Surface facies and sediment dispersal patterns: southeastern Gulf of Cadiz, Spanish continental margin

ELSEVIER

Marine Geology 155 (1999) 83–98

Surface facies and sediment dispersal patterns: southeastern Gulf of Cadiz, Spanish continental margin Alberto Lo´pez-Galindo Ł , Jesu´s Rodero, Andre´s Maldonado Instituto Andaluz de Ciencias de la Tierra. CSIC-Universidad de Granada, Avda. Fuentenueva s=n, 18002 Granada, Spain Received 4 March 1996; revised version received 20 November 1997

Abstract Grain size of 294 samples and mineralogical analysis of 364 surface samples from the Spanish continental shelf and upper slope of the Gulf of Cadiz delineate four sediment facies that derive mainly from siliciclastic sources. The northern area is formed by the mud-rich prodelta of the Guadalquivir River, parallel to the coast and progradational towards the southeast. It exhibits a typical bulk mineral association of illite × smectite > kaolinite C chlorite. In the north-central area, there is a smaller, coarser-grained prodelta in the Bay of Cadiz, linked to the Guadalete River, which offshore becomes connected to the Guadalquivir prodelta. Its mineral association is quartz × illite > kaolinite C chlorite. In the southern area, where fluvial input is negligible, the characteristic coarse-grained facies generally corresponds to relict, Late Pleistocene reworked deposits. The typical mineral association is quartz > calcite > aragonite × smectite–mixed layer clay minerals. The fourth area corresponds to the shelf break and upper slope offshore Cadiz Bay, where a westward grain-size coarsening differentiates the deposits of the last low sea-level stand and no typical mineral association exists. The mineralogy of the terrigenous deposits indicates a source area from Paleozoic sequences of the Sierra Morena to the north and the Subbetic Units and Gibraltar flysch to the east. The Neogene deposits of the Guadalquivir depression are an additional source of shelf sediments. The surface sediment distribution on the inner continental shelf is largely determined by the southeastern Atlantic water flow, the terrigenous input from rivers, and the shelf physiography. Sediment dispersal on the outer continental shelf and upper slope, in contrast, largely reflects the location of low sea-level stand, the development of palimpset deposits, and the location of the Mediterranean undercurrent over the margin.  1999 Elsevier Science B.V. All rights reserved. Keywords: Gulf of Cadiz; sediment facies; mineralogy; continental shelf processes; sediment dynamics

1. Introduction The margin of the Gulf of Cadiz between the mouth of the Guadalquivir River and the western entrance to the Strait of Gibraltar has a predominant NW–SE orientation, although locally the coastline displays an E–W trend (Fig. 1). These orientations are mainly controlled by tectonic lineations that have Ł Corresponding

author. Tel.: C34-58-246207; Fax: C34-58243075; E-mail: [email protected]

been recently active (Maldonado et al., 1999). The shelf analysed corresponds to a typical example of siliciclastic sedimentation, and is included in the Atlantic-type passive margins of the Iberian Peninsula (Maldonado and Nelson, 1988). This shelf is relatively wide, with an average width of 50 km, although there are important variations in the area close to the Straits of Gibraltar, where it narrows to 20–30 km. The shelf break occurs at 140–150 m water depth and shows significant variations north to south in cross section. Thus, in the northern area the

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

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Fig. 1. Simplified bathymetric map of the study area and location of samples. Depth in meters.

transition of the shelf to the continental slope is very gradual (average gradient of 1–2º), whereas in the southernmost area and near the Straits of Gibraltar, there is a narrow shelf break with a greater average gradient (3–4º) (Rodero et al., 1999). The slope is crossed by numerous canyons trending NE–SW, perpendicular to the main trend of the shelf (Emery and Uchupi, 1984; Nelson et al., 1993). The existence of a predominant southwest flow of Atlantic water over the shelf and the location of the two major sediment sources, the Guadalquivir and the Guadalete rivers, develops a sedimentological zonation in the study area (Segado et al., 1984; Gutie´rrez Mas et al., 1995; Nelson et al., 1999). This shelf, on the basis of the geomorphological and sedimentological (grain size and mineralogy) characteristics has traditionally been divided into three large areas (cf. Gutie´rrez Mas, 1992, and references therein): a coastal strip of gravel and sand; a southern sector, basically sandy, and a northern sector mainly composed of fine-grained deposits derived from the Guadalquivir River. In addition, the dis-

tribution of Holocene sediments shows significant thickness variations between these three areas. In this study we analyze the grain size and mineralogy of surface sediments from the shelf and upper slope of the southeastern Spanish margin of the Gulf of Cadiz. The mineralogical provinces are established, and the sources are determined. The Gulf of Cadiz, in addition, is a key area for the analysis of the water masses interactions between the Atlantic Ocean and the Mediterranean Sea (Ochoa and Bray, 1991; Nelson et al., 1993). The sediment dispersal patterns and the depositional processes controlled by these water-mass interactions are also examined, and the sedimentary consequences of the last cycle of eustatic sea-level rise the present distribution of surface sediments are finally discussed.

2. Methods A total of 370 samples have been analysed. These samples were collected aboard the B=O Garcia del

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Cid, cruise M-86-3 and M-86-4, June 1991 and October 1994, respectively. All the samples analysed were obtained from surface sediments collected by dredges, and gravity and rock cores (Fig. 1). The samples were accurately positioned with MAXIRAM and GPS satellite and the water depth recorded with echo-sounders. All the samples were positioned along the tracks of high-resolution reflection profiles (Rodero et al., 1999) The grain sizes of 294 samples were determined by standard sieve procedures for the coarse-grained fraction (>63 µm) and a computerized laser inspection system (CIS-1, GALAI) for the fine-grained fraction. The composition of the sand fraction of 265 samples were established by optical microscopy counting 300–400 grains in each sample. The mineralogical analysis of 364 samples was performed with a Philips PW1710 X-ray diffractometer, equipped with automatic slit, CuKÞ radiation, Ni filter, scan speed of 2º=minute, using the crystalline powder technique for the bulk mineralogy and oriented aggregates of the <2 µm for the clay mineralogy. Quantification of the different phases was carried out by the classic method of area measurement of peaks and reflective power (cf. Mellinger, 1979; Pevear and Mumpton, 1989). Factor analysis (Principal Components Analysis, PCA) is used to establish the relation between the different minerals, grain-size fractions, and their associations. The factors (principal components, PCs) have been selected for eigenvalues >1 (Kaiser, 1960), applying a Varimax rotation. According to Davis (1973), Reyment and Jo¨reskog (1993), and Swan and Sandilands (1995), among others, PCA is a technique for finding linear compounds of correlated variables, and it is used to characterize the data obtained from the analyses, thus reducing the complexity of the ‘natural model’ and classifying the mineralogical and granulometric variables into groups.

3. Results 3.1. Grain size Samples are mostly composed of, in order of abundance, sand and clay size, and subordinate

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amounts of silt and gravel. The grain-size distribution histograms and cumulative frequency diagrams reveal the existence of 7 main sediment types (Figs. 2 and 3). Fine-grained muds correspond to sediments located in isolated pockets off the Bay of Cadiz, with grain sizes between 8 and 1 µm (8-10) and small variance. Silty muds have a sand content of less than 10%, with cumulative curve shapes similar to the fine-grained muds, although the sorting is poorer. These type of samples cover a large part of the middle continental shelf in the northern area. Sandy muds have a grain-size distribution that is clearly bimodal, with 30% of the sample coarser than 62 µm (mode around 2:75) and the rest of the sample with an average grain size of less than 8 µm (mode around 9). They are distributed to the southwest of the silty muds, offshore of the Bay on Cadiz in the outer shelf. Muddy sands contain about 10% of material larger than 250 µm .2/, 60% between 250 and 62 µm (2-4), with the rest smaller than 8 µm .7/. They are largely located to the south and east of the silty muds, with a maximum presence on the outer shelf, off Cadiz. Silty sands are composed of 50% fraction larger than 125 µm, well sorted, 20% between 125 and 30 µm, poorly sorted, and the rest smaller than 30 µm. They are mostly located on the outer shelf and the upper slope, also near the coast in the northern area and in the Bay of Cadiz forming part of the prodelta lobe of the Guadalete River. Sands are well sorted with 90% of the grains between 500 and 200 µm. These sands occupy most of the southern part and they are also located inside the Bay of Cadiz. Gravels have 40% of the grains larger than 2000 µm and 50% of the grains between 2000 and 200 µm. They form pockets within the sands in the southern area, and is abundant in the inner continental shelf of the southern area and in two locations of the slope. The surface distribution of the grain sizes show that sand and clay size are the most abundant fractions (Fig. 4). Sand is more than 60% of the sediment throughout the study area, except in the middle shelf of the northern sector, where the prodelta lobe of the Guadalquivir River is located. The silt fraction, in contrast, is mostly represented in the Guadalquivir prodelta, where clay sizes constitutes, also, more than 60% of the sediment.

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Fig. 3. (A) Characteristic histogram distribution of grain size observed in the different sectors of the study area. (B) Surface distribution of sediment types in the Gulf of Cadiz.

Fig. 2. Characteristics cumulative frequency curves of grain size from the various sediment types identified. and ternary grain size fraction plots from samples analyzed. The shaded area in the cumulative curve shows the distribution of samples for each sediment type.

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Fig. 4. Maps of percentage of gravel, sand, silt, and clay size fractions in surface sediments of the study area.

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3.2. Composition of the sand fraction Table 1 shows the average composition of the sand fraction. The content of mica, detrital grains (quartz, feldspars, glauconite, etc.), planktonic and benthonic foraminifers, bivalves, bryozoans and others biogenic compounds (gastropods, echinoderms, ostracods, etc.) have been determined. 3.3. Mineralogy Minerals in practically all samples are phyllosilicates, quartz, calcite, and feldspars (Fig. 5A; Table 1). Dolomite and aragonite are occasionally present, though in small quantities. Other minerals found include gypsum, zeolites (clinoptiloliteheulandite, phillipsite), opal A, pyrolusite, ankerite, apatite, celestite, and strontianite. Quartz is the most abundant terrigenous mineral, with an average content of 37%. It is present in all samples, being most abundant in the southeast

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study area, off Barbate, where it forms up to 83% of the total (Fig. 6), in isolated parts of the middle shelf of this sector, and on the upper slope of the southern sector. Its average size exceeds 63 µm, and it is mainly found in the sand fraction. Phyllosilicates are most in abundance, forming on average 28% (Table 1). They are closely correlated to the silt .r D 0:81/ and clay sized .r D 0:88/ fractions and negatively correlated to quartz .r D 0:83/ (Table 2). The greatest concentrations occur in the northern part of the study area, where there is a large lobe lying parallel to the coast, NW–SE oriented, clearly related to the fine-grained sediments transported by the Guadalquivir River. A second area of accumulation is on the upper slope, off Cadiz, associated with the presence of diapiric marl (Fig. 3B). Finally, proportions of 20 to 40% are found on southern part of the study area. Feldspars (K-feldspar and plagioclase) are not particularly abundant, and the average content does not exceed 5%. However, K-feldspar may locally reach 19%, and plagioclases

Table 1 General univariate statistics from mineralogy, composition of sand fraction and grain size analysis Variable

Mean

Median

Mode

S.D.

Variance

Curtosis

Minerals Quartz Calcite Dolomite Aragonite K-feldspar Plagioclase Illite Mixed layers Smectites Kaol C Chlor Palygorskite

37.2 20.8 4.8 2.7 2.7 3.5 13.6 4.3 5.0 3.5 0.6

34.8 20.4 3.2 0.0 2.0 3.1 10.2 3.1 5.0 3.0 0.0

16.5 20.6 0.0 0.0 0.0 3.1 3.0 3.0 3.0 1.0 0.0

19.73 7.60 4.84 5.61 2.91 2.15 9.70 3.08 3.32 3.38 1.85

389.43 5775 23.44 31.47 8.44 4.60 94.16 9.50 11.00 11.44 3.44

0.90 3.93 7.58 12.61 6.60 2.49 0.75 4.59 1.44 2.96 13.94

Composition of sand fraction Mica Detrital grains Planktonic foram. Benthonic foram. Bivalves Bryozoans Others biogenic compounds

1.9 71.0 8.5 8.1 3.4 1.9 5.3

1.0 81.0 4.0 4.0 2.0 0.0 3.0

0.0 90.0 2.0 2.0 0.0 0.0 2.0

2.85 23.41 10.60 9.59 5.69 3.42 7.15

8.10 548.25 112.44 91.95 32.38 11.67 51.15

Grain-size fractions (%) Gravel Sand Silt Clay

7.78 63.23 8.28 20.48

0.50 65.39 1.60 3.20

13.12 31.45 10.11 25.00

172.18 989.11 102.26 625.04

0.00 0.50 0.50 1.60

Skewness

Minimum

Maximum

0.30 0.69 2.29 3.30 2.12 0.92 0.68 1.40 0.97 1.47 3.66

4 3 0 0 0 0 0 2 0 0 0

83 68 34 42 19 13 43 23 20 21 13

4.70 0.25 3.69 4.29 59.47 6.21 20.37

2.17 1.19 1.90 2.02 6.45 2.43 4.07

0 1 0 0 0 0 0

15 97 62 60 66 20 52

7.26 0.38 1.39 0.15

2.81 0.31 1.76 1.48

0.00 0.20 0.14 0.17

80.96 98.69 50.00 81.40

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Fig. 5. (A) Mineralogical composition of the surface samples from the Gulf of Cadiz. (B) Distribution of the several grain-size fractions vs. carbonates.

may rise to 13%. They are clearly detrital, and spatially associated with quartz, and therefore the southeastern sector is the richest in these components. Calcite occurs in all the samples, the average content being 21%, with maxima of up to 68%. Its dispersal is fairly irregular and the middle shelf of the southern sector is the area where calcite is most common (Fig. 6). It is basically of biogenic origin, with numerous bentonic and plantonic foraminifers, bryozoans, bivalve clasts, gastropods, and ostracods. There is an average of 5% dolomite, and range of 0 to 34%. The greatest concentrations occur on the middle and outer shelf, off the Bay of Cadiz, and in areas near the coast at the mouths of the Guadalquivir and Guadalete rivers. Aragonite is from fragments of bivalves shells and echinoderms, and significantly correlated .r D 0:67/ to gravel (Table 2). Although its average content is less than 5%, in some sectors of the middle shelf off Barbate it forms up to 40% of the sample. Most of the carbonates are associated with the gravel and sand fractions (Fig. 5B). Illite is the most abundant clay mineral (14% average content of the bulk sample), reaching the highest content (up to 43%) and a high degree of crystallinity in the central part of the lobe in the northern sector, and, further offshore, in the central and southern sectors of the outer shelf and continental slope (Fig. 7; Table 1). Smectite is of low crystallinity and, there-

fore, clearly detrital; it is concentrated (10–15%) in the Bay of Cadiz and surrounding areas, as well as on the upper slope of the central sector and is associated with the gravels of the central-south sector. Chlorite and kaolinite occur mainly in the Bay of Cadiz, attaining a concentration of 20%, and in the northern muddy lobe. Irregularly interstratified illite–smectite does not usually exceed 10–15%, and has different swelling phase proportions. Illite–smectite is concentrated in isolated pockets on the shelf, off Cadiz, and further offshore, related to the muddy and silty sands. Palygorskite occurs in isolated samples, with a content of less than 10%. Principal components analysis have been applied to the granulometric, sand compounds, and mineralogical variables. Four factors (PCs) explaining the 79% of the total variance were obtained (Fig. 8). These factors are lineal combination of original variables. Factor 1 (42% of total variance) is clearly interpretable as a grain size and mineralogical factor, which relates with loadings >0.75 the fine-grained fractions and clay minerals, and with negative loadings the coarser fractions and biogenic compounds. Factor 2 (14.5% of total variance) is also a grain size and mineralogical factor, which groups some carbonates (calcite and aragonite) with gravel and bivalves (negative loadings) and, on the other hand, the detrital silicates with the sand, silt and clay size frac-

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Fig. 6. Maps of percentage of phyllosilicates, quartz, feldspars, and calcite in the surface sediments of the study area.

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Fig. 7. Maps of percentage of illite, smectite, kaolinite C chlorite, and interstratified illite–smectite in the surface sediments of the study area.

1.00 0.13 1.00 0.58 0.09 1.00 0.52 0.44 0.21 1.00 0.58 0.61 0.78 0.28 1.00 0.02 0.00 0.00 0.03 0.00 1.00 0.02 0.32 0.26 0.33 0.27 0.14 1.00 0.12 0.20 0.27 0.01 0.03 0.19 0.13

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tions (positive loadings). Factors 3 (11.5% of total variance) and 4 (11% of total variance) discriminate among distinct types of carbonates, phyllosilicates and biogenic compounds. Geostatistical analysis (cf. Journel and Huijbregts, 1978), in particular the analysis of the variogram function, which represents the spatial variability of each granulometric and mineralogical element (or ‘regionalized variable’), showed that: (1) the sand, silt, and clay size fractions, together with quartz and illite, are stationary variables, i.e. they do not show significant variations in their spacial distribution at the scale of the sampling performed; (2) their respective variograms offer a large degree of spatial continuity of the data, with the presence of some variations depending on the direction (or ‘geometric anisotropy’), characterized by different ranges at two main directions: 28 km at the N30 direction and 53 km at the N125 direction (Fig. 9). This means that variance is minor at the N125 direction, which is in agreement with their respective spatial distribution maps; (3) on a small scale, of less than 10 km, these variables have an isotropic behavior. On a regional scale, the anisotropy of the phenomenon is visible at a greater intensity.

1.00 0.24 0.02 0.23 0.17 0.02 0.01 0.08 0.02 1.00 0.14 0.29 0.20 0.08 0.11 0.18 0.27 0.12 0.07 1.00 0.51 0.24 0.11 0.28 0.06 0.78 0.61 0.67 0.68 0.27 1.00 0.67 0.11 0.20 0.31 0.29 0.02 0.86 0.64 0.54 0.73 0.45 1.00 0.89 0.61 0.11 0.20 0.33 0.27 0.01 0.78 0.60 0.50 0.69 0.44 1.00 0.87 0.92 0.75 0.27 0.10 0.07 0.34 0.10 0.81 0.67 0.58 0.71 0.43 1.00 0.01 0.39 0.38 0.07 0.36 0.28 0.67 0.06 0.20 0.28 0.06 0.02 0.20 0.18 Gravel Sand Silt Clay Quartz Calcite Dolomite Aragonite K-feldspar Plagioclase Illite Mixed layers I-S Smectites Chlor C Kaol Palygorskite

Clay size Quartz Calcite Dolomite Aragonite K-feldspar Plagioclase Illite Silt Gravel Sand

Table 2 Pearson correlation coefficients for mineralogical and granulometric variables

Interstrati- Smectites Chlor C Kaol fied I-S

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4. Discussion and conclusions 4.1. Facies distribution Several studies have discussed aspects concerning sedimentary processes and facies distribution in the Gulf of Cadiz (Segado et al., 1984; Grousset et al., 1988; Maldonado and Nelson, 1988; Nelson et al., 1993). Perhaps the most comprehensive contribution describing the characteristics of the surface sediments is the study of Gutie´rrez Mas (1992), which analyzed grain-size, mineralogy and geochemical data from 180 samples obtained between the parallels of Chipiona and Trafalgar. In a later work, Gutie´rrez Mas et al. (1995) established three types of facies including bioclastic quartz-rich sands, bioclastic silty muds and bioclastic quartz-rich gravel, which show a distribution that broadly corresponded to the southern shelf, northern shelf, and Bay of Cadiz sectors. The results of our study indicate a more complex facies distribution pattern, which not

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Fig. 8. R-mode factor analysis showing the contribution of statistically dominant variables of grain size, mineralogy, and components of the sand fraction and binary diagram of factor 1 and 2.

only reflects the present sediment dynamics but also several characteristics of the surface sediment distribution inherited from the last low sea-level stand and Holocene sea-level rise (Fig. 3B). The type of facies better represented occupies most of the southern part, from south of Cadiz to Tarifa and it is characterized by the presence of rock outcrops, generally covered by a layer of sediments with variable thickness. Basement outcrops consist of Gibraltar flysch, which constitutes the external part of a tectonic arc (Malod, 1982; Maldonado et al., 1998). The overlying sediments are mainly composed of sand, with lesser amounts of gravel and with a high content of bioclastic remains. The side-scan records of this area depict a variety of bedforms, mostly several types of sand dunes, which indicate strong bottom currents (Nelson et al., 1993, 1998). Areas without a sedimentary cover generally correspond to morphological and structural highs, where strong currents sweep the bottom. A large area is located in the northeastern sector (Fig. 3B). This area occupies the middle shelf and delineates a southeastward trending lobe. The sediments are predominantly fine-grained muds that correspond to the submerged prodelta of the Guadalquivir River. Fine-grained sediments from the river mouth are advected by geostrophic currents

toward distal areas resulting in southward progradation of the mud over the sand layers. The sediment is distributed by the surface Atlantic water flowing southeastward towards the entrance of the Strait of Gibraltar, which carries mid-water turbid layers and surface-water sediment plumes. This relatively high-energy shelf, with strong tide and wave currents does not allow for the development of a fine-grained shallow prodelta, which is confined in the study area to water depths below 15–20 m. The finegrained deposits developed a thick prodelta wedge, characterized by acoustic maskings, probably due to accumulation of gases (Acosta, 1984; Rodero et al., 1998). Another smaller prodelta facies is located seaward of the entrance to the Bay of Cadiz (Fig. 3B). This facies derives from the fine-grained materials supplied by the Guadalete River and from resuspension of fine-grained material in the marsh-lands of the bay (Gutie´rrez Mas et al., 1994). A transitional area is delimited between the Guadalquivir prodelta and the sand and gravel facies to the south. This is a palimpsest facies characterized by the mixture of fine-grained materials advected to more distal areas from the prodeltas, mixed with the relict facies of the transgressive sands developed during the Holocene transgression (Rodero et al., 1998). There is a Holocene sandy and gravel coastal facies

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Fig. 9. Variogram function of variable ‘clay size fraction’ in the N30E and N125E directions. The adjust has been spherical.

that extends along the coast. This facies is overlapped by the Guadalete prodelta off the entrance to the Bay of Cadiz and it is transitional in the southern sector with the sand and gravel facies of the shelf. The outer shelf and slope in the northern area show a large variety of sediment facies. Near the shelf break are relict transgressive sands, developed during the initial stages of the last eustatic sea-level rise. These are muddy sands that probably represent a variety of coastal and estuarine deposits recording a complex transgressive history (Rodero et al., 1998). The facies distribution on the slope of this northern area is also very complex. The presence of diapiric linear ridges and submarine canyons perpendicular to the slope, and the development of large bedform fields and erosional surfaces by the contour-flow of the Mediterranean undercurrent, influence a large variety of sediment facies (Nelson et al., 1993, 1999). These facies include muddy sands, sand and gravel and marls, with a general distribution characterized by a westward increase of grain size. Although most of the coarse-grained facies on the slope represent palimpsest and relict deposits, the presence of gullies and canyons on the slope and the strong Mediterranean undercurrent facilitate the development of gravity and contour flows that are influencing the present sediment distribution patterns. 4.2. Sedimentary processes The sedimentary processes are largely controlled by two factors: the location of terrigenous sources

and the Mediterranean undercurrent and Atlantic surface-water flows. The main terrigenous source is the Guadalquivir River, where a large prodelta detached from the river mouth is developing below wave base (Maldonado and Nelson, 1988); and the output from the Bay of Cadiz where fine-grained terrigenous sediment plumes from the Guadalete River and bottom sediment resuspension are injected onto the shelf. Most of the coarse-grained material is retained in the estuaries of these rivers, or transported by waves, long-shore, and tidal currents to the sandy beaches of the coast. Most of the sediment supply of the Guadalete River is trapped, however, in the bay and only minor amounts are transported to the shelf, mostly by the currents generated by ebb tides and strong easterly winds. These materials generated a small detached prodelta offshore from the mouth of the bay, which is initially oriented westward and then diverts southeastward due to the influence of the surface Atlantic flow, becoming parallel to the Guadalquivir prodelta (Fig. 3B). South of Cadiz, the only terrigenous sediment source to the shelf are local ephemeral streams and erosion from beaches and coastal cliffs. In this area the present sediment dynamics are largely controlled by the Atlantic and Mediterranean water flows (Ochoa and Bray, 1991; Price et al., 1993). Sediment facies in this southern area are mostly of palimpsest type and they form a large variety of bedforms (Nelson et al., 1993, 1999). The outer shelf transgressive sands are also influenced by the present sediment dynamics as fine-

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Fig. 10. Source areas and sediment dynamic in the study area based on the distribution patterns of clay minerals. The size of letters is proportional to the quantity of phyllosilicates (I D illite; Sm D smectite; K D kaolinite; CH D chlorite; I-Sm D interstratified illite–smectite; Q D quartz).

grained sediments are advected from the coastal environments. On the slope the most active processes are developed by down-slope gravity sediment flows and the Mediterranean undercurrent (Nelson et al., 1993). The bluish-green marls cropping out in the axis of the linear diapirs are a source of fine-grained sediments as they are eroded by the Mediterranean undercurrent. 4.3. Source areas The clay mineralogy provides additional information about the nature of the source areas. In the northern sector of the shelf, the minerals include illite, smectite, kaolinite and chlorite, which is a similar association to that observed in Neogene and Quaternary rocks in the region of the Bay of Cadiz (Viguier, 1974). The greater crystallinity of illite compared to that observed in other sectors, together with the presence of chlorite and kaolinite in significant quantities, suggests, however, that there is in addition a source area basically composed of igneous and metamorphic rocks. The Paleozoic rocks of Sierra Morena, which are drained by the

Guadalquivir River are most probably the source rocks. The typical mineral association of the southern sector is, in contrast, quartz and illite of low crystallinity. These minerals indicate a source from the Subbetic and the Gibraltar flysch, which are drained by the Guadalete and Barbate rivers. The existence of minerals that are ultrastable in the heavy fraction (cf. Gutie´rrez Mas, 1992) also supports this conclusion. The presence in this area of smectite, interstratified clay minerals and kaolinite may also indicate a Mediterranean origin for the fine-grained sediments transported by the Mediterranean undercurrent (Fig. 10). 4.4. Sea-level changes and surface facies distribution During the last low sea-level stand, margin deltas developed in the northern area of the Cadiz outer shelf (Rodero et al., 1998). The nearshore and coastal deposits were reworked during the Flandrian transgression (14,000–6000 yr B.P.), which produced a layer of transgressive sands over most of the continental shelf. Some coastal deposits were abandoned

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relatively undisturbed near the shelf break, which are represented by the relatively coarser muddy sand facies with higher quartz content. A transgressive prodelta lobe facies began to develop about 6000 years ago, as a result of a slowing in the rate of sea-level rise, as in other margins of the Iberian Peninsula (Checa et al., 1988; Diaz and Maldonado, 1990; Farran and Maldonado, 1990). In the southern area, largely depleted from significant sediment supply, only reworked transgressive sands developed. When present sea level was reached, the two prodeltas of the Guadalquivir and Guadalete rivers prograded offshore over the transgressive sands in a southeasterly direction following the Atlantic surface-water flow. The sedimentary processes on the slope, in contrast, are controlled by the northwestard flowing Mediterranean outflow, which impinges over the margin and contributes with some fine-grained sediments.

Acknowledgements We thank the assistance at sea of the officers and crew of the B.O. Garcı´a del Cid. We also appreciate the critical review of an earlier version of the manuscript by J.M. Gutie´rrez Mas, as well as the constructive criticism of two anonymous reviewers. The statistics has been revised by Professor Mario Chica (Geostatistics, Univ. Granada) and Professor Francisco Torres (Statistic, Univ. Granada). This study was supported by Project 125=94 of the ITGE and group RNM-0179 of the Junta de Andalucı´a.

References Acosta, J., 1984. Occurrence of acoustic masking in sediments in two areas of the continental shelf of Spain: Ria de Muros (NW) and Gulf of Ca´diz (SW). Mar. Geol. 58, 427–434. Checa, A., Dı´az, J.I., Farra´n, M., Maldonado, A., 1988. Sistemas deltaicos holocenos de los rı´os Llobregat, Beso´s y Foix: modelos evolutivos transgresivos. Acta Geol. Hisp. 23, 241– 255. Davis, J.C., 1973. Statistical and Data Analysis in Geology. Wiley, New York, 550 pp. Diaz, J.I., Maldonado, A., 1990. Transgressive sand bodies on the maresme continental shelf, Western Mediterranean Sea. Mar. Geol. 91, 53–72.

97

Emery, M., Uchupi, E., 1984. The Geology of the Atlantic Ocean. Springer, New York, 1050 pp. Farran, M., Maldonado, A., 1990. The Ebro continental shelf: Quaternary seismic stratigraphy and growth patterns. Mar. Geol. 95, 289–312. Grousset, F.E., Joron, J.L., Biscaya, P.E., Latouche, C., Treuil, M., Faugeres, J.C., Goutier, E., 1988. Mediterranean outflow through Strait of Gibraltar since 18000 years B.P. Mineralogical and geochemical argument. Geomar. Lett. 8, 25–35. Gutie´rrez Mas, J.M., 1992. Estudio de los sedimentos recientes de la plataforma continental y Bahı´a de Ca´diz. Ph.D. Diss. Univ. Ca´diz, 364 pp. Gutie´rrez Mas, J.M., Herna´ndez-Molina, F.J., Lo´pez-Galindo, A., Lo´pez-Aguayo, F., 1994. Establecimiento de la traza de un flujo desde la bahı´a de Ca´diz a la plataforma continental. Gaia 8, 125–128. Gutie´rrez Mas, J.M., Lo´pez Galindo, A., Gonza´lez Caballero, J.L., Lo´pez Aguayo, F., 1995. Las facies detrı´ticas de la plataforma continental de Ca´diz (tramo Chipiona–Trafalgar) en relacio´n con la evolucio´n de la dina´mica sedimentaria reciente. Rev. Soc. Geol. Esp. 8 (12), 61–71. Journel, A., Huijbregts, C.H., 1978. Mining Geostatistics. Academic Press, New York, 600 pp. Kaiser, H.F., 1960. The application of electronic computers to factor analysis. Educ. Psychol. Meas. 20, 141–151. Maldonado, A., Nelson, C.H., 1988. Dos ejemplos de ma´rgenes continentales de la Penı´nsula Ibe´rica: El margen del Ebro y el Golfo de Ca´diz. Rev. Soc. Geol. Esp. 1, 317–325. Maldonado, A., Somoza, L., Lorenzo, P., 1999. The Betic orogen and the Iberian–African boundary in the Gulf of Cadiz: geological evolution (central North Atlantic). Mar. Geol. 155, 9–43. Malod, J.A., 1982. Comparaison de l’evolution des merges continentales au Nord et au Sud de la Peninsule Iberique. Ph.D. Diss., Univ. Pierre et Marie Curie, Paris, 235 pp. Mellinger, R.M., 1979. Quantitative X-ray diffraction analysis of clay minerals. An evaluation. SRC Rep. Saskatchewan Res. Counc. G-79, 1–46. Nelson, C.H., Baraza, J., Maldonado, A., 1993. Mediterranean undercurrent sandy contourites, Gulf of Cadiz, Spain. Sediment. Geol. 82, 103–131. Nelson, C.H., Baraza, J., Barber, J., Rodero, J., Maldonado, A., 1999. Influence of the Atlantic inflow and Mediterranean outflow currents on Late Pleistocene and Holocene sedimentary facies of Gulf of Cadiz continental margin. Mar. Geol. 155, 99–129. Ochoa, J., Bray, A., 1991. Water mass exchange in the Gulf of Cadiz. Deep-sea Res. 38, S465–S503. Pevear, D.R., Mumpton, D.R., 1989. Quantitative Mineral Analysis of clays. CMS Workshop Lectures 1. The Clay Mineral Society, Colorado. Price, J.F., O’Neil, 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. Reyment, R., Jo¨reskog, K.G., 1993. Applied Factor Analysis in the Natural Sciences. Cambridge Univ. Press, 369 pp.

98

A. Lo´pez-Galindo et al. / Marine Geology 155 (1999) 83–98

Rodero, J., Pallare´s, L., Maldonado, A., 1999. Late Quaternary sequence stratigraphy and continental shelf model controlled by eustatic and paleoceanographic events. Gulf of Cadiz, southwest Iberia. Mar. Geol. 155, 131–156. Segado, M., Gutierrez, J.M., Hidalgo, F., Martinez, J.M., Cepero, F., 1984. Estudio de los sedimentos recientes de la plataforma continental gaditana entre Chipiona y Cabo Roche. Bol. Geol.

Min. Esp. 95, 310–324. Swan, A.R.H., Sandilands, M., 1995. Introduction to Geological Data Analysis. Blackwell, Oxford, 446 pp. Viguier, C., 1974. Le Ne´oge´ne de l’Andalousie Nord-occidentale (Espagne). Histoire geologique du bassin du bas Guadalquivir. Ph.D. Diss. Univ. Bordeaux, 449 pp.