Sediment distribution patterns on the Galicia-Minho continental shelf

Progress in Oceanography 52 (2002) 215–231 www.elsevier.com/locate/pocean

Sediment distribution patterns on the Galicia-Minho continental shelf J.M.A. Dias a,∗, R. Gonzalez b, C. Garcia b, V. Diaz-del-Rio c a Universidade do Algarve, Campus de Gambelas, 8000 Faro, Portugal COACOMAR — Universidade do Algarve, Avenida das Forc¸as Armadas s/n, 8700-311 Olha˜o, Portugal Instituto Espan˜ol de Oceanografia, Centro Oceanografico de Mala´ga, Peurto Pesquero s/n, 29640 Fuengirola, Spain b

c

Abstract A sedimentological and bathymetric study of the Minho-Galicia Shelf shows a strong contrast between a southern shelf region with a thin partially relict cover of sands and gravel, and a northern region where fine-grained sediments predominate. This contrast is explained through differences in the sediment supply, the oceanographic environment (storms and ocean currents) and the morphology of the shelf which results from its underlying tectonic framework. Most sediment is supplied to the Galicia-Minho Shelf by river discharges onto the Minho Shelf, particularly that from the Douro River. In the northern part of the shelf the Galician Rı´as act as sediment traps rather than sediment suppliers. The bulk of the sediment washes out of the rivers during episodic storm events. While most of the coarse sediments remain deposited close to the coast, the fine-grained material is exported to the outer areas of the shelf. Subsequently, coarse sediments close to the coast are transported southwards by the littoral drift. Whereas the finegrained material is frequently resuspended through the action of the large swells who influence reaches deep into the water. This frequent resuspension has a long-term sorting effect on the sediments. Furthermore, resuspended sediments on the middle and outer regions of the shelf are transported northwards by a poleward flowing bottom current. As a consequence of the differential transport of coarse sediments to the south, and of the fine-grained sediments to the north, the outer reaches of the Minho Shelf are relatively poor in recent sediments. In many areas relict sediments as well as features associated with ancient coastlines and river mouths, still appear as seabed features. In contrast, the northern regions of the shelf are covered by a thin veneer of fine-grained material that smooth other most of these fossil features. The fine-grained sediment fractions (mostly very fine sands to coarse silts) are deposited in two large mud patches, the Douro and the Galicia Mud Patches, which are situated at water depths of around 100–120 m. These two mud patches are both controlled by the local hydrodynamics and morphology. The Beiral de Viana, to the west of the Douro Mud Patch is a plateau, up to 20 m high lying parallel to the shelf-break and is a morphological expression of an underlying horst system. This plateau acts as a barrier that prevents the drift of some of the fine-grained material to the west, out over the shelf-break and the continental slope. The Galicia Mud Patch is situated on the eastern part of the Galician Shelf to the north of the Douro Mud Patch. It is situated near the extension of the Porto–Tomar fault, which results in the shelf being usually steep in this region, down to a depth of about 100 m. West of this area the

∗

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 7 - 1

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slope is much more gentle. Northward transport of the sediment is strongly reduced by the E–W trending outcrops of plutonic and metamorphic rocks.  2002 Published by Elsevier Science Ltd.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 2.1. Grain size analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 2.2. Image treatment and production of maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 3. Study area . . . . . . . . . . . . . . 3.1. Geomorphology . . . . . . . . . 3.2. Tectonic framework . . . . . . . 3.3. Sediment supply to the shelf . 3.4. Oceanographic characterization 3.4.1. Wave regime . . . . . . . . . 3.4.2. Storm surges . . . . . . . . . 3.4.3. Tides . . . . . . . . . . . . . . 3.4.4. Currents . . . . . . . . . . . .

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Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

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Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

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Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

1. Introduction Continental shelves are places of transition between the continents and the deep oceanic basins. The distribution of sediments on a shelf is usually far from regular, and their occurrence is influenced by many environmental factors ranging from the characteristics of the sediment supply (e.g. riverine discharges, coastal erosion and accumulation of biogenic material) to the physical forces (wave climate, tidal regimes, currents, and up- and down-welling) that act as distributional processes (McManus, 1975). The geological history of a shelf is usually highly complex, and is documented in sedimentary sequences, faults, folding and erosional horizons that resulted from rifting and spreading events, sea-level fluctuations, subsidence and the effects of climate changes. In addition to sediment supply and environmental conditions and a region’s geological history are major influences on the prevailing sedimentary processes and environments over a shelf, through the determination of coastal and shelf morphology and differential subsidence. The importance of shelf sediment as a resource was recognized early on in Portugal. The first sediment samples collected off the western and south-western shelves of the Iberian peninsula were taken during the cruise of HMS Challenger while on passage between Great Britain and Gibraltar in 1872 (Murray & Renard, 1891). However, the first systematic analyses of shelf sediment were conducted under the auspices of the Portuguese Government around the turn of the 20th century as part of a programme to map shelf sediments. The first reliable maps of the sediment cover and the most salient morphological features of

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the Portuguese Shelf were published in 1913 (Cartas Litolo´ gicas Submarinhas). Although some important investigative work was carried out immediately after the Second World War (Pe´ res, Picard, & Ruivo, 1957; Berthois & Brenot, 1964; Allan, 1965; Berthois, Brenot, & Ailloud, 1965; Curray, Moore, Belderson, & Stride, 1966; Giesal & Seibold, 1968), a comprehensive systematic survey and analysis of the western margin of Iberia only began in the seventies and eighties. The shelf structure was described, amongst others by Boillot, Berthou, Dupeuple and Musellec (1972); Vanney and Mougenot (1981) and Lefort, Dias, Moteiro and Ribeiro (1981), and aspects of the sedimentary dynamics of the region were discussed in several papers (Dias, Monteiro, & Gaspar, 1980/81; Dias, 1987; Dias & Nittrouer, 1984). This intense first survey of the area was also synthesized in various theses (Musellec, 1974; Dias, 1987; Mougenot, 1989). It is in this context that the Minho Shelf was initially explored. The final phase of its investigation began in the late 1980s and has continued until the present time. This phase stimulated the production of a sediment cover map of the Minho Shelf (Institu´ to Hidrogra´ fico, 1999), which, in conjunction with several research projects, has lead to the publication of a series of papers, on sediment accumulation rates (Carvalho & Ramos, 1990), sediment distribution patterns (Magalha˜ es, Dias, Matos, & Carvalho, 1993), the post-glacial evolution of the shelf (Rodrigues, Magalha˜ es, & Dias, 1991), heavy minerals (Cascalho & Galopim de Carvalho, 1993), the Douro Mud Patch (Drago et al., 1994), the dynamics of suspended matter in the water column (Oliveira et al., 1998), the geochemistry of particulate matter in suspension (Almeida, Arau´ jo, Dias, Jouanneau, & Oliveira, 1993), the underlying geological structure (Rodrigues & Ribeiro, 1992/94) and possible active fault systems (Rodrigues, Dias, & Ribeiro, 1992). Work on the Galician Shelf has progressed at a much more moderate pace, and has been concentrated mainly on the Galicia Rı´as. The thesis of Rey (1993) is outstanding, in which the general morphology and depositional features of the continental shelf and the lower Rı´as, and a synthesis of the Quaternary and pre-Quaternary evolution of the shelf was presented. This uneven coverage in knowledge and understanding has partially been the result of the political border, which until recently has inhibited an integrated approach to the whole of the northern region of the western Iberian margin. The aim of this study, undertaken within the framework of the EU-funded Ocean Margin Exchange Programme (OMEX I and II) was to gather and synthesize detailed bathymetric and sedimentological data across the entire Minho-Galicia Shelf, in order to understand better the sedimentary environment and the control of large-scale factors (cf. Van Weering and McCave, 2002).

2. Methods 2.1. Grain size analyses Sediment samples were collected during a series of cruises conducted within the framework of OMEX I and II. Additional data from previous projects were also used, namely those collected during the Portuguese SEPLAT project and the Spanish cruise Breogan conducted on the Galician Shelf. The methods of sampling differed between the various cruises and included the use of a minicorer mark I, a Smith–McIntyre grab, a Reineck box-corer, a multicorer and a gravity interface corer. In general, all the material discussed here, was collected from the tops of the core samples, i.e. from the sediment surface. All samples have been analyzed for grain size using the classical methods of sieving for the coarse grained material and the pipette method for the fines (⬍63 µm). The sediments have been classified according to their gravel+sand/silt/clay ratios following the classification of Shepard (1954). Because some of the data used in this paper originated from research conducted during the 1980s and the original data are unavailable,

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the grain size boundaries used have the following lower limits: gravel — 2mm; sand — 63 µm; silt — 4 µm (rather than the modern clay/silt boundary at 2 Tm). 2.2. Image treatment and production of maps In the absence of a bathymetric map with adequate resolution and detail covering this shelf, a bathymetric map had been created based on maps produced by Rey (1993) and by Vanney and Mougenot (1981), which have been supplemented with unpublished data for the Minho Shelf. All these maps have been scanned at 300 dpi, imported into ERMapper 6.0 GIS and rectified (Datum Lisbon: Castelo de S. Jorge; Projection UTM). The maps were digitized using MAPInfo 5.5. Additional layers for sampling stations, outcrops, distribution of fines, distribution of mean grain sizes and Shepard classification were created. An additional layer containing information about the underlying tectonic fabric was created using data from Vanney and Mougenot (1981); Rey (1993) and Rodrigues, Dias & Ribeiro, 1992).

3. Study area 3.1. Geomorphology The area studied included the continental shelf off Minho and Galicia between 41°N and 42°50’N (roughly between Oporto and Cape Finisterre; Fig. 1). The shelf in this region is narrow (30–50 km wide). The shelf break occurs at depths of 160–180 m. Metamorphic and plutonic rocks dominate in the river basins discharging into the region. These rocks also form outcrops along the coast on large portions of the shelf to water depths of ~70 m. The southern limit of these outcrops lies approximately at 41°25’N (Fig. 2). They have their widest extension across the shelf at 41°45’N. The area immediately adjacent to the shelf-break is characterized by a series of narrow plateaux with elevations of up to 20 m bounded by cuestas or down-dip scarps (e.g. Beiral de Viana; Fig. 1). These rock plateaux are a barrier to the movement of sediment towards the shelf-break and beyond to the continental slope. The shelf-break itself is indented by a series of steep ravines, many of which converge into the Porto Canyon which is located around 41°20’N and is by far the most important morphological feature of the outer shelf and slope (Fig. 1). There are some notable differences between the northern and southern sectors of the study area, with a transition at about 42°N, just to the south of the Vigo Rı´a (Fig. 1). Throughout the southern sector all major relief lies roughly parallel to the coastline, giving the shelf a relatively simple, monoclinal structure. This monocline steepens towards the north. Five major rivers supply the coastal region with sediment (from south to north — the Douro, Ave, Ca´ vado, Lima and Minho. The mouth of each of these rivers is an estuary confined within a narrow valley. The northern sector of our study area is characterized by the Lower Galician Rı´as (from south to north — Vigo, Pontevedra, Arosa and Muros). The Rı´as are structurally controlled Tertiary river valleys, bounded by steep hills and mountains. They are valleys that were incised during the low sea-level stands of the Quaternary and then drowned during the last transgression (Rey, 1993). They are ~10 km wide at their mouths. This inner portion of the Galician Shelf is relatively steep. The outer shelf forms a re-entrant arc between 42° and 42°40’N. There are no plateaux bordering the outer edge of the shelf, and the shelf-break is generally poorly defined. However, north of 42°30’N the width of the shelf narrows to just 25 km off the Rı´a Pontevedra, and there the shelf-break is clearly defined. North of the Rı´a Arosa metamorphic and plutonic outcrops extend westwards across the shelf forming an E–W barrier.

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Fig. 1.

Bathymetry of the Galicia-Minho Shelf, including names of rivers and Rı´as as well as morphological features.

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Fig. 2.

Tectonic framework of the Galicia-Minho Shelf, including the location of the Douro and Minho Mud Patches.

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3.2. Tectonic framework A map of the regional tectonic framework (Fig. 2) was compiled from a variety of sources (Boillot et al., 1974; Musellec 1974; Lefort et al., 1981; Vanney & Mougenot 1981; Mougenot, 1989; Murillas et al., 1990; Rodrigues & Drago 1990; Drago, Magalha˜ es, Rodrigues, & Dias, 1991; Ribeiro, Rodrigues, & Dias, 1991; Rodrigues, 1991; Rodrigues, Dias & Ribeiro, 1992). The Minho-Galicia Shelf is segmented into small (tens of kilometres) tectonic units, mostly by dextral strike-slip faults striking in a WSW–ENE direction, roughly perpendicular to the coastline. The most prominent of these faults continue into the interior of the Iberian hinterland and often coincide with the river valleys. A second system of faults run parallel to the coast, the most prominent of these is the seaward extension of the Porto–Tomar fracture zone that strikes NNE–SSE and separates Precambrian and Paleozoic plutonic and metamorphic basement in the east from the Meso-Cenozoic sediments in the west (Lefort et al., 1981; Fig. 2). These two fault systems divide the shelf into small counterclockwise rotated ‘en echelon’ type tectonic blocks. Each of these blocks seems to have become independently adapted to changes in the hydro-isostatic equilibrium during the last sea-level rise (Vanney & Mougenot, 1981; Dias, 1987; Rey, 1993). Most of the shelf to the south of about 42°N is situated between outcrops of plutonic and metamorphic rocks in the eats and Mesozoic sediments in the west. This area forms a graben about 20 km wide, called the Pontal da Galega Graben (Rodrigues et al., 1992; Fig. 2), and the outcropping Mesozoic strata and part of the continental slope form a complex Horst system (Fig. 2). The northern boundary of this Horstgraben complex is more or less where the Mesozoic strata cease outcropping and the shelf limit forms a wide arc. Many of these fault systems are still active (Ribeiro et al., 1979, 1991). 3.3. Sediment supply to the shelf The two most important of the river basins in the region are the Douro (catchment area 95,682 km2) and the Minho (catchment area 17,081 km2). The Douro alone supplies about 79% of all the sediment reaching the shelf. The total annual sediment supply from all the rivers has been estimated to be about 2.25×106 tonnes (Oliveira, Valle, & Miranda, 1982). Rivers discharging to the south of 41°N supply very little sediment to the shelf (Dias & Nittrouer, 1984; Dias & Neal, 1990). The same is true for the Galician Rı´as, because they trap most of the sediment being discharged by their rivers (Rey, 1993; Jouanneau et al., 2002). 3.4. Oceanographic characterization 3.4.1. Wave regime Mean wave heights vary strongly with season (Vitorino, Oliveira, Jouanneau, & Drago, 2002a,b). In summer typical significant wave heights are 1–3 m, with periods of 11–13 s. Whereas during winter storms they often exceed 7 m, typically with periods of 13 s, but can exceed 18 s (Costa, 1994). On the west coast, the wave regime is dominated by swells from the NW (73%) with those from the W contributing 16% (Costa, 1994). 3.4.2. Storm surges Most storms occur during the winter months (October–March), and most come from the W or NWN. Most (57–91%) storm waves have heights of 5–6 m, but every 3–4 years a more violent storm occurs that generates waves 9–12 m in height (Pita & Santos, 1989). On average the Portuguese coast is exposed to three storms a year, each lasting around 4 days. The longest continuous period of stormy weather recorded was 14 days (Pita & Santos, 1989).

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3.4.3. Tides The tides along the Portuguese coast are semi-diurnal, with periods of about 12.25 h, and the tidal wave propagates from south to north. The tidal regime is mesotidal, with an average spring tide range of 3.02 m at Viana do Castelo and a neap tidal range of 1.37 m (Institu´ to Hidrogra´ fico, 1999). The maximum tidal range in the region is about 3.9 m and the minimum range is ⬍1 m. 3.4.4. Currents The main surface current in the region is the Portugal Current (also referred to as the Canaries Current), which flows southwards at average speeds of 0.5–0.75 knots (0.9–1.4 km/h) (Macmillan–Reeds Nautical Almanac, 2001). On the continental slope there is a significant bottom current that originates in the Mediterranean and flows northwards counter to the surface currents (Fraga, 1981). This slope current has a maximum velocity of 40 cm/s in the south of our study area, but its strength diminishes to the north of ~40°N (Ambar, 1984). Furthermore, the Galicia-Minho Shelf experiences seasonal upwelling of cool nutrient-rich water from depths of less than 500 to 1000 m along the coast and into the Rı´as (Rey, 1993). Fiu´ za (1982a) estimated the depth of this provenance to be about 60–120 m. The upwelling is the result of winddriven offshore advection of the surface waters (Madelain, 1967; Fraga, 1981), and may be influenced by the poleward flowing bottom currents (Miller, Gomez-Gallego, Tenore, & Parilla, 1974). It predominantly occurs during the summer (June–August) when winds blow intensely and persistently from the north (Fiu´ za, 1982b). According to Fiu´ za (1982b) the directions of the currents resulting in upwelling are strongly dependent on the bathymetric and morphological characteristics of the regions. A reversal of the usual circulation pattern can occur during storms, resulting in downwelling (Drago et al., 1998; Vitorino, Oliveira, Jouanneau, & Drago, 2000; Vitorino et al., 2002a,b). During such storm events, sediments can be resuspended so that the fine-grained material are transported polewards by the bottom currents (Drago et al., 1998).

4. Results The main results of this study are summarized in Figs. 3–6. The local areas within the Rı´as have been omitted because the high spatial variability of their sediments could not be depicted at the scale of these maps. Fig. 3 shows the distribution of silt and clay over the Minho-Galicia Shelf. Two areas stand out as being dominated (⬎90%) by sediments finer than 63 µm stand out: the Douro and Minho Mud Patches. The Douro Mud Patch lies at a depth of 90–120 m to the NW of the estuary of the Douro River and close to the head of the Porto Canyon. This patch has a maximum width of ~8 km and a length of ~35 km. If sediments with ⬎50% fines are included within the patch, then its width becomes 8–18 km and its length is 42 km. Thus its area is estimated to be 504 km2. To the west it is bounded by outcrops of Mesozoic and Cenozoic strata (Fig. 3). Including the area where the fines ⬎50% extends the patch (sensu strictu) northwards in the lee of the Mesozoic outcrops. The patch is a Holocene sedimentary body, the base of the sediment (at a depth of 3.1 m) was laid down ~1400 bp over consolidated sediments of Palaeocene and Eocene age (Drago et al., 1994). It seems likely that the Douro Mud Patch is under topographical control. Its geographical position is controlled by the Beiral de Viana (Fig. 1), which is of tectonic origin (Rodrigues & Ribeiro, 1992/94). This outcrop of bedrock forms a barrier to the transport of sediment westwards towards the shelf break and continental slope. There is evidence that the faults defining the barrier are still active (Rodrigues et al., 1992; Rodrigues & Drago, 1990), and its relief has probably increased in historic times, increasing its effectiveness as a sediment trap. The second area dominated by fines (90% ⬍63 µm) is the Galicia Mud Patch that occurs at a depth of

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Fig. 3.

Distribution of silt and clay on the Galicia-Minho Shelf.

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Fig. 4.

Classification of sediments according to Shepard (1954) on the Galicia-Minho Shelf.

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Fig. 5. Typical histograms of grain size observed along the Galicia Mud Patch. Similar to the case of the Washington Continental Shelf (Nittrouer, & Sternberg, 1981), the percentage of silt decreases northwards, documenting the increasing ‘downstream’ sorting of the sediments, and coarser particles are left behind.

110–120 m off the mouth of the Minho River. It is linear feature 50 km long and 2–3 km wide that is orientated N/S. In the north it terminates at a water depth of 120 m off the Rı´a de Pontevedra. Considering sediments with ⬎50% fines as being in the patch doubles its length and width (Fig. 3). These two mud patches are clearly identified using the sediment classification of Shepard (1954) (Fig. 4). The sediments within these mud patches are predominantly coarse silt (Fig. 5). Sediments, which are rich in clays, are only found on the shelf break, to the west of the Mesozoic outcrops that bound the Douro Mud Patch. The origins of these two types of fine sediment are probably not linked. Fig. 6 illustrates the distribution of the mean grain sizes of sediments in the study area. It again clearly shows the two Mud Patches, although the sediment in the Galicia Patch is slightly coarser than those in the Douro Patch (Jouanneau et al., 2002). According to these authors sedimentation rates are higher in the

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Fig. 6.

Mean grain size distributions on the Galicia-Minho Shelf.

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Douro Mud Patch — 0.17 cm/yr at its outer limit and 0.58 cm/yr on the eastern side closest to its source of sediment the Douro River. Accumulation rates in the Galicia Mud Patch are between 0.06 and 0.21 cm/yr. Elsewhere the shelf is mostly covered with very fine-grained sand (125–63µm). There are some notable exceptions in the south where there are several strips of coarse sands and gravels orientated roughly parallel to the coastline at depths of between 20 and 70 m (Fig. 5). These sediment bodies are associated with paleo-coastlines and so are relict (Dias, 1987; Rodrigues, Magalha˜ es & Dias, 1991). Gravel beds are also to be found in the mid-shelf region, where they are associated with the ancient mouths of the rivers Ave, Douro and Ca´ vado. Another area rich in gravel occurs close to the Beiral de Viana outcrop and may be related to the course of the Beiralis River, which prior to 16,000 bp linked the Rivers Minho, Ave, Ca´ vado and various other minor rivers (Rodrigues and Dias, 1992). At the shelf break and around the outcropping Mesozoic strata there are areas of slightly coarser fine-grained sands (125–250 µm). 5. Discussion The southern part of the Galicia-Minho shelf the sediments are rich in sands and some gravel containing ⬍5% silt and clay. In the north the sediments are richer in fines (there are virtually no sediment with ⬍5% fines), and fine-grained sediments dominate many areas. This contrast reflects the differing supply systems, the sorting effects of storms and currents, and the influence of control of topographical structure on the sedimentary regime. The river basins supplying the Minho shelf drain the nearby mountain ranges. The rivers have young profiles, narrow estuaries and are subject to frequent flooding. Hence they have high capacities for sediment transport. In contrast, the Rı´as along the Galician coastline function as efficient sediment traps, so there the sediment supplies are trivial. Most of the southern sector of the study area is relatively shallower than the northern sector. Water depths offshore from the Rı´as deepened quite rapidly to ⬎100 m within a few kilometers of the coast, and within the Rı´as depths are typically 50 m. The following scenario for sediment flux on the Minho-Galicia Shelf can be developed. Most sediment reaches the shelf via flood discharges from the river systems in the south, and its sediment export from the Rı´as is negligible. The bulk of the coarse-grained sediments discharged from the rivers is initially deposited close to the estuaries. The finer-grained sands are carried further offshore to where water depths are 20–30 m. Most of the silt and clay particles remain in suspension for much longer periods and so get transported much further offshore. The sand and gravel deposited close to the shore, tend to get moved southwards by the littoral drift during periods of fair weather. So the amounts of sand presently reaching the deeper areas are small. During periods of stormy weather swells predominantly coming from the NW remobilize the fines. The sands are only moved during the most extreme wave conditions, and once deposited the gravels remain in place (Magalha˜ es, Dias, & Taborda, 1991). Resuspended sediments are initially carried south and east by the swells. The sands are quickly re-deposited once conditions calm down, but the silts and clays tend to remain in suspension. The storms result in down-welling so that the suspended material comes under the sphere of influence of the poleward flowing bottom current, and hence is transported both northwards and offshore (Drago et al., 1998). After a sequence of re-suspension events a proportion of the clays and silts become deposited on the two Mud Patches. There they tend to remain for long periods, only being moved once again during the most extreme storms. Thus the coarser sediments are predominantly transported south and the fines to the north and west. As a result the superficial sediment cover of the Galicia Shelf region is predominantly fine material whereas over the Minho Shelf it is coarser. On the Galicia Shelf relict sediments laid down during the last glaciation have smoothed out some of the pre-existing topographical features. Whereas on the Minho Shelf where sand and gravel deposits are predominant mainly in shallow areas, in the deeper areas supplies of fine

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material have neither frequent enough nor abundant enough to have smoothed out the older topographical features or to cover relict sand and gravel deposits associated with old coastlines. Hence relict features are still clearly detectable in the southern region of the Minho Shelf (Dias & Nittrouer, 1984). The percentage of coarse silt in both mud patches is high, frequently reaching ⬎70% of the sediments (Fig. 5). Although similarly high percentages of silt on modern continental shelves, slopes and submarine canyons have been observed of the northwest coast of America (Nittrouer & Sternberg, 1981; Nittrouer, Sharara, & De Master, 1983; Thorbjarnarson, Nittrouer, & McMaster, 1986), they are relatively rare features. As on the Washington Shelf, the Minho-Galicia Shelf is relatively starved of clays because of its highly energetic wave climate, so that the deposition of clay-sized minerals is limited, frequently ⬍10% (Dias, 1987; Oliveira et al., 2002). The processes described above provide a very effective mechanism for separating fine-grained from coarser material. While the main controls of sediment remobilization and transport are certainly hydrodynamic, the locations of the mud patches are strongly influenced by the topography, with geomorphological features acting to trap the sediment. The two mud patches are separated by nearly 30 km (41°35’N–41°45’N), which argues against a purely hydrodynamic control (compare for example these features with that on the Washington Continental Shelf, where the mud patch is continuous (Nittrouer & Sternberg, 1981). While the Douro Mud Patch is set against the Mesozoic outcrops in the southern sector of the shelf, the Galicia Mud Patch lies to the west of plutonic and metamorphic outcrops in an area where the unusually steep dip of the Galicia Shelf decreases. This topography is controlled by an underlying graben system (Fig. 2).

6. Conclusions Our study has allowed the first bathymetric and sedimentological analysis of the entire Galicia-Minho Shelf. The Galicia Shelf is much richer in fine sediments than is the Minho Shelf where the sediments are predominantly fine sands. This is interpreted as being related to the sorting processes that result from oceanic forces interacting with the topography. Fines are transported northwards by the bottom currents, while the heavier sands are kept closer inshore where they are transported southwards during storms and heavy wave action. Relict sediments are to be found on the Minho Shelf, because they remain unobscured by recent deposition. In contrast on the Galicia Shelf most such features have been buried by a veneer of recent sediment, that has smoothed out the pre-existing morphology. Fine-grained sediment (mostly coarse silt and very fine-grained sand) are accumulated in the Douro and Galicia Mud Patches, as a result of a combination of regional morphology that acts as a sediment trap, and the transport mechanisms that move mainly fine-grained sediments into these regions. Coarser-grained material remains deposited close into shore on the Minho Shelf, and remain trapped within the Rı´as neighbouring the Galicia Shelf. The topography, which in fact controls sedimentation on the Galicia-Minho Shelf, is mainly a product of tectonic processes. A graben system dominates the underlying geological structure of the shelf. The geological structure changes at the latitude of the Minho River, so that the Douro Mud Patch in the south has become deposited against Mesozoic bedrock, whereas the Galicia Mud Patch further north has been deposited on the opposite side of the graben system.

Acknowledgements Financial support for this study was provided by the European Project OMEX. The authors wish to express their thanks to Isabel Mendes for putting the maps into GIS format. Fernando Magalha˜ es supplied data on the Portuguese Shelf and digitized the bathymetry of the Minho Shelf. The Portuguese Hydrographic

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Institute (Instituto Hidrogra´ fico) provided the results of grain size analyses of the CORVET cruise, while the sedimentology laboratory at the same institution carried grain size analyses for the CD 100 samples. This paper is DISEPLA contribution No. A209.

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