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Thalassas, 2009, 25 (2):27-40 An International Journal of Marine Sciences

HEAVY MINERALS IN MODERN SEDIMENTS OF THE BAY OF CADIZ AND THE ADJACENT CONTINENTAL SHELF (SOUTHWESTERN SPAIN): NATURE AND ORIGIN

M. ACHAB(1) & J.M.GUTIERREZ-MAS(2)

Key words: Heavy minerals, marine sediments, source area, Bay of Cadiz.

ABSTRACT The content of heavy minerals in modern sediments of the Bay of Cadiz and the adjacent continental shelf were analysed. Their nature and distribution were found to be related to the petrographic nature of the source areas and to the hydrodynamic behaviour of the mineral grains. The relationship between ultrastable and metastable minerals indicates the predominance of the first in sandy and muddy sand deposits. The distribution of this ratio permitted to distinguish both in the Bay of Cadiz and in the adjacent shelf sedimentary environments of variable degree of reworking and maturity. The factorial analysis has allowed the establishment of two mineralogical association (zircon>hornblende> andalusite and garnet> topaz> tourmaline> rutile) that are similar to those defined in different geological units close to the study area.

(1) Université Mohammed V-Agdal, Institut Scientifique, Département des Sciences de la Terre, B.P.703, Rabat, Maroc. E-mail : [email protected] (2) Universidad de Cadiz, Facultad de Ciencias del Mar, Departamento de Geologia, Apartado 40, 11510, Puerto Real (Cadiz).

The possible sources of heavy minerals to the study area are related with the Aljibe Sandstone and the calcarenites present in the Guadalete River basin, as well as to the igneous and metamorphic formations (Iberian Massif) of Guadalquivir river basin. The main transport agents are the Guadalquivir and the Guadalete s. Other possible source of supplies is related to the material eroded from exposed coastal cliffs and beaches. INTRODUCTION Heavy minerals have been traditionally used in sedimentological studies in order to determine source areas and conditions of erosional grain. They provide useful information about sediment transport trends and longshore drift processes (Li & Komar, 1992; Heikoop, 1993; Anfuso et al., 1999; Hoffman et al., 1999). Although their use is more restricted nowadays, the applications of modern techniques of mineralogical and statistical analysis allow approaching this procedure as an appropriate complement to wider sedimentological studies (Imbrie & Van Andel, 1964; Komar et al., 1989; Gutierrez Más et al., 1993).

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Figure 1: Upper Quaternary sea-level reconstructions (modified from Waelbroek et al. 2002).

The heavy fraction normally represents less than 5% of the total amount of detritic sediments; however, it can reach higher concentrations, especially in sandy beach sediments (Vatan, 1954; Perez-Mateos et al., 1982; Komar, 1998). They are composed by heterogeneous minerals groups, which are generally allochthonous and of terrigenous origin (Blatt et al., 1980). Heavy minerals are resistant to mechanical and chemical alteration processes; consequently, they are useful as natural tracers, so much for their hydrodynamic behaviour and their potential of preservation (Parfenoff et al., 1970; Pettijohn, 1975; Morton, 1985). The concentration and distribution of heavy minerals in detritic sediments is mainly controlled by the rate and the source type of sediment supply, the hydraulic processes, as well as by the grain size and the specific weight (Berquist et al., 1990; Haughton et al., 1991; Poppe, & Commeau, 1996; Dill, 1998; Ergin et

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al., 2007). Another factor controlling the distribution of the heavy minerals content is their mode of erosion, transport and sedimentation, due to their selective resistance to weathering and hydrodynamic processes (Flores & Shideler, 1978; Mezzadri & Saccani, 1988; Frihy & Dewidar, 2003). In this work, we analyse the content and distribution of heavy minerals found in the sandy fraction of modern sediments covering the Bay of Cadiz and adjacent shelf. The ratios amongst ultrastable and matastables heavy minerals, heavy minerals assemblages, as well as the relationships between them in different zones of the study area and their distribution patterns are used in order to recognise their nature and behaviour in the marine environment, source areas and transport agents, as well as their relationship with granulometric and hydrodynamic characteristics.

Heavy minerals in modern sediments of the Bay of Cadiz

STUDY AREA The study area is located in the northern sector of the Gulf of Cadiz (southwestern Spain) (Fig. 1). The Bay of Cadiz is about 28.5 km long and 13.5 km wide; the nearby continental shelf has an average width of 40 km, and the slope break occurs at 150-200 m water depth. The coastline and the continental shelf are oriented NNW-SSE, with E-W sections having a stepped profile because of the influence of both ancient and recent faults and diaclases, recognised both onshore and offshore (Gracia et al., 1999; Gutiérrez Mas et al., 2004) The tidal regime in the Bay of Cadiz is mesotidal and semidiurnal with an average amplitude of 2.39 m and maxima of 3.71 m; the associated tidal currents are considered to be responsible of fine sediment transport (Achab et al., 1998) In the nearby continental shelf the hydrodynamic system is dominated by southeastwardmoving North Atlantic Surface Water (NASW) and littoral currents, which lead to fine sediment dispersal from the Guadalquivir and Guadiana Rivers (Gutiérrez Mas et al., 2006). Basinward, the Mediterranean Outflow Water (MOW) moves westward contouring the slope (Baringer & Price, 1999). Westerly winds are the most frequent with 13.6% of occurrences. Easterly winds are also important with a frequency of 12.3% (Ramos, 1991) Sea waves (mean frequency of 6.96%) presents a relative predominance of east component, while for the Swell wave (10.26%) predominates the west component. Longshore currents are controlled essentially by northwestern surges, generating currents toward the southeast, whereas southwestern surges make it toward the north. Sedimentary strata outcropping in and around the Bay of Cadiz are of Plio-Quaternary age being constituted by clays, marls, sands, sandstones and some limestone and conglomerate levels. Other material outcropping in the Guadalete basin are upper Miocene calcarenites and marls, clays and gypsum of the Subbetic Trias (Fig.2). Upon all those materials,

Quaternary deposits are constituted by muddy marshes, beach sands and continental deposits (Zazo et al., 1983; Gutiérrez Mas et al., 1990; Dabrio et al, 2000; Achab, 2000). The recent marine sediments are siliciclastic, containing 23% of carbonates. Their origin is fundamentally biogenic (Achab et al., 2005), upon being constituted by marine skeletal remains. The most important mineral in the sand fraction is quartz (average of 45%), indicating that source areas are rich in this mineral (Achab et al., 2002). MATERIALS AND METHODS This study has been based on the analysis of modern sediments of the Bay of Cadiz and the adjacent continental shelf, collected with a Van Veen dredge (Fig. 1). Sample positioning was achieved by Differential Global Position System (DGPS). Granulometric (211 samples) and mineralogical (90 samples) analysis were carried out to establish the sediment facies distribution and the mineralogical composition. The grain size analysis has been executed in two steps: i) Humid separation of coarse and fine fractions using a sieve of 0.063 mm, ii) coarse sediments were dry-sieved during 15 minutes. The heavy fraction was separated by heavy liquids technique (bromoform, d=2.89 at 20°C), after the elimination of carbonates with HCl. The mineralogical composition was determined with a Philips PW-1710 X-ray diffractometer. Quantitative analysis of heavy minerals was calculated by the classic method of peak area measurements, taking into account the different reflection capacities of the minerals (Schultz, 1964). The Q-mode Factor analysis (Principal Components Analysis) was used to establish the relationship between the different heavy minerals and their associations (Mezzadri & Saccani., 1988). The method by Imbrie (1964) was used for this analysis, which is based on the samples similarity matrix. The associations obtained by this analysis are based on those variables showing the highest scores in each factor; the factor scores represent the weight or influence of each variable and components within the corresponding factor.

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Figure 2: Geologic map of the study area

RESULTS AND DISCUSSION Sediment and heavy fraction content We can divide the Bay of Cadiz in two sectors taking into account the grain size and deposit distribution (Fig.3): In the outer bay, sand is the dominant fraction, especially in the littoral zone. Muddy sands sediments occupy the central and eastern parts of the outer bay; those sediment facies extend into the 20-30 m deep inner shelf. In the inner bay, muds and sandy muds are the dominant fractions, which are agreement with dominance of low-energy processes active in this, sheltered setting (Achab et al., 1999). On the continental shelf, two sectors can be distinguised (Fig.3): a prodeltaic muddy zone to the north, related to supplies coming from the Guadalquivir River, and a southern zone covered by relict sands (Gutiérrez Mas et al.,

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1996). Those different depositional settings control the distribution of heavy minerals in sediments, in relation with selective classification processes and with the hydrodynamic behaviour of the mineral grains and their physical-chemical stability (Frihy & Dewidar, 2003) The study of the heavy fraction present in the modern sediments of the Bay of Cadiz and the adjacent continental shelf indicates relatively low concentrations (3%) in comparison with the total sample; it follows that their industrial exploitation is not economically profitable, in spite of previous exploratory studies (FOMAR, 1972). The low concentration of the heavy fraction in the sediments of the Bay of Cadiz indicates a variable degree of sediment reworking and maturity. In the study area, the lowest values of this fraction occur in the inner bay, in the middle part of the outer bay and to the north

Heavy minerals in modern sediments of the Bay of Cadiz

Figure 3: Grain-size distribution patterns in the study area

of the continental shelf between 30 and 70 m water depths, in coincidence with dominance of muddy and sandy-mud facies (Fig. 4). The intermediate and highest concentrations are found in coastal and sublittoral sandy facies of the bay at water depths ranging between 5 and 15 m, and to the south and deeper zones of the continental shelf. The relationship between ultrastable (Ult) and metastable (Met) minerals has been established in order to identify areas where the sediments show a high degree of reworking and mineralogical maturity. The relative abundance of ultrastable minerals in the sediment indicates prolonged erosion and a high degree of reworking; as a result, those minerals constitute the basis of many mineralogical assemblages (Morton & Hallsworth, 1994). In contrast, metastable minerals are less stable, and their occurrence may be related to the proximity of the mother rock or a high sedimentation

rate that would prevent from significant alteration (Pettijohn, 1975; Blatt et al., 1980). The distribution of the ultrastable/metastable ratio allows the distinction of several sectors, both in the Bay of Cadiz and in the adjacent shelf. (Fig. 5). In the Bay of Cadiz, ultrastable minerals present average concentrations of 24.5%, while metastable minerals reach 57%. The relationship between ultrastable and metastable minerals indicates the predominance of the first in sandy and sandy-muddy deposits in coastal and sublittoral areas. However, in the central sector of the outer bay, this ratio presents low values, due to the lower content of ultrastable minerals, indicating a less significant degree of sediment maturity. On the adjacent continental shelf, the relation Ult/Met indicates an intermediate and higher content of the first in sediments located in front of the

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Figure 4: Distribution patterns of heavy minerals fraction in the study area

Sanctipetri tidal creek mouth and in front of the Bay of Cadiz, at water depths ranging between 30 m and more than 70m, indicating the mineralogical maturity of these deposits (Fig.5) (Gutierrez Mas. et al., 1997). The low values of ultrastable/metastable ratio are given in the rest sectors of the continental shelf, indicating the influence of the Guadalquivir River supplies, which provides sediments with variable degrees of maturity and reworking. Heavy mineral distribution The results of the mineralogical analysis show that the most important minerals present in the heavy fraction are zircon (12.3%), garnet (8.6%), hornblende (8.5%) and rutile (6.6%). The rest of heavy minerals appears with contents between 2.2 and 5.6%. The distribution of the different heavy minerals is shown in Figure 6 & 7

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The Zircon presents an heterogeneous distribution, with average concentrations of 10%. The minimum values (<5%) occur in the continental shelf, while the maxima (20%) appear in the Bay of Cadiz and in front of the Sanctipetri tidal creek mouth. The tourmaline shows average concentrations of 5%, with maximum values of 15% in the eastern sector of the outer bay while the minimum values (<5%) are mainly found in the continental shelf. The rutile appears with an average content of 7%. The maximum values (15%) occur in the continental shelf up to 70 m water depth, whereas minimum values (<5%) are found in the Bay of Cadiz and in front of the Sanctipetri tidal creek mouth. The andalusite presents maximum values of 15%, generally inside the bay; the lower values (<5%) are found in the continental shelf and part of the outer bay. The average concentration in garnet (10%) is quite high if compared with the andalusite. The minimum contents occur in deep settings of the

Heavy minerals in modern sediments of the Bay of Cadiz

Figure 5: Distribution of ultrastable/metastable ratio of heavy minerals in the study area

continental shelf, while the maximum values appear to the north of the continental shelf and in front of Rota city. The topaz shows low average concentrations (<5%), with higher values (10%) in the central part of the continental shelf. The minimum values (<5%) are found in the outer bay and particularly to the north of the continental shelf. The hornblende presents an heterogeneous distribution, with an average content of 7%. The maximum values (15%) are found in the Bay of Cadiz and offshore. The minimum concentrations (<5%) occur in the continental shelf and part of the outer bay. The epidote shows an average content of 6%, with lower concentrations (<5%) on the central continental shelf, while the highest concentrations (20%) are found to the south of the continental shelf and part of the Cadiz Bay. With regard to the origin of the different heavy minerals described above, the zircon seems to be

related to the Guadalete River supplies, as this large river drains subbetic materials in its upstream areas (Mabesoone, 1966; Viguier, 1974), taking into account the high concentrations of zircon in the upper Miocene calcarenites and the Aljibe sandstones (Moral Cardona, 1994). The Guadalquivir basin also provides grains of zircon with low degree of reworking. These grains can reach the bay by the effect of littoral currents moving towards the southeast. The origin of the tourmaline also appears to be related to contributions of the Guadalete River, as high tourmaline concentrations are found in the Guadalete River terraces (Moral Cardona, 1994). The rutile was found to be related to fluvial inputs from the Guadalquivir and the Guadalete s. This last contributes with reworked grains coming from the erosion of their terraces (Moral Cardona, 1996). Another possible source of this mineral is the erosion of coastal cliffs and submarine outcrops.

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Figure 6: Distribution maps of zircon, hornblende and andalusite in the study area

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Figure 7: Distribution maps of garnet, rutile and tourmaline in the study area

Heavy minerals in modern sediments of the Bay of Cadiz

Table 1: Mineralogical assemblages defined in continental and marine areas close to the study area (A: Andalusite; E: Epidote; G: Garnet; R: Rutile; H: Hornblende; T: Tourmaline; Top: Topaz; S: Staurolite; Z: Zircon)

The origin of andalusite in the study area could be related to two different sources: by one hand from the Aljibe sandstones and the upper Miocene calcarenites, and from Pliocene sands and Plioquaternary materials which are rich in andalusite and are eroded by the Guadalete River amongst others (Mabesoone, 1963, 1966; Mateos Perez et al., 1982; Moral Cardona et al., 1996). On the other hand, the andalusite could be derived from metamorphic areas of the Iberian Massif drained by the Guadiana River. The distribution of garnet suggests an origin related to the erosion of upper Miocene calcarenites present in the rivers basins pouring into the study area and that also crop out in several coastline and shelf bottom locations (Mabesoone, 1966; Mateos Perez, 1982; Moral Cardona et al., 1996; Gutierrez Más. et al., 1997). The epidote appears in the drainage basin of the Guadalete River, mainly in its upstream part. This mineral is found in heavy mineral assemblages

of Plio-Quaternary red sands and in recent beach deposits near to Cadiz (Mabesoone, 1963, 1966; Moral Cardona, 1994). Another possible source of epidote is related with the Iberian Massif, from where they could reach the Gulf of Cadiz through the Guadalquivir River (Meliéres, 1974; Gutierrez Más. et al., 1993). Minerals assemblages and source areas Heavy minerals assemblages have been established to identify source areas of sediments, petrographic provinces and sediment maturity (Komar et al., 1989; Morton & Hallsworth, 1999). In the study area, Q-mode factor analysis results provide two factors explaining the 100% of the data variance (Fig.8). The Factor 1 (52% of variance) represents the heavy mineral association made up of Zirc on>Hornblende>Andalusite (Z>H>A). The second

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Figure 8: Factor scores (heavy minerals assemblages) of factor1 and factor2 obtained in the study area, from Q-mode Factor Analysis

association obtained by the factor 2 (48% of variance) is represented by Garnet>Topaz>Tourmaline>Ruti le (G>Top>T>R). Both mineralogical associations correspond to petrographic provinces that coexist in the study area and have been reported by several authors (Mabesoone, 1966; Viguier, 1974; Zazo, 1981; Gutiérrez Más et al., 1997). This fact has been related with the diversification of source areas and the ways of sediment transport. The first association (Zircon> Hornblende> Andalusite) appears to be representative of sources that pour directly into the Bay of Cadiz. The second one (Garnet> Topaz> Tourmaline> Rutile), is associated with the sediment supplies coming from allochthon areas to the Bay of Cadiz, discharging before into the Gulf of Cadiz, such as those coming from the contributions of the Guadalquivir and Guadiana Rivers, which reach the Bay through littoral currents.

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In the continental shelf, Gutierrez Más et al (1993, 1997) found the association constituted by E> G>> R (Table.1). In the alluvial deposits of the Guadalete river the predominant association is E> Z> G (Mabesoone, 1966). In the Plio-Quaternary materials of the Bay of Cadiz the mineralogical association is constituted by E> A> T> Z+G.; A> T> S predominates in red sands, while in the Guadalete river basin the established association is A > E> T+G (Zazo, 1981). On soils of alluvial terraces of the Guadalete river the association is made up of epidote, garnet, tourmaline, andalousite and rutile (E> G> T> A+R), similar to that defined by Moral Cardona (1994). The association A> T> S> Z+R predominated in ancient alluvial materials, similar to that found in the red sands and in the dunes formations (A> T> E+G+S). In the Pleistocene red sands of the Guadalete river (petrographic province of Jerez), the mineralogical association defined by

Heavy minerals in modern sediments of the Bay of Cadiz

Mabesoone (1963, 1966) is made up of epidote, garnet and andalousite (E> G> A), genetically related to the Betic Ranges and Sierra Morena (Iberain Massif). In the Neogene materials of the Guadalquivir Depression, Viguier (1974) divided the petrographic province of Jerez proposed by Mabesoone (1966) in two subprovinces: one close to Medina Sedonia (province of Cadiz) made up of zircon, garnet, and tourmaline (Z> T> G), and the other comprising the Guadalete basin, with epidote, garnet, andalusite and tourmaline (E> G> A+T). In coastal areas near to Cadiz, E> G> A is the dominant association (Mabesoone, 1963); towards the south between Trafalgar and Tarifa the mineralogical association is A> G> E according to Perez Mateos et al. (1982). On the basis of the heavy minerals assemblages defined in different geological units that crop out near to the Bay of Cadiz and the Guadalete River, some coincidences are observed amongst these and the associations found in the Bay of Cadiz sea floor (Table.1). The zircon is the predominant mineral in the first association found in the study area; it also appears as the first mineral in the association defined by Viguier (1974) in the petrographic province of Medina Sidonia., and it is the second mineral in the association defined by Mabesoone (1966) in the lower basin of the Guadalete River. The garnet, which is the dominant mineral in the second association, is also present as the second mineral in the majority of mineralogical assemblages established by other authors (Table.1) including that established by Gutiérrez Más et al (1997) in recent sediments of the continental shelf. The coexistence in the study area of two mineralogical associations with the same degree of significance could mean the existence of more than one sediment sources. We tentatively relate those associations to the present-day situation and to a previous fluvial network setting. These sediments were deposited in environments close to the bay and are now being eroded and deposited in the bay bottom, leading to the mixture of mineralogical associations,

as demonstrates the similarity of these associations with those found in sedimentary formations of the geological record near to the Bay of Cadiz. CONCLUSIONS The nature of heavy minerals identified in modern sediments of the Bay of Cadiz and the adjacent continental shelf was found to be related to the petrographic nature of the source areas and to the conditions of erosion-transport-deposition undergone by the mineral grains. Sedimentological and hydrodynamic factors have influenced the distribution of heavy minerals, which in turn is controlled by the geographical situation of different supply sources in relation to the hydrodynamic agents. The relationship between ultrastable and metastable minerals permitted to distinguish sedimentary environments of variable degree of reworking and maturity. A multivariate factorial analysis (Q-mode) applied to the heavy minerals of the Bay of Cadiz and nearby continental shelf sediments has allowed the establishment of two mineralogical associations that are similar to those defined in different geological units close to the study area. The first association (zircon> hornblende> andalusite) is more representative of sources pouring directly into the Bay of Cadiz. The second (garnet> topaz> tourmaline> rutile) represents supplies coming from allochthon areas to the Bay of Cadiz, such as those coming from the Guadalquivir and Guadiana Rivers and others. Regarding the origin of heavy minerals, we suggest several supplies sources: on the one hand, the Guadalete River, which supplies mainly reworked and very mature sediments, coming from the erosion of detritic sedimentary formations that crop out in its basin (Aljibe sandstones, upper Miocene calcarenites, Middle subbetic and Pliocene and Pleistocene sands). On the other hand, the Guadalquivir River in whose basin igneous and metamorphic formations (Iberian Massif) crop out. This river provides sediment of low degree of maturity that can reach the continental

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shelf and the Bay of Cadiz by littoral currents moving towards the southeast. Another possible source of supplies is related to the material eroded from exposed coastal cliffs and beaches. ACKNOWLEDGMENTS This paper was supported by the project MAR 980796 and 2002-01142/MAR of The Interministerial Commission of Science and Technology (CICYT) of the Spanish Government. REFERENCES

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(Received: January, 13, 2009; Accepted: April, 3, 2009)

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