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Clay minerals from the sedimentary cover from the Northwest Iberian shelf
Progress in Oceanography 52 (2002) 233–247 www.elsevier.com/locate/pocean
Clay minerals from the sedimentary cover from the Northwest Iberian shelf A. Oliveira a,∗, F. Rocha b, A. Rodrigues a, J. Jouanneau c, A. Dias d, O. Weber c, C. Gomes b a
b
Instituto Hidrogra´fico, Rua das Trinas, 49, 1249–093 Lisboa, Portugal Univ. Aveiro, Dep. Geocieˆncias, Campus de Santiago, 3810 Aveiro, Portugal c DGO–UMR , CNRS 5805, Av. des Facultes, 33405 Talence Cedex, France d UCTRA, Univ. do Algarve, 8000 Faro, Portugal
Abstract The Northern Iberian margin is a typical example of a continental margin subjected to seasonal highly energetic regime (waves and tides) and receiving inputs of continental sediments via riverine discharges. The principal goal of this study has been to use clay minerals as indicators of sedimentary dynamics in the open shelf system. The distributions of clay mineral in the top layer of the sedimentary cover are shown to be related to their continental sources, but also reflect the influences of winter storms and longshore currents in determining the pathways of sediment transport. The mineralogical composition of the material issuing from the rivers is very similar to the general mineralogical composition of the fine fractions of the seabed sediments. Those deposits that are directly influenced by riverine discharges have higher contents of kaolinite (⬎20%), whereas those that are not have higher contents of illite (⬎80%). The available data indicate no significant quantities of terrigenous particles are being discharged from the Spanish rias. Therefore, we conclude that physical processes are controlling the clay mineral distributions and that, despite contributions from the Minho River, the main source of fine detrital particles to the shelf region is the Douro River discharge. These particles settle on the middle shelf, below the 60 m isobath. During storm events these particles are re-suspended and advected northwards to the Galician shelf or into deeper domains. Thus the distributions of the clays indicate there is a net transport of fine sediments both northwards and off-shelf. 2002 Elsevier Science Ltd. All rights reserved.
Contents 1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
2. Regional setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 2.1. Morphology and geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 2.2. Oceanography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
∗
Corresponding author.
0079-6611/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 0 2 ) 0 0 0 0 8 - 3
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3.
Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
4.
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
5. Discussion . . . . . . . . . . . . . . . . 5.1. Illite . . . . . . . . . . . . . . . . . . 5.2. Kaolinite . . . . . . . . . . . . . . . 5.3. Chlorite . . . . . . . . . . . . . . . . 5.4. Smectite . . . . . . . . . . . . . . . . 5.5. Kaolinite (Kt) / Illite (Ill) ratio . . 5.6. General overview of clay minerals 6.
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Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
1. Introduction Clay minerals have been investigated worldwide using X-ray diffraction (XRD) techniques. Previous studies have shown that the modern clay mineral distribution in the Atlantic Ocean is controlled mainly by the zonation of climate and weathering over the adjacent land masses, implying that most of the clay minerals are of terrigenous origin (Biscaye, 1965; Griffin, Windom, & Goldberg, 1968; Chamley, 1989). Clay minerals have been widely used in studies of sedimentary dynamics studies. Their physical and chemical properties make them good indicators of sediment sources, and their distribution patterns in the sedimentary basins can be indicative of the main transport processes and pathways. Despite our detailed knowledge of the general distribution patterns of recent clay minerals in the N.E. Atlantic (Northern Europe and Gulf of Biscay) (Latouche, Jouanneau, Lapaquellerie, Maillet, & Weber, 1991), data on clay distributions on the Iberian continental margin has remained inadequate. This paper uses clay minerals as indicators of sedimentary dynamics on the open shelf system. The study area is the northwestern Iberian shelf, between 41°N and Cape Finisterre.
2. Regional setting The northwestern Iberian margin, especially in its shallowest domain, is characterised by complex sedimentary dynamics. This complexity arises because of the varied morphology and geology of the region, and the diversity of the oceanographic processes to which it is exposed. 2.1. Morphology and geology The continental shelf is narrow varying in width between 35 and 50 km, and has several unusual features, which interact with the sedimentary dynamics (Fig. 1). The coastal outline is very irregular because of its geology. It is characterised by an outcrop of old Hercynian formations, highly metamorphosed and fractured Palaeozoic granites, greywackes and schists with a general NNW–SSE orientation (Ribeiro, Antunes, Ferreira, Rocha, Soares, Zsbyszewski, Moitinho de Almeida, Carvalho, & Monteiro, 1979; Wilson, Hiscott, Willis, & Gradstein, 1989). Five main rivers discharge along this sector of the Iberian margin, the Douro, Ave, Ca´ vado, Lima and Minho Rivers. Fluvial drainage has been established, from the highlands in the interior down to the coastline, cutting through these old geological formations. The Douro is the largest of these rivers with an
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Fig. 1. Regional setting: (a) Bathymetry of the continental shelf; (b) Continental geological setting (adapted from Julivert et al., 1980, in Cascalho, 2000).
annual mean discharge of 8.2 x 109m3 and its catchment covers 98 x 103 km2. When these five Portuguese rivers flood during winter, the coastal waters are turned brown by the large amounts of suspended sediments being discharged in their plumes. In contrast in the Spanish sector there are four rias (Vigo, Pontevedra, Arosa and Muros) which are drowned river systems deeply incised into the coastline, but which, as we shall see below, tend to trap any fluvial supplies they receive rather than discharge them to the shelf seas. Basement rock outcrops extend out on to the inner shelf region, making the seabed rough and irregular (Vanney & Mougenot, 1981). These outcrops are bare of recent sediments. The Porto canyon and other more minor slope valleys are important features that have been created by geological structures, i.e. Mesozoic strike-slip faults (Boillot, Dupeuble, Hennequin-Marchand, Lamboy, Lepretre, & Musellec, 1974). Along the sector of the Portuguese continental shelf we studied, the distribution of the main deposits is well known (Dias & Nittrouer, 1984; Dias, 1987; Magalha˜ es & Dias, 1992). Generally the sedimentary cover is coarse-grained, but includes two well-defined muddy areas, where silt and clay components dominate (frequently having mud contents up to 90%), although their clay components (⬍2 µm) are small ⬍10% (Fig. 2). These two muddy areas (Douro and Galicia mud patches) experience the highest accumulation rates in the region (Jouanneau et al., 2002). The mineralogical composition of the silt fraction (⬍63 µm) has been described by Oliveira, Rocha, Rodrigues and Dias (2000) in non-orientated powders. They found that this fraction is dominated by particles of quartz, with average content of 40% on the Portuguese shelf, but
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Fig. 2. Location of the collected samples for CORVET 96 cruise (dots) and GAMINEX cruise (stars). The river sampling points are represented by triangles. Base map shows the distribution of fine sediment (⬍63µm) percentage (adapted from Dias et al., 2002).
increasing to a maximum of 85% (average of 60%) in the north, between Ria Muros and Ria Arosa. The other minerals in this fraction were mica (14%), plagioclase (12%), K-feldspar (10%) and calcite (9%), all of which occurred at higher concentrations on the Portuguese shelf (Oliveira et al., 2000). 2.2. Oceanography The northwestern Iberian margin is a typical example of a continental margin that is subjected to a highly energetic hydrodynamic regime of waves and tides, and is also strongly influenced by upwelling events in summer (Wooster, Bakun, & McLain, 1976; Fiu´ za, Macedo, & Guerreiro, 1982; Fiu´ za, 1983) and downwelling events in winter (Drago et al., 1998; Vitorino, Oliveira, Jouanneau, & Drago, 2000). It receives large riverine discharges particularly in winter, and is subject to poleward-flowing slope currents year-round (Frouin, Fiu´ za, Ambar, & Boyd, 1990; Haynes & Barton, 1990). Such phenomena generate
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considerable spatial and seasonal variability in particle fluxes and inverse circulation over the continental shelf (see Vitorino, Oliveira, Jouanneau, & Drago, 2002). Over the Portuguese shelf, the major river estuaries discharge very close to each other (⬇20 km), and so not only do the coastal waters tend to have relatively low salinities but are also quite turbid (Oliveira, Vitorino, Rodrigues, Jouanneau, Dias, & Weber, 2002). During winter the turbid plume of the most important river, the Douro, can extend for up to 14 km northwestwards out from the river mouth (Drago et al., 1998). On the other hand, the Rias to the north are at present important depositional areas, in which the majority of the fine particles they receive are trapped and there is no major transport out over the shelf (Rey Salgado, 1993). The importance of physical forcing in the sediment dynamics in the continental shelf has been quantified in several complementary studies (Vitorino, Oliveira, Jouanneau, & Drago, 2000; Jouanneau, Weber, Drago, Rodrigues, Oliveira, Dias, Garcia, Schmidt, & Reyss, 2002; Oliveira, Vitorino, Rodrigues, Jouanneau, Dias, & Weber, 2002). During stormy periods, with typically waves of 12s period and mean heights of 7.5m, these authors reported evidence of resuspension of the muddy deposits over the mid-shelf areas. Such events intensify the bottom turbid layer that covers the entire continental shelf and extending out over the upper slope region (Oliveira, Vitorino, Rodrigues, Jouanneau, Dias, & Weber, 2002).
3. Materials and methods Sediment samples from the northern Portuguese and Galician continental shelf were recovered during CORVET (November 1996, NRP Almeida Carvalho) and GAMINEX (June 1998, FV Coˆ te de la Manche) cruises (Fig. 2). Sediment samples were preferentially collected from the muddy deposits on the continental shelf. During the first cruise a Mark I multicorer (4 subcores) was used to retrieve undisturbed samples from the uppermost sediment layer, 1–2 cm thick. A Smith-McIntyre grab was also used and sub-samples taken using PVC tubes, which were then stored. During the GAMINEX cruise, gravity core and Reineck box core samples were collected from both the Portuguese and the Galician shelves (Fig. 2). Only the uppermost 1–2 cm of the surface sediments was analysed for this study, the amounts of sediment being used varying between 2 and 8g. Bottom sediment samples from northern Portuguese rivers had been collected near their outlets in February 1993, using a Petit-Ponar grab, and these samples were re-analysed with the shelf samples, using the same methods. Each sample was initially disaggregated using ultra-sound. After wet sieving of the sand (0.063–2 mm) and the silt (2-63 µm) fractions, the clay fractions (⬍2 µm) were separated by sedimentation according to Stoke’s law, using 1% sodium hexametaphosphate solution to avoid flocculation. For the preparation of preferentially oriented clay mounts of the ⬍2µm fraction, the suspension was placed on a thin glass plate and air-dried. XRD measurements were performed using a Philips diffractometer, with CuKα radiation. Scans were run between 2° and 18° 2θ in the air-dry state and after glycerol saturation and heat treatment (300 and 500°C). Peak areas of the basal reflections of the main clay minerals were calculated and weighted by empirically estimated factors (Schultz, 1964; Thorez, 1976; Rocha, 1993). The clay minerals identified were: illite ˚ peak, in natural specimen); kaolinite (7A ˚ peak, in natural specimen after removal of the chlorite (002) (10A ˚ peak, in 500°C heated specimen) and smectite (17A ˚ peak, in glycolated specimen). The peak), chlorite (14A ˚ peak area was divided by 4, the illite peak area by 0.5, the kaolinite 7 A ˚ peak area by 1 smectite 17 A ˚ (500°C) peak area by 0.75. and the chlorite 14A ˚ (002) reflection was also considered to estimate the ‘octahedral character’ using the 5A ˚ /10A ˚ The illite 5A peak intensity ratio. This quick method can be useful to identify clay mineral sources and transport paths ˚ /10A ˚ values of clay minerals in marine sediments (Moriarty, 1977). According to Esquevin (1969), high 5A
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(⬎0.4) correspond to Al-rich (muscovitic) illites whereas Fe and Mg- rich (biotitic) illites have values below 0.15. Kaolinite and illite crystallinity indices were determined. For the estimation of the kaolinite crystallinity, ˚ peak, and the height of this peak, in airthe ratio between the width measured at half-height of the 7A ˚ peaks. For illite crystallinity dry aggregates was used, after the decomposition of chlorite and kaolinite 7 A ˚ peak) was used. the Kubler (1964)/Segonzac (1969) index (the width measured at half-height of the 10A Well-ordered illite shows symmetrical and narrow basal reflections and low values of the Kubler/Segonzac index.
4. Results The surficial sediments studied on the continental shelf proved to be mainly siliciclastic, containing a low percentage of bioclastic carbonates (10% on average). Representative diffractograms of marine and river clay fractions are presented in Fig. 3. A rather uniform clay mineral association characterised the superficial sediments of the NW Iberian margin. The typical clay mineral association expressed in terms of illite+kaolinite+chlorite+smectite summed to 100%, was 70–85% illite, 15–25% kaolinite, 5% chlorite and traces of smectite (Table 1). Illite was identified in all marine and river samples. Its content exceeds 70% in all marine samples and 60% in all river samples except in one from the River Ca´ vado (only 24%). Kaolinite was also identified in all samples but at lower percentages than illite except in the one river sample (74%). Chlorite content of some of the river and marine samples was only a few percent, but was 5–10% in about half of the samples. It
Fig. 3.
Diffractograms of selected clay fraction samples (air-dry, glycolated and 500°C): (a) river samples; (b) marine samples.
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Table 1 Mineralogical composition of the clay fraction in surface marine and river sediments from the study area (NW Iberian Margin). Samples location in Fig. 2. ILL (Illite), KT (Kaolinite), SM (Smectite), CHL (Chlorite), C (Corvet cruise), G (Gaminex cruise) Marine samples
Water depth ILL % (m)
KT %
SM %
CHL %
ILL Crist. (2°θ)
ILL (002)/(001)
C1 C2 C3 C4 C6 C7 C8 C9 C11 C12 C13 C14 C15 C17 C18 C19 C21 C22 C23 C24 C25 C27 C29 C33 C34 C36 C40 C42 C44 G1 G4 G6 G7 G9 G10 G12 G15 G17 G18 G20 G21 G23 G26 G28 G32 G33 G34 G36 G42
67 80 100 148 68 67 87 106 122 94 79 75 76 104 83 72 68 77 94 73 81 86 96 115 115 95 121 61 40 128 142 211 103 115 49 102 125 169 133 115 113 39 99 138 117 108 115 98 90
16 16 17 17 16 19 22 18 23 22 19 25 24 17 16 16 15 16 18 15 15 17 15 14 16 19 14 15 19 13 17 16 20 23 24 16 24 23 12 13 18 17 14 19 20 15 16 19 18
0 0 ~1 2 ~1 ~1 ~1 2 0 ~1 0 0 ~1 ~1 2 ~1 ~1 ~1 2 2 5 ~1 2 2 3 ~1 10 3 ~1 0 0 0 0 0 0 0 0 ~1 0 0 0 0 0 0 0 0 0 0 0
4 4 0 3 4 4 5 5 5 4 3 3 4 7 7 6 7 3 5 5 4 6 5 5 4 5 4 5 5 7 1 4 2 2 2 5 3 2 4 5 6 5 6 6 4 3 4 6 4
0.15 0.16 0.28 0.30 0.18 0.15 0.16 0.15 0.13 0.15 0.15 0.18 0.18 0.15 0.15 0.16 0.20 0.15 0.20 0.15 0.20 0.16 0.15 0.20 0.15 0.15 0.20 0.20 0.20 0.20 0.20 0.20 0.15 0.20 0.15 0.20 0.20 0.20 0.15 0.15 0.20 0.15 0.15 0.20 0.12 0.20 0.15 0.15 0.15
0.51 0.05 0.53 0.04 0.65 0.27 0.55 0.27 0.44 0.06 0.50 0.04 0.57 0.04 0.48 0.03 0.50 0.04 0.58 0.04 0.58 0.04 0.55 0.05 0.44 0.06 0.49 0.09 0.53 0.05 0.47 0.07 0.46 0.05 0.47 0.12 0.43 0.08 0.46 0.04 0.45 0.05 0.42 0.09 0.44 0.08 0.54 0.05 0.46 0.10 0.51 0.05 0.48 0.13 0.38 0.15 0.47 0.15 0.44 0.08 0.73 0.17 0.76 0.05 0.57 0.11 0.56 0.17 0.50 0.28 0.77 0.06 0.50 0.08 0.29 0.43 0.07 0.38 0.04 0.52 0.06 0.48 0.08 0.57 0.09 0.50 0.07 0.49 0.04 0.47 0.05 0.48 0.06 0.50 0.05 0.41 0.07 (continued on next page)
80 80 82 77 79 76 72 75 72 73 78 72 71 75 78 77 77 79 75 78 76 76 78 79 77 75 72 77 75 80 82 80 78 72 74 79 73 74 84 82 76 78 80 75 76 82 80 75 78
KT Crist.
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Table 1 (continued) Marine samples
Water depth ILL % (m)
KT %
SM %
CHL %
ILL Crist. (2°θ)
ILL (002)/(001)
KT Crist.
G44 G46 G47 G50 G53 G54 G55 G56 River samples Minho 1 Minho 2 Lima 1 Lima 2 Ca´ vado 1 Ca´ vado 2 Ave 1 Ave 2 Douro 1 Douro 2
69 91 107 79 95 160 75 55
75 76 75 79 80 80 82 84
19 17 22 16 15 14 14 13
0 ~1 2 0 0 2 ~1 0
6 6 1 5 5 4 3 3
0.20 0.15 0.15 0.20 0.15 0.20 0.15 0.20
0.47 0.45 0.72 0.44 0.51 0.45 0.50 0.49
0.08 0.09 0.30 0.06 0.12 0.18 0.14 0.07
67 59 62 64 70 24 65 62 70 61
27 39 27 34 21 74 33 36 23 28
~1 0 2 0 ~1 ~1 ~1 ~1 ~1 ~1
5 2 9 2 8 ~1 ~1 ~1 6 10
0.20 0.30 0.25 0.30 0.25 0.25 0.30 0.20 0.20 0.20
0.44 0.57 0.35 0.35 0.38 0.46 0.32 0.27 0.38 0.42
0.02 0.12 0.16 0.20 0.07 0.08 0.18 0.40 0.06 0.10
was only in vestigial quantities in the two samples from River Ave. Smectite was definitely detected in two marine samples, ~10% in the Portuguese outer shelf (sample c40) and ⬍5% off the Lima river (sample c25). Its presence was suspected (⬍3%) in a few more marine samples but was completely absent from samples collected from the Galician shelf.
5. Discussion 5.1. Illite The sediments studied were characterised by very high contents of illite (⬎70%); this reflects the moderate climatic conditions prevailing in the source areas and the abundance of igneous (granitic rocks) and metamorphic rocks of Palaeozoic age (schists, gneisses, micaschists and greywackes) in the region. Such illite abundance, in particular if characterised by a well ordered structure, is considered to be evidence for a temperate climate, less warm and wet, that does not favour hydrolysis, so that physical alteration prevails over chemical alteration (Gala´ n, 1986; Chamley, 1989; Weaver, 1989). The illite showed an antiphase behaviour relative to kaolinite (Fig. 4 and 6) because of the low content of the other two clay minerals (a ˚ /10A ˚ ratio (Esquevin index) with values ⬎0.4, two clay locked system). All the marine samples have 5A corresponding to Al-rich illites (muscovite type) and reflecting a granitic provenance. The riverine samples were mostly in the range of 0.27–0.38 (Table 1). A clear east-west trend was observed between the rivers and outer shelf samples. The samples in general, showed low values of the Kubler crystallinity index for illite, indicating well-ordered structures and very low-grade chemical degradation both in the source-areas and during transportation and sedimentation. Fig. 5 shows that the sediments from the shelf contained illites displaying higher crystallinity than the illites from river sediments, so illite crystallinity increases within the marine environment. This can be explained by the capacity of illite in the marine environment, to fix new ions available in
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Fig. 4.
241
Illite distribution in surface sediments (0–1 cm) of the northwestern Iberian shelf.
seawater (Millot, 1964), since Fe and Mg tend be replaced by K and Al, increasing illite crystallinity (Nemecz, 1981). 5.2. Kaolinite Kaolinite is the dominant clay mineral in oceanic sediments at low latitudes where the continental sources are affected by intense chemical weathering. This led Griffin, Windom and Goldberg (1968) to describe kaolinite as the ‘low-latitude mineral’. Chemical weathering of continental rocks is favoured by warm climate (temperate to tropical) and high rainfall (⬎2000 mm/a). In the interior of northern Iberian, the extremely fractured basement (granite and gneiss) is affected by intense chemical weathering which yields important residual kaolinitic deposits, such as at Alvara˜ es, SE of Viana do Castelo, Sra. da Hora, north of Porto and Barqueiros, ~10 km from the Ca´ vado river mouth (Gomes, 1989; Gomes, Lopes Velho & Sa´ Delgado, 1990). These deposits are eroded by rain and carried
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Fig. 5.
Esquevin (1969) graphic applied to illites from sediments of the NW Iberian continental shelf.
via the streams and rivers into the oceanic systems. This explains the high kaolinite content of the clays found at the river mouths (especially in the Ca´ vado River). On the shelf, the kaolinite content of the sediments (Fig. 6) is closely dependent upon river discharges and distance from the sediment source at the river mouth. In fact, despite the low kaolinite contents (only 18%) generally occurring in the shelf sediments, higher values were found in the middle shelf in the Douro mud patch, and also north of Ria Arosa in sandy sediments (Fig. 6). Kaolinite crystallinity values (Table 1) are higher on the inner to middle shelf particularly, on the NW Portuguese shelf, but decrease both westwards and northwards to the Galician shelf. The areas where kaolinite shows higher crystallinity were directly supplied by kaolinite from the rivers. Kaolinite in the sediments from the Douro deposit, the southern mud patch at midshelf exhibited the same crystallinity values as the kaolinite in the sediments being transported by the rivers. The second sector, where kaolinite concentrations were high, was located on the Galician shelf, well away from any riverine source. There the kaolinite had low crystallinity, which can be explained by the chemical degradation processes occurring in the marine environment as a result of the higher pH and salinity, condition in which kaolinite is less stable. So once it reaches the oceanic environment, the crystallinity of kaolinite decreases as the residence time increases (Caille`re, He´ nin, & Rautureau, 1982; Gomes, 1989), which is in contrast to illite, whose crystallinity increases in the marine environment. It may be that the factors leading to the development of the two kaolin–rich areas are contrasting. In the Douro region the kaolin richness result from the entrapment of fresh riverine supplies, whereas in the more northern Galician area the enrichment results from the selective winnowing out of illite, leaving a richer residue of older kaolinite. 5.3. Chlorite Chlorite was a minor constituent of the clay fractions in the shelf sediments, with contents ⬍7%. The higher values in the inner and middle shelf were related to the proximity of the river mouths (Fig. 7). Generally chlorite is the dominant clay mineral at high latitudes (Griffin et al., 1968). It is usually less resistant to hydrolysis than illite (Chamley, 1989), so in warmer and wetter climates, chlorite is easily broken down by chemical weathering. Oceanic systems, outside high cold latitudes, are generally supplied
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Fig. 6.
243
Kaolinite distribution in surface sediments (0–1 cm) of the northwestern Iberian shelf.
with very low quantities of chlorite. Chlorite varies antagonistically to kaolinite (Windom, 1976; Chamley, 1989), and has been termed the ‘high-latitude clay mineral’ by Griffin et al. (1968). So high chlorite contents reflect conditions of low chemical alteration and close proximity to source rocks bearing high chlorite contents, such as low-grade metamorphic rocks (e.g. green schists and slates). In the Iberian land area, chlorite originates either from mechanical erosion of chloritic shales or from micaschists present in Palaeozoic igneous rocks and shales (e.g. in the Minho and Vigo river basins) (Silva, 1981), but most is subsequently broken down by weathering. 5.4. Smectite This clay mineral was present in only trace amounts in NW Portuguese shelf sediments (⬍2%) and was totally absent from the Galician shelf sediments. The maximum smectite content of 10% (in the outer shelf) was associated with low hydrodynamic
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Fig. 7.
Chlorite distribution in surface sediments (0–1 cm) of the northwestern Iberian shelf.
conditions. Smectite content of clays from the rivers studied was also very low (⬇1%). The gradual increase of smectite content observed towards the open ocean can be explained by segregation related to grain size (Gibbs, 1977). In terms of grain size, smectite is the finest of the clay minerals and so is preferentially deposited in low energy environments. 5.5. Kaolinite (Kt) / Illite (Ill) ratio The proportions of these two detrital minerals, transported by the local rivers, have a slight tendency to differ between depositional areas, reflecting the general circulation of particles depending on the energy of the principal erosive agents (tides and waves). The Kt/Ill ratio values varied within the range 0.15–0.3. In the Douro muddy deposit (see Fig. 2, for fine fraction distribution) this ratio increased to 0.3, possibly because of grain size segregation/differential
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setting (Gibbs, 1967, 1977; Tomadin & Borghini, 1987), but mainly, as suggested above, because kaolinite tends to accumulate where there is direct terrigenous supply (Chamley, 1989). Another area characterised by somewhat higher Kt/Ill ratios was located between the Ria Muros and the Ria Arosa in sandy sediments. In this case, the relatively higher concentration of kaolinite (as well as quartz) was quite probably related to the higher winnowing of the finer illite particles (note; the kaolinite particles from there exhibited lower crystallinity indicative of a higher degree of degradation). 5.6. General overview of clay minerals distribution patterns The distribution patterns of the four clay minerals discussed above show that the observed mineral assemblages are relatively homogeneous across the entire Iberian shelf, and there are no major discontinuities. This might be as a result of the homogenisation induced by action of hydrodynamic processes, mainly controlled by the waves and tidal currents. When sediments reach the shelf, they are constantly being remobilization by wave action that frequently disturbs the sediments of the seafloor and inhibits deposition. So the fine sediments remain in suspension and so tend to accumulate over the middle shelf region, where the wave action is weaker, so forming a ‘mid-shelf mud belt’ (McCave, 1972). At these mid-shelf depths fine sediments once deposited will be only re-suspended during very strong storms (Vitorino, Oliveira, Jouanneau, & Drago, 2001; Oliveira, Vitorino, Rodrigues, Jouanneau, Dias, & Weber, 2002), and will then be transported to northwards and into deeper waters by the general winter flow. Over the Portuguese sector of the shelf, the fine sediments are preferentially deposited at the mid shelf depths of around 60–100m, where there is a tectonically depressed trough, protected by rocky outcrops. At present the rivers are supplying new fine material that is of very similar make up to the overall clay mineral composition of bottom sediments. In the area of Douro mud patch the kaolinite content is slightly increased (where there is more direct supply from the river and the accumulation rate is higher) compared to the surrounding fine sand deposits that are richer in illite (with lower accumulation rates). The mineralogical composition of the Galician mud patch is very similar to the Douro mud deposits. The only difference is a slight increase in the chlorite content, possibly reflecting a local contribution by the River Minho and from the Ria Vigo (Arau´ jo, Marques, & Rocha, 2000). To the north, the seabed clay minerals become less crystalline as the supply of material from the rivers tends to dwindle. The mineralogical composition of the silt fractions shows a high degree of maturity, with higher contents of quartz relative to feldspars.
6. Conclusions The clay mineral distribution is mainly dependent upon riverine discharges and the wave regime. Illite is the predominant clay mineral in the ⬍2µm fraction, with a mean value of 77%, followed by kaolinite, chlorite and smectite. The material being discharged from the rivers is very similar in its general composition to the fine fractions of the seafloor sediments. Nevertheless, those deposits that are not directly influenced by river discharge have higher illite contents (⬎80%) whereas those receiving direct supplies of terrigenous materials have higher kaolinite contents (⬎20%). Detrital chlorite is low in abundance and is mainly related to river supply, being found preferentially at shallow depths. In summary, two main conclusions can be drawn: 1. There is clear evidence that the fine sediments exported by Portuguese rivers (mainly Douro and Minho) reach the Douro mud patch immediately offshore and the Galician mud area further to the north.
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2. In the Spanish area, north of ~42° 15’N, the majority of the clays remain trapped in the Galician Rias, although there may be a small contribution to the Galician mid-shelf mud belt from Ria Vigo.
Acknowledgements This study has been supported by the EU OMEX II-II program, Contract MAS3-CT97-0076. The authors would like to thank the crews and scientific team on board of N.O. Almeida Carvalho and Coˆ te de la Manche, during which sediments were collected. The first author also thanks the ‘Fundac¸ a˜ o da Cieˆ ncia e Tecnologia’ for a PhD grant.
References Arau´ jo, F., Marques, R. & Rocha, F. (2000). Caracterizac¸ a˜ o quı´mica e mineralo´ gica da fracc¸ a˜ o silto-argilosa de sedimentos dos Rios Minho, Lima, Ca´ vado, Ave e Douro. Book of abstract 3° Simpo´ sio Margem Continental Atlaˆ ntica Ibe´ rica (pp. 97-98). Faro. Biscaye, P. E. (1965). Mineralogy and sedimentation of recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans. Geological Society American Bulletin, 76, 803–832. Boillot, G., Dupeuble, P. A., Hennequin-Marchand, I., Lamboy, M., Lepretre, J. P., & Musellec, P. (1974). Le roˆ le des de´ crochements tardi-hercyniens dans l’evolution structurale de la marge continentale et dans la localization des grands canyons sous-marins de l’Ouest et au Nord de la Pe´ ninsule Ibe´ rique. Re´ vue de Ge´ ographie Physique et de Ge´ ologie Dynamique, XVI(I), 75–86. Caille`re, S., He´ nin, S., & Rautureau, M. (1982). Mine´ ralogie des argiles, Vol. 2. Paris: Masson. Cascalho, J.P.V. (2000). Mineralogia dos sedimentos arenosos da margem continental setentrional portuguesa. PhD. Thesis, Lisbon University, 400pp. Chamley, H. (1989). Clay sedimentology. Berlin: Springer. Dias, J. M. A., & Nittrouer, C. A. (1984). Continental shelf sediments of northern Portugal. Continental Shelf Research, 3, 147–165. Dias, J.M.A. (1987). Dinaˆ mica sedimentar e evoluc¸ a˜ o recente da plataforma continental portuguesa setentrional. PhD. Thesis, Lisbon University, 500pp. Dias, J. A., Joanneau, J. M., Arau´ jo, M. F., Drago, T., Garcia, C., Gonzalez, R., Oliveira, A., Rodrigues, A., Vitorino, J., & Weber, O. (2002). Present day sedimentary processes on the Northern Iberian shelf. Progress in Oceanography, 52(2-4), 249–259. Drago, T., Oliveira, A., Magalha˜ es, F., Cascalho, A., Jouanneau, J. M., & Vitorino, J. (1998). Some evidences of northward fine sediment in the northern Portuguese continental shelf. Oceanologica Acta, 21, 223–231. Esquevin, J. (1969). Influence de la composition chimique des argiles sur le cristallinite´ . Bulletin Centre Recherch Pau, S.N.P.A., 3, 147–154. Fiu´ za, A. F. G. (1983). Upwelling patterns off Portugal. In Coastal upwelling: its sediment record (pp. 85–98). New York: Plenum Press. Fiu´ za, A. F. G., Macedo, M. E., & Guerreiro, M. R. (1982). Climatological space and time variation of the Portuguese coastal upwelling. Oceanologica Acta, 5, 31–50. Frouin, R., Fiu´ za, A. F. G., Ambar, I., & Boyd, T. J. (1990). Observation of a poleward surface current off the coast of Portugal and Spain during the winter. Journal Geophysical Research, 95, 679–691. Gala´ n, E. (1986). Las arcillas como indicadores paleoambientales. Boletim Sociedade Espanhola de Mineralogia, 9, 11–22. Gibbs, R. J. (1967). The geochemistry of the Amazon River system. Part I: The factors that control the salinity and the composition of the suspended solids. Geological Society of America Bulletin, 78, 1203–1232. Gibbs, R. J. (1977). Clay mineral segregation in the marine environment. Journal of Sedimentary Petrology, 47, 237–243. Griffin, J. J., Windom, H., & Goldberg, E. D. (1968). The distribution of clay minerals in the world oceans. Deep-Sea Research, 15, 433–459. Gomes, C. (1989). Argilas. O que sa˜ o e para que servem. Lisboa: Fundac¸ a˜ o Calouste Gulbenkian. Gomes, C., Lopes Velho, J. A. G., & Sa´ Delgado, H. M. (1990). Kaolin deposits of Portugal. Geocieˆ ncias, 5, 75–89. Haynes, R., & Barton, E. D. (1990). A poleward flow along the Atlantic coast of the Iberian Peninsula. Journal Geophysical Research, 95, 11425–11441. Julivert, M., Fontbote, J. M., Ribeiro, A., & Conde, L. (1980). Mapa tectonico de la Penı´nsula Ibe´ rica y Baleares (scale 1/1 000 000) — explicative memory. Madrid: Instituto Geologico y Geominero de Espan˜ a. Jouanneau, J. M., Weber, O., Drago, T., Rodrigues, A., Oliveira, A., Dias, J. A., Garcia, C., Schmidt, S., & Reyss, J. L. (2002). Present day sedimentation and sedimentary budgets on the Northern Iberian Shelf. Progress in Oceanography, 52(2-4), 261–275.
A. Oliveira et al. / Progress in Oceanography 52 (2002) 233–247
247
Kubler, B. (1964). Les argiles, indicateurs de me´ tamorphisme. Revue Institute Franc¸ ais Pe´ trole, 19, 1093–1112. Latouche, C., Jouanneau, J. M., Lapaquellerie, Y., Maillet, N., & Weber, O. (1991). Re´ partition des mine´ raux argileux sur le plateau continental Sud-Gascogne. Oceanologica Acta, 11, 155–161. Magalha˜ es, F., & Dias, J. M. A. (1992). Depo´ sitos sedimentares da plataforma continental a norte de Espinho. Gaia, 5, 6–17. McCave, I. N. (1972). Transport and escape of fine-grained sediment from shelf areas. In D. Duane, & O. H. Pilkey (Eds.), Shelf sediment transport: processes and patterns (pp. 225–248). Stroudsburg, PA: D.J.P. Swift, Dowden, Hutchinson and Ross. Millot, G. (1964). Ge´ ologie des Argiles. Masson. Moriarty, K. C. (1977). Clay minerals in Southeast Indian Ocean sediments. Transport mechanisms and depositional environments. Marine Geology, 25, 149–174. Nemecz, E. (1981). Clay minerals. Budapeste: Ak. Kiado. Oliveira, A., Vitorino, J., Rodrigues, A., Jouanneau, J. M., Dias, J. M. A., & Weber, A. (2002). Nepheloid layer dynamics of the northern Portuguese shelf. Progress in Oceanography, 52(2-4), 195–213. Oliveira, A., Rocha, F., Rodrigues, A. & Dias, J.A. (2000). The fine sediments as dynamic sedimentary tracers (NW Iberian margin).Book of Abstract 3— Simpo´ sio Margem Continental Atlaˆ ntica Ibe´ rica (pp. 399-400). Faro. Rey Salgado, J. (1993). Relacion morphosedimentaria entre la plataforma continental de Galicia y las Rias bajas y su evolucion durante el Cuaternario. Publicaciones Especiales Instituto de Oceanografia, 17, 233. Ribeiro, A., Antunes, M. T., Ferreira, M. P., Rocha, R. B., Soares, A. F., Zsbyszewski, G., Moitinho de Almeida, F., Carvalho, D., & Monteiro, J. H. (1979). Introduction a` la ge´ ologie ge´ ne´ rale du Portugal. Servic¸ os Geolo´ gicos de Portugal. Rocha, F. (1993). Argilas Aplicadas a Estudos Litoestratigra´ ficos e Paleoambientais na Bacia Sedimentar de Aveiro. PhD Thesis, Aveiro University, 399pp. Segonzac, G. D. (1969). Les mineraux argileux dans la diagene`se. Passage au me´ tamorphisme. Service Carte Ge´ ology Alsace-Lorraine, Me´ moire, 29, 320. Schultz, L.G. (1964). Quantitative interpretation of mineralogical composition from X-ray and Chemical data for the Pierre shale. United States Geological Survey Professional Paper, 391-C, 1-31. Silva, J. M. (1981). Solos derivados de xistos da regia˜ o NW de Portugal. Caracterizac¸ a˜ o mineralo´ gica das fracc¸ o˜ es limo e argila. Pedologia, 16, 123–131. Tomadin, L. & Borghini, M. (1987). Source and dispersal of clay minerals from present and late Quaternary sediments of Southern Adriatic Sea. Proceedings 6th Meeting European Clay Groups (pp. 537-538). Sevilla. Thorez, J. (1976). In G. Lelotte (Ed.), Practical identification of clay minerals. Vanney, J. R., & Mougenot, D. (1981). La plateforme continentale du Portugal et les provinces adjacentes: analyse ge´ omorphologique. Memo´ rias dos Servicos Geolo´ gicos Portugal, 28, 145. Vitorino, J., Oliveira, A., Jouanneau, J.M., Drago, T. (2000). Winter dynamics and the transport of fine sediments on the northern Portuguese shelf. Book of Abstracts 3— Simpo´ sio Margem Continental Atlaˆ ntica Ibe´ rica, (pp.279-280). Faro. Vitorino, J., Oliveira, A., Jouanneau, J. M., & Drago, T. (2002). Winter dynamics on the northern Portuguese shelf: It’s importance to fine sediments transport. Progress in Oceanography, 52(2-4), 155–170. Weaver, C.E. (1989). Clays, muds, and shales. Developments in Sedimentology, 44, Elsevier, Amsterdam. Wilson, R.C.L., Hiscott R.N., Willis, M.G. & Gradstein, F.M. (1989). The Lusitanian Basin of West-Central Portugal: Mesozoic and Tertiary Tectonic, Stratigraphic and Subsidence History. In: A.J. Tankard and H.R. Balkwill (eds.), Extensional Tectonics and Stratigraphy of the North Atlantic Margins, AAPG Memo´ ire, 46, 341-361. Windom, H. L. (1976). Lithogenous material in marine sediments. In J. P. Riley, & R. Chester (Eds.), Chemical oceanography 5 (pp. 103–135). New York, London: Academic Press. Wooster, W. S., Bakun, A., & McLain, D. R. (1976). The seasonal upwelling cycle along the eastern boundary of the North Atlantic. Journal of Marine Research, 34, 131–141.
Clay minerals from the sedimentary cover from the Northwest Iberian shelf A. Oliveira a,∗, F. Rocha b, A. Rodrigues a, J. Jouanneau c, A. Dias d, O. Weber c, C. Gomes b a
b
Instituto Hidrogra´fico, Rua das Trinas, 49, 1249–093 Lisboa, Portugal Univ. Aveiro, Dep. Geocieˆncias, Campus de Santiago, 3810 Aveiro, Portugal c DGO–UMR , CNRS 5805, Av. des Facultes, 33405 Talence Cedex, France d UCTRA, Univ. do Algarve, 8000 Faro, Portugal
Abstract The Northern Iberian margin is a typical example of a continental margin subjected to seasonal highly energetic regime (waves and tides) and receiving inputs of continental sediments via riverine discharges. The principal goal of this study has been to use clay minerals as indicators of sedimentary dynamics in the open shelf system. The distributions of clay mineral in the top layer of the sedimentary cover are shown to be related to their continental sources, but also reflect the influences of winter storms and longshore currents in determining the pathways of sediment transport. The mineralogical composition of the material issuing from the rivers is very similar to the general mineralogical composition of the fine fractions of the seabed sediments. Those deposits that are directly influenced by riverine discharges have higher contents of kaolinite (⬎20%), whereas those that are not have higher contents of illite (⬎80%). The available data indicate no significant quantities of terrigenous particles are being discharged from the Spanish rias. Therefore, we conclude that physical processes are controlling the clay mineral distributions and that, despite contributions from the Minho River, the main source of fine detrital particles to the shelf region is the Douro River discharge. These particles settle on the middle shelf, below the 60 m isobath. During storm events these particles are re-suspended and advected northwards to the Galician shelf or into deeper domains. Thus the distributions of the clays indicate there is a net transport of fine sediments both northwards and off-shelf. 2002 Elsevier Science Ltd. All rights reserved.
Contents 1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
2. Regional setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 2.1. Morphology and geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 2.2. Oceanography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
∗
Corresponding author.
0079-6611/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 0 2 ) 0 0 0 0 8 - 3
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3.
Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
4.
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
5. Discussion . . . . . . . . . . . . . . . . 5.1. Illite . . . . . . . . . . . . . . . . . . 5.2. Kaolinite . . . . . . . . . . . . . . . 5.3. Chlorite . . . . . . . . . . . . . . . . 5.4. Smectite . . . . . . . . . . . . . . . . 5.5. Kaolinite (Kt) / Illite (Ill) ratio . . 5.6. General overview of clay minerals 6.
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Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
1. Introduction Clay minerals have been investigated worldwide using X-ray diffraction (XRD) techniques. Previous studies have shown that the modern clay mineral distribution in the Atlantic Ocean is controlled mainly by the zonation of climate and weathering over the adjacent land masses, implying that most of the clay minerals are of terrigenous origin (Biscaye, 1965; Griffin, Windom, & Goldberg, 1968; Chamley, 1989). Clay minerals have been widely used in studies of sedimentary dynamics studies. Their physical and chemical properties make them good indicators of sediment sources, and their distribution patterns in the sedimentary basins can be indicative of the main transport processes and pathways. Despite our detailed knowledge of the general distribution patterns of recent clay minerals in the N.E. Atlantic (Northern Europe and Gulf of Biscay) (Latouche, Jouanneau, Lapaquellerie, Maillet, & Weber, 1991), data on clay distributions on the Iberian continental margin has remained inadequate. This paper uses clay minerals as indicators of sedimentary dynamics on the open shelf system. The study area is the northwestern Iberian shelf, between 41°N and Cape Finisterre.
2. Regional setting The northwestern Iberian margin, especially in its shallowest domain, is characterised by complex sedimentary dynamics. This complexity arises because of the varied morphology and geology of the region, and the diversity of the oceanographic processes to which it is exposed. 2.1. Morphology and geology The continental shelf is narrow varying in width between 35 and 50 km, and has several unusual features, which interact with the sedimentary dynamics (Fig. 1). The coastal outline is very irregular because of its geology. It is characterised by an outcrop of old Hercynian formations, highly metamorphosed and fractured Palaeozoic granites, greywackes and schists with a general NNW–SSE orientation (Ribeiro, Antunes, Ferreira, Rocha, Soares, Zsbyszewski, Moitinho de Almeida, Carvalho, & Monteiro, 1979; Wilson, Hiscott, Willis, & Gradstein, 1989). Five main rivers discharge along this sector of the Iberian margin, the Douro, Ave, Ca´ vado, Lima and Minho Rivers. Fluvial drainage has been established, from the highlands in the interior down to the coastline, cutting through these old geological formations. The Douro is the largest of these rivers with an
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Fig. 1. Regional setting: (a) Bathymetry of the continental shelf; (b) Continental geological setting (adapted from Julivert et al., 1980, in Cascalho, 2000).
annual mean discharge of 8.2 x 109m3 and its catchment covers 98 x 103 km2. When these five Portuguese rivers flood during winter, the coastal waters are turned brown by the large amounts of suspended sediments being discharged in their plumes. In contrast in the Spanish sector there are four rias (Vigo, Pontevedra, Arosa and Muros) which are drowned river systems deeply incised into the coastline, but which, as we shall see below, tend to trap any fluvial supplies they receive rather than discharge them to the shelf seas. Basement rock outcrops extend out on to the inner shelf region, making the seabed rough and irregular (Vanney & Mougenot, 1981). These outcrops are bare of recent sediments. The Porto canyon and other more minor slope valleys are important features that have been created by geological structures, i.e. Mesozoic strike-slip faults (Boillot, Dupeuble, Hennequin-Marchand, Lamboy, Lepretre, & Musellec, 1974). Along the sector of the Portuguese continental shelf we studied, the distribution of the main deposits is well known (Dias & Nittrouer, 1984; Dias, 1987; Magalha˜ es & Dias, 1992). Generally the sedimentary cover is coarse-grained, but includes two well-defined muddy areas, where silt and clay components dominate (frequently having mud contents up to 90%), although their clay components (⬍2 µm) are small ⬍10% (Fig. 2). These two muddy areas (Douro and Galicia mud patches) experience the highest accumulation rates in the region (Jouanneau et al., 2002). The mineralogical composition of the silt fraction (⬍63 µm) has been described by Oliveira, Rocha, Rodrigues and Dias (2000) in non-orientated powders. They found that this fraction is dominated by particles of quartz, with average content of 40% on the Portuguese shelf, but
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Fig. 2. Location of the collected samples for CORVET 96 cruise (dots) and GAMINEX cruise (stars). The river sampling points are represented by triangles. Base map shows the distribution of fine sediment (⬍63µm) percentage (adapted from Dias et al., 2002).
increasing to a maximum of 85% (average of 60%) in the north, between Ria Muros and Ria Arosa. The other minerals in this fraction were mica (14%), plagioclase (12%), K-feldspar (10%) and calcite (9%), all of which occurred at higher concentrations on the Portuguese shelf (Oliveira et al., 2000). 2.2. Oceanography The northwestern Iberian margin is a typical example of a continental margin that is subjected to a highly energetic hydrodynamic regime of waves and tides, and is also strongly influenced by upwelling events in summer (Wooster, Bakun, & McLain, 1976; Fiu´ za, Macedo, & Guerreiro, 1982; Fiu´ za, 1983) and downwelling events in winter (Drago et al., 1998; Vitorino, Oliveira, Jouanneau, & Drago, 2000). It receives large riverine discharges particularly in winter, and is subject to poleward-flowing slope currents year-round (Frouin, Fiu´ za, Ambar, & Boyd, 1990; Haynes & Barton, 1990). Such phenomena generate
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considerable spatial and seasonal variability in particle fluxes and inverse circulation over the continental shelf (see Vitorino, Oliveira, Jouanneau, & Drago, 2002). Over the Portuguese shelf, the major river estuaries discharge very close to each other (⬇20 km), and so not only do the coastal waters tend to have relatively low salinities but are also quite turbid (Oliveira, Vitorino, Rodrigues, Jouanneau, Dias, & Weber, 2002). During winter the turbid plume of the most important river, the Douro, can extend for up to 14 km northwestwards out from the river mouth (Drago et al., 1998). On the other hand, the Rias to the north are at present important depositional areas, in which the majority of the fine particles they receive are trapped and there is no major transport out over the shelf (Rey Salgado, 1993). The importance of physical forcing in the sediment dynamics in the continental shelf has been quantified in several complementary studies (Vitorino, Oliveira, Jouanneau, & Drago, 2000; Jouanneau, Weber, Drago, Rodrigues, Oliveira, Dias, Garcia, Schmidt, & Reyss, 2002; Oliveira, Vitorino, Rodrigues, Jouanneau, Dias, & Weber, 2002). During stormy periods, with typically waves of 12s period and mean heights of 7.5m, these authors reported evidence of resuspension of the muddy deposits over the mid-shelf areas. Such events intensify the bottom turbid layer that covers the entire continental shelf and extending out over the upper slope region (Oliveira, Vitorino, Rodrigues, Jouanneau, Dias, & Weber, 2002).
3. Materials and methods Sediment samples from the northern Portuguese and Galician continental shelf were recovered during CORVET (November 1996, NRP Almeida Carvalho) and GAMINEX (June 1998, FV Coˆ te de la Manche) cruises (Fig. 2). Sediment samples were preferentially collected from the muddy deposits on the continental shelf. During the first cruise a Mark I multicorer (4 subcores) was used to retrieve undisturbed samples from the uppermost sediment layer, 1–2 cm thick. A Smith-McIntyre grab was also used and sub-samples taken using PVC tubes, which were then stored. During the GAMINEX cruise, gravity core and Reineck box core samples were collected from both the Portuguese and the Galician shelves (Fig. 2). Only the uppermost 1–2 cm of the surface sediments was analysed for this study, the amounts of sediment being used varying between 2 and 8g. Bottom sediment samples from northern Portuguese rivers had been collected near their outlets in February 1993, using a Petit-Ponar grab, and these samples were re-analysed with the shelf samples, using the same methods. Each sample was initially disaggregated using ultra-sound. After wet sieving of the sand (0.063–2 mm) and the silt (2-63 µm) fractions, the clay fractions (⬍2 µm) were separated by sedimentation according to Stoke’s law, using 1% sodium hexametaphosphate solution to avoid flocculation. For the preparation of preferentially oriented clay mounts of the ⬍2µm fraction, the suspension was placed on a thin glass plate and air-dried. XRD measurements were performed using a Philips diffractometer, with CuKα radiation. Scans were run between 2° and 18° 2θ in the air-dry state and after glycerol saturation and heat treatment (300 and 500°C). Peak areas of the basal reflections of the main clay minerals were calculated and weighted by empirically estimated factors (Schultz, 1964; Thorez, 1976; Rocha, 1993). The clay minerals identified were: illite ˚ peak, in natural specimen); kaolinite (7A ˚ peak, in natural specimen after removal of the chlorite (002) (10A ˚ peak, in 500°C heated specimen) and smectite (17A ˚ peak, in glycolated specimen). The peak), chlorite (14A ˚ peak area was divided by 4, the illite peak area by 0.5, the kaolinite 7 A ˚ peak area by 1 smectite 17 A ˚ (500°C) peak area by 0.75. and the chlorite 14A ˚ (002) reflection was also considered to estimate the ‘octahedral character’ using the 5A ˚ /10A ˚ The illite 5A peak intensity ratio. This quick method can be useful to identify clay mineral sources and transport paths ˚ /10A ˚ values of clay minerals in marine sediments (Moriarty, 1977). According to Esquevin (1969), high 5A
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(⬎0.4) correspond to Al-rich (muscovitic) illites whereas Fe and Mg- rich (biotitic) illites have values below 0.15. Kaolinite and illite crystallinity indices were determined. For the estimation of the kaolinite crystallinity, ˚ peak, and the height of this peak, in airthe ratio between the width measured at half-height of the 7A ˚ peaks. For illite crystallinity dry aggregates was used, after the decomposition of chlorite and kaolinite 7 A ˚ peak) was used. the Kubler (1964)/Segonzac (1969) index (the width measured at half-height of the 10A Well-ordered illite shows symmetrical and narrow basal reflections and low values of the Kubler/Segonzac index.
4. Results The surficial sediments studied on the continental shelf proved to be mainly siliciclastic, containing a low percentage of bioclastic carbonates (10% on average). Representative diffractograms of marine and river clay fractions are presented in Fig. 3. A rather uniform clay mineral association characterised the superficial sediments of the NW Iberian margin. The typical clay mineral association expressed in terms of illite+kaolinite+chlorite+smectite summed to 100%, was 70–85% illite, 15–25% kaolinite, 5% chlorite and traces of smectite (Table 1). Illite was identified in all marine and river samples. Its content exceeds 70% in all marine samples and 60% in all river samples except in one from the River Ca´ vado (only 24%). Kaolinite was also identified in all samples but at lower percentages than illite except in the one river sample (74%). Chlorite content of some of the river and marine samples was only a few percent, but was 5–10% in about half of the samples. It
Fig. 3.
Diffractograms of selected clay fraction samples (air-dry, glycolated and 500°C): (a) river samples; (b) marine samples.
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Table 1 Mineralogical composition of the clay fraction in surface marine and river sediments from the study area (NW Iberian Margin). Samples location in Fig. 2. ILL (Illite), KT (Kaolinite), SM (Smectite), CHL (Chlorite), C (Corvet cruise), G (Gaminex cruise) Marine samples
Water depth ILL % (m)
KT %
SM %
CHL %
ILL Crist. (2°θ)
ILL (002)/(001)
C1 C2 C3 C4 C6 C7 C8 C9 C11 C12 C13 C14 C15 C17 C18 C19 C21 C22 C23 C24 C25 C27 C29 C33 C34 C36 C40 C42 C44 G1 G4 G6 G7 G9 G10 G12 G15 G17 G18 G20 G21 G23 G26 G28 G32 G33 G34 G36 G42
67 80 100 148 68 67 87 106 122 94 79 75 76 104 83 72 68 77 94 73 81 86 96 115 115 95 121 61 40 128 142 211 103 115 49 102 125 169 133 115 113 39 99 138 117 108 115 98 90
16 16 17 17 16 19 22 18 23 22 19 25 24 17 16 16 15 16 18 15 15 17 15 14 16 19 14 15 19 13 17 16 20 23 24 16 24 23 12 13 18 17 14 19 20 15 16 19 18
0 0 ~1 2 ~1 ~1 ~1 2 0 ~1 0 0 ~1 ~1 2 ~1 ~1 ~1 2 2 5 ~1 2 2 3 ~1 10 3 ~1 0 0 0 0 0 0 0 0 ~1 0 0 0 0 0 0 0 0 0 0 0
4 4 0 3 4 4 5 5 5 4 3 3 4 7 7 6 7 3 5 5 4 6 5 5 4 5 4 5 5 7 1 4 2 2 2 5 3 2 4 5 6 5 6 6 4 3 4 6 4
0.15 0.16 0.28 0.30 0.18 0.15 0.16 0.15 0.13 0.15 0.15 0.18 0.18 0.15 0.15 0.16 0.20 0.15 0.20 0.15 0.20 0.16 0.15 0.20 0.15 0.15 0.20 0.20 0.20 0.20 0.20 0.20 0.15 0.20 0.15 0.20 0.20 0.20 0.15 0.15 0.20 0.15 0.15 0.20 0.12 0.20 0.15 0.15 0.15
0.51 0.05 0.53 0.04 0.65 0.27 0.55 0.27 0.44 0.06 0.50 0.04 0.57 0.04 0.48 0.03 0.50 0.04 0.58 0.04 0.58 0.04 0.55 0.05 0.44 0.06 0.49 0.09 0.53 0.05 0.47 0.07 0.46 0.05 0.47 0.12 0.43 0.08 0.46 0.04 0.45 0.05 0.42 0.09 0.44 0.08 0.54 0.05 0.46 0.10 0.51 0.05 0.48 0.13 0.38 0.15 0.47 0.15 0.44 0.08 0.73 0.17 0.76 0.05 0.57 0.11 0.56 0.17 0.50 0.28 0.77 0.06 0.50 0.08 0.29 0.43 0.07 0.38 0.04 0.52 0.06 0.48 0.08 0.57 0.09 0.50 0.07 0.49 0.04 0.47 0.05 0.48 0.06 0.50 0.05 0.41 0.07 (continued on next page)
80 80 82 77 79 76 72 75 72 73 78 72 71 75 78 77 77 79 75 78 76 76 78 79 77 75 72 77 75 80 82 80 78 72 74 79 73 74 84 82 76 78 80 75 76 82 80 75 78
KT Crist.
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Table 1 (continued) Marine samples
Water depth ILL % (m)
KT %
SM %
CHL %
ILL Crist. (2°θ)
ILL (002)/(001)
KT Crist.
G44 G46 G47 G50 G53 G54 G55 G56 River samples Minho 1 Minho 2 Lima 1 Lima 2 Ca´ vado 1 Ca´ vado 2 Ave 1 Ave 2 Douro 1 Douro 2
69 91 107 79 95 160 75 55
75 76 75 79 80 80 82 84
19 17 22 16 15 14 14 13
0 ~1 2 0 0 2 ~1 0
6 6 1 5 5 4 3 3
0.20 0.15 0.15 0.20 0.15 0.20 0.15 0.20
0.47 0.45 0.72 0.44 0.51 0.45 0.50 0.49
0.08 0.09 0.30 0.06 0.12 0.18 0.14 0.07
67 59 62 64 70 24 65 62 70 61
27 39 27 34 21 74 33 36 23 28
~1 0 2 0 ~1 ~1 ~1 ~1 ~1 ~1
5 2 9 2 8 ~1 ~1 ~1 6 10
0.20 0.30 0.25 0.30 0.25 0.25 0.30 0.20 0.20 0.20
0.44 0.57 0.35 0.35 0.38 0.46 0.32 0.27 0.38 0.42
0.02 0.12 0.16 0.20 0.07 0.08 0.18 0.40 0.06 0.10
was only in vestigial quantities in the two samples from River Ave. Smectite was definitely detected in two marine samples, ~10% in the Portuguese outer shelf (sample c40) and ⬍5% off the Lima river (sample c25). Its presence was suspected (⬍3%) in a few more marine samples but was completely absent from samples collected from the Galician shelf.
5. Discussion 5.1. Illite The sediments studied were characterised by very high contents of illite (⬎70%); this reflects the moderate climatic conditions prevailing in the source areas and the abundance of igneous (granitic rocks) and metamorphic rocks of Palaeozoic age (schists, gneisses, micaschists and greywackes) in the region. Such illite abundance, in particular if characterised by a well ordered structure, is considered to be evidence for a temperate climate, less warm and wet, that does not favour hydrolysis, so that physical alteration prevails over chemical alteration (Gala´ n, 1986; Chamley, 1989; Weaver, 1989). The illite showed an antiphase behaviour relative to kaolinite (Fig. 4 and 6) because of the low content of the other two clay minerals (a ˚ /10A ˚ ratio (Esquevin index) with values ⬎0.4, two clay locked system). All the marine samples have 5A corresponding to Al-rich illites (muscovite type) and reflecting a granitic provenance. The riverine samples were mostly in the range of 0.27–0.38 (Table 1). A clear east-west trend was observed between the rivers and outer shelf samples. The samples in general, showed low values of the Kubler crystallinity index for illite, indicating well-ordered structures and very low-grade chemical degradation both in the source-areas and during transportation and sedimentation. Fig. 5 shows that the sediments from the shelf contained illites displaying higher crystallinity than the illites from river sediments, so illite crystallinity increases within the marine environment. This can be explained by the capacity of illite in the marine environment, to fix new ions available in
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Fig. 4.
241
Illite distribution in surface sediments (0–1 cm) of the northwestern Iberian shelf.
seawater (Millot, 1964), since Fe and Mg tend be replaced by K and Al, increasing illite crystallinity (Nemecz, 1981). 5.2. Kaolinite Kaolinite is the dominant clay mineral in oceanic sediments at low latitudes where the continental sources are affected by intense chemical weathering. This led Griffin, Windom and Goldberg (1968) to describe kaolinite as the ‘low-latitude mineral’. Chemical weathering of continental rocks is favoured by warm climate (temperate to tropical) and high rainfall (⬎2000 mm/a). In the interior of northern Iberian, the extremely fractured basement (granite and gneiss) is affected by intense chemical weathering which yields important residual kaolinitic deposits, such as at Alvara˜ es, SE of Viana do Castelo, Sra. da Hora, north of Porto and Barqueiros, ~10 km from the Ca´ vado river mouth (Gomes, 1989; Gomes, Lopes Velho & Sa´ Delgado, 1990). These deposits are eroded by rain and carried
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Fig. 5.
Esquevin (1969) graphic applied to illites from sediments of the NW Iberian continental shelf.
via the streams and rivers into the oceanic systems. This explains the high kaolinite content of the clays found at the river mouths (especially in the Ca´ vado River). On the shelf, the kaolinite content of the sediments (Fig. 6) is closely dependent upon river discharges and distance from the sediment source at the river mouth. In fact, despite the low kaolinite contents (only 18%) generally occurring in the shelf sediments, higher values were found in the middle shelf in the Douro mud patch, and also north of Ria Arosa in sandy sediments (Fig. 6). Kaolinite crystallinity values (Table 1) are higher on the inner to middle shelf particularly, on the NW Portuguese shelf, but decrease both westwards and northwards to the Galician shelf. The areas where kaolinite shows higher crystallinity were directly supplied by kaolinite from the rivers. Kaolinite in the sediments from the Douro deposit, the southern mud patch at midshelf exhibited the same crystallinity values as the kaolinite in the sediments being transported by the rivers. The second sector, where kaolinite concentrations were high, was located on the Galician shelf, well away from any riverine source. There the kaolinite had low crystallinity, which can be explained by the chemical degradation processes occurring in the marine environment as a result of the higher pH and salinity, condition in which kaolinite is less stable. So once it reaches the oceanic environment, the crystallinity of kaolinite decreases as the residence time increases (Caille`re, He´ nin, & Rautureau, 1982; Gomes, 1989), which is in contrast to illite, whose crystallinity increases in the marine environment. It may be that the factors leading to the development of the two kaolin–rich areas are contrasting. In the Douro region the kaolin richness result from the entrapment of fresh riverine supplies, whereas in the more northern Galician area the enrichment results from the selective winnowing out of illite, leaving a richer residue of older kaolinite. 5.3. Chlorite Chlorite was a minor constituent of the clay fractions in the shelf sediments, with contents ⬍7%. The higher values in the inner and middle shelf were related to the proximity of the river mouths (Fig. 7). Generally chlorite is the dominant clay mineral at high latitudes (Griffin et al., 1968). It is usually less resistant to hydrolysis than illite (Chamley, 1989), so in warmer and wetter climates, chlorite is easily broken down by chemical weathering. Oceanic systems, outside high cold latitudes, are generally supplied
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Fig. 6.
243
Kaolinite distribution in surface sediments (0–1 cm) of the northwestern Iberian shelf.
with very low quantities of chlorite. Chlorite varies antagonistically to kaolinite (Windom, 1976; Chamley, 1989), and has been termed the ‘high-latitude clay mineral’ by Griffin et al. (1968). So high chlorite contents reflect conditions of low chemical alteration and close proximity to source rocks bearing high chlorite contents, such as low-grade metamorphic rocks (e.g. green schists and slates). In the Iberian land area, chlorite originates either from mechanical erosion of chloritic shales or from micaschists present in Palaeozoic igneous rocks and shales (e.g. in the Minho and Vigo river basins) (Silva, 1981), but most is subsequently broken down by weathering. 5.4. Smectite This clay mineral was present in only trace amounts in NW Portuguese shelf sediments (⬍2%) and was totally absent from the Galician shelf sediments. The maximum smectite content of 10% (in the outer shelf) was associated with low hydrodynamic
244
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Fig. 7.
Chlorite distribution in surface sediments (0–1 cm) of the northwestern Iberian shelf.
conditions. Smectite content of clays from the rivers studied was also very low (⬇1%). The gradual increase of smectite content observed towards the open ocean can be explained by segregation related to grain size (Gibbs, 1977). In terms of grain size, smectite is the finest of the clay minerals and so is preferentially deposited in low energy environments. 5.5. Kaolinite (Kt) / Illite (Ill) ratio The proportions of these two detrital minerals, transported by the local rivers, have a slight tendency to differ between depositional areas, reflecting the general circulation of particles depending on the energy of the principal erosive agents (tides and waves). The Kt/Ill ratio values varied within the range 0.15–0.3. In the Douro muddy deposit (see Fig. 2, for fine fraction distribution) this ratio increased to 0.3, possibly because of grain size segregation/differential
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setting (Gibbs, 1967, 1977; Tomadin & Borghini, 1987), but mainly, as suggested above, because kaolinite tends to accumulate where there is direct terrigenous supply (Chamley, 1989). Another area characterised by somewhat higher Kt/Ill ratios was located between the Ria Muros and the Ria Arosa in sandy sediments. In this case, the relatively higher concentration of kaolinite (as well as quartz) was quite probably related to the higher winnowing of the finer illite particles (note; the kaolinite particles from there exhibited lower crystallinity indicative of a higher degree of degradation). 5.6. General overview of clay minerals distribution patterns The distribution patterns of the four clay minerals discussed above show that the observed mineral assemblages are relatively homogeneous across the entire Iberian shelf, and there are no major discontinuities. This might be as a result of the homogenisation induced by action of hydrodynamic processes, mainly controlled by the waves and tidal currents. When sediments reach the shelf, they are constantly being remobilization by wave action that frequently disturbs the sediments of the seafloor and inhibits deposition. So the fine sediments remain in suspension and so tend to accumulate over the middle shelf region, where the wave action is weaker, so forming a ‘mid-shelf mud belt’ (McCave, 1972). At these mid-shelf depths fine sediments once deposited will be only re-suspended during very strong storms (Vitorino, Oliveira, Jouanneau, & Drago, 2001; Oliveira, Vitorino, Rodrigues, Jouanneau, Dias, & Weber, 2002), and will then be transported to northwards and into deeper waters by the general winter flow. Over the Portuguese sector of the shelf, the fine sediments are preferentially deposited at the mid shelf depths of around 60–100m, where there is a tectonically depressed trough, protected by rocky outcrops. At present the rivers are supplying new fine material that is of very similar make up to the overall clay mineral composition of bottom sediments. In the area of Douro mud patch the kaolinite content is slightly increased (where there is more direct supply from the river and the accumulation rate is higher) compared to the surrounding fine sand deposits that are richer in illite (with lower accumulation rates). The mineralogical composition of the Galician mud patch is very similar to the Douro mud deposits. The only difference is a slight increase in the chlorite content, possibly reflecting a local contribution by the River Minho and from the Ria Vigo (Arau´ jo, Marques, & Rocha, 2000). To the north, the seabed clay minerals become less crystalline as the supply of material from the rivers tends to dwindle. The mineralogical composition of the silt fractions shows a high degree of maturity, with higher contents of quartz relative to feldspars.
6. Conclusions The clay mineral distribution is mainly dependent upon riverine discharges and the wave regime. Illite is the predominant clay mineral in the ⬍2µm fraction, with a mean value of 77%, followed by kaolinite, chlorite and smectite. The material being discharged from the rivers is very similar in its general composition to the fine fractions of the seafloor sediments. Nevertheless, those deposits that are not directly influenced by river discharge have higher illite contents (⬎80%) whereas those receiving direct supplies of terrigenous materials have higher kaolinite contents (⬎20%). Detrital chlorite is low in abundance and is mainly related to river supply, being found preferentially at shallow depths. In summary, two main conclusions can be drawn: 1. There is clear evidence that the fine sediments exported by Portuguese rivers (mainly Douro and Minho) reach the Douro mud patch immediately offshore and the Galician mud area further to the north.
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2. In the Spanish area, north of ~42° 15’N, the majority of the clays remain trapped in the Galician Rias, although there may be a small contribution to the Galician mid-shelf mud belt from Ria Vigo.
Acknowledgements This study has been supported by the EU OMEX II-II program, Contract MAS3-CT97-0076. The authors would like to thank the crews and scientific team on board of N.O. Almeida Carvalho and Coˆ te de la Manche, during which sediments were collected. The first author also thanks the ‘Fundac¸ a˜ o da Cieˆ ncia e Tecnologia’ for a PhD grant.
References Arau´ jo, F., Marques, R. & Rocha, F. (2000). Caracterizac¸ a˜ o quı´mica e mineralo´ gica da fracc¸ a˜ o silto-argilosa de sedimentos dos Rios Minho, Lima, Ca´ vado, Ave e Douro. Book of abstract 3° Simpo´ sio Margem Continental Atlaˆ ntica Ibe´ rica (pp. 97-98). Faro. Biscaye, P. E. (1965). Mineralogy and sedimentation of recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans. Geological Society American Bulletin, 76, 803–832. Boillot, G., Dupeuble, P. A., Hennequin-Marchand, I., Lamboy, M., Lepretre, J. P., & Musellec, P. (1974). Le roˆ le des de´ crochements tardi-hercyniens dans l’evolution structurale de la marge continentale et dans la localization des grands canyons sous-marins de l’Ouest et au Nord de la Pe´ ninsule Ibe´ rique. Re´ vue de Ge´ ographie Physique et de Ge´ ologie Dynamique, XVI(I), 75–86. Caille`re, S., He´ nin, S., & Rautureau, M. (1982). Mine´ ralogie des argiles, Vol. 2. Paris: Masson. Cascalho, J.P.V. (2000). Mineralogia dos sedimentos arenosos da margem continental setentrional portuguesa. PhD. Thesis, Lisbon University, 400pp. Chamley, H. (1989). Clay sedimentology. Berlin: Springer. Dias, J. M. A., & Nittrouer, C. A. (1984). Continental shelf sediments of northern Portugal. Continental Shelf Research, 3, 147–165. Dias, J.M.A. (1987). Dinaˆ mica sedimentar e evoluc¸ a˜ o recente da plataforma continental portuguesa setentrional. PhD. Thesis, Lisbon University, 500pp. Dias, J. A., Joanneau, J. M., Arau´ jo, M. F., Drago, T., Garcia, C., Gonzalez, R., Oliveira, A., Rodrigues, A., Vitorino, J., & Weber, O. (2002). Present day sedimentary processes on the Northern Iberian shelf. Progress in Oceanography, 52(2-4), 249–259. Drago, T., Oliveira, A., Magalha˜ es, F., Cascalho, A., Jouanneau, J. M., & Vitorino, J. (1998). Some evidences of northward fine sediment in the northern Portuguese continental shelf. Oceanologica Acta, 21, 223–231. Esquevin, J. (1969). Influence de la composition chimique des argiles sur le cristallinite´ . Bulletin Centre Recherch Pau, S.N.P.A., 3, 147–154. Fiu´ za, A. F. G. (1983). Upwelling patterns off Portugal. In Coastal upwelling: its sediment record (pp. 85–98). New York: Plenum Press. Fiu´ za, A. F. G., Macedo, M. E., & Guerreiro, M. R. (1982). Climatological space and time variation of the Portuguese coastal upwelling. Oceanologica Acta, 5, 31–50. Frouin, R., Fiu´ za, A. F. G., Ambar, I., & Boyd, T. J. (1990). Observation of a poleward surface current off the coast of Portugal and Spain during the winter. Journal Geophysical Research, 95, 679–691. Gala´ n, E. (1986). Las arcillas como indicadores paleoambientales. Boletim Sociedade Espanhola de Mineralogia, 9, 11–22. Gibbs, R. J. (1967). The geochemistry of the Amazon River system. Part I: The factors that control the salinity and the composition of the suspended solids. Geological Society of America Bulletin, 78, 1203–1232. Gibbs, R. J. (1977). Clay mineral segregation in the marine environment. Journal of Sedimentary Petrology, 47, 237–243. Griffin, J. J., Windom, H., & Goldberg, E. D. (1968). The distribution of clay minerals in the world oceans. Deep-Sea Research, 15, 433–459. Gomes, C. (1989). Argilas. O que sa˜ o e para que servem. Lisboa: Fundac¸ a˜ o Calouste Gulbenkian. Gomes, C., Lopes Velho, J. A. G., & Sa´ Delgado, H. M. (1990). Kaolin deposits of Portugal. Geocieˆ ncias, 5, 75–89. Haynes, R., & Barton, E. D. (1990). A poleward flow along the Atlantic coast of the Iberian Peninsula. Journal Geophysical Research, 95, 11425–11441. Julivert, M., Fontbote, J. M., Ribeiro, A., & Conde, L. (1980). Mapa tectonico de la Penı´nsula Ibe´ rica y Baleares (scale 1/1 000 000) — explicative memory. Madrid: Instituto Geologico y Geominero de Espan˜ a. Jouanneau, J. M., Weber, O., Drago, T., Rodrigues, A., Oliveira, A., Dias, J. A., Garcia, C., Schmidt, S., & Reyss, J. L. (2002). Present day sedimentation and sedimentary budgets on the Northern Iberian Shelf. Progress in Oceanography, 52(2-4), 261–275.
A. Oliveira et al. / Progress in Oceanography 52 (2002) 233–247
247
Kubler, B. (1964). Les argiles, indicateurs de me´ tamorphisme. Revue Institute Franc¸ ais Pe´ trole, 19, 1093–1112. Latouche, C., Jouanneau, J. M., Lapaquellerie, Y., Maillet, N., & Weber, O. (1991). Re´ partition des mine´ raux argileux sur le plateau continental Sud-Gascogne. Oceanologica Acta, 11, 155–161. Magalha˜ es, F., & Dias, J. M. A. (1992). Depo´ sitos sedimentares da plataforma continental a norte de Espinho. Gaia, 5, 6–17. McCave, I. N. (1972). Transport and escape of fine-grained sediment from shelf areas. In D. Duane, & O. H. Pilkey (Eds.), Shelf sediment transport: processes and patterns (pp. 225–248). Stroudsburg, PA: D.J.P. Swift, Dowden, Hutchinson and Ross. Millot, G. (1964). Ge´ ologie des Argiles. Masson. Moriarty, K. C. (1977). Clay minerals in Southeast Indian Ocean sediments. Transport mechanisms and depositional environments. Marine Geology, 25, 149–174. Nemecz, E. (1981). Clay minerals. Budapeste: Ak. Kiado. Oliveira, A., Vitorino, J., Rodrigues, A., Jouanneau, J. M., Dias, J. M. A., & Weber, A. (2002). Nepheloid layer dynamics of the northern Portuguese shelf. Progress in Oceanography, 52(2-4), 195–213. Oliveira, A., Rocha, F., Rodrigues, A. & Dias, J.A. (2000). The fine sediments as dynamic sedimentary tracers (NW Iberian margin).Book of Abstract 3— Simpo´ sio Margem Continental Atlaˆ ntica Ibe´ rica (pp. 399-400). Faro. Rey Salgado, J. (1993). Relacion morphosedimentaria entre la plataforma continental de Galicia y las Rias bajas y su evolucion durante el Cuaternario. Publicaciones Especiales Instituto de Oceanografia, 17, 233. Ribeiro, A., Antunes, M. T., Ferreira, M. P., Rocha, R. B., Soares, A. F., Zsbyszewski, G., Moitinho de Almeida, F., Carvalho, D., & Monteiro, J. H. (1979). Introduction a` la ge´ ologie ge´ ne´ rale du Portugal. Servic¸ os Geolo´ gicos de Portugal. Rocha, F. (1993). Argilas Aplicadas a Estudos Litoestratigra´ ficos e Paleoambientais na Bacia Sedimentar de Aveiro. PhD Thesis, Aveiro University, 399pp. Segonzac, G. D. (1969). Les mineraux argileux dans la diagene`se. Passage au me´ tamorphisme. Service Carte Ge´ ology Alsace-Lorraine, Me´ moire, 29, 320. Schultz, L.G. (1964). Quantitative interpretation of mineralogical composition from X-ray and Chemical data for the Pierre shale. United States Geological Survey Professional Paper, 391-C, 1-31. Silva, J. M. (1981). Solos derivados de xistos da regia˜ o NW de Portugal. Caracterizac¸ a˜ o mineralo´ gica das fracc¸ o˜ es limo e argila. Pedologia, 16, 123–131. Tomadin, L. & Borghini, M. (1987). Source and dispersal of clay minerals from present and late Quaternary sediments of Southern Adriatic Sea. Proceedings 6th Meeting European Clay Groups (pp. 537-538). Sevilla. Thorez, J. (1976). In G. Lelotte (Ed.), Practical identification of clay minerals. Vanney, J. R., & Mougenot, D. (1981). La plateforme continentale du Portugal et les provinces adjacentes: analyse ge´ omorphologique. Memo´ rias dos Servicos Geolo´ gicos Portugal, 28, 145. Vitorino, J., Oliveira, A., Jouanneau, J.M., Drago, T. (2000). Winter dynamics and the transport of fine sediments on the northern Portuguese shelf. Book of Abstracts 3— Simpo´ sio Margem Continental Atlaˆ ntica Ibe´ rica, (pp.279-280). Faro. Vitorino, J., Oliveira, A., Jouanneau, J. M., & Drago, T. (2002). Winter dynamics on the northern Portuguese shelf: It’s importance to fine sediments transport. Progress in Oceanography, 52(2-4), 155–170. Weaver, C.E. (1989). Clays, muds, and shales. Developments in Sedimentology, 44, Elsevier, Amsterdam. Wilson, R.C.L., Hiscott R.N., Willis, M.G. & Gradstein, F.M. (1989). The Lusitanian Basin of West-Central Portugal: Mesozoic and Tertiary Tectonic, Stratigraphic and Subsidence History. In: A.J. Tankard and H.R. Balkwill (eds.), Extensional Tectonics and Stratigraphy of the North Atlantic Margins, AAPG Memo´ ire, 46, 341-361. Windom, H. L. (1976). Lithogenous material in marine sediments. In J. P. Riley, & R. Chester (Eds.), Chemical oceanography 5 (pp. 103–135). New York, London: Academic Press. Wooster, W. S., Bakun, A., & McLain, D. R. (1976). The seasonal upwelling cycle along the eastern boundary of the North Atlantic. Journal of Marine Research, 34, 131–141.