; Caralp, 1992;Stevenson, 1997). As a consequence, nearsurfaceshelf mud blanket facies develop asymmetricallytoward the southeast (Gutierrez-Mas et al., 1996;Rodero et al., 1999, this issue) and bedforms developon the eastern end of the shelf (Lobo et al., 1996).Below 300 m water depth there is a significant de-velopment of bottom-current deposited sediment andbedforms because the deep Mediterranean OutflowWater (MOW — hereafter called Mediterranean out-flow) shears northwestward from Gibraltar along theCadiz continental slope as the Mediterranean un-dercurrent (Heezen and Johnson, 1969; Kenyon andBelderson, 1973). The main deposits associated withthe Mediterranean outflow end with the developmentof the large Faro Drift sediment body off Portugal(Fig. 3) (Faugeres et al., 1985c; Stow et al., 1986).

Previous papers describe the influence of strongMediterranean outflow currents on the late Quater-nary history of the Cadiz continental slope (Nelsonet al., 1993) and sea level effects on the Holocenesedimentary history of the easternmost end of theCadiz shelf (Gutierrez-Mas et al., 1996; Rodero etal., 1999). A number of technical reports (IGME,1974; ITGE, 1997) and an excellent coastal zonephysiographic map series (Vanney and Menanteau,1985) provide detailed information on the complexsurficial geology of the inner shelf and littoral zoneof less than 50 m water depth. This paper focuses onthe Atlantic inflow current effects on the entire Span-

ish continental shelf and integrates these new datawith the Mediterranean outflow current effects onthe Spanish continental slope in the eastern Gulf ofCadiz. This paper provides new detailed informationon the stratigraphy, bedforms, lithology and sedi-ment facies that develop as a result of the opposingAtlantic inflow and Mediterranean outflow currents.

We outline the influence of the tectonic and mor-phologic setting on the bottom-current patterns andthen describe the acoustic stratigraphy of nearsurfaceshelf and slope deposits. Next, we characterize eastto west and upslope to downslope gradations in bed-forms and surface lithology of deposits. Finally, wedefine the subsurface lithologic gradations, ages ofnearsurface sediments and sedimentation rates. Weuse this detailed set of data to map the variation ofCadiz margin sediment facies related to the inflowand outflow currents and then explain the resultantlate Quaternary sedimentary history of the easternGulf of Cadiz.

2. Methods

The eastern Gulf of Cadiz continental margin wastraversed by 3400 km of tracklines of high-resolu-tion seismic profiles taken with 3.5 kHz, Geopulse,and single-channel 20–40 cu. in. air guns (Fig. 1).Over the continental shelf area we obtained 1200km of tracklines with a 100 kHz sidescan sonarusing a 100 m swath width. Another 400 km ofdeep-tow records (35 kHz sidescan sonar with a1 km swath and 3.5 kHz high-resolution profiler)were collected in selected locations with bedforms(Fig. 1). The trackline and core locations were ac-quired with MAXIRAN, Loran C, transit satelliteand GPS navigation systems.

A total of 490 dredges and gravity cores, measur-ing between 10 and 277 cm in length, were taken

C.H. Nelson et al. / Marine Geology 155 (1999) 99–129 101

Fig. 1. Map showing 3.5 kHz, Geopulse, single-channel airgun and sidescan sonar tracklines with thin lines. Thick line segments indicatemultichannel seismic profile tracklines. Numbered line indicates the trackline location for a 3.5 kHz seismic profile as illustrated inFig. 6.

from the Gulf of Cadiz continental shelf and slope,at water depths ranging from 15 to 959 m (Fig. 2).Subsampling intervals were selected to obtain themaximum lithologic information from the prograd-ing shelf muds and sand–silt rich contourite layers.Grain-size analyses were conducted with the SEDI-GRAPH 5000D (<63 µm), and by sieving (>63µm). The sand fraction composition was studiedwith the binocular microscope. A minimum of 250grains were counted in each of the 460 surficial andsubsurface samples. Conventional radiocarbon ageswere obtained from carbonate carbon in transgres-

sive shell lag layers and from bulk organic carbon inwhole sediment samples of shelf and slope mud.

3. Oceanographic setting

A general water mass transport pattern of Atlanticinflow into the Mediterranean Sea at the surface anda Mediterranean outflow at depth from the Strait ofGibraltar are driven by the net evaporation of theMediterranean Sea (Ochoa and Bray, 1991) and me-teorological forcing (Candela et al., 1989). Atlantic


Fig. 2. General bathymetric map in meters showing gravity-core locations. The bathymetric chart is based on our echosounder andhigh-resolution seismic profiles synthesized with bathymetry from the Hydrographer of the Navy (1969) Chart G6101 and from Vanneyand Mougenot (1981). Shelf paleo-valley locations are after Vanney and Menanteau (1985)

inflow currents flow eastward to southeastward overthe Gulf of Cadiz continental shelf as is shown bythe sand dune crests in the inner shelf areas (Fig. 3)(Vanney and Menanteau, 1985). Extensive sand dunefields across the entire southeastern end of the shelfindicate that current speeds accelerate significantlytoward Gibraltar Strait (Fig. 3) (Lobo et al., 1996).Because almost no current speed measurements existin the published literature, these geologic featuresare the main indicators of current velocities for theAtlantic inflow over the Cadiz shelf.

The deeper Mediterranean outflow water travelsas the Mediterranean undercurrent northwestwardfrom Gibraltar Strait below 300 m and acceler-ates down the Cadiz continental slope because ofgravity-driven currents of the denser water (Fig. 3)(Ochoa and Bray, 1991). Because of density differ-ences with respect to the surrounding Atlantic wa-

ter, the warm, saline, and dense (>12ºC; salinity D>36.2‰) Mediterranean outflow progressively sinksas it flows northwestward at varying depths be-tween 300 and 1800 m as an independent stream,the geostrophic Mediterranean undercurrent (Fig. 3)(Madelain, 1970; Ambar and Howe, 1979a,b). TheMediterranean undercurrent maintains contact withthe seafloor at up to 1000 m water depth on the east-ern region of the Gulf of Cadiz and approximately to1400 m depth on the western side of our study area(Kenyon and Belderson, 1973; Gardner and Kidd,1983).

Maximum undercurrent speeds are 80 cm=s in theeastern part of our study area (Kenyon and Belder-son, 1973; Ambar and Howe, 1979b) but decreaseto 75–40 cm=s on the central slope where the un-dercurrent subdivides into several branches becauseof interaction with the rugged ridge and valley mor-


Fig. 3. Current system of the Gulf of Cadiz, showing the area of the surficial Atlantic inflow current over the continental shelf, the areawhere the subsurface Mediterranean outflow current impinges on the seafloor, and current speeds (modified from Thorpe, 1972, 1976;Kenyon and Belderson, 1973; Ambar and Howe, 1979a,b).

phology (Fig. 3) (Madelain, 1970; Fernandez-Lopezand Ortega-Serrano, 1986). Current ribbons splaysouthwestward through the valleys producing fasterchannelized bottom-current flows downslope that av-erage 80 cm=s in a region where the contour-parallelcurrents of the Mediterranean outflow average only40 cm=s (Thorpe, 1972, 1976). Along the westernCadiz slope, current speed decreases to 10–20 cm=sand speed in channels declines to 25 cm=s (Fig. 3)(Faugeres et al., 1985b). Current speeds at the south-ern fringes are considerably slower than immedi-ately upslope because the Mediterranean outflow hashigher speed cores there (Ambar and Howe, 1979a,b;Price et al., 1993; Price and O’Neil-Baringer, 1994).

Previous studies recognize the variability of currentspeed east to west, upslope to downslope and valleyto intervalley areas in the patterns of bedforms ob-served on the Cadiz continental slope (Kenyon andBelderson, 1973; Gardner and Kidd, 1983; Nelson etal., 1993).

4. Geologic setting

4.1. Morphology

As a result of the complex geological evolution,the bathymetry of the continental shelf and slope


of the eastern Gulf of Cadiz is quite irregular andexhibits widely varying gradients. The shelf area canbe divided into four morphological provinces; (a) theinner shelf, (b) the smooth mid-shelf, (c) the south-east shelf with bedforms, and (d) the outer shelf. Theinner shelf contains a complex morphology includ-ing irregular bedrock outcrops, Pleistocene sea levelstillstand terraces, smooth mud-filled depressions,paleo-valleys and bedform fields (Fig. 2) (IGME,1974; Vanney and Menanteau, 1985; Gutierrez-Maset al., 1996; Lobo et al., 1996). The mid-shelf areasare smoothed by prograding mud derived from thenumerous river mouths (Gutierrez-Mas et al., 1996;Rodero et al., 1999). Large areas of the southeasternshelf are covered with complex sets of bedforms(Fig. 2) (Lobo et al., 1996). The outer shelf regionoften begins with a slight break in slope at about100 š 20 m water depth, has a steeper gradientuntil about 120 š 20 m water depth, and then flat-tens out to the shelf edge (Fig. 2) (Rodero et al.,1999).

The shelf break occurs at about 130 š 20 mwater depth and the continental slope below canbe differentiated into four morphologic provinces:(a) a narrow belt between 130 and 400 m formedby the steeper (2–3º) upper slope, (b) two gentlydipping (<1º) wide terraces located between 400and 700 m water depth on both the southeast andnorthwest sides of the middle slope, (c) a centralsection between the terraces in which several, steep(3–30º), narrow curvilinear ridges (3–10 km width)and valleys (1–3 km width) trend NE–SW to E–W,and (d) a smooth lower slope from 900 to 1500 mthat is more steeply dipping (2–4º) and incised byshallow valleys in a NE–SW direction (Fig. 2).

4.2. Tectonics and stratigraphy

The opening of the Strait of Gibraltar and a signif-icant increase in the subsidence rate during the EarlyPliocene caused a major change in the depositionalpatterns of the Cadiz continental margin (Maldon-ado and Nelson, 1999). Deposition was controlledby the diapiric activity, the extensional collapse ofthe margin and the location of tectonic depressions.Large roll-over extensional basins with strong marlydiapirism phenomena occurred in the central regionof the Gulf of Cadiz. From Early to Late Pliocene

time there was a rapid decrease in basement subsi-dence rates; consequently, eustatic sea level changesand bottom current distribution gained importancefor controlling depositional patterns (Maldonado andNelson, 1999).

During the Pliocene and Quaternary, the Mediter-ranean outflow current has been the main influenceon the facies distribution of the continental slope;large drift depositional bodies are associated withthe diapirs, and erosional surfaces have developedon the upper slope (Nelson et al., 1993). The LatePliocene–Pleistocene eustatic sea level fluctuationschanged the effective cross section at the Strait ofGibraltar which controls the Atlantic and Mediter-ranean water mass exchange. During eustatic lowsea-levels, the connection between the Atlantic andMediterranean waters has been shallower and re-stricted; during the highstands, such as the presentHolocene time, the connection has been deeper andless restricted (Huang and Stanley, 1972; Diester-Haass, 1973; Sonnenfeld, 1974; Vergnaud-Grazziniet al., 1986; Grousset et al., 1988). As the strengthand volume of the Mediterranean outflow currentwas significantly modified, the facies distributionover the margin experienced major changes (Nelsonet al., 1993: Rodero et al., 1999).

4.3. Age and acoustic character of slope sedimentdrift sequences

Sediment drift bodies associated with the diapiricridges are the thickest deposits (up to 0.75 s of sedi-ment) related to the Mediterranean outflow and thosethat exhibit the most complete seismic stratigraphicrecord on the Cadiz slope (Fig. 4) (Nelson et al.,1993; Maldonado and Nelson, 1999). Previous stud-ies interpret that sediment drape reflectors of LateMiocene age are replaced by sediment drift reflectorsof Early Pliocene age because the Mediterranean out-flow current commenced with the opening of Gibral-tar Strait and flooding of the western MediterraneanSea. The underlying Miocene beds are generallydraped layers of sediment without acoustic charac-teristics of bottom-current processes. The older bedsare intruded and thrust upward by the diapiric struc-tures suggesting that the diapirism began during theMiocene, prior to deposition of the embankments ofsediment drift that onlap the diapirs. Uplift, however,

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Fig. 4. Geomorphic features of the eastern Gulf of Cadiz continental margin based on interpretations of seismic profile and sidescan sonograph data collected along tracklinesshown in Fig. 1. Modified from ITGE (1997). Pattern 1 D shelf surface of Atlantic inflow current erosion, 2 D surface of Mediterranean outflow current erosion, 3 D bedrockoutcrops, 4 D continental slope valleys, 5 D slope area with dune bedforms, 6 D slope area with smooth sediment cover, 7 D slope sediment failure areas, 8 D slope diapirareas, 9 D slope gullied areas, 10 D shelf areas with gas-charged sediment, 11 D Holocene upper sedimentary unit, 12 D Holocene lower sedimentary unit, 13 D Guadalate riverprodelta unit, 14 D Late Quaternary undifferentiated shelf sediment, 15 D shelf large-scale dune area, 16 D shelf small-scale dune area, 17 D shelf megaripple area, 18 D shelfsmall-scale ripple area, 19 D outcropping faults, 20 D undifferentiated scarps.


may continue to be active at present (Maldonado andNelson, 1999; Rodero et al., 1999).

The sediment drift deposits are characterized bythick wedges of sediment with irregular erosionalsurfaces, broad-scale low-angle unconformities andinternal reflectors that are generally continuouswithin the wedges (Nelson et al., 1993; Maldonadoand Nelson, 1999). Locally, the drift wedges exhibitalternating sequences of sediment drape depositsand irregular, discontinuous high-energy depositslike sand dunes. The channelized deposits alongthe valleys between diapiric ridges in upper slopeareas often are asymmetric with bedrock flankingone margin and sediment drift deposited on the othermargin. In other locations, sediment drift may flankboth margins of the channel floor that sometimes hasremained stable and other times has migrated withtime creating large cut and fill sections.

4.4. Age and acoustic character of Quaternary shelfsequences

Based on distribution of late Quaternary depositsin the Gulf of Cadiz, two types of regional deposi-tional sequences can be defined on the margin. Inthe first type located on the northwestern Cadiz mar-gin, Quaternary deposits form part of the foredeepbasins of the Guadalquivir Depression (Maldonadoand Nelson, 1999). This region is characterized bysubsidence, thick sedimentary units, and develop-ment of numerous prograding shelf margin deltas(Rodero et al., 1999). The second type located onthe eastern margin, in contrast, is part of the exter-nal zone of the Gibraltar Arch affected by tectonicmovements during the Quaternary. Because this re-gion has been a structural and morphologic highduring the Quaternary, only tidal flat sequences havedeveloped. Between these two regions, there is atransition zone with intermediate characteristics.

The upper Quaternary deposits of the Gulf ofCadiz contain nine units and four types of depo-sitional bodies. These sequences formed between740 000 yr BP to the present and are characterizedby 5th order cycles (Rodero et al., 1999). The se-quences in each cycle are shallowing upward andconsist of a littoral prism and tidal flats on the innerand mid-shelf, prodelta lobes and marginal deltasat the shelfbreak and upper slope, and hemipelagic

low-stand wedges. The wedges also contain limitedsequences of turbidites, bedforms and slumps on themiddle–lower slope (Fig. 4). In general, regressiveand lowstand system tract deposits predominate, andare separated by significant sedimentary hiatuses thatcorrespond to erosional surfaces. Because these de-velop during phases of eustatic sea-level rise andhigh stands of sea-level, the preservation potential oftransgressive deposits in the stratigraphic record ofthe margin is low.

Diapirism on the slope, linked in part to tec-tonic episodes, also influences the development ofQuaternary sedimentary sequences on the margin(Maldonado and Nelson, 1999). The distribution ofdiapirs influences the current patterns which in turndetermine the location of depocenters and currentscour (Fig. 4) (Nelson et al., 1993)

5. Distribution and acoustic character ofHolocene mud

Gutierrez-Mas et al. (1996) and Rodero et al.(1999) recognize a highstand system tract layer ofHolocene transparent mud in the mid-shelf regionof the southern shelf. This study of the entire Span-ish shelf finds that the Holocene layer is poorlydeveloped at the northern and southern ends, but in-between off the largest Guadiana and Guadalquivirriver mouths, it is well-developed and covers nearlythe entire shelf (Fig. 5). The thickest part of theHolocene layer off the Guadiana river mouth isoffset to the southeast of the river mouth, has anelongate taper to the southeast and all the mudbeltmargins except the southeast are interrupted by non-depositional areas. At its thickest point in the centralshelf, the apparent Guadiana-derived Holocene layeris about 15 m thick and 25 km wide. The maximumthickness of the Holocene layer off the Guadalquivirriver is approximately 25 m and its width reaches 45km to cover the nearly the entire shelf. The thick-est region develops southwest of the river mouthand a tongue in the central shelf extends over 60km to the southeast of the river mouth (Fig. 5).Beyond this point, irregular-shaped Holocene mudpatches of decreasing size are found toward theStrait of Gibraltar where only bedrock outcrops arefound.


Fig. 5. Map showing isopach thickness of the Holocene mid-shelf mud layer or highstand systems tract mud deposits. Isopachs areshown in milliseconds (thickness in meters is approximately 75% of millisecond value). The pattern with thick horizontal lines showsareas where acoustic basement outcrops. Isopach and outcrop data are based on seismic profile tracklines shown in Fig. 1.

The smaller river systems also influence the dis-tribution and thickness of the Holocene acousticlayer. Northwest of the Guadalquivir river mouth aninshore isopach lobe leads toward the Tinto rivermouth (Fig. 5). The Tinto contributes to this part ofthe Holocene layer according to seismic reflectorsand heavy metal contamination in sediment that istraced from this source (Nelson and Lamothe, 1993;Palanques et al., 1995; Van Geen et al., 1997). Bothsurface and subsurface heavy metal distribution alsoshow that Holocene mud deposited from the Tintosource extends as far west as the Guadiana and as

far southeast as the outboard Holocene isopach lobeto the northwest of the Guadalquivir (Palanques etal., 1995; Van Geen et al., 1997). The Guadaleteriver mouth also appears to be associated with anextension of a slightly thicker isopach portion of theHolocene layer that extends into the river mouth anda thickened patch to the west of the river mouth(Fig. 5).

A typical example of the nearshore to offshorechanges in the acoustic facies character of theHolocene highstand mud layer is found off theGuadalete river mouth. Distinct proximal reflectors


prograde westward offshore from the river mouthand downlap onto the underlying late Pleistocene toearly Holocene transgressive sand surface (Gutier-rez-Mas et al., 1996). Offshore, the distal mud be-comes increasingly transparent and lacks distinct re-flectors. The same nearshore characteristics of strat-ified reflections are found in the northern shelf offthe Guadiana and Tinto river mouths; in this case,however, the transition to transparent distal mudfacies continues toward the southeast because ofthe Atlantic inflow current advection of distal muds(Fig. 6). The development of the proximal acousticfacies with reflections grading to offshore and along-shelf transparent Holocene mud facies is similar to

Fig. 6. A 3.5 kHz seismic profile showing onshore to offshore change in acoustic facies of the Holocene mid-shelf mud layer. Profilelocation is shown in Fig. 1.

the well-defined Holocene acoustic facies for theEbro river prodelta mud layer (Diaz et al., 1990).

In addition to the acoustic facies that show asoutheast progradation of the Holocene river prodeltamuds off the Tinto, the trend to thickest isopachssouth and southeastward off both the largest rivermouths, the Guadiana and Guadalquivir, shows thesame pattern of progradation (Figs. 5 and 6). Thisthickness trend is most pronounced off the Guadianawhere the main depocenter is offset 20 km to thesoutheast (Fig. 5), however, both older and youngerHolocene sedimentary units of the Guadalquivir riverprograde more than 75 km southeastward (Fig. 4)(Gutierrez-Mas et al., 1996; Rodero et al., 1999).


6. Characteristics of bedform fields

6.1. Shelf bedform distribution

A complex pattern of bedforms is found in boththe continental shelf and slope regions of the easternGulf of Cadiz. In the northwestern inner shelf region,a generally regular distribution of submarine duneshas been found between 10 and 20 m water depthwith crests that are perpendicular to the eastwardAtlantic inflow shelf currents paralleling the coast-line (Fig. 3) (Vanney and Menanteau, 1985). West ofthe Tinto estuary mouth, the dune field is deeper andwider; east of the Guadalquivir estuary mouth, it is ab-sent because of the Holocene mud layer that extendsinto the estuary mouth (Figs. 5 and 7) (Gutierrez-Maset al., 1996; Lopez-Galindo et al., 1999). Deeper onthe inner shelf, a number of shallow paleo-valleys,of approximately 5 km width and up to 40 m depth,have been formed oblique to the coast (Fig. 2) (Van-ney and Menanteau, 1985). They drain offshore flowdown valley and develop submarine dune fields whosecrests are transverse to this flow.

Much of the southeast shelf is covered by a widevariety of bedform fields with diverse orientations ofbedform crests (Fig. 4) (Lobo et al., 1996; ITGE,1997). In the inner shelf zone between Cadiz andConil, transverse dune crests are perpendicular toand asymmetric toward an Atlantic inflow currentdirection to the southeast (Lobo et al., 1996). FromConil south to Barbate, nearly the entire inner to mid-dle shelf exhibits transverse dune ridges suggestingsoutheast Atlantic inflow currents across the entireregion. In contrast, south of Barbate to the Strait ofGibraltar and in the middle to the outer shelf area be-tween Conil and Cadiz, the transverse dune ridges areasymmetric toward a reversed Mediterranean outflowto the northwest. In the reversed outflow current regionwest from Gibraltar, the bedforms grade from large-scale dunes to megaripples; further west on the outershelf, small-scale dunes are found (Fig. 4). In the mid-shelf region west of Barbate, inflow current bedformsgrade from small-scale ripples to large-scale dunes.

6.2. Slope bedform distribution

The main areas with bedforms on the Cadiz con-tinental slope coincide with the general pattern of

bottom currents described previously (Fig. 3). Themain area of sand dunes is found in the deeper andflatter terrace of the upstream middle slope (300–800m) (Fig. 4) (Kenyon and Belderson, 1973; Nelson etal., 1993). Where this main sand dune region in-tersects the steeper upper slope (300–500 m waterdepth), the dune area often evolves into local areaswith erosional truncation of beds or downstream intoareas with small dunes (Fig. 4); where the dune re-gion encounters diapiric ridges downstream in themiddle slope, the dune area evolves into sedimentdrift bodies; where the dune region reaches the deepdistal southwestern part of the slope (about 800m) large mudwaves are found. The valleys that cutdownslope through all of the aforementioned upperto lower slope regions with bedforms, have addi-tional bedform sets of their own that are associatedwith the down-valley Mediterranean outflow currents(Fig. 4) (Nelson et al., 1993).

When describing the bedforms types in the afore-mentioned slope environments we conform to thesuggested terminology of the SEPM Bedform Re-search Group (Ashley, 1990) and rely on the detailedsonographs published previously in Nelson et al.(1993) and Baraza et al. (1999). In the upper slope,small 2D transverse sand dunes (0.5 m wave heightand 10 m in wave length) are developed in muddysand (65–75% sand) of the middle to eastern area inthe Gulf of Cadiz (Fig. 7B) (Baraza et al., 1999, seefig. 8A).

The main sand dune field of the middle slope ischaracterized by long-crested, 2D large sand dunes(heights <10 m, usually 3–5 m and lengths of30–150 m) with straight crests transverse to themain contour-parallel, Mediterranean outflow cur-rent (Nelson et al., 1993; Baraza et al., 1999, seefig. 8C). Within the sand dune field, however, localvariation in topography and downslope bottom cur-rents in valleys can result in change in orientation ofsand dunes and a complex interplay of superimposedbedforms. For example, in the transition zone from avalley toward the main sand dune area, the apparentinterplay of downslope and alongslope currents andtopographic disruption of the valley wall results in aregion dominated by compound dunes with 3D largebarchans combined with large and small-scale 2Dsand dunes (Baraza et al., 1999, see fig. 8B). Bottomphotos of Heezen and Johnson (1969) and Melieres


Fig. 7. Maps of (A) percentage gravel, (B) percentage sand in surface sediment of the Cadiz shelf and continental slope.

et al. (1970) show that complex sets of sand ripples(wavelengths of 10–30 cm) also may be superim-posed on the sand dunes just as they often are onshallow water sand dunes in unidirectional currents(Nelson et al., 1982).

As current speeds slow from about 40 to 75 cm=sin the sand dune area to 15–20 cm=s toward thewestern edge of the Mediterranean outflow (Fig. 3)(Ambar and Howe, 1979b), the mid-slope (ca. 400–800 m) sand dune area evolves into the lower-slope


Fig. 7 (continued). (C) Percentage clay in surface sediment of the Cadiz shelf and continental slope.

(ca. 800–1200 m) mud wave area (heights 10–40 mand lengths approximately 500 m) of Kenyon andBelderson (1973). In the transition zone, we observesand-dunes that appear to be superimposed on largemud waves and sediment texture changes graduallyfrom muddy fine sand (>65% sand) to sandy mud(<45% sand) (Fig. 7) (Nelson et al., 1993).

A wide variation of bedforms characterize the val-ley floor and adjacent wall areas of the major slopevalleys with down-valley Mediterranean outflow cur-rents (Fig. 3). Straight-crested dunes on valley floorsare transverse to the valley floor and down-valleyflow, but these crest orientations are perpendicular tothe direction of those formed in the sand dune andmud wave regions of contour-parallel flow from theMediterranean outflow (Fig. 3) (Nelson et al., 1993).Small to large sand dunes are found on the chan-nel floors of valleys that cut through the sand duneor mud wave area and coarser-grained sediment oc-curs on the valley floor compared to the surroundingmuddy or sandy contourite areas (Fig. 7A). Bed-form types may change from proximal to distal partsof the same valley (Nelson et al., 1993). Bedformcharacteristics also vary from southern valleys withstrong currents (80 cm=s) to northern valleys withweak currents (25 cm=s) (Fig. 3).

7. Surface sediment lithology

7.1. Complex inner shelf outcrops, sands and muds

The innermost shelf area has a complex vari-ety of sediment lithologies consisting of consoli-dated bedrock outcrops, beach rock, shoreface sandand gravel deposits, and mud patches. Pleistocenebeach rock deposits developed at sea level lowstandsare the most common type of consolidated rock(IGME, 1974; Vanney and Menanteau, 1985; ITGE,1997). Toward the southeast, bedrock outcrops be-come widespread over the shelf and predominatewest of Tarifa (Fig. 4) (Lopez-Galindo et al., 1999).Bedrock consists of a wide variety of lithologictypes and ages, but the Gibraltar flysch predomi-nates (Maldonado and Nelson, 1988; ITGE, 1997;Lopez-Galindo et al., 1999).

Sporadic local gravel deposits, typically of co-quina, are associated with the beach rock depositsthroughout innermost shelf areas (Fig. 7A) (IGME,1974; Vanney and Menanteau, 1985; ITGE, 1997).The inner shelf and extending to the outer shelf areasouth of the Spain=Portugal border exhibits suchareas of basement outcrop with associated coquina(Figs. 5 and 8). Midway between the Tinto and


Fig. 8. General lithology of cores from the Gulf of Cadiz Spanish shelf and slope.

the Guadalquivir river mouths there is a large areawhere coquina occurs as shown by bottom sampleswith shell hash, carbonate contents up to 70%, andgravel content of 10–20% (Figs. 7A and 9). Fromthe Guadalete river mouth and to the southeast onthe inner shelf, gravels associated with both coquinadeposits and bedrock outcrops become increasinglycommon (Fig. 7A). Carbonate contents and bottomsamples show that coquinas are the main source ofgravel near the Guadalete, although carbonate co-quinas become less predominate west and south ofBarbate where gravel content is consistently highest(20–40%) and associated with an increasing numberof bedrock outcrops (Figs. 4, 5, 7A and 9) (Gutier-rez-Mas et al., 1996; Lopez-Galindo et al., 1999).

Sand is the predominant sediment texture in theinner shelf, except near the previously mentionedgravel-rich deposits associated with beach rock andbedrock outcrops as well as the mud rich areasassociated with river mouths. Sand content generallyis 60–70% with increasingly higher percentage nearthe coast in the north or outcrops in the southerninner shelf (Fig. 7). Toward river mouth areas thesand content becomes increasingly diluted by siltand clay content and sediment lithology grades tosilt and clay patches. Silt texture predominates inthe inner shelf mud patch off the Guadiana rivermouth, clay predominates in the mud patch off theGuadalquivir, and both silt and clay patches arefound in Cadiz bay at the mouth of the Guadalete


Fig. 9. Bulk percent calcium carbonate content in surface sediment from the Gulf of Cadiz Spanish shelf and slope. West-central slopearea labeled ‘planktonic tests’ is a region where surface sand deposits (Fig. 7B) contain >60% planktonic tests in the sand-sized sediment(Nelson et al., 1993). ‘Valley floor’ label shows where sediment of the continental slope valley floor is dominated by bioclastic lagdeposits.

river (Fig. 7). Local patches of silty and clayey mudare found on the shelf west of Barbate that appear tobe associated with local topographic depressions.

7.2. Mid-shelf mud layer

The middle shelf between the inner shelf sandand the outer shelf muddy sand is covered by a mudlayer with greater than 50% clay grain size exceptnear Gibraltar (Fig. 7C). The mud layer extendstowards the mouths of the two largest rivers, theGuidiana and Guadalquivir. The mud layer extensiontoward the Guadiana is narrower and less clay dom-

inated (50–60%) compared to the wider and moreclay dominated (60–70%) mud layer that enters theGuadalquivir river estuary.

Off both the Guadiana and the Guadalquivir riversthere is a proximal to distal offshore and southeasterngradation to finer grain size. The Guadiana gradessouthward for 20 km offshore to 50% clay and for50 km southeastward along-shelf in the mud layerto 70% clay (Fig. 7C). The Guadalquivir gradeswestward for 30 km offshore to over 70% clay andfor 50 km southeastward along-shelf in the mudlayer to 80% clay. The much smaller Guadalete rivergrades to a 50% maximum clay patch in the center


of Cadiz Bay, but does not show the continuousgradation southeastward into the mud belt (Fig. 7C)(Gutierrez-Mas et al., 1996).

7.3. Outer and southeastern shelf sand

The mid-shelf mud layer grades offshore into aclayey sand at the shelf edge and southeastwardinto a gravelly sand (Fig. 7). The shelf edge clayeysand averages 50% sand and up to 40% clay. In thesoutheastern middle to outer shelf west northwest ofBarbate, the sand content reaches over 90%. South-eastward from this clean sand maximum, depositsgrade to gravel contents of 30–40% and can reachover 80% gravel. Further southeastward west of Tar-ifa and near the Strait of Gibraltar, mainly bedrockoutcrops are found (Figs. 4 and 5) (Lopez-Galindo etal., 1999).

7.4. Upper slope and ridge and valley sands

In contrast to the shelf that has a significant areacovered with silty clay except near the Strait ofGibraltar, most of the slope is covered by sand tomuddy sand (Fig. 7B). From south to north alongthe upper slope, there is a tongue-like gradation ofsurface sand that is 40 km wide at the southern endwhere it contains 65–80% sand; the northern endnarrows to 10 km and contains 55–65% sand.

In the ridge and valley area of the central slope,the textural patterns become more complex. Thesoutheastern valley floors contain gravelly sand, thecentral valley floors contain coarse bioclastic sandand the northwestern channel floors contain fine sand(Figs. 7A and 9) (Nelson et al., 1993). Similar to thevalley floors, the surface deposits of the sedimentdrift bodies along the ridges also vary from coarser-grained sediment on the southeast bodies (>80%sand) to finer-grained sediment on the northwestbodies (<50% sand) (Fig. 7B). On the crests of theridges, Miocene marls outcrop (Fig. 8) (Maldonadoet al., 1989; Maldonado and Nelson, 1999).

7.5. Western and lower slope sandy muds

Northwest of the high relief of the ridge andvalley area in the central slope (Fig. 2), a leewardshadow of finer sediment is found with only 10–

40% sand (Fig. 7B, ca. 36º300 lat., 7º120 long.). Onthe large topographic high to the west of this basinwith sandy mud, the sand percentage again increasesto over 65%, but as carbonate contents show, thesand-size particles are mainly planktonic tests, notsiliciclastic grains (Fig. 9, ca. 36º300 lat., 7º250 long.)(Nelson et al., 1993). In the deepest southwesternpart of the study area dominated by large mud waves,the sand content generally is 55% or less except invalley-floor areas, and the clay content may exceed30% (Fig. 7). In contrast to the muddy sand andsilt of the distal mud wave area, the sand dune areaon the deep slope toward the Strait of Gibraltar ischaracterized by silty sand to clean medium sandwith greater than 65% sand and less than 20% clay.

8. Subsurface sediment lithology

8.1. Complex inner shelf outcrops, sands and muds

The isopach map of Holocene sediment thicknesshelps to explain the subsurface sediment distributionand the lithology obtained from cores (Figs. 5 and 8).A transect of cores across the margin just north of themouth of the Bay of Cadiz also provides a generalsummary of the inner shelf to upper slope subsurfacelithology (Fig. 10). In the inner shelf zone, except offthe mouth of the Guadalquivir river, most cores wereless than 0.5 m length and consisted of bioclasticsand and shell lag layers (Figs. 8 and 10). In deeperparts of the inner shelf with >20 m of water, muddysand occurs (Fig. 11) and off the Guadalquivir rivermouth, mud thickness reaches greater than 15 m(Fig. 5). These inner shelf muds, however containnumerous shell and sand lag layers (Figs. 8 and 10)that appear to be storm lag layers similar to thosenoted in other deltaic settings (Nelson, 1982a,b; Diazet al., 1990).

8.2. Mid-shelf mud layer

Subsurface core lithology of the mid-shelf mudlayer conforms to the Holocene isopach distribution.In the central shelf areas from the Guadiana toGuadalete rivers, the mud layer generally is 10–20 m thick, but elsewhere it is only a few metersthick and can be penetrated by shallow gravity cores

C.H

.N

elsonet

al./Marine

Geology

155(1999)

99–129115

Fig. 10. General lithology of a core transect profile 1 across the Gulf of Cadiz shelf southwest of the Guadalquivir river mouth. Location of Profile 1 is shown in Fig. 2.


Fig. 11. Vertical distribution of grain size in sand and coarse silt layers in nearsurface sediment of the Cadiz shelf and slope.

(Fig. 5). At the northwestern end of the mud near thePortuguese border and directly south of the mouth ofthe Guadiana river, the mud is thin and patchy andonly approximately 1 m thick. By 80 m water depthat the western offshore edge in the central part of theGulf of Cadiz, the mud layer has thinned to less than1 m over an underlying sand (Fig. 10, see core 33).The inner margin where the mid-shelf mud is lessthan 1 m thick is quite variable. It extends offshoreto greater than 50 m water depth on the northwest,whereas toward the Guadalquivir river it extendsprogressively inshore to reach nearly 30 m waterdepth near Cadiz (Figs. 8 and 10, see core G-164).South of Cadiz, the mud rapidly thins to exposeunderlying sand across the entire shelf (Fig. 8).

In all the fringe areas of the mid-shelf mud,except the southern margin, the mud layer is clayeysilt (Fig. 11). Away from the fringes, the upper mudis silty clay size, but grades to coarser clayey siltdown section. At the southern edge of the mid-shelfmud, the entire section is silty clay.

8.3. Outer and southeastern shelf sand

The outer shelf is characterized by bioclasticmuddy sand and silt with common shell lag lay-ers in the subsurface sediment. In the northwesternpart, where the outer shelf is wider from approx-imately 100–120 m depth, muddier sandy silt ischaracteristic and thin interbedded silt and mud lay-


ers occur (Fig. 11). In the central outer shelf west ofthe Guadalquivir river, the mud belt extends nearlyto the shelf edge, but again muddy silt and shellbeds are common on the outer shelf (Fig. 10, seecores G30–33). The southeastern outer-middle shelfis characterized by cleaner silty sand that has in-creasing amounts of bioclastic layers and internalcross-lamination toward Gibraltar (Figs. 4 and 8).The entire shelf south of Cadiz grades southeastwardfrom interbedded silt and mud beds, to increasinglythinner and cleaner silty bioclastic sand and thento scoured bedrock with no unconsolidated depositsnear Gibraltar (Figs. 5, 8 and 11).

8.4. Slope sand beds

The lithology of the upper 2 m of sedimentshows interbedded sand and coarse silt layers withhemipelagic silty clay (Fig. 8). Throughout our studyarea of the continental slope there is a surface sandlayer that varies from 0.5 to 1.5 m in thickness andfrom gravelly or clean sand in the southeast to sandysilt in the northwest (Figs. 8 and 11). In some lo-cations, a second sand layer was penetrated that hasthe same general southeast to northwest gradationsas the upper layer. The thickness (generally >0.5 m)and net sand percent (typically >50% sand layers tomud layers) in nearsurface sediment are greatest inthe upper slope (Fig. 8) (Nelson et al., 1993). Below550 m water depth, the net sand percentage usuallyranges from 20 to 45%.

The surface and deeper sand layer are generallycharacterized by reverse graded bedding (Fig. 11).Mean grain size varies from 3.5 phi at the top oflayers to 6 phi at the base. The coarsest layers andgreatest vertical gradation occur in the southeasternarea. Within the layers, trough cross lamination,flat lamination, or massive bedding are observed asinternal sedimentary structures, but they do not occurin any vertical sequence (Nelson et al., 1993).

9. Surface and subsurface sediment composition

9.1. Carbonate

The shelf sediment contains 10–20% planktonictests in the sand fraction, except in the northwest-

ern shelf edge area where >30% tests are found(Nelson et al., 1993). A low content of planktonicconstituents is found in the southeastern and upperslope regions, but these also grade to high quantitiesin the northwest mid-slope area where >60% testsare found and high carbonate content is observed(Fig. 9). In general, the bulk carbonate content ofthe surface sediment over the northwestern margin isgreater (20–30%) than the southeastern margin (10–20%) (Fig. 9). In the northwest mid-shelf mud, theaverage surface and subsurface (up to 2.3 m depth)amount of carbonate in the total clayey-silt sedimentwas about 30%. South of the Guadalquivir rivermouth in the finest silty-clay part of the mid-shelfmud up to 2 m depth, however, carbonate contentswere higher (37–43%). In both the northwest andsoutheast regions of the shelf, the maximum car-bonate contents are associated with the inner shelfPleistocene beach rock where carbonate content ex-ceeds 50% (Fig. 9) (Lopez-Galindo et al., 1999).

The maximum carbonate content on the continen-tal slope also reaches greater than 50% carbonatein the central and southeastern valley floors betweenslope ridges because floors contain gravel to coarsesand-sized bioclastic lag deposits (Fig. 9) (Nelson etal., 1993). The surface and subsurface (up to 1.7 m)slope sediment in general contains more carbonatethan the shelf and has consistent values between 37and 41% except in the southeastern sand dune areawhere content decreases to 37%.

9.2. Organic carbon

One main transect of samples for organic carbonanalysis has been collected across the northern mar-gin from the mouth of the Tinto river directly southto the slope base (Fig. 9). The surface mid-shelf siltyclay consistently contains 1.5% organic carbon. Thecontent drops to about half of this amount (0.8%) inthe outer shelf sandy silt and to minimum contents of0.3% in the upper slope sand sheet. The middle slopesandy silts of the sediment drift deposits consistentlycontain 0.5–0.6% organic carbon. The organic car-bon content shows a clear correlation with grain sizewhich is consistent in the Cadiz region as elsewhere(Nelson and Lamothe, 1993)


10. Sediment age and sedimentation rates

10.1. Continental shelf

On the continental shelf, the oldest ages occur inshell lags of apparent transgressive sand and gravel.Both the 12 900 yr BP age at 99 m and the 12 640 yrBP age at 77 m are co-incident with the most well-established eustatic sea level curves from Barba-dos and Huon Peninsula (Figs. 12 and 13) (Elias etal., 1996). Ages of about 8–9000 yrs BP for twoother shell lags at 50 m are in general agreementwith established curves, but are slightly younger, per-

Fig. 12. Sedimentation rates and age of the nearsurface sediment on the Cadiz continental margin. Ages of mid-shelf and slope mudare based on conventional radiocarbon dates made from bulk organic carbon in whole sediment samples. Ages of transgressive shell laglayers are from carbonate carbon. Locations of radiocarbon subsamples are shown by small dots along core.

haps because of contaminating younger shells affect-ing surface sample 26B and=or subsidence of theGuadalquivir depression affecting sample 63 (Fig. 12)(Maldonado and Nelson, 1999; Rodero et al., 1999).The age data from the northwestern shelf indicatesthat this area of the shelf has been tectonically stablesince the late Pleistocene sea level transgression.

Sedimentation rate information based on radio-carbon ages is only available for the younger mid-shelf mud layer, because the coarser grained sand,gravel and shell lags deposited by the late Quater-nary sea level transgression were not penetrated byour gravity cores. Sedimentation rates are in general


Fig. 13. Eustatic sea level curve comparison between Gulf of Cadiz basal transgressive shell lag radiocarbon ages with those fromBarbados and Huon Peninsula. Barbados and Huon Peninsula data are modified from Elias et al. (1996).

agreement with Holocene sediment isopach thick-ness derived from high-resolution seismic profiling(Fig. 5). As expected at the fringes of the mudlayer, the sedimentation rates were low, about 11cm =kyr at core site 82 on the northwestern edgeand 18 cm=kyr at core site 63 on the southeasternedge (Fig. 12). These rates are in close agreementwith rates estimated from ages determined using wa-ter depth, mud thickness from seismic isopachs, andeustatic sea level curve ages (Figs. 5 and 13).

The highest sedimentation rates are found in thethickest central mud layer associated with majorriver sources (Fig. 5). On the northwestern shelf atcore site 25 located southeast of the Guadiana rivermouth at mid-shelf water depths, the sedimentationrate is 131 cm=kyr based on the eustatic sea levelcurve plus isopach thickness deposited during thelast 10 300 years (Figs. 5 and 13), 64 cm=kyr basedon 14C ages during the past 8000 years (Fig. 11), and80–120 cm=kyr based on 210Pb activity of the past100 years (Van Geen et al., 1997). These data showa pattern of earlier Holocene higher sedimentationrates, middle to late Holocene decrease in rates and apast 100 year increase in rates at this location wherethe shelf appears to have been tectonically stable.

In the outer central shelf region fed by the Tintoriver source, lower, but similar patterns of sedimenta-tion rate change are apparent at core site 22 (Fig. 12).The average sedimentation rate for the past 12 000

years based on eustatic sea level curves and sedi-ment isopach thickness is 74 cm=kyr (Figs. 5 and13). Similar to the northwest mud layer, the sedi-mentation rate of 22.6 cm=kyr (or 53 cm=kyr with a5000 yr correction for surface contamination shownin cores 25 north and 15 south of site 22) for thepast 9000 years based on 14C ages is lower than therate for the entire isopach thickness of the mud. Thischange in sedimentation rate from the early to laterhistory of the mud layer may be expected because itis typical for sediment input to decrease during thelater Holocene as vegetation cover increased (Nel-son, 1990). The sedimentation rates based on 210Pbactivity of the past 100 years vary from 110 to 160cm=kyr in core 22 (Van Geen et al., 1997), and likedata from core 25 on the northwest shelf, suggestthat human deforestation on the Iberian Peninsuladuring the last few centuries has caused increasedsedimentation rates on the Cadiz shelf as it has onthe Ebro shelf off Spain (Nelson, 1990).

The highest sedimentation rates in the mud layerare found at station 15 near the Guadalquivir riversediment source. The average sedimentation rate atthis site for the past 10 300 years based on eustaticsea level curves and sediment isopach thickness is110 cm=kyr (Figs. 5 and 13). Transgressive sedimentages at nearby station 63, however, show that depo-sition of the mud layer from the Guadalquivir sourcehas reached this area only about 8000 years ago.


Using this age for initiation of mud deposition atsite 15 results in average mud blanket sedimentationrates of 141 cm=kyr. Based on 14C ages, the highestsedimentation rates (234 cm=kyr) encountered on theshelf are found during the past 6000 years at site 15(Fig. 12). This is not surprising considering that thelargest sediment source in the Gulf of Cadiz feedsthis region and that, after initial aggradation near theriver mouth to fill accommodation space (Gutierrez-Mas et al., 1996), rapid southeastward progradationof the mud layer from the Guadalquivir source hasoccurred more recently near site 15.

10.2. Continental slope

From previous studies of seismic profiles fromthe Cadiz margin and from the Mediterranean Sea,we interpret that deposition of the sediment driftsequence began at the end of the Messinian (Nelsonet al., 1993; Maldonado and Nelson, 1999). It isalso apparent that within the drift there have beenalternating cycles of bedform and sediment driftdeposits or sediment drape. In this paper, the agesof these major cycles is not determined and weonly consider ages of specific cycles in the latePleistocene and Holocene nearsurface sediment.

The surface sand layer on the slope is Holocenein age because the youngest age determined fromthe mud below the sand layer is about 10 000 years(Fig. 12). In the most southeasterly and upstreamupper slope core, where age of the mud immediatelybelow the surface sand layer is 25 260 years, we in-terpret that the strong contour currents of this regionhave eroded the underlying poorly consolidated mudprior to sand deposition (Figs. 3 and 12). Currentreworking of the sand layer itself is suggested byupstream surface ages older than 5000 years com-pared to downstream ages younger than 500 years(Fig. 12). In the core with the oldest surface sandlayer age of 5380 yr, a second sand layer youngerthan 22 400 years old is found. Ages in this and thecores immediately south of it show that more thanone sand layer developed in sediment drift depositsof the central slope ridge and valley area duringthe latest Pleistocene and Holocene time, similar tothose that developed in the Faro Drift (Fig. 12) (Stowet al., 1986). In contrast, radiocarbon ages of 25 260to >30 000 yr suggest that the deeper sand layers in

the upper slope area are much older than mid-slopelayers and may be correlated with the previous highsea level stand at about 120 000 yr B.P. (Chappelland Shackleton, 1986).

As a consequence of the old nearsurface ages ofthe sediment, the Cadiz slope sedimentation ratesin general are low (1–12 cm=kyr) compared evento normal Holocene hemipelagic mud sedimentationrates of 10–35 cm=kyr on the slope (Fig. 12) (Nel-son, 1976; Nelson, 1990). The sedimentation rates(1–5 cm=kyr) are lowest in the Cadiz upper slope,however, these rates are similar to those Holoceneupper slope rates elsewhere (Fig. 12) (Nelson, 1990).Considering that sediment drift bodies exhibit asmuch as 657 m thickness, deposited over about thepast 5 million years since the end of the Messinian,or an average sedimentation rate of 13.1 cm=kyr, therates of the sediment drift deposits appear to haveremained relatively constant throughout their history(Fig. 12) (Nelson et al., 1993).

11. Shelf and slope facies associations andprocesses

11.1. Holocene highstand inner shelf sand facies

A number of sediment facies are found in theinner shelf area between 0 and 50 m, but becausewe collected few data in this area we will not makea detailed definition of the several types of faciesthere. From 0–10 m a shoreface coarse to fine sandfacies formed by shoaling waves is described byothers (IGME, 1974; Vanney and Menanteau, 1985;ITGE, 1997; Lopez-Galindo et al., 1999). Between10 and 20 m water depth and extending locallydown paleo-valleys off the Tinto and Guadalquivirrivers to more than 40 m, a sand dune facies withintermittent rock outcrops of beach rock is mapped(Fig. 14) (Vanney and Menanteau, 1985). We findshell lag layers within this facies that are less than3000 years old showing the Holocene age of thisfacies (Fig. 12). Southeastward Atlantic inflow cur-rents and south to southwestward veering flow downpaleo-valleys appear to control the development ofbedforms (Fig. 15). A sandy-silt mud patch facieswith storm shell lag layers is developed at 20–50 mbetween the paleo-valleys. Its formation appears to


Fig. 14. Distribution of current-deposited and other shelf and slope facies of the Cadiz continental margin. Types of bedforms are shownin Fig. 4, and sediment texture is presented in Figs. 7, 8, 10 and 11. Location of unsampled shelf paleo-valley facies is after Vanney andMenanteau (1985).

be controlled by a combination of lower energy At-lantic inflow currents and reworking by storm waves.

11.2. Holocene highstand mid-shelf mud facies

The mid-shelf clayey silt to silty clay mud layerbegins inshore at 40–50 m, extends offshore to about100 m and has Holocene radiocarbon ages (Figs. 12and 14). The inshore boundary appears to be con-trolled by wave action and paleo-valley offshore cur-rents, except where the large sediment source of theGuadalquivir river causes the mud blanket to extendinto the Guadalquivir estuary mouth (Figs. 14 and15). The offshore edge extends to about 100 m wherecomplex shelf-edge current and internal wave pro-cesses (e.g. Karl et al., 1983; Pietrafesa, 1983; Stan-ley et al., 1983; Cacchione and Drake, 1986) limit itsoffshore extent, except west of the large Guadalquivirsediment source. The northwestern edge of the mudlayer at the Spanish–Portuguese border is limited bya lack of a nearby significant river source to the west

and erosion of the Atlantic inflow currents. The ab-sence of the mud belt to the southeast of Cadiz Bay inpart may be attributed to the lack of significant riversources, but most importantly, morphologic and tex-tural gradations (Figs. 4 and 7) show that accelerationof the Atlantic inflow currents near Gibraltar preventsdeposition of silt and clay (Figs. 3 and 15).

We find the same sequence stratigraphic patternthroughout the entire Gulf of Cadiz shelf area as thatnoted by other studies for the southern shelf area(Gutierrez-Mas et al., 1996; Rodero et al., 1999).Near river mouths a Holocene highstand systemstract has prograded offshore with sigmoidal clino-forms (Fig. 6). Isopach thicknesses that are off-set (Guadiana) and elongated toward the southeast(Figs. 4 and 5) and textural gradations toward thesoutheast (Fig. 7), however, show that the mud layeralso has prograded southeastward from the majorriver sources. We attribute this significant southeast-ward progradation alongshelf to the Atlantic inflowcurrent system (Figs. 3 and 15).


Fig. 15. Sedimentary processes of current-deposited facies of the Cadiz margin. Strength of flow in slope environments is based oncurrent speeds illustrated in Fig. 3. General current speeds and direction for shelf processes are based on estimates from distribution andorientation of sediment bedform features shown in Vanney and Menanteau (1985); the formation of active inner shelf dune field featuresrequires medium strength currents, and the lack of bedforms in the Holocene mid-shelf fine-grained mud layer suggests weak currents.The inner shelf region affected by shoaling waves begins at about 35 m water depth where the onset of stratified acoustic facies isobserved (e.g. Fig. 6).

11.3. Late Quaternary outer shelf transgressive sandfacies

Data from seismic profiles, radiocarbon ages andgrain size all show that a transgressive sand depositunderlies the Holocene mud facies (Figs. 6, 7 and12). Because of shelf-edge current dynamics andincreasing bed shear from Atlantic inflow currentsnear Gibraltar, this transgressive sand has not beencovered by Holocene mud at the outer shelf edgefrom about 100 to 120 m nor across the entiresoutheast shelf region (Figs. 3, 5 and 15). Sedimentlithology and Holocene mud isopach data, however,do show that the central outer shelf facies west of theGuadalquivir source has some mixing and overlapwith Holocene mud facies (Figs. 10 and 14). Thelate Pleistocene age and coarse-grained nature of theouter shelf facies shows that they are relict and notin equilibrium with the present wave and currentregime (Figs. 12 and 15).

A number of current and wave processes appearto interact at the Cadiz shelf edge to prevent depo-sition of modern muds over the relict transgressiveouter shelf sand. On the southern shelf edge, in-teraction of the Atlantic inflow and Mediterraneanoutflow currents advect and prevent deposition offine-grained sediment. Further north, the focus oftypical boundary, tidal, internal wave, surface waveand other meteorologically induced currents at theshelf edge combine to prevent mud deposition (Karlet al., 1983; Cacchione and Drake, 1986), exceptwhere the Guadalquivir mud source appears to over-whelm oceanic processes.

11.4. Palimpsest southeast shelf sand facies

The sands on the southeastern shelf, althoughoriginating as transgressive shoreline deposits havenow been reworked into sand dunes by the Atlanticinflow currents inshore and Mediterranean outflow


currents offshore (Figs. 3, 4, 6 and 15). Because ofthe combination of modern and ancient processes,they are categorized as a palimpsest sand dune faciesthat extends across the middle shelf from south ofCadiz to Tarifa (Figs. 4 and 14) (Lobo et al., 1996).Beach rock and bedrock outcrops increasingly dis-rupt the dune facies toward the south until eventuallythe constriction and increased speed of the Atlanticinflow and Mediterranean outflow currents result in anon-depositional scoured bedrock seafloor from Tar-ifa to Gibraltar Strait (Figs. 4 and 15) (Lobo et al.,1996; Lopez-Galindo et al., 1999).

The complex interplay of the Atlantic inflow andMediterranean outflow currents near Gibraltar re-sults in a variety of sand dune types and orientationof flow directions (Fig. 4) (Lobo et al., 1996). Ingeneral, the inshore dunes are transverse and asym-metric toward the southeast with larger bedformstoward Gibraltar, coincident with the intensified At-lantic inflow toward the Strait (Figs. 3 and 4). Theoffshore northwestern and southern dunes are trans-verse and asymmetric with smaller bedforms towardthe west, coincident with slacking Mediterraneanoutflow from the Strait. The transverse crests of theinshore dunes face southeasterly in the north andprogressively become more easterly as Atlantic in-flow turns more easterly to enter the Strait of Gibral-tar.

11.5. Upper slope facies

The upper slope facies develops in the steep upperslope region of smooth topography from 300 to 500m water depth (Fig. 14). This facies is characterizedby flat-lying bedded sequences, except for small-scale sand dune fields that occur in central and east-ern locations of stronger proximal currents (Figs. 3and 15) (Nelson et al., 1993). A gradual decrease incurrent speeds away from the Strait of Gibraltar hasdeposited sand layers that are about a meter thickand have an even gradation from fine clean sand onthe southeast to muddy silt on the northwest upperslope (Figs. 7 and 11). This even gradational faciesdevelops because of slacking currents, the smoothtopography and straight slope. When the significantbend in the coast occurs near the Portuguese bor-der, the current regime changes (Ambar and Howe,1979b) resulting in deposition of the large Faro Drift

sediment body in the upper slope region (Fig. 3)(Faugeres et al., 1985c; Stow et al., 1986).

11.6. Sand dune facies

The sand dune facies dominates most of the up-stream middle to lower slope region between thescour=sand ribbon area to the east near the Straitof Gibraltar (Kenyon and Belderson, 1973) and theridge=trough area to the northwest in the centralGulf of Cadiz (Figs. 2 and 14). The sand dune fa-cies is best developed on the upstream flatter lyingmid-slope terraces where there are strong currentspeeds and sand contents generally greater than 70%(Figs. 4, 7 and 15). The downstream regions de-velop more irregular sand dunes with finer grain size(50–70% sand) as current speeds gradually decrease.Near slope valleys, sand dunes exhibit variable orien-tation, crest length, and intermixing with compound3D barchan dune and ripple fields all of which ap-pear to be related to down-valley currents (Figs. 3and 15) (Nelson et al., 1993).

Present-day Mediterranean outflow currents in thedune facies area actively transport sand in suspen-sion as much as 11 m off the bottom (Thorpe,1972). Current activity, facies boundaries, and bed-form development may vary significantly because oftopographic changes, related variation in local down-valley currents and a number of general oceano-graphic processes (Figs. 2, 3 and 15) (Kenyon andBelderson, 1973). Small-scale turbulence may causethe patterns of small ripples superimposed on sanddunes (Melieres et al., 1970). Medium-term currentfluctuations and bedform variation can be caused byebb, flood, spring and neap tidal forcing (Boyum,1967) as well as by atmospheric pressure changesand strong east or west winds on either side of theGibraltar Strait (Lacombe and Tchernia, 1960; Priceet al., 1993; O’Neil-Baringer and Price, 1999).

11.7. Large mud wave facies

Similar to the sand dune facies, nearsurface lithol-ogy of the mud wave facies on the lower slopeconsists of alternating coarse-grained and mud in-terbeds, but mud waves are characterized by muchlarger wave-lengths >500 m, >30% clay, and <50%sand (Figs. 7, 8 and 14). Texture grades from a


median grain size of 0.60 mm on the east to 0.20mm on the west (Fig. 7) (Faugeres et al., 1985b).Grain size, however, is quite variable and there aresome transitional mud wave areas with a higher sandcontent and compound dunes and=or ripples; otherareas exhibit a smooth muddy sea floor with currentlineations (Heezen and Johnson, 1969; Melieres etal., 1970). High sand contents in some parts of themud wave facies result from valleys that dissect thearea and provide sand sources for local sand dunepatches in the mud wave area (Nelson et al., 1993).

The complex sets of bedforms and grain-sizevariation that characterize the mud wave area resultfrom the aforementioned variations in current forcingmechanisms, the gradual diminishing of Mediter-ranean outflow current speeds with depth, and thetemporal fluctuation in water depth of the undercur-rent (Figs. 3 and 15) (O’Neil-Baringer and Price,1999). In addition to the local down-valley flow thatwe believe modifies the large-scale waves (Nelson etal., 1993), Kenyon and Belderson (1973) suggestedthat major fluctuations in depth and velocity of theundercurrent in the mud wave area (i.e. š100 m,and <20–37 cm=s; Thorpe, 1972, 1976) may ex-plain the large wave form development and grainsize variation.

11.8. Sediment drift facies

Sediment drift bodies occur banked againstbedrock ridges in the central slope area and mostcommonly exhibit low angle unconformities in thesubsurface (Fig. 14) (Nelson et al., 1993). The driftis scoured and modified by cut and fill sequenceswhere it is intersected by bottom-currents flowingdownslope in valleys (Figs. 3 and 15). Interbeddedsand and mud layers make up the nearsurface lithol-ogy, but these vary from reversely-graded fine sandbeds in eastern to muddy silt in western drift bodies(Fig. 11). The trapping of current-suspended sand inupstream drift bodies apparently causes developmentof foraminiferal-rich drift deposits consisting of win-nowed planktonic tests deposited on topographic el-evations in the leeside ‘current shadow’ downstreamfrom the proximal bedrock ridges (Figs. 2, 9 and 15)(Nelson et al., 1993). Further downstream, the FaroDrift sediment body located off Portugal is charac-terized by deposits of silt beds interbedded with clay

(Gonthier et al., 1984; Faugeres et al., 1985a; Stowet al., 1986).

Sedimentation rates are consistently low through-out the entire history of the Gulf of Cadiz sedimentdrift deposits. Late Quaternary sedimentation ratesremain generally constant at 6–12 cm=kyr in bothupstream Cadiz and downstream Faro sediment drift,but this is reduced to <4 cm=kyr in erosive marginsalong the drift bodies (Fig. 12) (Stow et al., 1986).These late Quaternary rates are similar to those of12 cm=kyr for the thickest part (600 m) of the FaroDrift deposit during the entire post-Messinian periodof the last 4.8 million years (Faugeres et al., 1985a)and to post-Messinian rates of 14 cm=kyr for thethickest part (600–700 m) of the eastern Gulf ofCadiz sediment drift bodies (Fig. 12).

The general southeast to northwest change tofiner texture and greater planktonic test content ofthe sediment drift facies is explained by the down-stream decrease in the Mediterranean outflow cur-rents (Figs. 3, 7, 9 and 15). The scour and cut and fillsequences in sediment drift facies adjacent to valleywalls is associated with down-valley currents of theMediterranean outflow (Nelson et al., 1993). Thereare not sufficient data on sedimentation rates, grainsize and seismic stratigraphy of the drift deposits inthe eastern Gulf of Cadiz, however, to decipher theinterplay of bottom-current processes and generalgrowth patterns of the sediment drift facies.

11.9. Valley facies

Valley facies are downslope bottom-current de-posited facies that cut perpendicularly across thegeneral southeast to northwest along-slope con-tourite facies of sand dunes and sediment drift onthe Cadiz slope (Fig. 14). Bedform crests of thevalley areas strike SE to NW whereas those of thecontourite sand dune, mud wave and upper slopefacies strike NE to SW (Nelson et al., 1993). Similarto the contourites, the texture and scale of bedformsprogressively diminish from the southeast channelsin the region of stronger Mediterranean outflow cur-rents to the northwest region of weaker currents;however, the grain size and scale of bedforms in val-leys always are larger in any valley compared to thelaterally associated contourite facies that they tran-sect (Fig. 7) (Nelson et al., 1993). As a consequence


of the strong down-valley currents, bioclastic sedi-ment lags and lower sedimentation rates characterizethe valley floors compared to associated contouritesediment drift deposits (Figs. 9 and 12) (Faugeres etal., 1985a,b; Stow et al., 1986).

In contrast to the complex interplay of currentsdepositing sediment drift facies, the valley facies canbe readily accounted for by the strong down-valleybottom currents. The down-valley measured currentsin any locality are generally stronger than the contourcurrents (Fig. 3), which explains the coarser textureof valley facies compared to the adjacent contouritesand dune or sediment drift facies (Figs. 7A, 14and 15). The currents along a valley floor are nor-mally stronger than those along the walls and theshearing of down-valley flow with contour parallelflow should slow valley wall currents even more thanusual (Fig. 3). This difference between floor andwall currents explains the changes in bedforms andtexture between valley floor and wall environments(Nelson et al., 1993) The general weakening of theMediterranean outflow currents from the southeasttoward the northwest also affects the current rib-bons that flow down valley, and consequently, thesoutheastern valleys contain larger-scale bedforms,coarser texture and more bioclastic shell lags thanthe northwestern valleys (Figs. 3, 7, 9 and 15).

12. Factors controlling facies development

The initial control on the shelf and slope faciesdevelopment in the Gulf of Cadiz has been the open-ing of the Strait of Gibraltar at the beginning ofPliocene time. Since the opening of the strait, highsea level, equivalent to the present or greater waterlevel over the Gibraltar sill has permitted circulationthrough the strait and formation of strong Atlantic in-flow and Mediterranean outflow currents. Althoughtectonic change in sill depth can also control thedevelopment of these currents, data is insufficient toevaluate this affect prior to the Late Pliocene (Mal-donado and Nelson, 1999). The fluctuating glacial tointerglacial climates and eustatic sea levels (Chappelland Shackleton, 1986; Haq et al., 1987) seem to havebeen the dominant influence on the currents since theLate Pliocene, because prior to that time cyclic de-position of dune and sediment drape sequences is not

apparent. The Quaternary series of alternating duneand drape sequences appear to correlate with highand low sea levels, respectively (Nelson et al., 1993;Maldonado and Nelson, 1999). Another significantcontrolling effect of eustatic sea levels upon shelfand slope facies development is the formation of thelowstand prograding slope wedge off major rivers,transgressive sand sheet across the shelf, and high-stand prograding mud layer in the mid-shelf (Figs. 12and 15) (Gutierrez-Mas et al., 1996; Rodero et al.,1999).

During high sea-level times of sediment faciesdevelopment, the Atlantic inflow and Mediterraneanoutflow currents have been influenced by thermoha-line circulation, Coriolis force, gravity, lunar tides,storm tides and sea floor morphology. The result-ing basic inflow and outflow gradation from highvelocities on the southeastern areas of the Cadizmargin to weak currents in the northwest areas,causes a change in along-shelf facies as well asalong- and down-slope facies based on decreasinggrain size and bedform development (Figs. 3, 14 and15). The along-shelf gradation of shelf prodeltaicmud facies is similar to that of the Ebro prodeltaicmuds (Diaz et al., 1990). The down stream grada-tion of sandy contourite facies on the Cadiz slopeis similar to the bedrock to sand ribbon to sanddune facies sequences controlled by tidal currentsin the English Channel (Belderson et al., 1971) andMessinian Strait (Colella, 1990) or by unidirectionalgeostrophic currents in the Bering Strait (Nelson,1982a; Nelson et al., 1982). Tracing the bedrock tosand wave to prodeltaic mud facies (in the case ofshelves) sequences can provide a tool to define an-cient sequences and their controlling current patternsin deep or shallow water.

Morphologic control is an important influenceon the development of both shelf and slope fa-cies. Bathymetric elevations of beach- and bed-rockoutcrops contribute significantly to the patchy de-velopment of facies on the inner shelf (Figs. 4,5 and 14) (Vanney and Menanteau, 1985; Lopez-Galindo et al., 1999). The Guadalquivir structuraldepression (Maldonado and Nelson, 1999) providesaccommodation space that allows the encroachmentof the mud belt into the Guadalquivir inner shelfarea (Figs. 7C and 14). Shelf paleo-valleys controlthe offshore encroachment of inner shelf sand dune


facies (Fig. 14) (Vanney and Menanteau, 1985). Thediapiric ridges and intervening valleys of the conti-nental slope change the basic pattern of the Mediter-ranean contour-parallel outflow and produce acceler-ating down-valley currents (Fig. 3) that modify thedevelopment of the general southeast to northwestpattern of contourite facies (Fig. 14). As a result,down-valley facies trends are perpendicular to, andlarger scale than those of the laterally associatedcontourite system (Figs. 7, 8 and 14) (Nelson et al.,1993). In addition, large sediment drift bodies aredeposited because perpendicular ridges disrupt thecontour-parallel flow (Figs. 2, 14 and 15).

The interplay of morphology and the Mediter-ranean outflow currents described in the precedingparagraph emphasizes that varying combinations ofcontrolling factors often contribute to the resultantsediment facies development. Shelf paleo-valleys(Fig. 2) and apparent down-valley Atlantic inflowcurrents are a shelf analog of the slope valley andMediterranean outflow current system that controldevelopment of slope facies (Fig. 15). The mid-shelf mud blanket facies is another example forwhich river mouth mud sources are one importantcontrol combined with the southeastward prograda-tion caused by Atlantic inflow currents (Figs. 4, 5and 7C) (Gutierrez-Mas et al., 1996). At the mouthof the Guadalquivir river, the additional factor ofa structural depression is associated with the rivermouth source and Atlantic inflow currents to de-termine the final mid-shelf mud layer facies distri-bution. The mud layer progrades directly from theGuadalquivir estuary mouth perpendicularly acrossnearly the entire shelf width paralleling the depres-sion, and also progrades southeastward over 50 kmfrom the river mouth paralleling the inflow currentdirection (Figs. 5, 7C and 14) (Rodero et al., 1999).

13. Conclusions

(1) The opening of the Strait of Gibraltar afterMessinian time has resulted in development of theAtlantic inflow currents over the Spanish continentalshelf and Mediterranean outflow currents over theslope. Consequently, during late Quaternary time,shelf prodeltaic deposits have prograded southeast-ward and sand dunes have developed in the shelf

region of strongest inflow currents near Gibraltar.As a result of outflow currents, a 200 km longsandy contourite sequence, that varies gradationallyfrom sand dune fields in the southeast to sandy-siltysediment drift banked against diapiric ridges in thenorthwest has deposited over the eastern Gulf ofCadiz continental slope.

(2) Data from the Cadiz margin and elsewhere(Belderson et al., 1971; Kenyon and Belderson,1973; Nelson, 1982a; Colella, 1990) now show thattracing the bedrock to sand ribbon, sand wave andsediment drift sequences can provide a tool to defineancient straits and their controlling current patternsin deep or shallow water.

(3) Because the shelf inflow or slope outflowbottom currents accelerate down valleys and cre-ate interference with contour-parallel flow (Fig. 3):grain size in valleys is greater than that in adja-cent prodeltaic muds or contourites (Figs. 7A and9); bedforms occur in shelf paleo-valleys as wellas central and northern slope valleys whereas theyare absent in the surrounding prodelta or continen-tal slope contourite mud (Fig. 14); valley bedformcrests are perpendicular to those of associated shelfor slope contourite sand dune fields; and complexsuperimposed bedforms develop along continentalslope valley margins (Nelson et al., 1993; Baraza etal., 1999).

(4) These current parallel and perpendicular vari-ations show that unidirectional bottom-current con-tourite systems may exhibit complex sand-prone fa-cies and bedform patterns that are not always con-tour parallel and may be associated with adjacentmud-prone continental shelf or slope facies. A widevariety of bottom-current facies result: (a) On theshelf these vary from complex inner shelf shorefaceand paleo-valley sand dune facies, to current pro-graded prodeltaic mid-shelf mud facies, to currentwinnowed outer shelf, and to southeast shelf trans-gressive and reworked sand dune facies; (b) On theslope these vary from a simple current-graded up-per slope contourite facies to a complex contourparallel and perpendicular middle and lower slopesand facies, and to distal along slope sediment driftsandy-silt facies or down slope mud-wave facies.

(5) The bedforms and the surface sand to mud fa-cies on the Cadiz margin are in equilibrium with pre-sent-day bottom currents and commenced with the


Holocene time, except for the outer shelf transgres-sive sand facies. This suggests that during high sealevel periods, when the present Gibraltar sill is notconstricted, strong Atlantic inflow currents depositmid-shelf contourite mud sequences or Mediter-ranean outflow currents deposit continental slopecontourite sand sheets.

Acknowledgements

We thank the many scientists, officers, and crewaboard the Garcia del Cid for assistance with ship-board data collection. Jose Ignacio Diaz and MarcelliFarran provided valuable additional surface sedimenttextural data. Marta Ezpeleta, Gita Dunhill and JimMcRea assisted with data compilation. The fundingfor this study was provided by the Joint Commit-tee of Science and Technology of the U.S. SpainTreaty of Friendship (Project CA 83=047 and theJoint Project no. 125=94 of the ITGE=CSIC). Themanuscript benefited from reviews by Dave Drakeand Steve Eittreim.

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Documents

Influence of the Atlantic inflow and Mediterranean outflow currents on Late Quaternary sedimentary facies of the Gulf of Cadiz continental margin