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

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Marine Geology 155 (1999) 99–129

Influence of the Atlantic inflow and Mediterranean outflow currents on Late Quaternary sedimentary facies of the Gulf of Cadiz continental margin C. Hans Nelson a,Ł , Jesus Baraza b , Andre´s Maldonado c , Je´sus Rodero c , Carlota Escutia a , John H. Barber, Jr. a a US Geological Survey, 345 Middlefield Rd. MS999, Menlo Park, CA 94025, USA Instituto de Ciencias del Mar, CSIC, Paseo Juan de Borbo´n s=n. 08039, Barcelona, Spain c Instituto Andaluz de Ciencias de la Tierra, CSIC=Universidad Granada, 18071, Granada, Spain b

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 from an interaction between a number of controlling factors that are dominated by the Atlantic inflow currents flowing southeastward across the Cadiz shelf toward the Strait of Gibraltar. An inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10–20 m that extends deeper and obliquely down paleo-valleys as a result of southward down-valley flow. A mid-shelf Holocene mud facies progrades offshore from river mouth sources, but Atlantic inflow currents cause extensive progradation along shelf toward the southeast. Increased inflow current speeds near the Strait of Gibraltar and the strong Mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf. At the outer shelf edge and underlying the mid-shelf mud and inner shelf sand facies is a late Pleistocene to Holocene transgressive sand sheet formed by the eustatic shoreline advance. The late Quaternary pattern of contourite deposits on the Spanish Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the Mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges. Coincident with the northwestward decrease in outflow current speeds from the Strait there is the following northwestward gradation of contourite sediment facies: (1) upper slope sand to silt bed facies, (2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against the diapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar contains medium–fine sand beds interbedded with mud. The adjacent valley floor facies is composed of gravelly, shelly coarse to medium sand lags and large sand dunes on the valley margins. By comparison, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Because of the complex pattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean outflow current, the larger-scale bedforms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grained contourite 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 of 0.2–1.2 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

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in maximum water depth over the Gibraltar sill and full development of the Atlantic inflow and Mediterranean outflow currents. The transgressive sand sheet of the shelf, and the mud layer underlying the surface contourite sand sheet of the slope, correlate, respectively, with the late Pleistocene sea level lowstand and apparent weak Mediterranean outflow current.  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 slope facies; sedimentation rates; sediment ages; contourite facies

1. Introduction The Gulf of Cadiz is located northwest of Gibraltar Strait in the eastern Atlantic Ocean along a reentrant of the Spanish coast (Fig. 1). Above 300 m water depth there is a strong southeastward inflow of North Atlantic Surficial Water (NASW — hereafter called Atlantic inflow) over the Gulf of Cadiz shelf that intensifies toward the Strait of Gibraltar (Figs. 2 and 3) (Ambar and Howe, 1979a; Caralp, 1992; Stevenson, 1997). As a consequence, nearsurface shelf mud blanket facies develop asymmetrically toward the southeast (Gutierrez-Mas et al., 1996; Rodero et al., 1999, this issue) and bedforms develop on the eastern end of the shelf (Lobo et al., 1996). Below 300 m water depth there is a significant development of bottom-current deposited sediment and bedforms because the deep Mediterranean Outflow Water (MOW — hereafter called Mediterranean outflow) shears northwestward from Gibraltar along the Cadiz continental slope as the Mediterranean undercurrent (Heezen and Johnson, 1969; Kenyon and Belderson, 1973). The main deposits associated with the Mediterranean outflow end with the development of the large Faro Drift sediment body off Portugal (Fig. 3) (Faugeres et al., 1985c; Stow et al., 1986). Previous papers describe the influence of strong Mediterranean outflow currents on the late Quaternary history of the Cadiz continental slope (Nelson et al., 1993) and sea level effects on the Holocene sedimentary history of the easternmost end of the Cadiz shelf (Gutierrez-Mas et al., 1996; Rodero et al., 1999). A number of technical reports (IGME, 1974; ITGE, 1997) and an excellent coastal zone physiographic map series (Vanney and Menanteau, 1985) provide detailed information on the complex surficial geology of the inner shelf and littoral zone of less than 50 m water depth. This paper focuses on the Atlantic inflow current effects on the entire Span-

ish continental shelf and integrates these new data with the Mediterranean outflow current effects on the Spanish continental slope in the eastern Gulf of Cadiz. This paper provides new detailed information on the stratigraphy, bedforms, lithology and sediment facies that develop as a result of the opposing Atlantic inflow and Mediterranean outflow currents. We outline the influence of the tectonic and morphologic setting on the bottom-current patterns and then describe the acoustic stratigraphy of nearsurface shelf and slope deposits. Next, we characterize east to west and upslope to downslope gradations in bedforms and surface lithology of deposits. Finally, we define the subsurface lithologic gradations, ages of nearsurface sediments and sedimentation rates. We use this detailed set of data to map the variation of Cadiz margin sediment facies related to the inflow and outflow currents and then explain the resultant late Quaternary sedimentary history of the eastern Gulf of Cadiz.

2. Methods The eastern Gulf of Cadiz continental margin was traversed by 3400 km of tracklines of high-resolution 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 1200 km of tracklines with a 100 kHz sidescan sonar using a 100 m swath width. Another 400 km of deep-tow records (35 kHz sidescan sonar with a 1 km swath and 3.5 kHz high-resolution profiler) were collected in selected locations with bedforms (Fig. 1). The trackline and core locations were acquired with MAXIRAN, Loran C, transit satellite and GPS navigation systems. A total of 490 dredges and gravity cores, measuring between 10 and 277 cm in length, were taken

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Fig. 1. Map showing 3.5 kHz, Geopulse, single-channel airgun and sidescan sonar tracklines with thin lines. Thick line segments indicate multichannel seismic profile tracklines. Numbered line indicates the trackline location for a 3.5 kHz seismic profile as illustrated in Fig. 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 the maximum lithologic information from the prograding shelf muds and sand–silt rich contourite layers. Grain-size analyses were conducted with the SEDIGRAPH 5000D (<63 µm), and by sieving (>63 µm). The sand fraction composition was studied with the binocular microscope. A minimum of 250 grains were counted in each of the 460 surficial and subsurface samples. Conventional radiocarbon ages were obtained from carbonate carbon in transgres-

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

3. Oceanographic setting A general water mass transport pattern of Atlantic inflow into the Mediterranean Sea at the surface and a Mediterranean outflow at depth from the Strait of Gibraltar are driven by the net evaporation of the Mediterranean Sea (Ochoa and Bray, 1991) and meteorological forcing (Candela et al., 1989). Atlantic

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

inflow currents flow eastward to southeastward over the Gulf of Cadiz continental shelf as is shown by the sand dune crests in the inner shelf areas (Fig. 3) (Vanney and Menanteau, 1985). Extensive sand dune fields across the entire southeastern end of the shelf indicate that current speeds accelerate significantly toward Gibraltar Strait (Fig. 3) (Lobo et al., 1996). Because almost no current speed measurements exist in the published literature, these geologic features are the main indicators of current velocities for the Atlantic inflow over the Cadiz shelf. The deeper Mediterranean outflow water travels as the Mediterranean undercurrent northwestward from Gibraltar Strait below 300 m and accelerates down the Cadiz continental slope because of gravity-driven currents of the denser water (Fig. 3) (Ochoa and Bray, 1991). Because of density differences with respect to the surrounding Atlantic wa-

ter, the warm, saline, and dense (>12ºC; salinity D >36.2‰) Mediterranean outflow progressively sinks as it flows northwestward at varying depths between 300 and 1800 m as an independent stream, the geostrophic Mediterranean undercurrent (Fig. 3) (Madelain, 1970; Ambar and Howe, 1979a,b). The Mediterranean undercurrent maintains contact with the seafloor at up to 1000 m water depth on the eastern region of the Gulf of Cadiz and approximately to 1400 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 the eastern part of our study area (Kenyon and Belderson, 1973; Ambar and Howe, 1979b) but decrease to 75–40 cm=s on the central slope where the undercurrent subdivides into several branches because of interaction with the rugged ridge and valley mor-

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Fig. 3. Current system of the Gulf of Cadiz, showing the area of the surficial Atlantic inflow current over the continental shelf, the area where 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-Lopez and Ortega-Serrano, 1986). Current ribbons splay southwestward through the valleys producing faster channelized bottom-current flows downslope that average 80 cm=s in a region where the contour-parallel currents of the Mediterranean outflow average only 40 cm=s (Thorpe, 1972, 1976). Along the western Cadiz slope, current speed decreases to 10–20 cm=s and speed in channels declines to 25 cm=s (Fig. 3) (Faugeres et al., 1985b). Current speeds at the southern fringes are considerably slower than immediately upslope because the Mediterranean outflow has higher speed cores there (Ambar and Howe, 1979a,b; Price et al., 1993; Price and O’Neil-Baringer, 1994).

Previous studies recognize the variability of current speed east to west, upslope to downslope and valley to intervalley areas in the patterns of bedforms observed on the Cadiz continental slope (Kenyon and Belderson, 1973; Gardner and Kidd, 1983; Nelson et al., 1993).

4. Geologic setting 4.1. Morphology As a result of the complex geological evolution, the bathymetry of the continental shelf and slope

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of the eastern Gulf of Cadiz is quite irregular and exhibits widely varying gradients. The shelf area can be divided into four morphological provinces; (a) the inner shelf, (b) the smooth mid-shelf, (c) the southeast shelf with bedforms, and (d) the outer shelf. The inner shelf contains a complex morphology including irregular bedrock outcrops, Pleistocene sea level stillstand terraces, smooth mud-filled depressions, paleo-valleys and bedform fields (Fig. 2) (IGME, 1974; Vanney and Menanteau, 1985; Gutierrez-Mas et al., 1996; Lobo et al., 1996). The mid-shelf areas are smoothed by prograding mud derived from the numerous river mouths (Gutierrez-Mas et al., 1996; Rodero et al., 1999). Large areas of the southeastern shelf are covered with complex sets of bedforms (Fig. 2) (Lobo et al., 1996). The outer shelf region often begins with a slight break in slope at about 100 š 20 m water depth, has a steeper gradient until about 120 š 20 m water depth, and then flattens out to the shelf edge (Fig. 2) (Rodero et al., 1999). The shelf break occurs at about 130 š 20 m water depth and the continental slope below can be differentiated into four morphologic provinces: (a) a narrow belt between 130 and 400 m formed by the steeper (2–3º) upper slope, (b) two gently dipping (<1º) wide terraces located between 400 and 700 m water depth on both the southeast and northwest sides of the middle slope, (c) a central section 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 m that is more steeply dipping (2–4º) and incised by shallow valleys in a NE–SW direction (Fig. 2). 4.2. Tectonics and stratigraphy The opening of the Strait of Gibraltar and a significant increase in the subsidence rate during the Early Pliocene caused a major change in the depositional patterns of the Cadiz continental margin (Maldonado and Nelson, 1999). Deposition was controlled by the diapiric activity, the extensional collapse of the margin and the location of tectonic depressions. Large roll-over extensional basins with strong marly diapirism phenomena occurred in the central region of the Gulf of Cadiz. From Early to Late Pliocene

time there was a rapid decrease in basement subsidence rates; consequently, eustatic sea level changes and bottom current distribution gained importance for controlling depositional patterns (Maldonado and Nelson, 1999). During the Pliocene and Quaternary, the Mediterranean outflow current has been the main influence on the facies distribution of the continental slope; large drift depositional bodies are associated with the diapirs, and erosional surfaces have developed on the upper slope (Nelson et al., 1993). The Late Pliocene–Pleistocene eustatic sea level fluctuations changed the effective cross section at the Strait of Gibraltar which controls the Atlantic and Mediterranean water mass exchange. During eustatic low sea-levels, the connection between the Atlantic and Mediterranean waters has been shallower and restricted; during the highstands, such as the present Holocene time, the connection has been deeper and less restricted (Huang and Stanley, 1972; DiesterHaass, 1973; Sonnenfeld, 1974; Vergnaud-Grazzini et al., 1986; Grousset et al., 1988). As the strength and volume of the Mediterranean outflow current was significantly modified, the facies distribution over the margin experienced major changes (Nelson et al., 1993: Rodero et al., 1999). 4.3. Age and acoustic character of slope sediment drift sequences Sediment drift bodies associated with the diapiric ridges are the thickest deposits (up to 0.75 s of sediment) related to the Mediterranean outflow and those that exhibit the most complete seismic stratigraphic record on the Cadiz slope (Fig. 4) (Nelson et al., 1993; Maldonado and Nelson, 1999). Previous studies interpret that sediment drape reflectors of Late Miocene age are replaced by sediment drift reflectors of Early Pliocene age because the Mediterranean outflow current commenced with the opening of Gibraltar Strait and flooding of the western Mediterranean Sea. The underlying Miocene beds are generally draped layers of sediment without acoustic characteristics of bottom-current processes. The older beds are intruded and thrust upward by the diapiric structures suggesting that the diapirism began during the Miocene, prior to deposition of the embankments of sediment drift that onlap the diapirs. Uplift, however,

C.H. Nelson et al. / Marine Geology 155 (1999) 99–129 Fig. 4. Geomorphic features of the eastern Gulf of Cadiz continental margin based on interpretations of seismic profile and sidescan sonograph data collected along tracklines shown 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 bedrock outcrops, 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 diapir areas, 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 river prodelta 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 shelf small-scale ripple area, 19 D outcropping faults, 20 D undifferentiated scarps. 105

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may continue to be active at present (Maldonado and Nelson, 1999; Rodero et al., 1999). The sediment drift deposits are characterized by thick wedges of sediment with irregular erosional surfaces, broad-scale low-angle unconformities and internal reflectors that are generally continuous within the wedges (Nelson et al., 1993; Maldonado and Nelson, 1999). Locally, the drift wedges exhibit alternating sequences of sediment drape deposits and irregular, discontinuous high-energy deposits like sand dunes. The channelized deposits along the valleys between diapiric ridges in upper slope areas often are asymmetric with bedrock flanking one margin and sediment drift deposited on the other margin. In other locations, sediment drift may flank both margins of the channel floor that sometimes has remained stable and other times has migrated with time creating large cut and fill sections.

low-stand wedges. The wedges also contain limited sequences of turbidites, bedforms and slumps on the middle–lower slope (Fig. 4). In general, regressive and lowstand system tract deposits predominate, and are separated by significant sedimentary hiatuses that correspond to erosional surfaces. Because these develop during phases of eustatic sea-level rise and high stands of sea-level, the preservation potential of transgressive deposits in the stratigraphic record of the margin is low. Diapirism on the slope, linked in part to tectonic episodes, also influences the development of Quaternary sedimentary sequences on the margin (Maldonado and Nelson, 1999). The distribution of diapirs influences the current patterns which in turn determine the location of depocenters and current scour (Fig. 4) (Nelson et al., 1993)

4.4. Age and acoustic character of Quaternary shelf sequences

5. Distribution and acoustic character of Holocene mud

Based on distribution of late Quaternary deposits in the Gulf of Cadiz, two types of regional depositional sequences can be defined on the margin. In the first type located on the northwestern Cadiz margin, Quaternary deposits form part of the foredeep basins of the Guadalquivir Depression (Maldonado and Nelson, 1999). This region is characterized by subsidence, thick sedimentary units, and development of numerous prograding shelf margin deltas (Rodero et al., 1999). The second type located on the eastern margin, in contrast, is part of the external zone of the Gibraltar Arch affected by tectonic movements during the Quaternary. Because this region has been a structural and morphologic high during the Quaternary, only tidal flat sequences have developed. Between these two regions, there is a transition zone with intermediate characteristics. The upper Quaternary deposits of the Gulf of Cadiz contain nine units and four types of depositional bodies. These sequences formed between 740 000 yr BP to the present and are characterized by 5th order cycles (Rodero et al., 1999). The sequences in each cycle are shallowing upward and consist of a littoral prism and tidal flats on the inner and mid-shelf, prodelta lobes and marginal deltas at the shelfbreak and upper slope, and hemipelagic

Gutierrez-Mas et al. (1996) and Rodero et al. (1999) recognize a highstand system tract layer of Holocene transparent mud in the mid-shelf region of the southern shelf. This study of the entire Spanish shelf finds that the Holocene layer is poorly developed at the northern and southern ends, but inbetween off the largest Guadiana and Guadalquivir river mouths, it is well-developed and covers nearly the entire shelf (Fig. 5). The thickest part of the Holocene layer off the Guadiana river mouth is offset to the southeast of the river mouth, has an elongate taper to the southeast and all the mudbelt margins except the southeast are interrupted by nondepositional areas. At its thickest point in the central shelf, the apparent Guadiana-derived Holocene layer is about 15 m thick and 25 km wide. The maximum thickness of the Holocene layer off the Guadalquivir river is approximately 25 m and its width reaches 45 km to cover the nearly the entire shelf. The thickest region develops southwest of the river mouth and a tongue in the central shelf extends over 60 km to the southeast of the river mouth (Fig. 5). Beyond this point, irregular-shaped Holocene mud patches of decreasing size are found toward the Strait of Gibraltar where only bedrock outcrops are found.

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Fig. 5. Map showing isopach thickness of the Holocene mid-shelf mud layer or highstand systems tract mud deposits. Isopachs are shown in milliseconds (thickness in meters is approximately 75% of millisecond value). The pattern with thick horizontal lines shows areas 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 distribution and thickness of the Holocene acoustic layer. Northwest of the Guadalquivir river mouth an inshore isopach lobe leads toward the Tinto river mouth (Fig. 5). The Tinto contributes to this part of the Holocene layer according to seismic reflectors and heavy metal contamination in sediment that is traced from this source (Nelson and Lamothe, 1993; Palanques et al., 1995; Van Geen et al., 1997). Both surface and subsurface heavy metal distribution also show that Holocene mud deposited from the Tinto source extends as far west as the Guadiana and as

far southeast as the outboard Holocene isopach lobe to the northwest of the Guadalquivir (Palanques et al., 1995; Van Geen et al., 1997). The Guadalete river mouth also appears to be associated with an extension of a slightly thicker isopach portion of the Holocene layer that extends into the river mouth and a thickened patch to the west of the river mouth (Fig. 5). A typical example of the nearshore to offshore changes in the acoustic facies character of the Holocene highstand mud layer is found off the Guadalete river mouth. Distinct proximal reflectors

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prograde westward offshore from the river mouth and downlap onto the underlying late Pleistocene to early Holocene transgressive sand surface (Gutierrez-Mas et al., 1996). Offshore, the distal mud becomes increasingly transparent and lacks distinct reflectors. The same nearshore characteristics of stratified reflections are found in the northern shelf off the Guadiana and Tinto river mouths; in this case, however, the transition to transparent distal mud facies continues toward the southeast because of the Atlantic inflow current advection of distal muds (Fig. 6). The development of the proximal acoustic facies with reflections grading to offshore and alongshelf transparent Holocene mud facies is similar to

the well-defined Holocene acoustic facies for the Ebro river prodelta mud layer (Diaz et al., 1990). In addition to the acoustic facies that show a southeast progradation of the Holocene river prodelta muds off the Tinto, the trend to thickest isopachs south and southeastward off both the largest river mouths, the Guadiana and Guadalquivir, shows the same pattern of progradation (Figs. 5 and 6). This thickness trend is most pronounced off the Guadiana where the main depocenter is offset 20 km to the southeast (Fig. 5), however, both older and younger Holocene sedimentary units of the Guadalquivir river prograde more than 75 km southeastward (Fig. 4) (Gutierrez-Mas et al., 1996; Rodero et al., 1999).

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

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6. Characteristics of bedform fields 6.1. Shelf bedform distribution A complex pattern of bedforms is found in both the continental shelf and slope regions of the eastern Gulf of Cadiz. In the northwestern inner shelf region, a generally regular distribution of submarine dunes has been found between 10 and 20 m water depth with crests that are perpendicular to the eastward Atlantic inflow shelf currents paralleling the coastline (Fig. 3) (Vanney and Menanteau, 1985). West of the Tinto estuary mouth, the dune field is deeper and wider; east of the Guadalquivir estuary mouth, it is absent because of the Holocene mud layer that extends into the estuary mouth (Figs. 5 and 7) (Gutierrez-Mas et al., 1996; Lo´pez-Galindo et al., 1999). Deeper on the 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) (Vanney and Menanteau, 1985). They drain offshore flow down valley and develop submarine dune fields whose crests are transverse to this flow. Much of the southeast shelf is covered by a wide variety of bedform fields with diverse orientations of bedform crests (Fig. 4) (Lobo et al., 1996; ITGE, 1997). In the inner shelf zone between Cadiz and Conil, transverse dune crests are perpendicular to and asymmetric toward an Atlantic inflow current direction to the southeast (Lobo et al., 1996). From Conil south to Barbate, nearly the entire inner to middle shelf exhibits transverse dune ridges suggesting southeast Atlantic inflow currents across the entire region. In contrast, south of Barbate to the Strait of Gibraltar and in the middle to the outer shelf area between Conil and Cadiz, the transverse dune ridges are asymmetric toward a reversed Mediterranean outflow to the northwest. In the reversed outflow current region west from Gibraltar, the bedforms grade from largescale dunes to megaripples; further west on the outer shelf, small-scale dunes are found (Fig. 4). In the midshelf region west of Barbate, inflow current bedforms grade from small-scale ripples to large-scale dunes. 6.2. Slope bedform distribution The main areas with bedforms on the Cadiz continental slope coincide with the general pattern of

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bottom currents described previously (Fig. 3). The main area of sand dunes is found in the deeper and flatter terrace of the upstream middle slope (300–800 m) (Fig. 4) (Kenyon and Belderson, 1973; Nelson et al., 1993). Where this main sand dune region intersects the steeper upper slope (300–500 m water depth), the dune area often evolves into local areas with erosional truncation of beds or downstream into areas with small dunes (Fig. 4); where the dune region encounters diapiric ridges downstream in the middle slope, the dune area evolves into sediment drift bodies; where the dune region reaches the deep distal southwestern part of the slope (about 800 m) large mudwaves are found. The valleys that cut downslope through all of the aforementioned upper to lower slope regions with bedforms, have additional bedform sets of their own that are associated with the down-valley Mediterranean outflow currents (Fig. 4) (Nelson et al., 1993). When describing the bedforms types in the aforementioned slope environments we conform to the suggested terminology of the SEPM Bedform Research Group (Ashley, 1990) and rely on the detailed sonographs 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 height and 10 m in wave length) are developed in muddy sand (65–75% sand) of the middle to eastern area in the Gulf of Cadiz (Fig. 7B) (Baraza et al., 1999, see fig. 8A). The main sand dune field of the middle slope is characterized by long-crested, 2D large sand dunes (heights <10 m, usually 3–5 m and lengths of 30–150 m) with straight crests transverse to the main contour-parallel, Mediterranean outflow current (Nelson et al., 1993; Baraza et al., 1999, see fig. 8C). Within the sand dune field, however, local variation in topography and downslope bottom currents in valleys can result in change in orientation of sand dunes and a complex interplay of superimposed bedforms. For example, in the transition zone from a valley toward the main sand dune area, the apparent interplay of downslope and alongslope currents and topographic disruption of the valley wall results in a region dominated by compound dunes with 3D large barchans combined with large and small-scale 2D sand dunes (Baraza et al., 1999, see fig. 8B). Bottom photos of Heezen and Johnson (1969) and Melieres

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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 superimposed on the sand dunes just as they often are on shallow water sand dunes in unidirectional currents (Nelson et al., 1982).

As current speeds slow from about 40 to 75 cm=s in the sand dune area to 15–20 cm=s toward the western 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

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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 m and lengths approximately 500 m) of Kenyon and Belderson (1973). In the transition zone, we observe sand-dunes that appear to be superimposed on large mud waves and sediment texture changes gradually from muddy fine sand (>65% sand) to sandy mud (<45% sand) (Fig. 7) (Nelson et al., 1993). A wide variation of bedforms characterize the valley floor and adjacent wall areas of the major slope valleys with down-valley Mediterranean outflow currents (Fig. 3). Straight-crested dunes on valley floors are transverse to the valley floor and down-valley flow, but these crest orientations are perpendicular to the direction of those formed in the sand dune and mud wave regions of contour-parallel flow from the Mediterranean outflow (Fig. 3) (Nelson et al., 1993). Small to large sand dunes are found on the channel floors of valleys that cut through the sand dune or mud wave area and coarser-grained sediment occurs on the valley floor compared to the surrounding muddy or sandy contourite areas (Fig. 7A). Bedform types may change from proximal to distal parts of the same valley (Nelson et al., 1993). Bedform characteristics also vary from southern valleys with strong currents (80 cm=s) to northern valleys with weak 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 variety of sediment lithologies consisting of consolidated bedrock outcrops, beach rock, shoreface sand and gravel deposits, and mud patches. Pleistocene beach rock deposits developed at sea level lowstands are the most common type of consolidated rock (IGME, 1974; Vanney and Menanteau, 1985; ITGE, 1997). Toward the southeast, bedrock outcrops become widespread over the shelf and predominate west of Tarifa (Fig. 4) (Lo´pez-Galindo et al., 1999). Bedrock consists of a wide variety of lithologic types and ages, but the Gibraltar flysch predominates (Maldonado and Nelson, 1988; ITGE, 1997; Lo´pez-Galindo et al., 1999). Sporadic local gravel deposits, typically of coquina, are associated with the beach rock deposits throughout innermost shelf areas (Fig. 7A) (IGME, 1974; Vanney and Menanteau, 1985; ITGE, 1997). The inner shelf and extending to the outer shelf area south of the Spain=Portugal border exhibits such areas of basement outcrop with associated coquina (Figs. 5 and 8). Midway between the Tinto and

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

the Guadalquivir river mouths there is a large area where coquina occurs as shown by bottom samples with shell hash, carbonate contents up to 70%, and gravel content of 10–20% (Figs. 7A and 9). From the Guadalete river mouth and to the southeast on the inner shelf, gravels associated with both coquina deposits and bedrock outcrops become increasingly common (Fig. 7A). Carbonate contents and bottom samples show that coquinas are the main source of gravel near the Guadalete, although carbonate coquinas become less predominate west and south of Barbate where gravel content is consistently highest (20–40%) and associated with an increasing number of bedrock outcrops (Figs. 4, 5, 7A and 9) (Gutierrez-Mas et al., 1996; Lo´pez-Galindo et al., 1999).

Sand is the predominant sediment texture in the inner shelf, except near the previously mentioned gravel-rich deposits associated with beach rock and bedrock outcrops as well as the mud rich areas associated with river mouths. Sand content generally is 60–70% with increasingly higher percentage near the coast in the north or outcrops in the southern inner shelf (Fig. 7). Toward river mouth areas the sand content becomes increasingly diluted by silt and clay content and sediment lithology grades to silt and clay patches. Silt texture predominates in the inner shelf mud patch off the Guadiana river mouth, clay predominates in the mud patch off the Guadalquivir, and both silt and clay patches are found in Cadiz bay at the mouth of the Guadalete

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Fig. 9. Bulk percent calcium carbonate content in surface sediment from the Gulf of Cadiz Spanish shelf and slope. West-central slope area 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 lag deposits.

river (Fig. 7). Local patches of silty and clayey mud are found on the shelf west of Barbate that appear to be associated with local topographic depressions. 7.2. Mid-shelf mud layer The middle shelf between the inner shelf sand and the outer shelf muddy sand is covered by a mud layer with greater than 50% clay grain size except near Gibraltar (Fig. 7C). The mud layer extends towards the mouths of the two largest rivers, the Guidiana and Guadalquivir. The mud layer extension toward the Guadiana is narrower and less clay dom-

inated (50–60%) compared to the wider and more clay dominated (60–70%) mud layer that enters the Guadalquivir river estuary. Off both the Guadiana and the Guadalquivir rivers there is a proximal to distal offshore and southeastern gradation to finer grain size. The Guadiana grades southward for 20 km offshore to 50% clay and for 50 km southeastward along-shelf in the mud layer to 70% clay (Fig. 7C). The Guadalquivir grades westward for 30 km offshore to over 70% clay and for 50 km southeastward along-shelf in the mud layer to 80% clay. The much smaller Guadalete river grades to a 50% maximum clay patch in the center

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of Cadiz Bay, but does not show the continuous gradation 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 a clayey sand at the shelf edge and southeastward into a gravelly sand (Fig. 7). The shelf edge clayey sand averages 50% sand and up to 40% clay. In the southeastern middle to outer shelf west northwest of Barbate, the sand content reaches over 90%. Southeastward from this clean sand maximum, deposits grade to gravel contents of 30–40% and can reach over 80% gravel. Further southeastward west of Tarifa and near the Strait of Gibraltar, mainly bedrock outcrops are found (Figs. 4 and 5) (Lo´pez-Galindo et al., 1999).

40% sand (Fig. 7B, ca. 36º300 lat., 7º120 long.). On the large topographic high to the west of this basin with sandy mud, the sand percentage again increases to over 65%, but as carbonate contents show, the sand-size particles are mainly planktonic tests, not siliciclastic grains (Fig. 9, ca. 36º300 lat., 7º250 long.) (Nelson et al., 1993). In the deepest southwestern part of the study area dominated by large mud waves, the sand content generally is 55% or less except in valley-floor areas, and the clay content may exceed 30% (Fig. 7). In contrast to the muddy sand and silt of the distal mud wave area, the sand dune area on the deep slope toward the Strait of Gibraltar is characterized by silty sand to clean medium sand with greater than 65% sand and less than 20% clay.

8. Subsurface sediment lithology

7.4. Upper slope and ridge and valley sands

8.1. Complex inner shelf outcrops, sands and muds

In contrast to the shelf that has a significant area covered with silty clay except near the Strait of Gibraltar, most of the slope is covered by sand to muddy sand (Fig. 7B). From south to north along the upper slope, there is a tongue-like gradation of surface sand that is 40 km wide at the southern end where it contains 65–80% sand; the northern end narrows to 10 km and contains 55–65% sand. In the ridge and valley area of the central slope, the textural patterns become more complex. The southeastern valley floors contain gravelly sand, the central valley floors contain coarse bioclastic sand and the northwestern channel floors contain fine sand (Figs. 7A and 9) (Nelson et al., 1993). Similar to the valley floors, the surface deposits of the sediment drift bodies along the ridges also vary from coarsergrained sediment on the southeast bodies (>80% sand) to finer-grained sediment on the northwest bodies (<50% sand) (Fig. 7B). On the crests of the ridges, Miocene marls outcrop (Fig. 8) (Maldonado et al., 1989; Maldonado and Nelson, 1999).

The isopach map of Holocene sediment thickness helps to explain the subsurface sediment distribution and the lithology obtained from cores (Figs. 5 and 8). A transect of cores across the margin just north of the mouth of the Bay of Cadiz also provides a general summary of the inner shelf to upper slope subsurface lithology (Fig. 10). In the inner shelf zone, except off the mouth of the Guadalquivir river, most cores were less than 0.5 m length and consisted of bioclastic sand and shell lag layers (Figs. 8 and 10). In deeper parts of the inner shelf with >20 m of water, muddy sand occurs (Fig. 11) and off the Guadalquivir river mouth, mud thickness reaches greater than 15 m (Fig. 5). These inner shelf muds, however contain numerous shell and sand lag layers (Figs. 8 and 10) that appear to be storm lag layers similar to those noted in other deltaic settings (Nelson, 1982a,b; Diaz et al., 1990).

7.5. Western and lower slope sandy muds Northwest of the high relief of the ridge and valley area in the central slope (Fig. 2), a leeward shadow of finer sediment is found with only 10–

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

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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.

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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 the Portuguese border and directly south of the mouth of the Guadiana river, the mud is thin and patchy and only approximately 1 m thick. By 80 m water depth at the western offshore edge in the central part of the Gulf of Cadiz, the mud layer has thinned to less than 1 m over an underlying sand (Fig. 10, see core 33). The inner margin where the mid-shelf mud is less than 1 m thick is quite variable. It extends offshore to greater than 50 m water depth on the northwest, whereas toward the Guadalquivir river it extends progressively inshore to reach nearly 30 m water depth near Cadiz (Figs. 8 and 10, see core G-164). South of Cadiz, the mud rapidly thins to expose underlying 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 clayey silt (Fig. 11). Away from the fringes, the upper mud is silty clay size, but grades to coarser clayey silt down section. At the southern edge of the mid-shelf mud, the entire section is silty clay. 8.3. Outer and southeastern shelf sand The outer shelf is characterized by bioclastic muddy sand and silt with common shell lag layers in the subsurface sediment. In the northwestern part, where the outer shelf is wider from approximately 100–120 m depth, muddier sandy silt is characteristic and thin interbedded silt and mud lay-

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ers occur (Fig. 11). In the central outer shelf west of the Guadalquivir river, the mud belt extends nearly to the shelf edge, but again muddy silt and shell beds are common on the outer shelf (Fig. 10, see cores G30–33). The southeastern outer-middle shelf is characterized by cleaner silty sand that has increasing amounts of bioclastic layers and internal cross-lamination toward Gibraltar (Figs. 4 and 8). The entire shelf south of Cadiz grades southeastward from interbedded silt and mud beds, to increasingly thinner and cleaner silty bioclastic sand and then to scoured bedrock with no unconsolidated deposits near Gibraltar (Figs. 5, 8 and 11). 8.4. Slope sand beds The lithology of the upper 2 m of sediment shows interbedded sand and coarse silt layers with hemipelagic silty clay (Fig. 8). Throughout our study area of the continental slope there is a surface sand layer that varies from 0.5 to 1.5 m in thickness and from gravelly or clean sand in the southeast to sandy silt in the northwest (Figs. 8 and 11). In some locations, a second sand layer was penetrated that has the same general southeast to northwest gradations as the upper layer. The thickness (generally >0.5 m) and net sand percent (typically >50% sand layers to mud layers) in nearsurface sediment are greatest in the upper slope (Fig. 8) (Nelson et al., 1993). Below 550 m water depth, the net sand percentage usually ranges from 20 to 45%. The surface and deeper sand layer are generally characterized by reverse graded bedding (Fig. 11). Mean grain size varies from 3.5 phi at the top of layers to 6 phi at the base. The coarsest layers and greatest vertical gradation occur in the southeastern area. Within the layers, trough cross lamination, flat lamination, or massive bedding are observed as internal sedimentary structures, but they do not occur in any vertical sequence (Nelson et al., 1993).

9. Surface and subsurface sediment composition 9.1. Carbonate The shelf sediment contains 10–20% planktonic tests in the sand fraction, except in the northwest-

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ern shelf edge area where >30% tests are found (Nelson et al., 1993). A low content of planktonic constituents is found in the southeastern and upper slope regions, but these also grade to high quantities in the northwest mid-slope area where >60% tests are found and high carbonate content is observed (Fig. 9). In general, the bulk carbonate content of the surface sediment over the northwestern margin is greater (20–30%) than the southeastern margin (10– 20%) (Fig. 9). In the northwest mid-shelf mud, the average surface and subsurface (up to 2.3 m depth) amount of carbonate in the total clayey-silt sediment was about 30%. South of the Guadalquivir river mouth in the finest silty-clay part of the mid-shelf mud up to 2 m depth, however, carbonate contents were higher (37–43%). In both the northwest and southeast regions of the shelf, the maximum carbonate contents are associated with the inner shelf Pleistocene beach rock where carbonate content exceeds 50% (Fig. 9) (Lo´pez-Galindo et al., 1999). The maximum carbonate content on the continental slope also reaches greater than 50% carbonate in the central and southeastern valley floors between slope ridges because floors contain gravel to coarse sand-sized bioclastic lag deposits (Fig. 9) (Nelson et al., 1993). The surface and subsurface (up to 1.7 m) slope sediment in general contains more carbonate than the shelf and has consistent values between 37 and 41% except in the southeastern sand dune area where content decreases to 37%. 9.2. Organic carbon One main transect of samples for organic carbon analysis has been collected across the northern margin from the mouth of the Tinto river directly south to the slope base (Fig. 9). The surface mid-shelf silty clay consistently contains 1.5% organic carbon. The content drops to about half of this amount (0.8%) in the outer shelf sandy silt and to minimum contents of 0.3% in the upper slope sand sheet. The middle slope sandy silts of the sediment drift deposits consistently contain 0.5–0.6% organic carbon. The organic carbon content shows a clear correlation with grain size which is consistent in the Cadiz region as elsewhere (Nelson and Lamothe, 1993)

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10. Sediment age and sedimentation rates 10.1. Continental shelf On the continental shelf, the oldest ages occur in shell lags of apparent transgressive sand and gravel. Both the 12 900 yr BP age at 99 m and the 12 640 yr BP age at 77 m are co-incident with the most wellestablished eustatic sea level curves from Barbados and Huon Peninsula (Figs. 12 and 13) (Elias et al., 1996). Ages of about 8–9000 yrs BP for two other shell lags at 50 m are in general agreement with established curves, but are slightly younger, per-

haps because of contaminating younger shells affecting surface sample 26B and=or subsidence of the Guadalquivir depression affecting sample 63 (Fig. 12) (Maldonado and Nelson, 1999; Rodero et al., 1999). The age data from the northwestern shelf indicates that this area of the shelf has been tectonically stable since the late Pleistocene sea level transgression. Sedimentation rate information based on radiocarbon ages is only available for the younger midshelf mud layer, because the coarser grained sand, gravel and shell lags deposited by the late Quaternary sea level transgression were not penetrated by our gravity cores. Sedimentation rates are in general

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

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

agreement with Holocene sediment isopach thickness derived from high-resolution seismic profiling (Fig. 5). As expected at the fringes of the mud layer, the sedimentation rates were low, about 11 cm =kyr at core site 82 on the northwestern edge and 18 cm=kyr at core site 63 on the southeastern edge (Fig. 12). These rates are in close agreement with rates estimated from ages determined using water depth, mud thickness from seismic isopachs, and eustatic sea level curve ages (Figs. 5 and 13). The highest sedimentation rates are found in the thickest central mud layer associated with major river sources (Fig. 5). On the northwestern shelf at core site 25 located southeast of the Guadiana river mouth at mid-shelf water depths, the sedimentation rate is 131 cm=kyr based on the eustatic sea level curve plus isopach thickness deposited during the last 10 300 years (Figs. 5 and 13), 64 cm=kyr based on 14 C ages during the past 8000 years (Fig. 11), and 80–120 cm=kyr based on 210 Pb activity of the past 100 years (Van Geen et al., 1997). These data show a pattern of earlier Holocene higher sedimentation rates, middle to late Holocene decrease in rates and a past 100 year increase in rates at this location where the shelf appears to have been tectonically stable. In the outer central shelf region fed by the Tinto river source, lower, but similar patterns of sedimentation 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 sediment isopach thickness is 74 cm=kyr (Figs. 5 and 13). Similar to the northwest mud layer, the sedimentation rate of 22.6 cm=kyr (or 53 cm=kyr with a 5000 yr correction for surface contamination shown in cores 25 north and 15 south of site 22) for the past 9000 years based on 14 C ages is lower than the rate for the entire isopach thickness of the mud. This change in sedimentation rate from the early to later history of the mud layer may be expected because it is typical for sediment input to decrease during the later Holocene as vegetation cover increased (Nelson, 1990). The sedimentation rates based on 210 Pb activity of the past 100 years vary from 110 to 160 cm=kyr in core 22 (Van Geen et al., 1997), and like data from core 25 on the northwest shelf, suggest that human deforestation on the Iberian Peninsula during the last few centuries has caused increased sedimentation rates on the Cadiz shelf as it has on the Ebro shelf off Spain (Nelson, 1990). The highest sedimentation rates in the mud layer are found at station 15 near the Guadalquivir river sediment source. The average sedimentation rate at this site for the past 10 300 years based on eustatic sea level curves and sediment isopach thickness is 110 cm=kyr (Figs. 5 and 13). Transgressive sediment ages at nearby station 63, however, show that deposition of the mud layer from the Guadalquivir source has reached this area only about 8000 years ago.

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Using this age for initiation of mud deposition at site 15 results in average mud blanket sedimentation rates of 141 cm=kyr. Based on 14 C ages, the highest sedimentation rates (234 cm=kyr) encountered on the shelf are found during the past 6000 years at site 15 (Fig. 12). This is not surprising considering that the largest sediment source in the Gulf of Cadiz feeds this region and that, after initial aggradation near the river mouth to fill accommodation space (GutierrezMas et al., 1996), rapid southeastward progradation of the mud layer from the Guadalquivir source has occurred more recently near site 15. 10.2. Continental slope From previous studies of seismic profiles from the Cadiz margin and from the Mediterranean Sea, we interpret that deposition of the sediment drift sequence began at the end of the Messinian (Nelson et al., 1993; Maldonado and Nelson, 1999). It is also apparent that within the drift there have been alternating cycles of bedform and sediment drift deposits or sediment drape. In this paper, the ages of these major cycles is not determined and we only consider ages of specific cycles in the late Pleistocene and Holocene nearsurface sediment. The surface sand layer on the slope is Holocene in age because the youngest age determined from the mud below the sand layer is about 10 000 years (Fig. 12). In the most southeasterly and upstream upper slope core, where age of the mud immediately below the surface sand layer is 25 260 years, we interpret that the strong contour currents of this region have eroded the underlying poorly consolidated mud prior to sand deposition (Figs. 3 and 12). Current reworking of the sand layer itself is suggested by upstream surface ages older than 5000 years compared to downstream ages younger than 500 years (Fig. 12). In the core with the oldest surface sand layer age of 5380 yr, a second sand layer younger than 22 400 years old is found. Ages in this and the cores immediately south of it show that more than one sand layer developed in sediment drift deposits of the central slope ridge and valley area during the latest Pleistocene and Holocene time, similar to those that developed in the Faro Drift (Fig. 12) (Stow et al., 1986). In contrast, radiocarbon ages of 25 260 to >30 000 yr suggest that the deeper sand layers in

the upper slope area are much older than mid-slope layers and may be correlated with the previous high sea level stand at about 120 000 yr B.P. (Chappell and Shackleton, 1986). As a consequence of the old nearsurface ages of the sediment, the Cadiz slope sedimentation rates in general are low (1–12 cm=kyr) compared even to normal Holocene hemipelagic mud sedimentation rates of 10–35 cm=kyr on the slope (Fig. 12) (Nelson, 1976; Nelson, 1990). The sedimentation rates (1–5 cm=kyr) are lowest in the Cadiz upper slope, however, these rates are similar to those Holocene upper slope rates elsewhere (Fig. 12) (Nelson, 1990). Considering that sediment drift bodies exhibit as much as 657 m thickness, deposited over about the past 5 million years since the end of the Messinian, or an average sedimentation rate of 13.1 cm=kyr, the rates of the sediment drift deposits appear to have remained relatively constant throughout their history (Fig. 12) (Nelson et al., 1993).

11. Shelf and slope facies associations and processes 11.1. Holocene highstand inner shelf sand facies A number of sediment facies are found in the inner shelf area between 0 and 50 m, but because we collected few data in this area we will not make a detailed definition of the several types of facies there. From 0–10 m a shoreface coarse to fine sand facies formed by shoaling waves is described by others (IGME, 1974; Vanney and Menanteau, 1985; ITGE, 1997; Lo´pez-Galindo et al., 1999). Between 10 and 20 m water depth and extending locally down paleo-valleys off the Tinto and Guadalquivir rivers to more than 40 m, a sand dune facies with intermittent rock outcrops of beach rock is mapped (Fig. 14) (Vanney and Menanteau, 1985). We find shell lag layers within this facies that are less than 3000 years old showing the Holocene age of this facies (Fig. 12). Southeastward Atlantic inflow currents and south to southwestward veering flow down paleo-valleys appear to control the development of bedforms (Fig. 15). A sandy-silt mud patch facies with storm shell lag layers is developed at 20–50 m between the paleo-valleys. Its formation appears to

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

be controlled by a combination of lower energy Atlantic inflow currents and reworking by storm waves. 11.2. Holocene highstand mid-shelf mud facies The mid-shelf clayey silt to silty clay mud layer begins inshore at 40–50 m, extends offshore to about 100 m and has Holocene radiocarbon ages (Figs. 12 and 14). The inshore boundary appears to be controlled by wave action and paleo-valley offshore currents, except where the large sediment source of the Guadalquivir river causes the mud blanket to extend into the Guadalquivir estuary mouth (Figs. 14 and 15). The offshore edge extends to about 100 m where complex shelf-edge current and internal wave processes (e.g. Karl et al., 1983; Pietrafesa, 1983; Stanley et al., 1983; Cacchione and Drake, 1986) limit its offshore extent, except west of the large Guadalquivir sediment source. The northwestern edge of the mud layer at the Spanish–Portuguese border is limited by a lack of a nearby significant river source to the west

and erosion of the Atlantic inflow currents. The absence of the mud belt to the southeast of Cadiz Bay in part may be attributed to the lack of significant river sources, but most importantly, morphologic and textural gradations (Figs. 4 and 7) show that acceleration of the Atlantic inflow currents near Gibraltar prevents deposition of silt and clay (Figs. 3 and 15). We find the same sequence stratigraphic pattern throughout the entire Gulf of Cadiz shelf area as that noted by other studies for the southern shelf area (Gutierrez-Mas et al., 1996; Rodero et al., 1999). Near river mouths a Holocene highstand systems tract has prograded offshore with sigmoidal clinoforms (Fig. 6). Isopach thicknesses that are offset (Guadiana) and elongated toward the southeast (Figs. 4 and 5) and textural gradations toward the southeast (Fig. 7), however, show that the mud layer also has prograded southeastward from the major river sources. We attribute this significant southeastward progradation alongshelf to the Atlantic inflow current system (Figs. 3 and 15).

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Fig. 15. Sedimentary processes of current-deposited facies of the Cadiz margin. Strength of flow in slope environments is based on current speeds illustrated in Fig. 3. General current speeds and direction for shelf processes are based on estimates from distribution and orientation of sediment bedform features shown in Vanney and Menanteau (1985); the formation of active inner shelf dune field features requires 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 is observed (e.g. Fig. 6).

11.3. Late Quaternary outer shelf transgressive sand facies Data from seismic profiles, radiocarbon ages and grain size all show that a transgressive sand deposit underlies the Holocene mud facies (Figs. 6, 7 and 12). Because of shelf-edge current dynamics and increasing bed shear from Atlantic inflow currents near Gibraltar, this transgressive sand has not been covered by Holocene mud at the outer shelf edge from about 100 to 120 m nor across the entire southeast shelf region (Figs. 3, 5 and 15). Sediment lithology and Holocene mud isopach data, however, do show that the central outer shelf facies west of the Guadalquivir source has some mixing and overlap with Holocene mud facies (Figs. 10 and 14). The late Pleistocene age and coarse-grained nature of the outer shelf facies shows that they are relict and not in equilibrium with the present wave and current regime (Figs. 12 and 15).

A number of current and wave processes appear to interact at the Cadiz shelf edge to prevent deposition of modern muds over the relict transgressive outer shelf sand. On the southern shelf edge, interaction of the Atlantic inflow and Mediterranean outflow currents advect and prevent deposition of fine-grained sediment. Further north, the focus of typical boundary, tidal, internal wave, surface wave and other meteorologically induced currents at the shelf edge combine to prevent mud deposition (Karl et al., 1983; Cacchione and Drake, 1986), except where the Guadalquivir mud source appears to overwhelm oceanic processes. 11.4. Palimpsest southeast shelf sand facies The sands on the southeastern shelf, although originating as transgressive shoreline deposits have now been reworked into sand dunes by the Atlantic inflow currents inshore and Mediterranean outflow

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currents offshore (Figs. 3, 4, 6 and 15). Because of the combination of modern and ancient processes, they are categorized as a palimpsest sand dune facies that extends across the middle shelf from south of Cadiz to Tarifa (Figs. 4 and 14) (Lobo et al., 1996). Beach rock and bedrock outcrops increasingly disrupt the dune facies toward the south until eventually the constriction and increased speed of the Atlantic inflow and Mediterranean outflow currents result in a non-depositional scoured bedrock seafloor from Tarifa to Gibraltar Strait (Figs. 4 and 15) (Lobo et al., 1996; Lo´pez-Galindo et al., 1999). The complex interplay of the Atlantic inflow and Mediterranean outflow currents near Gibraltar results in a variety of sand dune types and orientation of flow directions (Fig. 4) (Lobo et al., 1996). In general, the inshore dunes are transverse and asymmetric toward the southeast with larger bedforms toward Gibraltar, coincident with the intensified Atlantic inflow toward the Strait (Figs. 3 and 4). The offshore northwestern and southern dunes are transverse and asymmetric with smaller bedforms toward the west, coincident with slacking Mediterranean outflow from the Strait. The transverse crests of the inshore dunes face southeasterly in the north and progressively become more easterly as Atlantic inflow turns more easterly to enter the Strait of Gibraltar. 11.5. Upper slope facies The upper slope facies develops in the steep upper slope region of smooth topography from 300 to 500 m water depth (Fig. 14). This facies is characterized by flat-lying bedded sequences, except for smallscale sand dune fields that occur in central and eastern locations of stronger proximal currents (Figs. 3 and 15) (Nelson et al., 1993). A gradual decrease in current speeds away from the Strait of Gibraltar has deposited sand layers that are about a meter thick and have an even gradation from fine clean sand on the southeast to muddy silt on the northwest upper slope (Figs. 7 and 11). This even gradational facies develops because of slacking currents, the smooth topography and straight slope. When the significant bend in the coast occurs near the Portuguese border, the current regime changes (Ambar and Howe, 1979b) resulting in deposition of the large Faro Drift

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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 upstream middle to lower slope region between the scour=sand ribbon area to the east near the Strait of Gibraltar (Kenyon and Belderson, 1973) and the ridge=trough area to the northwest in the central Gulf of Cadiz (Figs. 2 and 14). The sand dune facies is best developed on the upstream flatter lying mid-slope terraces where there are strong current speeds and sand contents generally greater than 70% (Figs. 4, 7 and 15). The downstream regions develop more irregular sand dunes with finer grain size (50–70% sand) as current speeds gradually decrease. Near slope valleys, sand dunes exhibit variable orientation, crest length, and intermixing with compound 3D barchan dune and ripple fields all of which appear to be related to down-valley currents (Figs. 3 and 15) (Nelson et al., 1993). Present-day Mediterranean outflow currents in the dune facies area actively transport sand in suspension as much as 11 m off the bottom (Thorpe, 1972). Current activity, facies boundaries, and bedform development may vary significantly because of topographic changes, related variation in local downvalley currents and a number of general oceanographic processes (Figs. 2, 3 and 15) (Kenyon and Belderson, 1973). Small-scale turbulence may cause the patterns of small ripples superimposed on sand dunes (Melieres et al., 1970). Medium-term current fluctuations and bedform variation can be caused by ebb, flood, spring and neap tidal forcing (Boyum, 1967) as well as by atmospheric pressure changes and strong east or west winds on either side of the Gibraltar Strait (Lacombe and Tchernia, 1960; Price et al., 1993; O’Neil-Baringer and Price, 1999). 11.7. Large mud wave facies Similar to the sand dune facies, nearsurface lithology of the mud wave facies on the lower slope consists of alternating coarse-grained and mud interbeds, but mud waves are characterized by much larger wave-lengths >500 m, >30% clay, and <50% sand (Figs. 7, 8 and 14). Texture grades from a

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median grain size of 0.60 mm on the east to 0.20 mm on the west (Fig. 7) (Faugeres et al., 1985b). Grain size, however, is quite variable and there are some transitional mud wave areas with a higher sand content and compound dunes and=or ripples; other areas exhibit a smooth muddy sea floor with current lineations (Heezen and Johnson, 1969; Melieres et al., 1970). High sand contents in some parts of the mud wave facies result from valleys that dissect the area and provide sand sources for local sand dune patches in the mud wave area (Nelson et al., 1993). The complex sets of bedforms and grain-size variation that characterize the mud wave area result from the aforementioned variations in current forcing mechanisms, the gradual diminishing of Mediterranean outflow current speeds with depth, and the temporal fluctuation in water depth of the undercurrent (Figs. 3 and 15) (O’Neil-Baringer and Price, 1999). In addition to the local down-valley flow that we believe modifies the large-scale waves (Nelson et al., 1993), Kenyon and Belderson (1973) suggested that major fluctuations in depth and velocity of the undercurrent in the mud wave area (i.e. š100 m, and <20–37 cm=s; Thorpe, 1972, 1976) may explain the large wave form development and grain size variation. 11.8. Sediment drift facies Sediment drift bodies occur banked against bedrock ridges in the central slope area and most commonly exhibit low angle unconformities in the subsurface (Fig. 14) (Nelson et al., 1993). The drift is scoured and modified by cut and fill sequences where it is intersected by bottom-currents flowing downslope in valleys (Figs. 3 and 15). Interbedded sand and mud layers make up the nearsurface lithology, but these vary from reversely-graded fine sand beds in eastern to muddy silt in western drift bodies (Fig. 11). The trapping of current-suspended sand in upstream drift bodies apparently causes development of foraminiferal-rich drift deposits consisting of winnowed planktonic tests deposited on topographic elevations in the leeside ‘current shadow’ downstream from the proximal bedrock ridges (Figs. 2, 9 and 15) (Nelson et al., 1993). Further downstream, the Faro Drift sediment body located off Portugal is characterized by deposits of silt beds interbedded with clay

(Gonthier et al., 1984; Faugeres et al., 1985a; Stow et al., 1986). Sedimentation rates are consistently low throughout the entire history of the Gulf of Cadiz sediment drift deposits. Late Quaternary sedimentation rates remain generally constant at 6–12 cm=kyr in both upstream Cadiz and downstream Faro sediment drift, but this is reduced to <4 cm=kyr in erosive margins along the drift bodies (Fig. 12) (Stow et al., 1986). These late Quaternary rates are similar to those of 12 cm=kyr for the thickest part (600 m) of the Faro Drift deposit during the entire post-Messinian period of the last 4.8 million years (Faugeres et al., 1985a) and to post-Messinian rates of 14 cm=kyr for the thickest part (600–700 m) of the eastern Gulf of Cadiz sediment drift bodies (Fig. 12). The general southeast to northwest change to finer texture and greater planktonic test content of the sediment drift facies is explained by the downstream decrease in the Mediterranean outflow currents (Figs. 3, 7, 9 and 15). The scour and cut and fill sequences in sediment drift facies adjacent to valley walls is associated with down-valley currents of the Mediterranean outflow (Nelson et al., 1993). There are not sufficient data on sedimentation rates, grain size and seismic stratigraphy of the drift deposits in the eastern Gulf of Cadiz, however, to decipher the interplay of bottom-current processes and general growth patterns of the sediment drift facies. 11.9. Valley facies Valley facies are downslope bottom-current deposited facies that cut perpendicularly across the general southeast to northwest along-slope contourite facies of sand dunes and sediment drift on the Cadiz slope (Fig. 14). Bedform crests of the valley areas strike SE to NW whereas those of the contourite sand dune, mud wave and upper slope facies strike NE to SW (Nelson et al., 1993). Similar to the contourites, the texture and scale of bedforms progressively diminish from the southeast channels in the region of stronger Mediterranean outflow currents to the northwest region of weaker currents; however, the grain size and scale of bedforms in valleys always are larger in any valley compared to the laterally associated contourite facies that they transect (Fig. 7) (Nelson et al., 1993). As a consequence

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of the strong down-valley currents, bioclastic sediment lags and lower sedimentation rates characterize the valley floors compared to associated contourite sediment drift deposits (Figs. 9 and 12) (Faugeres et al., 1985a,b; Stow et al., 1986). In contrast to the complex interplay of currents depositing sediment drift facies, the valley facies can be readily accounted for by the strong down-valley bottom currents. The down-valley measured currents in any locality are generally stronger than the contour currents (Fig. 3), which explains the coarser texture of valley facies compared to the adjacent contourite sand dune or sediment drift facies (Figs. 7A, 14 and 15). The currents along a valley floor are normally stronger than those along the walls and the shearing of down-valley flow with contour parallel flow should slow valley wall currents even more than usual (Fig. 3). This difference between floor and wall currents explains the changes in bedforms and texture between valley floor and wall environments (Nelson et al., 1993) The general weakening of the Mediterranean outflow currents from the southeast toward the northwest also affects the current ribbons that flow down valley, and consequently, the southeastern valleys contain larger-scale bedforms, coarser texture and more bioclastic shell lags than the northwestern valleys (Figs. 3, 7, 9 and 15).

12. Factors controlling facies development The initial control on the shelf and slope facies development in the Gulf of Cadiz has been the opening of the Strait of Gibraltar at the beginning of Pliocene time. Since the opening of the strait, high sea level, equivalent to the present or greater water level over the Gibraltar sill has permitted circulation through the strait and formation of strong Atlantic inflow and Mediterranean outflow currents. Although tectonic change in sill depth can also control the development of these currents, data is insufficient to evaluate this affect prior to the Late Pliocene (Maldonado and Nelson, 1999). The fluctuating glacial to interglacial climates and eustatic sea levels (Chappell and Shackleton, 1986; Haq et al., 1987) seem to have been the dominant influence on the currents since the Late Pliocene, because prior to that time cyclic deposition of dune and sediment drape sequences is not

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apparent. The Quaternary series of alternating dune and drape sequences appear to correlate with high and low sea levels, respectively (Nelson et al., 1993; Maldonado and Nelson, 1999). Another significant controlling effect of eustatic sea levels upon shelf and slope facies development is the formation of the lowstand prograding slope wedge off major rivers, transgressive sand sheet across the shelf, and highstand prograding mud layer in the mid-shelf (Figs. 12 and 15) (Gutierrez-Mas et al., 1996; Rodero et al., 1999). During high sea-level times of sediment facies development, the Atlantic inflow and Mediterranean outflow currents have been influenced by thermohaline circulation, Coriolis force, gravity, lunar tides, storm tides and sea floor morphology. The resulting basic inflow and outflow gradation from high velocities on the southeastern areas of the Cadiz margin to weak currents in the northwest areas, causes a change in along-shelf facies as well as along- and down-slope facies based on decreasing grain size and bedform development (Figs. 3, 14 and 15). The along-shelf gradation of shelf prodeltaic mud facies is similar to that of the Ebro prodeltaic muds (Diaz et al., 1990). The down stream gradation of sandy contourite facies on the Cadiz slope is similar to the bedrock to sand ribbon to sand dune facies sequences controlled by tidal currents in the English Channel (Belderson et al., 1971) and Messinian Strait (Colella, 1990) or by unidirectional geostrophic currents in the Bering Strait (Nelson, 1982a; Nelson et al., 1982). Tracing the bedrock to sand wave to prodeltaic mud facies (in the case of shelves) sequences can provide a tool to define ancient sequences and their controlling current patterns in deep or shallow water. Morphologic control is an important influence on the development of both shelf and slope facies. Bathymetric elevations of beach- and bed-rock outcrops contribute significantly to the patchy development of facies on the inner shelf (Figs. 4, 5 and 14) (Vanney and Menanteau, 1985; Lo´pezGalindo et al., 1999). The Guadalquivir structural depression (Maldonado and Nelson, 1999) provides accommodation space that allows the encroachment of the mud belt into the Guadalquivir inner shelf area (Figs. 7C and 14). Shelf paleo-valleys control the offshore encroachment of inner shelf sand dune

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facies (Fig. 14) (Vanney and Menanteau, 1985). The diapiric ridges and intervening valleys of the continental slope change the basic pattern of the Mediterranean contour-parallel outflow and produce accelerating down-valley currents (Fig. 3) that modify the development of the general southeast to northwest pattern of contourite facies (Fig. 14). As a result, down-valley facies trends are perpendicular to, and larger scale than those of the laterally associated contourite system (Figs. 7, 8 and 14) (Nelson et al., 1993). In addition, large sediment drift bodies are deposited because perpendicular ridges disrupt the contour-parallel flow (Figs. 2, 14 and 15). The interplay of morphology and the Mediterranean outflow currents described in the preceding paragraph emphasizes that varying combinations of controlling factors often contribute to the resultant sediment facies development. Shelf paleo-valleys (Fig. 2) and apparent down-valley Atlantic inflow currents are a shelf analog of the slope valley and Mediterranean outflow current system that control development of slope facies (Fig. 15). The midshelf mud blanket facies is another example for which river mouth mud sources are one important control combined with the southeastward progradation caused by Atlantic inflow currents (Figs. 4, 5 and 7C) (Gutierrez-Mas et al., 1996). At the mouth of the Guadalquivir river, the additional factor of a structural depression is associated with the river mouth source and Atlantic inflow currents to determine the final mid-shelf mud layer facies distribution. The mud layer progrades directly from the Guadalquivir estuary mouth perpendicularly across nearly the entire shelf width paralleling the depression, and also progrades southeastward over 50 km from the river mouth paralleling the inflow current direction (Figs. 5, 7C and 14) (Rodero et al., 1999).

13. Conclusions (1) The opening of the Strait of Gibraltar after Messinian time has resulted in development of the Atlantic inflow currents over the Spanish continental shelf and Mediterranean outflow currents over the slope. Consequently, during late Quaternary time, shelf prodeltaic deposits have prograded southeastward and sand dunes have developed in the shelf

region of strongest inflow currents near Gibraltar. As a result of outflow currents, a 200 km long sandy contourite sequence, that varies gradationally from sand dune fields in the southeast to sandy-silty sediment drift banked against diapiric ridges in the northwest has deposited over the eastern Gulf of Cadiz continental slope. (2) Data from the Cadiz margin and elsewhere (Belderson et al., 1971; Kenyon and Belderson, 1973; Nelson, 1982a; Colella, 1990) now show that tracing the bedrock to sand ribbon, sand wave and sediment drift sequences can provide a tool to define ancient straits and their controlling current patterns in deep or shallow water. (3) Because the shelf inflow or slope outflow bottom currents accelerate down valleys and create interference with contour-parallel flow (Fig. 3): grain size in valleys is greater than that in adjacent prodeltaic muds or contourites (Figs. 7A and 9); bedforms occur in shelf paleo-valleys as well as central and northern slope valleys whereas they are absent in the surrounding prodelta or continental slope contourite mud (Fig. 14); valley bedform crests are perpendicular to those of associated shelf or slope contourite sand dune fields; and complex superimposed bedforms develop along continental slope valley margins (Nelson et al., 1993; Baraza et al., 1999). (4) These current parallel and perpendicular variations show that unidirectional bottom-current contourite systems may exhibit complex sand-prone facies and bedform patterns that are not always contour parallel and may be associated with adjacent mud-prone continental shelf or slope facies. A wide variety of bottom-current facies result: (a) On the shelf these vary from complex inner shelf shoreface and paleo-valley sand dune facies, to current prograded prodeltaic mid-shelf mud facies, to current winnowed outer shelf, and to southeast shelf transgressive and reworked sand dune facies; (b) On the slope these vary from a simple current-graded upper slope contourite facies to a complex contour parallel and perpendicular middle and lower slope sand facies, and to distal along slope sediment drift sandy-silt facies or down slope mud-wave facies. (5) The bedforms and the surface sand to mud facies on the Cadiz margin are in equilibrium with present-day bottom currents and commenced with the

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Holocene time, except for the outer shelf transgressive sand facies. This suggests that during high sea level periods, when the present Gibraltar sill is not constricted, strong Atlantic inflow currents deposit mid-shelf contourite mud sequences or Mediterranean outflow currents deposit continental slope contourite sand sheets.

Acknowledgements We thank the many scientists, officers, and crew aboard the Garcia del Cid for assistance with shipboard data collection. Jose Ignacio Diaz and Marcelli Farran provided valuable additional surface sediment textural data. Marta Ezpeleta, Gita Dunhill and Jim McRea assisted with data compilation. The funding for this study was provided by the Joint Committee of Science and Technology of the U.S. Spain Treaty of Friendship (Project CA 83=047 and the Joint Project no. 125=94 of the ITGE=CSIC). The manuscript benefited from reviews by Dave Drake and Steve Eittreim.

References Ambar, I., Howe, M.R., 1979a. Observations of the Mediterranean outflow — I. Mixing in the Mediterranean outflow. Deep-Sea Res. 26A, 535–554. Ambar, I., Howe, M.R., 1979b. Observations of the Mediterranean outflow — II. The deep circulation in the vicinity of the Gulf of Cadiz. Deep-Sea Res. 26A, 555–568. Ashley, G.M., 1990. Classification of large-scale subaqueous bedforms: a new look at an old problem. J. Sediment. Petrol. 60, 160–172. Baraza, J., Ercilla, G., Nelson, C.H., 1999. Potential geologic hazards on the eastern Gulf of Cadiz slope (SW Spain). Mar. Geol. 155, 191–215. Belderson, R.H., Kenyon, N.H., Stride, A.H., 1971. Holocene sediments on the continental shelf west of the British Isles. Inst. Geol. Sci. Rep. 70 (14), 157–170. Boyum, G., 1967. Hydrological observations of the M=S Helland-Hansen and current measurements in the area west of Gibraltar — May 1965. NATO Sub-Comm. Oceanogr. Res. Tech. Rep. 34, 35, 36. Cacchione, D.A., Drake, D.E., 1986. Nepheloid layers and internal waves over continental shelves and slopes. Geo-Mar. Lett. 6, 147–152. Candela, J., Winant, C.D., Bryden, H.L., 1989. Meteorogically forced subinertial flows though the Strait of Gibraltar. J. Geophys. Res. 94, 12667–12679.

127

Caralp, M., 1992. Paleohydrologie des bassins profunds nordmoracain (est et ouest Gibraltar) au Quaternarire terminal: apport des foraminieres benthiques. Bull. Soc. Geol. Fr. 163, 169–178. Chappell, J., Shackleton, J.H., 1986. Oxygen isotopes and sea level. Nature 32, 137–140. Colella, A., 1990. Active tidal sand waves at bathyal depths observed from submersible and bathysphere (Messina Strait, southern Italy). IAS 13th Int. Sedimentol. Congr., Nottingham, England, August 26–31, 1990, Abstr., pp. 98–99. Diaz, J.I., Nelson, C.H., Barber, J.H., Giro, S., 1990. Late Pleistocene and Holocene sedimentary facies on the Ebro continental shelf. In: Nelson, C.H., Maldonado, A. (Eds.), The Ebro continental margin, northwestern Mediterranean Sea. Mar. Geol. 95, 333–352. Diester-Haass, L., 1973. No current reversal at 10,000 B.P. in the straits of Gibraltar. Mar. Geol. 15, M1–M9. Elias, S.A., Short, S.K., Nelson, C.H., Birks, H.H., 1996. Life and times of the Bering land bridge. Nature 382, 60–63. Faugeres, J.C., Cremer, M., Monteiro, H., Gaspar, L., 1985a. Essai de reconstitution des processus d’edification de la ride sedimentaire de Faro (Marge Sud-Portugaise). Bull. Inst. Geol. Bassin Aquit. 37, 229–258. Faugeres, J.C., Frappa, M., Gonthier, E., Grousset, F., 1985b. Impact de la veine d’eau Mediterraneenne sur la sedimentation de la marge sud et ouest Iberique au Quaternaire recent. Bull. Inst. Geol. Bassin Aquit. 37, 259–287. Faugeres, J.C., Frappa, M., Gonthier, E., Resseguiet, A., Stow, D.A.V., 1985c. Modele et facies de type contourite a la surface d’une ride sedimentaire edifiee par des courants issus de la vieine d’eau Mediteraneanne (ride du Faro, Golfe de Cadix). Bull. Soc. Geol. Fr. 8 (t. I, n. 1), 35–47. Fernandez-Lopez, J.M., Ortega-Serrano, E., 1986. Initial analysis of some measurements taken in the Gulf of Cadiz during ‘Donde Va?’ Project. Oct. 1982. Gardner, J.V., Kidd, R.B., 1983. Sedimentary processes on the Iberian continental margin viewed by long-range side-scan sonar Part I: Gulf of Cadiz. Oceanol. Acta 6, 245–254. Gonthier, E.G., Faugeres, J.C., Stow, D.A.V., 1984. Contourite facies of Faro Drift, Gulf of Cadiz. In: Stow, D.A.V., Piper, D.J.W. (Eds.), Fine-grained Sediment: Deep Water Processes and Facies. Geol. Soc. Spec. Publ. 15, 275–292. Grousset, F.E., Joron, J.L., Biscaye, P.E., Latouche, C., Treuil, M., Maillet, N., Faugeres, J.C., Gonthier, E., 1988. Mediterranean outflow through the strait of Gibraltar since 18,000 years, mineralogical and geochemical arguments. Geo-Mar. Lett. 8, 25–34. Gutierrez-Mas, J.M., Hernandez-Molina, F.J., Lopez-Aguayo, F., 1996. Holocene sedimentary dynamics on the Iberian continental shelf of the Gulf of Cadiz (SW Spain). Cont. Shelf Res. 16 (13), 1635–1653. Haq, B.U., Hardenbol, J., Vail, P.R., 1987. Chronology of fluctuating sea levels since the Triassic. Science 235, 1156–1167. Heezen, B.C., Johnson, G.L., 1969. Mediterranean undercurrent and microphysiography west of Gibraltar. Bull. Inst. Oceanogr. Monaco 67, 1–95. Hydrographer of the Navy, 1969. Western approaches to the

128

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

strait of Gibraltar. Bathymetric and free air anomaly gravity maps. Charts C6101 and C6101 A, The Royal Navy, Taunton, England. Huang, T.C., Stanley, D.J., 1972. Western Alboran sea: sedimentary dispersal ponding and reversal currents. In: Stanley, J. (Ed.), The Mediterranean Sea: a Natural Sedimentation Laboratory. Bowden, Hutchinson and Ross, Inc., Stroudsburg, Penn., pp. 521–559. Instituto Geologico y Minero de Espan˜a (IGME), 1974. Investigacion Minera Submarina en el Subsector ‘Huelva 1’ Golfo de Cadiz. Serv. Publ. Minist. Ind. Madr., 89 pp. Instituto Tecnolo´gico y Geominero de Espan˜a (ITGE), 1997. Mapa Geolo´gico de la Plataforma Continental Espan˜ola y Zonas Adyacentes. Escala 1:200 000. Hoja no. 86-86W (CADIZ). Inst. Tecnol. Geomin. Esp., Serv. Publ. Minist. Ind. Madr. Karl, H.A., Carlson, P.R., Cacchione, D.A., 1983. Factors that influence sediment transport at the shelfbreak. SEPM Spec. Publ. 33, 219–231. Kenyon, N.H., Belderson, R.H., 1973. Bedforms of the Mediterranean undercurrent observed with side-scan sonar. Sediment. Geol. 9, 77–99. Lacombe, H., Tchernia, T., 1960. Liste des stations M.O.P. Calypso 241 234 (campagne 1957) pour servir l’etude des echanges entre la mer Mediterrane´e et l’ocean Atlantique. Cah. Oceanogr. Paris 12 (3), 204–235. Lobo, F.J., Hernandez-Molina, J., Maldonado, A., Rodero, J., 1996. Los campos de ondas de arena en la plataforma continental del Golfo de Cadiz entre Chipiona y Zahara de los Atunes. Geogaceta 20, 420–423. Lo´pez-Galindo, A., Rodero, J., Maldonado, A., 1999. Surface facies and sediment dispersal patterns. Southeastern Gulf of Cadiz Spanish margin. Mar. Geol. 155, 83–97. Madelain, F., 1970. Influence de la topographie du fond sur l’ecoulement mediterrane´en entre le detroit de Gibraltar et le Cap Saint-Vicent. Cah. Oceanogr. Paris 22 (1), 43–61. Maldonado, A., Nelson, C.H., 1988. Dos ejemplos de margenes continentales de la Peninsula Iberica: el margen del Ebro y el Golfo de Cadiz. Rev. Soc. Geol. Esp. 1, 317–325. Maldonado, A., Nelson, C.H., 1999. Interaction of tectonic and depositional processes that control the evolution of the Iberian Gulf of Cadiz margin. Mar. Geol. 155, 217–242. Maldonado, A., Baraza, J., Checa, A., Nelson, C.H., Barber, J.H., Hampton, M.H., Kayen, R.E., Lee, H.J., 1989. Tectonic framework, pattern of sedimentation, and potential environmental problems of the Cadiz Continental Margin, Spain. 28th Int. Geol. Congr. Wash. 3, 2–356. Melieres, F., Nesteroff, W.D., Lancelot, Y., 1970. Etude photographique des fonds du Golfe de Cadix. Cah. Oceanogr. Paris 22, 63–72. Nelson, C.H., 1976. Late Pleistocene and Holocene depositional trends, processes and history of Astoria deep-sea fan, northeast Pacific. Mar. Geol. 20, 129–173. Nelson, C.H., 1982. Late Pleistocene–Holocene transgressive sedimentation in deltaic and non-deltaic areas of the Northeastern Bering epicontinental shelf. In: Nelson, C.H., Nio, S.D. (Eds.), The Northeastern Bering Shelf: New Perspectives

of Epicontinental Shelf Process and Depositional Products. Geol. Mijnbouw 61, 5–18. Nelson, C.H., 1982b. Modern shallow-water graded sand layers from storm surges — a mimic of Bouma sequences and turbidite systems. J. Sediment. Petrol. 52, 537–545. Nelson, C.H., 1990. Estimated post-Messinian sediment supply and sedimentation rates on the Ebro continental margin, Spain. In: Nelson, C.H., Maldonado, A. (Eds.), Ebro Continental Margin, Northwestern Mediterranean Sea. Mar. Geol. 95, 395– 418. Nelson, C.H., Lamothe, P.J., 1993. Heavy Metal Anomalies in the Tinto and Odiel River and Estuary system. In: Kuwabara, J.S. (Ed.), Trace Contaminants and Nutrients in Estuaries. Estuaries 16, 496–511. Nelson, C.H., Dupre´, W.R., Field, M.E., Howard, J.D., 1982. Variation in sand body types on the eastern Bering epicontinental shelf. In: Nelson, C.H., Nio, S.D. (Eds.), The Northeastern Bering Shelf: New Perspectives of Epicontinental Shelf Processes and Depositional Products. Geol. Mijnbouw 61, 37– 40. Nelson, C.H., Baraza, J., Maldonado, A., 1993. Mediterranean undercurrent sandy contourites, Gulf of Cadiz, Spain. In: Stow, D.A.V., Faugeres, J.C. (Eds.), Contourites and Hemipelagites in the Deep Sea. Sediment. Geol. 82, 103–131. Ochoa, J., Bray, N.A., 1991. Water mass exchange in the Gulf of Cadiz. Deep-Sea Res. 38 (1), s465–s503. O’Neil-Baringer, M., Price, J.F., 1999. A review of the physical oceanography of the Mediterranean outflow. Mar. Geol. 155, 63–82. Palanques, A., Diaz, J.I., Farran, M., 1995. Contamination of heavy metals in the suspended and surface sediment of the Gulf of Cadiz (Spain): the role of sources, currents, pathways and sinks. Oceanol. Acta 18, 469–478. Pietrafesa, L.J., 1983. Shelfbreak circulation, fronts and physical oceanography: East and west coast perspectives. In: Stanley, D.J., Moore, G.T. (Eds.), The Shelfbreak: Critical Interface on Continental Margins. Soc. Econ. Paleontol. Mineral. Spec. Publ. 33, 233–250. Price, J.F., O’Neil-Baringer, M., 1994. Outflows and deep water production by marginal seas. Prog. Oceanogr. 33, 161–200. Price, J.F., O’Neil-Baringer, M., Lueck, R.G., Johnson, G.C., Ambar, I., Parrilla, G., Cantos, A., Kennelly, M.A., Sanford, T.B., 1993. Mediterranean outflow mixing and dynamics. Science 259, 1277–1282. Rodero, J., Pallare´s, L., Maldonado, A., 1999. Late Quaternary seismic facies of the Gulf of Cadiz Spanish margin: depositional processes influenced by sea-level change and tectonic controls (Central North-Atlantic). Mar. Geol. 155, 131–156. Sonnenfeld, P., 1974. No current reversal at 10,000 B.P. in the Strait of Gibraltar — a discussion. Mar. Geol. 17, 339–340. Stanley, D.J., Addy, S.K., Behrens, E.W., 1983. The mudline: variability of its position relative to shelfbreak. In: Stanley, D.J., Moore, G.T. (Eds.), The Shelfbreak: Critical Interface on Continental Margins. Soc. Econ. Paleontol. Mineral. Spec. Publ. 33, 279–298. Stevenson, R.E., 1997. Huelva front and Malaga, Spain, eddy

C.H. Nelson et al. / Marine Geology 155 (1999) 99–129 chain as defined by satellite and oceanographic data. Dtsch. Hydrograph. Z. 30 (2), 51–56. Stow, D.A.V., Faugeres, J.C., Gonthier, E., 1986. Facies distinction and textual variation in Faro Drift contourites. Velocity fluctuation and drift growth. Mar. Geol. 72, 71–100. Thorpe, S.A., 1972. A sediment cloud below the Mediterranean outflow. Nature 236, 326–327. Thorpe, S.A., 1976. Variability of the Mediterranean undercurrent in the Gulf of Cadiz. Deep-Sea Res. 23, 711–727. Van Geen, A., Adkins, J.F., Boyle, E.A., Nelson, C.H., Palanques, A., 1997. A 120 year record of metal contamination on an unprecedented scale from mining of the Iberian Pyrite Belt. Geology 25, 291–294.

129

Vanney, J.R., Menanteau, L., 1985. Physiographic map of the Atlantic littoral of Andalusia, M.F. 02 Punta Umbria– Matalascanas and M.F. 03 Matalascanas–Chipiona, Junta de Andalucia, Consejeria de Politica Territorial, Agencia de Medio Ambiente, Casa de Velazquez, Servicio de Publicaciones y BOJA Aptdo Correos 100,000, 41071 Sevilla, Spain. Vanney, J.R., Mougenot, D.L., 1981. La plateforme continentale du Portugal et les provinces adjacentes: Analyse geomorphologique. Mem. Serv. Geol. Port. 28. Vergnaud-Grazzini, C., Devaux, M., Znaidi, J., 1986. Stable isotope ‘anomalies’ in Mediterranean Pleistocene records. Mar. Macropaleontol. 10, 35–69.