Present day sedimentary processes on the northern Iberian shelf

Progress in Oceanography 52 (2002) 249–259 www.elsevier.com/locate/pocean

Present day sedimentary processes on the northern Iberian shelf J.M.A. Dias a,∗, J.M. Jouanneau b, R. Gonzalez c, M.F. Arau´jo d, T. Drago e, C. Garcia c, A. Oliveira f, A. Rodrigues f, J. Vitorino f, O. Weber b a

Universidade do Algarve, Campus de Gambelas, 8000 Faro, Portugal DGO-UMR 5805 CNRS, Av. des Facultes, 33405 Talence Cedex, France c CIACOMAR — Universidade do Algarve, Avenida das Forc¸as Armadas s/n, 8700—311 Olha˜o, Portugal Departamento de Quı´mica, Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686—953 Sacave´m, Portugal e IPIMAR, Av. de Brasilia, 1449-006 Lisboa, Portugal f Instituto Hidrogra´fico, Rua das Trinas, 49, 1200 Lisboa, Portugal b

d

Abstract The Galicia-Minho Shelf features two large mud patches, the Douro and the Galicia Mud patches. These are recent sediment bodies that have accumulated under a combination of conditions including: (1) abundant supplies of sediment; (2) morphological barriers that act as sediment traps; and (3) hydrographic conditions that favour the accumulation of fine sediment in these sinks. This paper describes the mechanisms controlling the deposition of the fine-grained sediment depositions and the processes that result in resuspension processes on the Galicia-Minho Shelf. Fine-grained sediments are provided from discharges from the river basins on the southern sector of the shelf, mainly the Douro and Minho rivers. Sediments are exported from river estuaries onto the shelf during episodic flood events. In contrast, most of the sediments originating from the Galician hinterland fail to contribute significantly to sedimentation on the shelf, because they are retained in the Galician Rı´as, which function as sediment traps. Sediments deposited on the shelf are frequently remobilized, particularly during southwesterly storms that coincide with downwelling conditions. Once in suspension the fine-grained sediments are transported northwards by the poleward flowing bottom currents and are eventually deposited on the Douro and Galicia Mud patches after a series of resuspension events. The locations of the two mud patches are strongly influenced by the shelf morphology. Fines already deposited on the mud patches are occasionally reintroduced into the system by large storm events. Some material from the Douro Mud patch and adjacent areas is re-deposited in the Galicia Mud patch. It is probable that sediments re-suspended from the Galicia Mud patch are carried off the shelf when storm events coincide with downwelling conditions.  2002 Published by Elsevier Science Ltd.

∗

Corresponding author.. E-mail address: [email protected] (J.M.A. Dias).

0079-6611/02/$ - see front matter  2002 Published by Elsevier Science Ltd. PII: S 0 0 7 9 - 6 6 1 1 ( 0 2 ) 0 0 0 0 9 - 5

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Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

2.

Methods

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

3. Regional setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 3.1. Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 3.2. Oceanography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 4.

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

5.

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

6.

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

1. Introduction Continental shelves are highly complex systems influenced by continental as well as oceanic factors. No single quantitative model can describe the exchange of particles and dissolved matter across a continental margin because of the diversity of processes occurring at the margins and the variability of coastal systems (Van Weering et al. 2000). Still, knowledge of fine-grained sedimentation in coastal zones is essential to establish sedimentological and biogeochemical budgets on the shelves and to define sediment exchange from the shelf (Jouanneau, Weber, & Lesueur, 2000). This paper provides a synthesis of part of the results of the European Ocean Margin Exchange II programme (OMEX) (Van Weering and McCave, 2002). It will focus on the role of the morphology and hydrodynamic conditions in affecting and controlling the fine-grained sediment dispersal over the MinhoGalicia Shelf (NW Iberia), and particular on the two deposits of fine-grained material, the Douro and Galicia Mud patches.

2. Methods The methods used to analyze sediments and establish oceanographic conditions are described elsewhere in this issue in detail (Araujo et al., 2002; Dias et al., 2002; Jouanneau et al., 2002; Oliveira et al., 2002; Vitorino, Oliveira, Jouanneau, & Drago, 2002a,b) and so will only be summarized briefly here. Sampling of sediments from the Minho-Galicia Shelf was mostly carried out during the cruises ‘CORVET’ (October 1996, NRP Almeida de Carvalho), ‘SEDMAR’ (November 1997, Mestre Costeiro), and ‘GAMINEX’ (July 1998, FV Coˆ te de la Manche). A Minicorer Mark I (KTB) was used to retrieve undisturbed samples from the top sediment layer, and a Smith–McIntyre grab for other samples during the first cruise. Cores with a maximum length of 0.7 m were collected with a Reineck box corer (KR). A multicorer was used during the SEDMAR cruise to sample undisturbed sediments from the top 0.2–0.3 m of the seabed. A gravity interface corer (KI) was used during the GAMINEX cruise. Wave measurements using a directional wave buoy located at 41°19 N, 8°59 W were carried out by the Instituto Hidrogra´ fico between July 1996 and June 1999. Coastal wind records at Porto (41°00 N) were obtained during the same period from daily weather reports published by the Instituto de Meteorologia. Measurements of pressure, temperature, salinity, and current speed and direction were carried out between

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November 1996 and January 1997 as well as from February to May 1998. Additional hydrographic and sedimentological observations were carried out by the Instituto Hidrogra´ fico during the CORVET96, CLIMA97 and OMEX99 cruises. Laboratory analyses included grain size (Dias et al., 2002; Jouanneau et al., 2002), water content, calcium carbonate and organic carbon contents, radiography, X-ray diffractometry, and radioisotopic measurements (Jouanneau et al., 2002). The methods and instruments used varied between the laboratories, and we refer to the original sources for detailed descriptions.

3. Regional setting 3.1. Morphology The Galicia-Minho Continental Shelf is located on the northwestern Iberian margin (Fig.1). The area studied included the Iberian continental shelf between 41°N and 42°50 N roughly between Oporto and Cape Finisterra (Fig. 2) where the shelf is rather narrow (30 km) off the Rı´as but slightly broader (50 km) elsewhere. The shelf break is usually at around 160 and 180 m water depth. The Galicia-Minho Shelf is an estuary-dominated system in the south, but is dominated by the Spanish

Fig. 1.

Localisation of the study area on the northwestern Iberian continental shelf margin.

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Fig. 2. Study area with the sampled sites and the location of the mud patches (areas with more than 50% fines).

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Rı´as in the north. The transition between the two types of shelf system is at the Minho Estuary at about 42°00 N, which although it is broader and deeper than its southern counterparts, but cannot be considered to be a Rı´a (Fig. 2). The series of estuaries along the Minho Shelf (Douro, Ave, Ca´ vado, Lima, and Minho) are the main suppliers of the sediments, with the Douro and Minho supplying the main bulk. The Douro alone introduces ~87% of all fluvial sediments into the area (Dias, 1987). Inputs of continental sediments from rivers occur episodically when floods flush sediments out of the estuaries after storm over the Iberian hinterland, usually during the winter. The northern Galician part of the study area is characterized by the Lower Galician Rı´as, which are structurally controlled Tertiary river valleys, bound by steep hills and mountains which became drowned during the last transgression (Rey, 1993). They trend roughly WSW–ENE and have mouths that are up to 10 km wide. Water depth inside the Rı´as can typically reach 40–50 m. The gradient of the inner reaches of the Galicia-Minho Shelf is low in the area of the Douro Estuary (Fig. 2). In the area immediately adjacent to the shelf break there is a series of narrow steps with heights of up to 20 m, which mark the western edge of an underlying tectonic graben system (e.g. Beiral de Viana; Fig. 2, profile C–D: e.g. Mougenot, 1989; Dias et al., 2002. These steps form a barrier towards the shelf break and continental slope. The shelf break is locally indented by a series of steep fractures and incisions to the shelf edge. These merge to form the Porto Canyon system, which is located at around 41°20 N, and is by far the most important feature on the outer shelf. North of the Minho Estuary the Galician Shelf slopes relatively steeply down to a depth of approximately 100 m (Fig. 2, profile A–B). Here, the outer shelf forms a wide re-entrant arc between 42°00 N and 42°30 N. The northern boundary of this arc is a continuation of the northern rim of the Arosa Rı´a, which is formed by outcrops of metamorphic and plutonic rocks down to a depth of approximately 100 m (Fig. 2; Dias et al., 2002. There are no steps on elevation similar to those limiting the outer shelf of the southern sector, nor is the shelf break as well defined as in the other areas. 3.2. Oceanography The wave and tidal regime is highly energetic (e.g. Vitorino et al., 2002a,b) The dominant regional surface current (‘Portugal Current’) flows southwards, and there are significant slope bottom currents that flow northwards (Fraga, 1981). The region of the NW Iberian shelf experiences upwelling events of nutrient rich bottom waters along the shelf edge and over the shelf from July to October and also to some extent from January to February, as a consequence of intense and persistent winds from the North (Fiu´ za, 198). In contrast, storm systems, which generally blow from the southwest drive downwelling events (e.g. Fiu´ za, Maredo, & Guerreiro, 1982; Vitorino et al., 2000, 2002a,b). During the latter, sediments are often resuspended and the finegrained fraction is transported polewards by the persisting bottom currents (Drago et al., 1999). Wave heights in the region are strongly seasonal. Significant wave heights during the summer are between 1–3 m, with periods below 10 s. During winter storms they often exceed 7 m with typical periods around 13 s and eventually can reach 18 s (PO-WAVES Group, 1994; Vitorino et al., 2002a,b). Most storm waves are about 5–6 m in height, but wave heights of 9–12 m have been recorded (Pita & Santos 1989).

4. Results Two large mud patches occur on the Minho-Galicia Shelf (Fig. 2; Dias & Nittrouer 1984; Dias 1987; Drago et al., 1994). The Douro Mud Patch lies at a depth of 65–130 m to the northwest of the mouth of the Douro River (Fig. 2). It is about 8–18 km wide and 42 km long, and its western boundary is delimited

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by the outcropping Mesozoic and Cenozoic strata. The Galicia Mud Patch is situated further north off the Minho River estuary at a water depth of 110–120 m (Fig. 2). It is a narrow strip orientated north/south that is 50 km long and 2–3 km wide and about 50 km. It deepens to 120 m at its northern extremity off the Rı´a de Pontevedra. The Douro Mud Patch consists mostly of silty material, whereas the Galicia Mud Patch is constituted of finer grained silty clayey material at its core (Drago, Jouanneau, Weber, Naughton, & Rodrigues, 2000; Dias et al., 2002; Oliveira et al., 2002). The sediments of both mud patches become coarser with increasing distance from their axis. Both are recent sedimentary features; the Douro Mud Patch is dated at ~1400 years B.P. (ICEN–962) at 3.1 m sediment depth (Drago et al., 1994) and has been deposited over strata of Paleocene and Eocene age. Sedimentation rates have been estimated to be 0.1–0.23 cm yr-1 for the Galicia Mud Patch, and between 0.17 and 0.4 cm yr-1 for the Douro Mud Patch (Drago et al., 2000), although data from two cores suggest local accumulation rates can be as high as 0.57–0.58 cm yr-1. The mechanisms controlling the development of the mud patches are only now beginning to be understood, and are summarized as a conceptual model in Fig. 3. The Douro Mud Patch seems to have developed as a consequence of a high amount of sediment input from rivers to the shelf, favourable hydrodynamic conditions, and a morphological setting of the outer shelf edge that acts as a sediment trap (e.g. Dias et al., 2002). Fines are transported to the inner and sometimes middle shelf when flood waters discharge from the rivers and supply large amounts of sediment to the shelf (Fig. 3). Initially these sediments usually remain relatively close to their sources. They are then resuspended during winter storms from southwest, which drive downwelling conditions, and are carried further offshore (Fig. 3; Jouanneau et al., 2002; Vitorino et al., 2002a,b). These resuspended sediments are dispersed over a wide area, so that material discharged from the various rivers become mixed. Only some of this resuspended sediment remains is retained on the shelf. As an example, it has been estimated that annually during the last 2000 years after a series of resuspension events ~475×103 tons of sediment from the Douro River have ultimately been incorporated into the Douro Mud Patch (Drago et al., 1994), which is only about 4% of the total annual sediment discharge of the Douro River onto the shelf (Drago et al., 1994). In order to resuspend grains of similar size at greater depths, only larger, more violent storms can generate the sufficient energy at the seabed. These occur less frequently and so this process acts as a sorting mechanism that leaves the coarser fractions behind successively as the sediments are carried offshore (Dias et al., 1980). While mixing and reworking of surface sediments in water depths shallower than 100 m occurs frequently, in the deeper areas sediments are only resuspended as a result of exceptionally large storm events (Jouanneau et al., 2002), which in part is responsible for the formation of the mud patches. The geochemical analysis of shelf sediments shows that most sediment within the mud patches is so thoroughly mixed that often downprofile elemental signatures in cores show no or little variation (Araujo, Dias, & Jouanneau, 1994; Araujo et al., 2002). Jouanneau et al. (2000, 2002) estimated that the upper 7–9 cm of sediment is constantly subjected to remobilization. The frequency and intensity of this remobilization decreases with increasing depth on the shelf (Magalha˜ es, Dias, Taborda, 1991; Vitorino, 2000, 2002a,b). The morphology of the outer shelf forms an excellent trap for fines being transported to the deeper areas of the shelf (Dias et al., 2002). The series of narrow steps, which are of tectonic origin (the underlying fault systems are still active; Rodrigues, Dias, & Ribeiro, 1992), form a barrier towards the slope, particularly in the area of the Beiral de Viana (Fig. 2, profile C–D; Rodrigues et al., 1992). It is against this barrier that the fine-grained sediment being advected from the shallower areas of the shelf becomes trapped forming the Douro Mud Patch. The main original sediment source for the Galicia Mud Patch was probably the Minho River. However, the sediment supplies from both the Douro and Minho Rivers have declined during the course of this century, mainly because of the construction of dams in the river basins (e.g. Oliveira, Valle, & Miranda, 1982; Dias, 1990). At present, a large component of the sediment arriving at the Galicia Mud Patch has

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Fig. 3.

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Conceptual model of main sedimentation patterns of fine-grained sediments and oceanic forces influencing them during storm events.

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been remobilized from the Douro Mud Patch and adjacent shelf areas, which is carried northwards by bottom currents. During storms current velocities at the bottom are high enough to re-suspend material within the Douro Mud Patch (Vitorino et al., 2000). Between three and nine storms occur each year, which in total span 2–13 days. The Galician Rias contribute very little sediment to the shelf (Rey, 1993). They are relatively deep, with water depths of up to 50 m and so act as sediment traps. Furthermore, they are sheltered from large storm events by rocky outcrops and small islands situated near their mouths (e.g. Dias et al., 2002; Jouanneau et al., 2002). The few sediments that are exported from the Rias probably are deposited directly in the vicinity of the Galicia Mud Patch, as to the north of 42°N the shelf slopes down to about 100 m depth within a few kilometres of the mouths of the Rı´as. (Fig. 2, Profile A–B). This scenario is supported by mineralogical data. The clay mineral composition of the Galician Mud Patch is very similar to that of the Douro Mud Patch. The only notable difference is the slightly higher chlorite content of the clays in the Galicia Mud Patch, which possibly originates as a contribution from the Minho River and the Vigo Rı´a. From south to north along the shelf there is a decline in the fresh material coming from rivers and the clay minerals tend to become less crystalline (Oliveira et al, 2002). The mineralogical composition of silt fractions shows a higher degree of maturation with higher levels of quartz in relation to feldspar and mica. This clearly shows that the composition of the Galicia Mud Patch is not entirely fresh sediment from the Minho River, but includes inputs from the south, including the Douro Mud Patch, of sediment that has been in the shelf system for some time (Oliveira, Rocha, Rodrigues, & Dias, 2000). The origins of the Galicia Mud Patch, like those of the Douro Mud Patch, at least in part, have been determined by morphological features. The outer slope between depths of 100 and 160–180 m to the west of the steep initial shelf slope, is a much more regular and softer in character. The outer western margin of the Minho Shelf lacks the outcrops of Meso-and Cenozoic strata that are characteristic of the Galician Shelf, so that there is no offshore barrier to sediment transport. Hence, material re-suspended from the Galicia Mud Patch during storm events is probably advected off the shelf by the characteristic downwelling currents that occur during winter storms, and so are lost to deeper lying oceanic areas (Fig. 3). North of the Arosa Ria an east/west barrier of outcropping plutonic and metamorphic rocks extends out on to the shelf greatly reducing any further sediment transport northwards (Jouanneau et al., 2002).

5. Discussion A comparison of the Douro and Galicia Mud Patches with other recent mud patches on the Western European Atlantic Margin (the Grande Vasiere, off the Loire River; the Gironde Mud Patches off the Gironde Estuary; the Tagus Mud Patch off the Tagus Estuary) shows several similarities: 1. their sediment accumulation rates average of ~0.2 cm yr-1 (the range of their sedimentation rates is between 0.05 and 0.4 cm yr-1, (Lesueur, Jouanneau, Weber, & Tastet, 1997; Jouanneau et al., 1998); 2. all are located to the west and north of their respective source river, except the Tagus Mud Patch, which is located to the west of its source, because the way the Portuguese mainland extends westwards just north of the mouth of Tagus estuary; 3. all are situated at similar depths, although for the Gironde patches are slightly shallower; 4. all are currently still active (Jouanneau et al., 2002). These similarities are remarkable considering that the shelf bounding the Bay of Biscay off the Gironde Estuary has a width of up to 300 km and has a very different morphology from the Portuguese Shelf, which is much narrower and the river basins that supply its patches are different in size and sediment supply. This implies that it is larger scale forcing provided by storms, swell and bottom currents common

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to the entire Western European Margin is to a large extent determining the locations, as well as the mechanisms of deposition and erosion over these the mud patches. However, local factors, such as sediment supply and shelf morphology, will determine the shape and sediment make up of each patch; shelf sedimentation always depends on both sediment supply and the distribution processes (McManus, 1975). However, sediment supplies to the northern Iberian shelf are being substantially modified by anthropogenic activity, since 85% of all Iberian river basins are now located upriver of barrages and dams, which have reduced the amount of sand being supplied to the shelf by ~80% (Dias, 1990). Quantities of sediments being exported from the Douro River, the main supplier of sediment t the region, have fallen from an original estimated average of 1.8×106 m3/year to 0.25×106 m3/year (Oliveira et al., 1982). In addition, extensive dredging of sands and gravel inside the estuaries and even on beaches is increasingly depriving the inner shelf of its modern sediment (Dias, 1990). Although the fine-grained sediments are only partially retained by dams, the storage of large amounts of water in lakes has greatly altered the characteristics of the floods. They now occur much less frequently, and the occasional opening of barrage floodgates produces artificial floods with different characteristics from natural ones. The problem of sediment supply to the shelf is further accentuated by the relative isolation of the GaliciaMinho Shelf. The offshore transport northwards, supplies from the north are insignificant, not only because there are no significant rivers supplying sediments in the north, but also the movement of any sediments that are being supplied via the littoral drift is blocked by the rocky outcrops that dominate the coastline. Furthermore, the main orientation of the coastline of the Iberian Peninsula changes at Cape Finisterra to an east–west direction, which marks the northern limit of the Galicia-Minho Shelf. The extension southwards of the Galicia-Minho Shelf between the Douro River and the Nazare´ Canyon (Fig. 1) is dominated by relic sediments (Dias et al., 2002), and there are few important sources of sediment supplying the shelf in this region. There are only two rivers of any size, the Vouga, which empties into the Aveiro Lagoon, and the relatively small Mondego discharges into an estuary, which acts as a sediment trap. The Nazare´ Canyon that bounds this shelf to the south is so effective as a conduit transporting sediment off the shelf that it prevents any sediment from being supplied from further south (Dias, 1987). Therefore, the only two substantial sources of sediment for the shelf are the rivers Minho and Douro, which are located in the central portion of extent of shelf. The hydrodynamic forces acting along this central region effectively sort sediments, carrying sands southwards via the littoral drift, and fines northward through large scale forcing mechanisms resulting from storms, poleward bottom currents, and downwelling. This system is currently undergoing substantial change as a result of changes in the shelf sediment supply resulting from the construction of barrages and other anthropogenic influences; change which is likely to have large-scale socio-economic consequences, through, for example, alterations to fishing grounds, increases in coastal erosion, and changes in the dispersal of contaminants.

6. Conclusions The integration of all observations leads to the following conceptual model for the transport and deposition of fine-grained sediments on the Northern Portuguese and Galician Shelf: Sediments provided by the Portuguese River basins, mainly the Douro and Minho are distributed throughout the shelf area. The main mechanism regulating the supply of sediment input is floods. Sediments provided from the Galician hinterland are trapped in the Galician Rı´as. Sediments deposited on the shelf are frequently remobilized by storm waves as confirmed by the presence of intense bottom nepheloid layers. Fine-grained sediments in suspension are transported northwards by poleward flowing bottom currents, and some are deposited in the Douro and Galicia Mud Patches, where a combination of structural features result in these areas acting as sediment traps. Transport by the north-

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ward flowing bottom currents explains why both Mud Patches lie to the north of their main sources of sediment. Extreme storm events such can through resuspension, re-introduce fines already deposited on one of the mud patches back into the system. Some sediments resuspended from the Douro Mud Patch are redeposited in the Galicia Mud Patch. Transverse rocky outcrops act as barriers greatly reducing the transport of fine sediment to the north of the Arosa Rı´a. There is evidence for the transport of sediments off the shelf when storm winds from the southwest generate downwelling conditions.

Acknowledgements This study has been supported by the EU OMEX II-II programme, Contract MAS3-CT97-0076. This paper is DISEPLA contribution number 110.

References Araujo, M. F., Dias, J. M. A., & Jouanneau, J. -M (1994). Chemical characterisation of the main fine sedimentary deposit at the northwestern Portuguese shelf. Gaia, 9, 59–65. Araujo, M. F., Jouanneau, J. M., Valerio, P., Barbosa, T., Gouveia, A., Weber, O., Oliveira, A., Rodrigues, A., & Dias, J. M. A. (2002). Geochemical tracers of northern Portuguese estuarine sediments on the Iberian Shelf. Progress in Oceanography, 52(24), 277–297. Dias, J.M.A. (1987). Dinaˆ mica sedimentar e evoluc¸ a˜ o recente da plataforma continental portuguesa setrentrional. PhD. Thesis, Lisboa. Dias, J. M. A. (1990). A evoluc¸ a˜ o actual do litoral Portugueˆ s. Protecc¸ a˜ o Civil, 10(3), 2–16. Dias, J. M. A., Gonzalez, R., Garcia, C., & Diaz-del-Rio, V. (2002). Sediment distribution patterns on the Galicia-Minho continental shelf. Progress in Oceanography, 52(2-4), 215–231. Dias, J. M. A., & Nittrouer, C. A. (1984). Continental shelf sediments of northern Portugal. Continental Shelf Research, 3, 147–165. Dias, J. M. A., Gaspar, L. C., & Monteiro, J. H. (1980). Sedimentos recentes da plataforma continental Portuguesa a norte do canha˜ o submarino da Nazare´ . Boletim da Sociedad Geolo´ gica de Portugal, 22/81, 181–185. Drago, T., Jouanneau, J. -M., Dias, J. M. A., Prud’homme, R., Kuel, S., & Soares, A. M. (1994). La vasie`re Ouest-Douro et le piegeage des sediments estuariens re´ cents. Gaia, 9, 53–58. Drago, T., Jouanneau, J. M., Weber, O., Naughton, F., & Rodrigues, A. (2000). Environmental factors controlling the depositional facies characteristics of Minho, Douro and Tejo mud patches. 3rd Symposium on the Iberian Atlantic Margin, Faro, 217–218. Drago, T., Weber, O., Jouanneau, J. M., Arau´ jo, F., & Vale´ rio, P. (1999). Geomorphological control of fine sedimentation of the northern Portuguese shelf. Boletim Espanol de Oceanografia, 15, 111–122. Fiu´ za, A. F. G. (1983). Upwelling patterns off Portugal. In Coastal Upwelling: Its Sediment Record (pp. 85–98). Plenum Press: New York. 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. Fraga, F. (1981). Upwelling off the Galician coast, North west Spain. In F. Richards (Ed.), Coastal Upwelling (pp. 176–182). Washington: American Geophysical Union. Jouanneau, J. M., Garcı´a, C., Oliveira, A., Rodrigues, A., Dias, J. M. A., & Weber, O. (1998). Dispersal and deposition of suspended sediment off Tagus and Sado on the adjacent continental shelf. Progress in Oceanography, 42, 233–257. Jouanneau, J. M., Weber, O., & Lesueur, P. (2000). Comparison of shelf mud patch sedimentation rates on the NE Atlantic margin (Biscay Bay - Western Iberia). In 3rd Symposium on the Iberian Atlantic Margin (pp. 259–260). Faro. Jouanneau, J. M., Weber, O., Drago, T., Gonzalez, R., Oliveira, A., Dias, J. A., Garcia, C., Schmidt, S., & Reyss, J. -C. (2002). Present day sedimentation and sedimentary budgets on the Northern Iberian Shelf. Progress in Oceanography, 52(2-4)(this issue), Lesueur, P., Jouanneau, J. M., Weber, O., & Tastet, J. P. (1997). Evaluation des flux continentaux se´ dimente´ s dans les vasie`res de la plate-forme atlantique franc¸ aise. In 6e`me Congre`s Franc¸ ais de Se´ dimentologie (ASF), 17–19 Novembre 1997, Montpellier (p. 293). Paris, 27: ASF. Magalha˜ es, F., Dias, J. M. A., & Taborda, R. (1991). Caracterizac¸ a˜ o e dinaˆ mica dos depo´ sitos inertes da plataforma continental Norte Portuguesa. Memo´ rias e Notı´cias do Museu e Laborato´ rio Mineralo´ gico e Geolo´ gico da Universidade de Coimbra, 112, 285–298. McManus, D. A. (1975). Modern versus relict sediments on continental shelves. Geological Society of America Bulletin, 86, 1154–1160.

J.M.A. Dias et al. / Progress in Oceanography 52 (2002) 249–259

259

Mougenot, D. (1989). Geologie de la Marge Portugaise. Documentos Te´ cnicos do Instituto Hidrogra´ fico, 32, 259. Oliveira, I.M., Valle, A.F. & Miranda, F. (1982). Littoral problems in the Portuguese west coast. Coastal Engineering 1982 proceedings, III, 1951-1969. Oliveira, A., Rocha, F., Rodrigues, A., Dias, J. A. (2000). The fine sediments as dynamic sedimentary tracers (NW Iberian margin). 3rd Symposium on the Iberian Atlantic Margin, Faro, pp. 399–400. Oliveira, A., Rocha, F., Rodrigues, A., Jouanneau, J. M., Dias, A., & Weber, O. (2002). Clay minerals from the sedimentary cover of the North Iberian Margin. Progress in Oceanography, 52(2-4), 233–247. Pita, C. & Santos, J. (1989). Ana´ lise dos temporais da costa oeste de Portugal Continental. Relato´ rio PO-WAVES 1/89-A, IH/LNEC, 29pp. PO-WAVES Group (1994). Final report of sub project A: Wave climatology of the Portuguese coast. NATO PO-WAVES, 6/94A, 80pp. Rey, J. (1993). Relacion morphose dimentaria entre la platforma continental de Galicia y las Rias bajas y su evolucion durante el Cuaternario. Publicaciones Especiales Instituto de Oceanografia, 17, 1–233. Rodrigues, A., Dias, J. M. A., & Ribeiro, A. (1992). First appraisal of active faults in the north Portuguese continental shelf. Gaia, 4, 25–30. Van Weering, Tj., & McCave, I. N. (2002). Benthic processes and dynamics at the NW Iberian margin: An introduction. Progress in Oceanography, 52(2-4), 123–128. Van Weering, T. C. E., Jouanneau, J. M., de Stigter, H. C., Schmidt, S., Thomsen, L., & Weber, O.Omex Partners (2000). Recent sediment transport processes at the NW Iberian Continental Margin. In 3rd Symposium on the Iberian Atlantic Margin, Faro (pp. 275–276). Vitorino, J., Oliveira, A., Jouanneau, J. M., & Drago, T. (2000). Winter dynamics and the transport of fine sediments on the northern Portuguese shelf. In 3rd Symposium on the Iberian Atlantic Margin, Faro (pp. 279–280). Vitorino, J., Oliveira, A., Joanneau, J. M., & Drago, T. (2002a). Winter dynamics on the northern Portugese Shelf. 1: Physical processes. Progress in Oceanography, 52(2-4), 129–153. Vitorino, J., Oliveira, A., Joanneau, J. M., & Drago, T. (2002b). Winter dynamics on the northern Portugese Shelf. 2: Bottom boundary layers and sedimental dispersal. Progress in Oceanography, 52(2-4), 155–170.