Sediment dynamics and post-glacial evolution of the continental shelf around the Blanes submarine canyon head (NW Mediterranean)

Progress in Oceanography 118 (2013) 28–46

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Sediment dynamics and post-glacial evolution of the continental shelf around the Blanes submarine canyon head (NW Mediterranean) Ruth Durán a, Miquel Canals a,⇑, Galderic Lastras a, Aaron Micallef a, David Amblas a, Rut Pedrosa-Pàmies a, José Luis Sanz b a b

GRC Geociències Marines, Departament d’Estratigrafia, Paleontologia i Geociències Marines, Facultat de Geologia, Universitat de Barcelona, E08028 Barcelona, Spain Instituto Español de Oceanografía, E28002 Madrid, Spain

a r t i c l e

i n f o

Article history: Available online 9 August 2013

a b s t r a c t The Blanes submarine canyon (BC) deeply incises the Catalan continental shelf in the NW Mediterranean Sea. As a consequence of the closeness (only 4 km) of its head to the coastline and the mouth of the Tordera River, the canyon has a direct influence on the shelf dispersal system as it collects large amounts of sediment, mainly during high-energy events. Multibeam bathymetry, backscatter imagery and very-high resolution seismic reflection profiles have allowed characterizing the morphology of the continental shelf around the canyon head, also identifying sediment sources and transport pathways into the canyon. The morphological data have also been used to reconstruct the evolution of the continental shelf during the last sea-level transgression so that the current understanding of shelf-to-canyon sediment exchanges through time could be improved. The continental shelf surrounding the BC consists of both depositional and erosional or non-depositional areas. Depositional areas display prominent sediment bodies, a generally smooth bathymetry and variable backscatter. These include: (i) an area of modern coarse-grained sediment accumulation that comprises the inner shelf; (ii) a modern fine-grained sedimentation area on the middle shelf offshore Tossa de Mar; and (iii) a modern sediment depleted area that covers most of the middle and outer shelf to the west of the canyon head. Erosional and non-depositional areas display a rough topography and high backscatter, and occur primarily to the east of the canyon head, where the arrival of river-fed inputs is very small. In agreement with this pattern, the continental shelf north and west of the canyon head likely is the main source of shelf sediment into the canyon. To the north, a pattern of very high backscatter extends from the coastline to the canyon head, suggesting the remobilization and off-shelf export of fines. Additionally, relict near-shore sand bodies developed over the Barcelona shelf that extend to the canyon head rim constitute a source of coarse sediment. High-energy processes, namely river floods and coastal storms, are the main controls over the river-shelf-canyon sediment exchange. River floods increase the delivery of terrigenous particles to the coastal system. Storms, mainly from the east, remobilize the sediment temporarily accumulated on the shelf towards the canyon head, so that the finer fractions are preferentially removed and a coarse lag is normally left on the shelf floor. Exceptionally, very strong storms also remove the coarse fractions from the shelf drive them into the canyon. Processes like dense shelf water cascading, which is much more intense in canyons to the north of BC, and the Northern Current also contribute to the transport of suspended sediment from far distant northern sources. During the last post-glacial transgression the BC had a strong influence on the evolution of the inner continental margin, as it interrupted the shelf sediment dispersal system by isolating the shelves to its north and south, named La Planassa and Barcelona shelves, respectively. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Sediment dynamics in many continental shelves depends on the balance between sediment supply by rivers and its dispersal across the shelf (Nittrouer and Wright, 1994). Among the general factors that influence the across shelf transport of particles towards the ⇑ Corresponding author. Tel.: +34 934021360; fax: +34 934021340. E-mail address: [email protected] (M. Canals). 0079-6611/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pocean.2013.07.031

shelf break are the shelf morphology and the hydrodynamic regime. Irregular shelves, with promontories and changing width, can modify the along-shelf circulation thus increasing the off-shelf transport of particles such as off Cap de Creus promontory (Canals et al., 2006; Puig et al., 2008; Ribó et al., 2011). Similarly, submarine canyons incising the continental shelf may enhance the offshelf transport of sediment, either by funnelling fluvial sediment to the deep sea, like in Sepik (Kineke et al., 2000; Walsh and Nittrouer, 2003), Monterey (Xu et al., 2002; Paull et al., 2003) and

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Kao-ping (Liu et al., 2002; Liu and Lin, 2004) canyons, or by intercepting the shelf sediment dispersal system, like in La Jolla (Shepard and Dill, 1966), Quinault (Cutshall et al., 1986), Eel (Mullenbach and Nittrouer, 2000; Puig et al., 2003; Mullenbach et al., 2004), Cap de Creus (Canals et al., 2006), Nazaré (Oliveira et al., 2007) and Bari (Turchetto et al., 2007) canyons. The across and off-shelf transport of sediment also changes as a function of short-term high-energy events such as river floods, storms or dense shelf water cascading (DSWC). River floods increase the transfer of terrigenous material to the coastal area (Granata et al., 1999; Ogston et al., 2000; Liu et al., 2002; Zúñiga et al., 2009) that can be subsequently remobilized during storms (Xu et al., 2002; Liu and Lin, 2004; Palanques et al., 2008). When a storm coincides with a period of high river discharge, suspended sediment concentrations are considerably enhanced, leading to an increased export of shelf sediment to submarine canyon and the continental slope at large (Mullenbach and Nittrouer, 2000; Ogston et al., 2004; Ulses et al., 2008). DSWC has also been identified as an efficient mechanism in promoting the dispersal of sediment over the shelf and off-shelf through submarine canyons mainly (Canals et al., 2006; Turchetto et al., 2007; Bourrin et al., 2008; Puig et al., 2008; Sanchez-Vidal et al., 2008; Pasqual et al., 2011; Ribó et al., 2011). The characterization of the sediment dynamics on the continental shelf adjacent to a submarine canyon head allows improving the understanding of the continent-shelf-canyon system functioning. Continental shelves are both sinks and major sources of sediment, organic matter and pollutants that can be transported towards deep areas after being trapped by submarine canyons (Canals et al., 2006; Palanques et al., 2008; Puig et al., 2008; Salvadó et al., 2012). Several authors have reported on the effects of submarine canyon hydrosedimentary processes over marine biodiversity and living resources, also in the study area (Gili et al., 1999; Company et al., 2008; Sardà et al., 2009). The magnitude and nature of shelf-canyon sediment exchanges have important implications for the morphological and stratigraphical development of continental shelves and slopes too (Walsh and Nittrouer, 2003). The analysis of the fine scale geomorphology of the continental shelf can potentially provide significant insight into sediment dynamics. The shape, orientation and distribution of large-scale depositional features and superimposed bedforms are strong indicators of the effects of cumulated sediment transport and the processes behind (Belderson and Stride, 1969; Dalrymple et al., 1978; Flemming, 1980; Allen, 1982; Ashley, 1990; Li and King, 2007; Barnard et al., 2011). High-resolution multibeam bathymetry systems constitute nowadays a fundamental tool to achieve such an analysis because of their ability to yield high quality and density seafloor data that could be displayed in various forms such as bathymetry, slope gradient or backscatter maps (Barnard et al., 2012; Hughes Clarke, 2012). Swath mapping shelf areas requires a stronger effort in terms of time and cost as the coverage per swath is less because of its shallower depth compared to slope and deeper ocean regions. In this paper we present a detailed geomorphologic analysis of a narrow canyon-incised continental shelf using multibeam bathymetry, including backscatter data, complemented by wide-spaced very-high resolution seismic reflection profiles, in order to: (i) identify the potential sources and transport pathways of sediment across the shelf and into the canyon, and (ii) reconstruct the evolution of the continental shelf since the Last Glacial Maximum to understand how shelf-to-canyon sediment processes have changed in space and time. The continental shelf around the Blanes submarine canyon (BC), in the NW Mediterranean Sea, was chosen for this study because: (i) the shelf is very narrow where it is incised by the canyon head, which itself is located in the vicinity of the coastline with a fluvial source ; and (ii) previous studies of the BC revealed an intense shelf to canyon transport of sediment, particularly

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during episodic high-energy events (Ulses et al., 2008; Zúñiga et al., 2009; Sanchez-Vidal et al., 2012; Pedrosa-Pàmies et al., this issue).

2. General setting 2.1. Geological setting The study area comprises the segment of the Catalan continental shelf around the BC head, between 41°300 N and 41°450 N (Fig. 1). The BC is 184 km long and has a nearly N–S trending course in its shelf-incised section (Amblas et al., 2006; Lastras et al., 2011). The continental shelf neighbouring the canyon head includes the southernmost part of the La Planassa shelf to the east, the northernmost part of the Barcelona shelf to the west, and a narrow shelf stretch between the coastline and the tip of the canyon head to the north (Fig. 1). Hercynian granitoids outcrop along the southern Costa Brava and the Maresme coastline, with colluvial and alluvial deposits of Quaternary age forming a narrow coastal fringe along the Maresme coastal stretch and in the Tordera delta (IGC-ICC, 2010) (Fig. 2A). According to the onshore geological structure, the Barcelona and La Planassa shelves in the neighbourhood of the Blanes canyon head are dominated by NE–SW structural directions (Fig. 2A). In the Barcelona shelf, a large listric fault named Barcelona Fault and other parallel to subparallel faults bound the NE–SW oriented Malgrat High off the Maresme coast (IGC-ICC, 2010) (Fig. 2A). A vertical slip rate of 0.02–0.04 mm yr1 has been estimated for the Barcelona Fault during the Plio-Quaternary (Perea et al., 2006, 2012). The coastline parallel Costa Brava Fault cutting the innermost La Planassa shelf represents the north-eastern extension of the Barcelona Fault (IGC-ICC, 2010; Durán et al., 2012) (Fig. 2A). The main morphological expressions of the structural offset associated to this fault are the coastal cliffs of the Costa Brava, the steepness of the inner shelf, and the lithological contrast between the coastal Hercynian granites and the rocky outcrops and the overconsolidated Plio-Quaternary sediment of the middle and outer shelf (Serra, 1976; ITGE, 1989). The Plio–Quaternary architecture of the continental shelf near the BC consists of a vertical stacking of sequences separated by major discontinuities (Serra, 1976; ITGE, 1989; Liquete et al., 2008). Within these sequences, forced-regressive deposits (FRST; Fig. 2B) are the predominant element comprising the outer shelf sequences, whilst transgressive deposits (TST; Fig. 2B) are limited to thin units of reworked sands (Liquete et al., 2008). The most recent sequence (unit E; Fig. 2C) overlies an erosional surface (SB 4; Figs. 2C and D) developed during the last sea-level lowstand of Marine Isotope Stage (MIS) 2 and comprises the transgressive (TST) and highstand (HST) system tracts (Liquete et al., 2008). In the Barcelona shelf, this TST is defined by a series of prograding sediment bodies (Díaz and Maldonado, 1990; Serra and Sorribas, 1993; Liquete et al., 2007), whereas in the La Planassa shelf it is almost absent, with large rocky outcrops dominating the middle shelf (Serra, 1976; ITGE, 1989; García et al., 2011). In both shelf areas, the HST is mostly restricted to the inner shelf modern prodeltas and infralittoral prograding wedges (IPW) (Serra, 1976; ITGE, 1989; Serra et al., 2003, 2007). General descriptions of the morphology of the studied continental shelf can be found in Serra (1976) and ITGE (1989). Such works are based on single-beam echo sounder and side scan sonar data, seismic reflection profiles and bottom samples and describe large seafloor features such as prodeltas, IPW, middle-shelf sediment bodies and rocky outcrops. The sparse bathymetric coverage did not allow these authors to describe adequately these features in the study area. Recently, new multibeam bathymetry data from

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Fig. 1. Location and general bathymetric map of the BC area. The shadowed area on the continental shelf indicates the swath coverage area available for this study. Contours are every 100 m. The location of Figs. 4–11 is also shown.

the northern Catalan continental shelf have revealed a complex seafloor, particularly near canyon heads, where large seafloor features such as modern and relict sediment bodies and numerous rocky outcrops have been identified (Lastras et al., 2011; Durán et al., 2012). The high-resolution swath bathymetry data presented in this study allows precisely defining the morphology and shape of the above-mentioned seafloor features and helps recognizing many other previously unknown and poorly known features. 2.2. Sediment input The Tordera River and several torrents, such as the Tossa de Mar and Lloret de Mar ones, fed the continental shelf of the BC area (Fig. 2A). Their sediment input depends directly of the rainfall

regime, which is characterized by long dry periods interrupted by short, strong events that can result in floods within a few hours, especially in the case of eastern storms carrying wet air against the coastal relieves (Martín-Vide, 1982; Martin-Vide et al., 2008). The Tordera River catchment area covers 879 km2 and has a maximum altitude of 1684 m and a mean slope of 3.8° (Liquete et al., 2009). The Tordera River headwaters are located in the MontsenyGuilleries Massif and the Pre-Littoral Chain. The drainage system is incised to a large extent in the granites of the Catalan Coastal Ranges (Fig. 2A). The Tordera River releases to its mouth high amounts of coarse immature sands carried as bedload and fine sediment in suspension. Sand sizes represent approximately 83% of the total sediment discharge of the river (Rovira et al., 2005; Liquete et al., 2009). The immature character of these sediments

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Fig. 2. (A) Geological map of the Blanes coastal area. The offshore structural elements have been synthesized from Serra (1976), Bartrina et al. (1992), Mauffret et al. (1995) and Maillard and Mauffret (1999) and the onshore structure is adapted from IGC-ICC (2010). BF: Barcelona Fault. CBF: Costa Brava Fault. MF: Montseny Fault. (B) Across-shelf and (C) along-shelf and across-canyon sections showing the seismostratigraphic configuration of the study area synthesized from Serra (1976) and Liquete et al. (2008). FRST: Forced Regressive System Tracts. TST: Transgressive System Tracts. HST: Highstand System Tracts. (D) Main sequence boundaries (SB1 to SB4) and units (A to E) correlated to Marine Isotope Stages MIS1 to MIS10 (Liquete et al., 2008).

is due to the nature of source rocks, to the relatively short distances travelled and to the dominant fast flood torrential regime of the river.

2.3. Hydrodynamic setting The Catalan continental shelf is a wave-dominated, microtidal (<0.2 m) environment that has a seasonal wave climate with high-energy events occurring during fall and winter mostly. The cyclonic Northern Current (NC; Millot, 1999) is a quasi-permanent geostrophic current flowing along the continental slope and shelf break that often develops meanders or eddies eventually invading the continental shelf (Font et al., 1995; Rubio et al., 2005). The 30 km wide stream of the south-westwards flowing NC moves at speeds of up to 35 cm s1 near the surface (Durrieu de Madron et al., 1990; Monaco et al., 1990), thus generating a dominant south-westward transport of suspended sediment from the north-east (Canals et al., 1995; Flexas et al., 2002; Arnau et al., 2004; Ulses et al, 2005; Heussner et al., 2006).

Northerly and easterly winds prevail over the Blanes shelf. Strong northerly winds mostly occur during December and January, whilst easterly winds are more frequent during February, March, April and November (Bolaños et al., 2009). Cold and dry northerly winds are responsible for the formation of dense shelf water over the Gulf of Lion and the northern Catalan continental shelf. This dense water mass flows southward along the shelf and cascades down the continental slope through submarine canyons, thus resuspending and transporting large amounts of sediment (Dufau-Julliand et al., 2004; Canals et al, 2006; Puig et al., 2008; Ulses et al., 2008; Palanques et al, 2009; Ribó et al, 2011). Because of the short fetch, northerly winds trigger only small waves over the inner-shelf (Gómez et al., 2005; Bolaños et al., 2009). In contrast, humid marine winds blowing from the east are associated to large swells resulting from some hundreds of kilometres fetch that cause intense sediment resuspension along the coastline (Monaco et al., 1990; Mendoza and Jiménez, 2008; Sanchez-Vidal et al., 2012). The storm wave base is located at 20 m depth (Calafat, 1986), but it can move down to at least 30 m during the strongest storms (Sorribas et al., 1993). Because waves approach the coast at

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oblique angles during eastern storms, they generate an intense south-westward alongshore transport of sediment that can be up to 45,000 m3 yr1 (DGPC, 1986) or even 83,000 m3 yr1 (Copeiro, 1982) off the Maresme Coast (Fig. 3).

3. Methods The morphosedimentary analysis of the continental shelf near the Blanes submarine canyon head is based on the integration of multibeam swath bathymetry data, including backscatter, with widely-spaced sub-bottom seismic reflection profiles (Fig. 1). Swath bathymetry data cover an area of about 525 km2 and were acquired with two Simrad systems, the EM-3000D and the EM1002S, during several cruises carried out between 2004 and 2010. The EM-3000D is a dual system with two sonar heads, each of them with a swath width of 130°. This system uses 254 beams and operates at a frequency of 300 kHz with a maximum ping rate of 40 Hz. The EM-1002S system operates at a frequency of 95 kHz with a maximum ping rate of 10 Hz, and it forms 111 beams. Measurement accuracies are 5 cm and 10 cm of root mean square for the EM-3000D and EM-1002S, respectively. Deep-water EM-120 multibeam data, on the other hand, were acquired over the continental slope. Multibeam bathymetry data were supplemented with single beam data from various sources (e.g. nautical charts, former bathymetric maps and open access bathymetric databases) in the shelf areas lacking swath bathymetry coverage (Fig. 1). Positioning during multibeam data acquisition was by differential GPS. Multibeam bathymetry data processing was carried out with CARIS HIPS and SIPS software. Backscatter strength, originally measured in decibels (dB), represents the amount of energy that is scattered from the seafloor

back to the receiver transducer. Backscatter is influenced by several factors including surface roughness, impedance contrast and volumetric heterogeneity, showing a good correlation with the mean grain size (Goff et al., 2005; Collier and Brown, 2005). Despite the limitations of backscatter intensity in predicting seafloor texture, high backscatter intensities can be associated to larger grain sizes and low backscatter intensities to smaller grain sizes (Collier and Brown, 2005; Amblas et al., 2006; Micallef et al., 2012). In parallel to the multibeam bathymetry data acquisition, very high-resolution seismic reflection data were collected with a Simrad 018 TOpographic PArametric Sonar (TOPAS). The TOPAS transmitted each 1–2 s, with a beam angle of approximately 5°, and a modulated frequency sweep (chirp) ranging between 3 and 5 kHz.

4. Results 4.1. Morphology of the continental shelf The continental shelf shoreward of the BC tip is just 4 km wide. Subsequently, shelf width increases towards the Barcelona and La Planassa shelves (up to 15 and 18 km, respectively). The shelf of the study area comprises: (i) a narrow (0.4–0.7 km) inner shelf, extending from the coastline to 30–50 m depth, (ii) a wider middle shelf from 30–50 to 90 m depth, and (iii) an outer shelf from 90 m depth to the shelf edge. The shelf edge is located at 125–150 m water depth along the western rim of the Blanes Canyon, and at less than 100 m water depth along the eastern rim (Fig. 3). The inner shelf shows a relatively steep seafloor (1.5° average slope gradient; Fig. 4A) that ends seaward in an abrupt step (up to 20°). The inner shelf displays numerous relieves that correspond

Fig. 3. Shaded relief and bathymetry map of the study area showing the main seafloor features identified on the continental shelf adjacent to the BC head. Contours are at 10 m intervals down to 200 m and every 100 m in the slope and canyon. Onland orthophotomap from ‘‘Institut Cartogràfic de Catalunya’’. See Fig. 1 for location.

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to IPWs, small prodeltas and rocky outcrops (Fig. 3). To the west of the canyon head, the general bathymetric trend of the Barcelona middle and outer shelf is disrupted by several morphological steps and narrow ridges that show gradients in excess of 2° and up to 14° (Fig. 4A). To the east of the canyon head, the La Planassa shelf shows a rugged seafloor that extends down to 90 m depth showing gradients of about 2.8°. This uneven topography results from a large, complex rocky outcrop and several narrow ridges (Figs. 3 and 4A). The middle La Planassa shelf offshore Tossa de Mar and the outer shelf show a smooth seafloor (Figs. 3 and 4A). The backscatter imagery reveals marked along- and across-shelf variations around the canyon head (Fig. 4B). The inner shelf shoreward of the north of the canyon head shows very-high backscatter intensities (brighter areas) with elongate, shore-normal regions of low backscatter returns (darker areas) that extend from the coastline down to 40–50 m depth (Fig. 4B). Backscatter intensities are medium to high over most of the middle shelf surrounding the canyon head, except for small areas on the Barcelona shelf and a large area of very low backscatter offshore Tossa de Mar (Fig. 4B). Backscatter decreases from the middle Barcelona shelf towards the outer shelf, whilst the La Planassa shelf shows an opposite trend with higher values on the outer shelf. 4.2. Seafloor features Bathymetric and backscatter data and seismic reflection profiles led to the identification of distinctive seafloor features over the studied continental shelf. These comprise prodeltas, IPWs, rocky outcrops and sorted bedforms in the inner shelf; and a widespread rocky outcrop, large morphological steps, narrow ridges, featureless seafloor zones and sediment waves in the middle and outer shelf. 4.2.1. Prodeltas The Tordera prodelta extends almost 5 km along-shelf and 0.4 km seawards ending in a steep slope down to 40 m depth (Figs. 3 and 4A). The very high backscatter it shows is most probably indicative of coarse sediment (Fig. 4B). To the north of the Tordera River mouth, smaller prodelta-like wedges (0.7 km wide along-shelf) have been recognized at the mouth of Tossa de Mar and Lloret de Mar torrents (Fig. 3).

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4.2.2. Infralittoral prograding wedges IPW develop from the lower edge of the shoreface to a strong break in slope at 30–35 m water depth (Fig. 5A). Seawards, the gradient increases to a relatively steep slope down to 40–50 m depth (Fig. 4A). To the north of the Tordera River mouth, an IPW appears in the form of isolated bodies that are best developed in bays and pocket beaches (Fig. 5A). In contrast, south of the Tordera River mouth the IPW consists of a set of continuous, coast-parallel to coast-oblique sediment bodies that extend along the Maresme coastline (Fig. 3). There, the IPW is characterized by very-high backscatter with elongate patches of lower backscatter (Fig. 4B). 4.2.3. Sorted bedforms Sorted bedforms appear in the bathymetry as slightly depressed (0.3–1 m in depth) and elongate (50–250 m) features with a patchy distribution (Fig. 5A). Most of bedforms lay normal to oblique to the general trend of the isobaths and develop at water depths between 10 and 40 m depth (Fig. 5A). The largest of these bedforms can exceed 700 m in length. Backscatter data display an abrupt transition between high intensities in the shallow depressions and low values in the adjacent shallower areas (Fig. 5B). 4.2.4. Rocky outcrops Rocky outcrops appear sparsely in the inner shelf but become dominant on the middle shelf to the north and east of the canyon head (Figs. 3 and 6A). In the inner shelf, zones with rocky outcrops show a variable relief (up to 8–10 m high) and mean gradient (0.2– 7°), and high backscatter (Figs. 4A and B). To the north and east of the canyon head, a large, complex rocky outcrop occupies the entire middle shelf down to 90 m depth (Figs. 3 and 6A). It shows an uneven appearance determined by WNW-ESE trending, 2–3 m high and 1–4 km long linear crests almost normal to the isobaths (Fig. 6A). It is characterized by intermediate backscatter, with crests showing higher intensities (Fig. 6B). Seismic reflection profiles show an almost opaque, high amplitude seafloor reflection, with faint south-west dipping outcropping strata, which are locally draped by a very thin transparent unit in the deepest zones (Fig. 6C). 4.2.5. Morphological steps Seven morphological steps, S1 to S7, have been identified in the continental shelf adjacent to the canyon head. The shallower steps,

Fig. 4. (A) Shaded relief and slope gradient map of the study area. (B) Shaded relief and backscatter intensity map of the study area. Onland orthophotomap from ‘‘Institut Cartogràfic de Catalunya’’. See Fig. 1 for location.

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Fig. 5. (A) Shaded relief and bathymetry map showing the IPW offshore Tossa de Mar. Note the presence of sorted bedforms normal to contours atop of the IPW. Contours every 2 m. (B) Backscatter intensity map showing elongated patches of low backscatter normal to contours. Onland orthophotomap from ‘‘Institut Cartogràfic de Catalunya’’. See Fig. 1 for location.

Fig. 6. (A) Shaded-relief and bathymetry map showing the large rocky outcrop to the east of the BC head occupying the La Planassa middle shelf. Contours every 10 m. (B) Backscatter intensity map of the same area. Onland orthophotomap from ‘‘Institut Cartogràfic de Catalunya’’. (C) Very-high resolution seismic reflection profile across the La Planassa middle shelf large rocky outcrop. Vertical scale in milliseconds two-way travel time (TWTT). See Fig. 1 for location.

S1 to S3, show a general E-W orientation and appear shoreward of the canyon head to the north, at water depths of 30–45 m, 30– 50 m and 35–65 m (Fig. 7A). The step S4, at 74–78 m depth, is NE-SW oriented and limits a very smooth depression along its shoreward side (Fig. 8). A notch located 2.5 km away from the

modern Tordera River mouth, likely corresponding to the remnant of a narrow fluvial channel, cuts S4. The deeper steps S5 and S6 are almost parallel and appear along the outer Barcelona shelf at 78– 90 m and 105–115 m of water depth, respectively (Fig. 9A). While S5 and S6 are ENE-WSW oriented offshore the Maresme coast, they

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change to NNE-SSW shoreward of the canyon head. S3 to S6 run across the shelf and die at the rim of the canyon head. To the east of the BC, a small step, S7, delineates a 1.5 m thick sediment body that extends 3 km along the La Planassa outer shelf at 94–95 m depth (Fig. 10). Backscatter data show very high values with small patches of very low reflectivity in the continental shelf between the coastline and S3 (Fig. 4B and 7B). The same step S3 marks an abrupt change in backscatter from very high to low reflectivity. This low backscatter zone extends eastwards from the Tordera prodelta to the Blanes canyon rim (Fig. 7B). Seaward from S3, the continental shelf displays medium backscatter down to S6, which marks another change to low intensities in backscatter (Fig. 4B). Limited acoustic penetration hinders the interpretation of TOPAS profiles in this zone. However, a semi-transparent unit of up to 20 m in thickness overlies a discontinuous and uneven high amplitude reflector that locally crops out (Figs. 7B and 9C). 4.2.6. Narrow ridges Narrow ridges are 40–100 m wide positive relieves up to 2.5 m above the surrounding seafloor, which are either parallel or oblique to the isobaths (Fig. 3). They are straight elongate features in plan view, except close to the canyon tip, where they become arcuate (Figs. 6A and 7A). Their length ranges from 0.5 to 8 km. Within the study area, twelve narrow ridges have been identified showing different shapes and orientations: (i) seven NE-SW straight ridges at depths of 48–59 m, 56–58 m, 63–65 m, 69–72 m, 74 m, 74– 77 m, and 77–78 m; (ii) a ENE-WSW straight ridge at 35–45 m depth (Fig. 7A); and (iii) four arcuate ridges at 52–57 m (Figs 6A

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and 7A). The ridges yield medium to high backscatter (Figs. 6B and 7B). In sub-bottom penetrator profiles they are associated to an irregular strong reflector that prevents acoustic penetration (Fig. 7C).

4.2.7. Featureless shelf floor areas A large almost flat (0.3–0.5° of slope gradient) and featureless area with very low backscatter intensities extends along the middle La Planassa shelf offshore Tossa de Mar. It covers 61 km2 at depths ranging from 50 to 102 m (Fig. 4B) though it probably extends beyond the swath bathymetry data limit (Fig. 1). East of the canyon, the outer La Planassa shelf is characterized by a relatively flat, gently inclined (0.2° on average) bottom down to 98 m depth (Fig. 10A). It shows medium to high backscatter (Fig. 4B), which probably corresponds to relatively coarse-grained sediment.

4.2.8. Sediment waves To the west of the canyon, between steps S5 and S6, a field of almost contour parallel sediment waves, 6 km2 in areal extent, drapes the outer shelf at 95–115 m of water depth (Fig. 9B). The sediment waves are 0.2–0.5 m high with mean wavelengths of 400 m. Backscatter data are of medium intensities both in the crests and troughs (Fig. 4B).

Fig. 7. (A) Shaded-relief and bathymetry map showing steps S1 to S4 identified in the middle shelf stretch between the canyon rim and the coast of Blanes and Lloret de Mar. Contours every 10 m. (B) Backscatter intensity map of the same area. Onland orthophotomap from ‘‘Institut Cartogràfic de Catalunya’’. (C) Very-high resolution seismic profile across steps S3 and S4. Vertical scale in milliseconds two-way travel time (TWTT). See Fig. 1 for location.

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Fig. 8. (A) Bathymetry map showing the location and morphology of step S4 identified off the Tordera River mouth. Note the pronounced notch cut in the step. Contours every 10 m. Bathymetric sections across (B and C) and along (D) S4. See Fig. 1 for location.

Fig. 9. (A) Shaded relief and bathymetry map showing steps S4 and S5 in the middle shelf and S6 in the outer shelf, close to the western rim of the BC. Contours every 10 m. (B) Detailed map of sediment waves on the outer shelf. Contours every 2 m. (C) Very-high resolution seismic reflection profile across steps S4 and S5. Vertical scale in milliseconds two-way travel time (TWTT). See Fig. 1 for location.

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Fig. 10. (A) Detailed shaded-relief bathymetry map of the La Planassa outer shelf. Contours every 2 m. (B) Very-high resolution seismic reflection profile across step S7 identified in the outer shelf. Vertical scale in milliseconds two-way travel time (TWTT). See Fig. 1 for location.

5. Discussion 5.1. Seafloor morphology The detailed multibeam bathymetry and backscatter data presented in this study yield a new accurate image of the seabed that provides a comprehensive overview of the geomorphology of the continental shelf and reveals a variety of seafloor features, some of which had not been previously recognized in the study area. The interpretation of these features together with the backscatter pattern provides useful insights about past and present sediment dynamics. 5.1.1. Interpretation of seafloor features 5.1.1.1. Prodeltas. Sediment deposition by the Tordera River and smaller streams, such as Lloret de Mar and Tossa de Mar torrents, has led to the formation of small submarine prodeltas off the river mouths (Fig. 11). They display very high backscatter, which is indicative of the coarse nature of the sediments supplied by these water courses (Fig. 2A). The bulging to elongate shape of these features results from the predominant southwards littoral drift that distributes the sediment delivered by these streams southwards along the inner shelf. This alongshore transport is also evidenced by the formation of a large submerged sand spit that extends southwards off the Tordera River mouth, as described by Serra et al. (2003, 2007). 5.1.1.2. Infralittoral prograding wedge. Near the BC, the IPW develops laterally forming continuous prodeltaic wedges such as those along the Maresme coastline (Liquete et al., 2007; Ercilla et al., 2010), or isolated bodies, such as those to the north of the Tordera River mouth (Fig. 11). As the IPW forms just below the wave-face depth of major storms (Hernández-Molina et al., 2000), it is actively involved in the present day littoral sedimentary processes. This is confirmed by very high backscatter corresponding to coarse-grained facies that reflect the dominant influence of stormy hydrodynamic conditions along the infralittoral belt. The IPWs show a predominant stretched geometry that is parallel to the coastline as a consequence of the dominant south-westward littoral drift.

5.1.1.3. Sorted bedforms. The elongate patches in backscatter images observed on the IPW have been interpreted as sorted bedforms (Murray and Thieler, 2004) or ‘‘rippled scour depressions’’ (Cacchione et al., 1984). As commented above, sorted bedforms are slightly depressed features filled with coarse sediments, as evidenced by backscatter data (Fig. 5). They appear between 10 and 40 m depth and their longitudinal axis is normal to oblique to the coastline (Fig. 11). The generation and maintenance of these features arise primarily from a sediment sorting feedback in shelf environments dominated by alongshore rather than cross-shore currents (Murray and Thieler, 2004; Coco et al., 2007; Lo Iacono and Guillén, 2008), which is in accordance to the hydrodynamic regime deduced from the prodeltas and IPWs. According to their location in relation to the storm wave base level, these sorted bedforms possibly become active during high-energy events. The storm wave base along the Maresme coast has been estimated at 20–30 m water depth (Calafat, 1986; Sorribas et al., 1993), but several works reported sediment remobilization down to 60 m depth under very strong conditions (Puig et al., 2001; Palanques et al., 2002). 5.1.1.4. Rocky outcrops. Rocky outcrops appear in the inner and middle shelf primarily to the north and east of the canyon head (Fig. 11). In the inner shelf, rocky outcrops appear as the submerged feet of coastal cliffs showing continuity with the Hercynian granites outcropping inland (Fig. 2A). In the middle shelf, a widespread rocky outcrop extends down to 90 m water depth covering an area of 106 km2 (Fig. 11). Although it is partially capped by sediment, probably coarse grained as suggested by backscatter data and sub-bottom seismic reflection profiles, the structural features of the rock outcrop are clearly visible (Figs. 3 and 11). 5.1.1.5. Morphological steps. The large morphological steps observed in the middle and outer shelf near the BC belong to three main groups, according to their location and orientation (Fig. 11): (i) a E–W oriented set that comprise steps S1 to S3 at 30–45, 35– 55 and 35–65 m depth, respectively; (ii) a NE–SW step (S4) cut by a shallow channel at 74–78 m depth; and (iii) a set made of NNE-SSW oriented steps S5 and S6 at 105–115 and 78–90 m depth and an arcuate step (S7) at 94–95 m depth.

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Fig. 11. General shaded relief map of the study area showing the main geomorphic elements commented in the text. Onland orthophotomap from ‘‘Institut Cartogràfic de Catalunya’’. See Fig. 1 for location.

The E-W oriented steps appear to the north of the canyon head (Fig. 11). They are characterized by very high backscatter corresponding to coarse sand (see Pedrosa-Pàmies et al., this issue, for more details). Step S3 marks an abrupt change in backscatter to low intensities corresponding to an elongate patch of medium to fine sand (Pedrosa-Pàmies et al., this issue) that extends from the Tordera prodelta to the canyon head rim (Figs. 4B and 11). Further south, the seafloor is characterized by medium backscatter associated to medium sand (Pedrosa-Pàmies et al., this issue). Based on the seismic profiles and backscatter characteristics, the morphological steps S1 to S3 to the north of the canyon head have been interpreted as the edges of sediment bodies, most probably of sandy nature. S4 shows a particular morphology determined by a narrow channel that likely corresponds to an old bed of the Tordera River. Samples from the top of these relict sediment bodies consist of medium to coarse sand (ITGE, 1989; Díaz and Maldonado, 1990), which is in agreement with the observed medium to high backscatter (Fig. 4B). The basal reflector underlying these relict sand bodies corresponds to a ravinement surface developed during the Versilian transgression (i.e. younger than 18,000 yr BP), which can be correlated across the entire study area and beyond (Serra, 1976; Díaz and Maldonado, 1990; Liquete et al., 2007). Therefore, the Barcelona shelf relict sediment bodies were built during the last postglacial sea-level rise. 5.1.1.6. Narrow ridges. Narrow ridges, oriented NE–SW or ENE– WSW, appear at depths between 35 and 78 m, but mostly at 48–58 m and 74–78 m (Fig. 11). Based on the bathymetric and sub-bottom seismic reflection data, the narrow ridges could be ascribed to relict beach-rock alignments. Although no sediment cores were collected, the narrow ridges in the study area show identical morphology and appear at the same position than the cemented beach-rocks described further south in the same Barcelona shelf by Liquete et al. (2007). Surface sediment is composed by bioclastic

to siliciclastic-bioclastic, rounded to well-rounded sands and gravels (Liquete et al., 2007) yielding high backscatter intensities (Fig. 7B). However, further investigations are required to constrain their origin more accurately. 5.1.1.7. Featureless seafloor. Two areas of featureless seafloor have been identified in the La Planassa shelf (Fig. 11). Offshore Tossa de Mar, the middle shelf displays very low backscatter suggesting that the shelf floor there is covered by fines. This area corresponds to the southernmost limit of a large patch of low backscatter that extends along the La Planassa middle shelf (Durán et al., 2012). This pattern contrasts with the La Planassa outer shelf, where backscatter is high, which would be indicative of coarse sediment pointing to erosion or non-deposition and sorting of fines. 5.1.1.8. Sediment waves. Contour parallel sediment waves have been observed in a restricted area of the Barcelona outermost shelf at 95–115 m depth (Fig. 11). The morphology of the sediment waves and the local hydrodynamics suggest that they could be reactivated during energetic conditions, particularly under strong storms. In the Gulf of Lion, numerical modelling showed that strong energy events are able to remobilize and transport sand over the outermost shelf and shelf edge (Bassetti et al., 2006). Recent observations in the BC head also noticed that large storms were able to produce enough bottom shear stress to resuspend shelf sediment including the transport of coarse sand down to 50–60 m water depth, which could explain the arrival of this sediment fraction into the canyon head (Sanchez-Vidal et al., 2012; Pedrosa-Pàmies et al., this issue). Such sediment remobilization over the outer shelf would contribute to the activation of these sediment waves. However, the morphology of these bedforms is inconclusive about their degree of present activity, so that additional observations would be necessary to confirm or deny this interpretation.

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5.1.2. Shelf zonation The interpretation of the shelf floor features allows distinguishing four zones dominated by erosion (or non-deposition) and deposition that illustrate the sediment dynamics of the study area (Fig. 12). These are: (i) a littoral belt of modern coarse sediment transport and accumulation; (ii) a zone of modern fine sediment deposition; (iii) a zone dominated by erosion or non-deposition; and (iv) a relict, modern sediment depleted zone. The littoral belt of modern coarse sediment transport and accumulation covers almost the whole inner shelf, i.e. 30 km2 or 5.7% of the study area (Fig. 12). It is characterized by large depositional features, such as prodeltas, IPWs and sorted bedforms (Fig. 11). Backscatter is very high along this belt, which fits with the coarse nature of sediment inputs from the Tordera River and coastal torrents, and the action of eastern waves against this exposed coastal stretch. Storm waves resuspend the finest fractions of sediment deposited on the inner shelf that can be transported offshore towards the middle-outer shelf and slope, and south-westward by the induced littoral drift. Modern fine sediment deposition occurs at the middle shelf offshore Tossa de Mar, as evidenced by backscatter data, covering an area of 61 km2 or 11.6% of the study area (Fig. 12). Middle shelf fine deposition zones, like the one observed in the La Planassa middle shelf, have been recognized worldwide and described in the adjacent continental shelves too, such as the Ebro shelf (Palanques and Drake, 1990; Puig et al., 2001), the Barcelona shelf further south (Liquete et al., 2010) and the Gulf of Lion shelf (Got and Aloisi, 1990; Durrieu de Madron et al., 2008). A large zone of long-term erosion or non-deposition measuring 226 km2, or 43.1% of the study area, has been identified over the La Planassa middle and outer shelf, near the canyon head (Fig. 12). This zone displays a rough bathymetry and high backscatter that corresponds to rocky outcrops and coarse sediment (Fig. 11). The scarce fluvial sediment input reaching La Planassa shelf, which is fed (only by short ephemeral torrents (Fig. 2A), and the bottom

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shear stress resulting from high-energy storms leads to a sediment-starved shelf. The erosive or non-depositional character of the La Planassa shelf in the vicinity of BC extends to a sharp canyon rim there, which further indicates erosion and sediment depletion (Lastras et al., 2011). The relict zone, markedly starved of modern sediment accumulation, extends over the middle and outer shelf to the north and west of the canyon head, covering an area of 208 km2, or 39.6% of the study area (Fig. 12). It includes six large-scale transgressive sediment bodies that occupy the entire middle and outer shelf (Fig. 11). At present, the relict character of this shelf zone is attested by (i) the relatively rough topography of the seafloor with isolated rocky outcrops and narrow ridges; and (ii) the medium to high values of backscatter intensity corresponding to coarse sand (Pedrosa-Pàmies et al., this issue). 5.2. Modern sediment dynamics 5.2.1. Sediment sources and transport processes The variability of seafloor morphology and sediment type, and subseafloor configuration across the studied continental shelf provides significant insight on sediment sources, dynamics and transport pathways from the shelf to the canyon. The deeply incised into the continental shelf BC intercepts the alongshore and along-shelf sediment dispersal paths, which results in the escape of sediment from the shelf into the canyon head (Fig 12). The sorting and preferential escape of fines makes the sedimentary cover of the continental shelf around the canyon head to be mainly sandy, as shown by the high backscatter pattern observed there, which also indicates reworking of shelf deposits finally feeding into the canyon (Figs. 4B and 12). Previous works have documented a dominant south-westward littoral and inner shelf drift along the North Catalan continental shelf in previous works (Copeiro, 1982; DGPC, 1986) (Fig. 12). Evidences of sediment transport over the shelf floor are provided by:

Fig. 12. 3D image of the study area illustrating the main domains in terms of sediment dynamics and sediment transport pathways across the shelf and into the canyon. See text for explanation.

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(i) the elongated morphology of modern prodeltas and IPWs; (ii) the orientation of sorted bedforms; and (iii) a complex backscatter pattern (Figs. 3 and 4). The pattern of very high backscatter between the coastline and the canyon head and the elongate patch of low backscatter to the south of step S3 indicate the remobilization and south-westward along-shelf transport of fines that are deposited southwards on the lee side of S3 or off-shelf into the canyon head (Fig. 12). This predominant south-westward transport of sediment along the shelf would also explain the limited contribution of river inputs directly into the canyon head. Due to the closeness of the Blanes Canyon to the Tordera River mouth, a direct input from this river into the canyon head could be expected. However, recent works have noticed that only a small amount of the sediment volume released by the Tordera River enters the BC during high-energy events (Zúñiga et al., 2009; Sanchez-Vidal et al., 2012; PedrosaPàmies et al., this issue). This is attributed to the predominant along-shelf sediment transport that favours the dispersal of the coarse sediment delivered by the Tordera River along the shore and inner shelf towards the south-west, thus feeding river prodelta itself and the beaches and IPWs off the Maresme coast. This view is supported by the seafloor morphology and backscatter imagery (Fig. 12). The finest fractions supplied by the river bypass the inner shelf and disperse over the middle shelf and beyond, as indicated by medium backscatter there (Figs. 4B and 12). This interpretation is further supported by bottom samples collected in the continental shelf offshore the Tordera River mouth that show a high contribution of terrestrial sediment in the middle shelf that decreases toward the outer shelf and slope (see Pedrosa-Pàmies et al., this issue). According to this, the most probable shelf sources of sediment to the BC responding to present day hydrodynamics are located to the north and west of its head. Furthermore, the large relict sediment bodies that occupy almost the entire middle and outer shelf, to the north (i.e. shoreward) and west of the canyon also represent a potential source of coarse sediment into its head and upper course (Fig. 12). This would be in agreement with the massive arrival of sand into the canyon during the severe storm of December 2008, as observed by Sanchez-Vidal et al. (2012) and other morphological evidences within the canyon head that reported a retrogradation of the BC western rim seaward of the Maresme sediment bodies (Lastras et al., 2011) at the head of a gully system that cut the whole canyon wall and reach the shelf edge. East of the canyon head, the continental shelf is sediment starved and mostly erosional or non-depositional, so that a minor sediment contribution into the canyon should be expected from there (Fig. 13). 5.2.2. Hydrodynamic forcings Short-lived hydrodynamic processes such as floods, storms and DSWC largely drive the transport and distribution of sediment over the Catalan continental shelf and deep margin. In the Blanes Canyon, the eastern storms are the most prominent oceanographic processes controlling the shelf-to-canyon transport of sediment, as noticed in recent works (Sanchez-Vidal et al., 2012; PedrosaPàmies et al., this issue). Storm waves generate bed shear stresses sufficient to resuspend fine sediment on the inner shelf, which can be subsequently deposited beyond the wave-base level forming a mid-shelf mud belt, as observed offshore Tossa de Mar (Fig. 4B). The most energetic storms can resuspend again these fines and force their transport further offshore and also off-shelf into the submarine canyon (Guillén et al., 2006; Palanques et al., 2008; Pedrosa-Pàmies et al., this issue). This would explain the lack of fines over the continental shelf adjacent to the canyon head, particularly shoreward (Fig. 4B). Extreme storms are also able to remobilize sediment from the shelf and supply sand sizes to the canyon head, as demonstrated by Sanchez-Vidal et al. (2012) and

Pedrosa-Pàmies et al. (this issue). The role of up and down currents along the canyon over the resuspension of fines and, in general, sediment remobilization in the BC area is not known at present. DSWC likely contributes to the transfer of shelf sediment into the submarine canyon too (Fig. 12). Dense shelf waters form mainly over the Gulf of Lion shelf and the Roses shelf to the north, and to a much lesser extent over the La Planassa shelf, which southern limit marks the boundary of the span of the cold, dry and persistent northern Tramuntana wind (Canals et al., 2006; Ulses et al., 2008). Direct observations and numerical simulations have shown that dense shelf water is transferred to the deep basin mostly through Cap de Creus, La Fonera and Blanes canyons, with a decreasing trend from north to south (e.g. Canals et al., 2006; Ulses et al., 2008; Palanques et al., 2009; Lastras et al., 2011; Ribó et al., 2011). Accordingly, the BC traps dense shelf water formed over La Planassa shelf and in northernmost areas. Because of topographical constrictions these waters and the sediment load they carry after resuspension and turbulent bed load transport enter BC mostly through its northern flank, as supported by direct observations too (Zúñiga et al., 2009). However, the volume of water and sediment carried out by DSWC into the BC likely is much less than in La Fonera Canyon and, especially, Cap de Creus Canyon, which is by far the main outlet for these waters into the deep margin and basin along the North Catalan margin. Sediment volumes entering the BC because of DSWC remain to be precisely quantified. The permanent mesoscale NC also brings fines to the BC area (Fig. 12). Interaction of the NC with the canyon topography may lead to eddy formation and enhanced resuspension of fines. Such eddies may enter the continental shelf, eventually guided by the canyon topography, thus further resuspending the finest particles accumulated on the outer shelf (Arnau, 2000; Flexas et al., 2002; Arnau et al., 2004) and easing their southward transport along the shelf and trapping into the canyon, probably contributing to the development of a canyon fill sedimentary body observed at the shelf edge (Lastras et al., 2011). 5.3. Evolution of the continental shelf during the last transgression Past studies of submarine canyons established that during sealevel low-stands they are particularly efficient conduits for the transport of sediment from the continental shelf to the deep margin and basin (May et al., 1983). Recent studies have also shown that canyons incising narrow continental shelves receive large amount of sediment regardless of sea-level conditions (e.g. Cutshall et al., 1986; Kineke et al., 2000; Xu et al., 2002; Puig et al., 2003; Mullenbach et al., 2004). With this background in mind, we have investigated the evolution of the Blanes continental shelf in terms of sediment transport pathways from terrestrial and shelf sources into the canyon head during the last transgression. 5.3.1. Seafloor features as sea-level indicators The evolution of the continental shelf around the BC can be reconstructed after several geomorphological features, such as narrow ridges and morphological steps, which can be used as sea-level indicators (Fig. 11). Narrow ridges have been ascribed to beachrocks and thus they are excellent indicators of paleo-coastlines. Near the canyon head, narrow ridges are mainly located at 52– 58 m, 63–35 m, 69–72 and 74–78 m depth. Morphological steps corresponding to the front of relict sediment bodies appear at 30–65 m (S1 to S3), 74–78 m (S4), 78–90 m (S5), 94–95 m (S7), and 105–115 m depth (S6). They are also considered as indicators of ancient sea-level positions during the last transgression, although some caution is required. Present-day depositional breaks in slope such as prodelta fronts or IPWs form below the storm-wave base, i.e., some meters below the current sea-level.

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Fig. 13. (A) Relative sea-level curves for the past 120 kyr (Waelbroeck et al., 2002; Siddall et al., 2003; Imbrie et al., 1990). (B) Global (Siddall et al., 2003) and Mediterranean relative sea-level curves of the last transgression (Aloïsi et al., 1978; Lambeck and Bard, 2000). Vertical scale is in meters with respect to present sea-level (zero value). The steps and narrow ridges identified in the study area have been placed in the sea-level plot according to their present water depth.

Consequently, a difference in water depth between the morphological step and the relative sea-level must be taken into account. The morphological features observed on the Blanes shelf area have been correlated to global (Siddall et al., 2003; Deschamps et al., 2012) and Mediterranean (Aloïsi et al., 1978; Lambeck and Bard, 2000) sea-level curves. We naturally focus on the last sea-level cycle, as most of the geomorphological features on the studied shelf floor formed during the last transgression and subsequent highstand. Though studies of the Barcelona shelf reported differential subsidence across and along the shelf, with maximum values at the shelf break and off Besòs and Llobregat river mouths (Liquete et al., 2008), tectonic subsidence in the study area is of little relevance for the purpose of our paper. Perea et al. (2006, 2012) have estimated a vertical slip rate of 0.02–0.04 mm yr1 along the Barcelona Fault (Fig. 2A) during the Plio-Quaternary, which is equivalent to 0.4–0.8 m for the last 20 kyr. In other deltaic nearby shelves, such as the Gulf of Lion, Quaternary subsidence has been inferred at 0.25 mm yr1 near the shelf edge (Rabineau et al., 2006). This subsidence is attributed to sediment compaction in a delta environment, which cannot be directly transposed to the continental shelf near the BC. Therefore, we consider reasonable to assume that the study area has been fairly stable tectonically during the last 20 kyr. Sediment compaction likely is negligible given the different setting with respect to the study cases above and also the dominant coarse nature and limited thickness of post-glacial units and the short time elapsed since the beginning of the transgression. However, because of the lack of correction, even if minor, for tectonic vertical movements and sediment compaction, our results should be considered in terms of relative sea levels. During the last glacial cycle, the Mediterranean Sea was connected to the global ocean and therefore followed a similar pattern of sea-level changes. The sea level reached its lowest position during the LGM between 26.5 and 19 kyr (Fairbanks, 1989; Lambeck and Chappell, 2001). In the Western Mediterranean, a minimum sea-level at about 105–115 m below present sea-level (bpsl) was inferred (Lambeck and Bard, 2000; Jouet et al., 2006; Berné et al., 2007). Since the LGM, the sea-level rise was not steady (Fig. 13);

instead relatively short periods of rapid sea level rise were followed by periods of slower rise with occasional brief stillstands (Fairbanks, 1989). Intervals of rapid sea-level rise occurred at 14.65 kyr and 11.3 kyr, referred to as Meltwater Pulse 1A (MWP1a) and Meltwater Pulse 1B (MWP1b), respectively (Bard et al., 2010; Deschamps et al., 2012). Periods of short stillstands or slow sea-level rise occurred during the early deglacial, after MWP1a, during the Younger Dryas cold climatic event (12.8–11.5 kyr) and at the 8.2 kyr cold event (Lambeck et al., 2002; Bard et al., 2010). By comparing the depths of the main seafloor features of the Blanes shelf with the timing of these well-known sea-level changes during the last post-glacial transgression, we found noteworthy correlations to sea-level rise, even though our water depths are uncorrected, as explained above (Fig. 13B). The depth of the narrow ridges at 52–65 m bpsl would likely correspond to: (i) the onset of the Younger Dryas, when the sea-level was located at a water depth of approximately 50–55 m bpsl in the Western Mediterranean (Siddall et al., 2003; Berné et al., 2007); and (ii) a phase of decreased sea-level rise after MWP1a when sea level was at about 77 m bpsl (Fairbanks, 1989; Bard et al., 1990). The depths of the morphological steps, however, fall at or below three short intervals of slow sea-level rise or stillstand (Fig. 13B): (i) the decrease in sea level rise during the deglacial onset; (ii) the short-lived stillstands or slowdowns in sea-level rise during the Younger Dryas; and (iii) the 8.2 kyr cold event, when sea level was at about 20 bpsl (Siddall et al., 2003; Camoin et al., 2004) or 30 m bpsl in the Mediterranean (Aloïsi, 1986; Lambeck et al., 2002). The difference between water depth of sediment bodies and the relative sea-level would be related to the storm-wave base level or other local factors, as has been commented above. Our interpretation on the relation between sea-level rise and shelf floor features in the BC area would largely benefit from age dating so that they could be confirmed or better adjusted. 5.3.2. Morphological development of the continental shelf under a rising sea-level The identification of the various morphologies left mainly during relative stillstands or slowdowns is critical in reconstructing

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the evolution of the submerged landscape of the continental shelf near the BC since the LGM. Such reconstruction should help understanding the varying influence of the canyon on the shaping of the shelf and how the nature of their interactions has changed through time (Fig. 14). The morphology of the continental shelf model reveals a major shift in sediment dynamics likely related to the flooding of the shelf stretch shoreward of the canyon head, which determined the reestablishment of the littoral drift and the coastline parallel circulation over the inner shelf. We hypothesize

that the achievement of modern conditions occurred in three stages corresponding to the lowest LGM sea-level (18 kyr BP, 105–115 m) and to two intermediate stillstands or slowdowns during the transgression placed before (14.1 kyr BP, 74–77 m) and after (8.2 kyr BP, 30 m) the flooding of the continental shelf shoreward of the BC (Fig. 14). Each stage is plotted as a function of water depth of the main morphological sea-level indicators on the shelf floor over the relative sea-level curve (Fig. 13B).

Fig. 14. Reconstruction of the evolution of the continental shelf in the BC area during the last transgression. Schematic diagrams illustrate the coastline configuration and the main processes shaping the continental shelf at three different relative sea-level stages: (A) during the last lowstand and initial phases of the transgression (105–115 m); (B) at about 74–77 m bpsl; and (C) at about 30 m bpsl.

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The lowest sea-level position corresponds to the maximum exposure of the continental shelf, when the BC head was deeply incised in the paleo-coastline, thus preventing a shallow water connection between the La Panassa and Barcelona shelves (Fig. 14A). East of the BC, the relatively small volumes of sediment delivered by the northern coastal streams, and the sediment resuspended on the inner shelf were very likely transported southwards by the littoral drift till directly entering the submarine canyon. West of the canyon, the Tordera River mouth opened into the canyon rim or very close to it. Consequently, the paleo-Tordera River discharged directly into the canyon, which trapped most of the fluvial input with only small amounts that could be transported southwards by the littoral drift (Fig. 14A). With the subsequent rise of sea-level and the landward migration of the coastline, extensive transgressive sediment bodies began to develop on the Barcelona shelf, shown by the reconstruction corresponding to a short stillstand or slowdown at about 74–77 m bpsl (Fig. 14B). At this stage, the narrow shelf stretch landwards of the BC head was still totally emerged. Small amounts of sediment were supplied by coastal torrents to the La Planassa shelf, where fines could be easily resuspended during storms and transported towards the canyon by the dominant circulation, thus leaving an essentially sediment-starved shelf. West of the canyon, the increasing distance between the Tordera River mouth and the canyon head with the sea-level rise favoured the development of large sediment bodies in the half flooded Barcelona shelf. However, the relative closeness of the river mouth to the canyon rim still favoured the off-shelf transport of some sediment, mainly the finest fractions, to the western canyon flank. The notch attributed to a fluvial channel cutting S4 corresponds to this stage. The most significant change in the sediment dynamics of the Blanes shelf took place when the sea-level raised enough to flood of the continental shelf stretch landwards of the shallowest part of the canyon head (Fig. 14C). This allowed the establishment of an alongshore sediment transport between the La Planassa and Barcelona shelves uninterrupted by the canyon head. Such a situation favoured the development of new morphosedimentary features on the shelf stretch north of the canyon head. The change in the orientation of these features with regard to the sand bodies in the Barcelona shelf can be tentatively attributed to a westward migration of the Tordera delta and prodelta from the canyon rim at S3 to its current position, combined with a promontory effect by the same delta that favoured the accumulation of sediment to the east under the effective action of the newly established littoral drift carrying loose IPW deposits along the coastline of La Planassa towards the southwest while redistributing the inputs by Tordera River. In the Barcelona shelf, the steady landward migration of the coastline contributed to a reduction of the off-shelf export of sediment till reaching the present situation with a south-westward sediment transport belt attached to the shoreline. 6. Conclusions 1. The detailed study of the geomorphology and uppermost sediment cover of the continental shelf surrounding the Blanes submarine canyon yields insight into the past and present shelf sediment dynamics and the shelf-to-canyon sediment exchanges. 2. The continental shelf near the canyon head consists of mosaic where erosional, or non-depositional, and depositional zones coexist. East of the canyon and offshore Tossa de Mar, the modern sediment deposition is mostly confined to the inner and middle shelf, whilst most of the La Planassa shelf is sediment depleted with numerous relict morphosedimentary features cropping out. Rocky outcrops, narrow ridges and relict coarse

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sand deposits suggesting erosion or non-deposition of fine sediments in modern times occupy the middle and outer shelf floor east and northeast of the canyon head. In contrast, north and west of the canyon head, the middle and outer shelf comprises several large relict sand bodies that point out to long-term deposition. However, the lack of modern sediments on top of these bodies supports active erosion or by-pass in present times. 3. The morphology of the continental shelf near the canyon head records the imprint of the main factors controlling the shelf sediment-dispersal system and provides evidence for the main sources and transport pathways of sediment from the shelf into the canyon. The depletion of fine sediments on the continental shelf, as evidenced by backscatter data, suggests that the Blanes Canyon acts as a sediment trap collecting the finest fractions resuspended primarily from the adjacent shelf to the north. The main processes that control the shelf-to-canyon transfer of sediment are eastern storms, which enhance the off-shelf export of mainly fine sediment from the shelf. Particularly severe storms are also able to remobilize and transport coarse sediment from the shelf and also from the relict sand bodies into the canyon. Other processes, such as DSWC and the Northern Current, contribute to a lesser extent to the transport of sediment along the shelf and into the canyon. 4. During the last post-glacial transgression, the BC played a crucial role in the shaping of the continental shelf surrounding it by cutting the littoral drift of sediment between the shelf areas to the north and south, thus severely modifying the across- and along-shelf sediment pathways. As a result, to the east of the canyon, the poor development of transgressive deposits indicates the prevalence of erosion and non-deposition associated to a limited sediment supply and an effective action of the littoral drift leading to a south-westward transport of sediment towards the canyon head. To the north and west of the canyon the morphology of the continental shelf changed significantly during the sea-level rise. At the early stage of the transgression, the sediment supplied by the Tordera River was discharged directly into the canyon, thus preventing deposition over the shelf. Later, the progressive sea-level rise favoured the development of large depositional bodies on the Barcelona shelf favoured by the increase of accommodation space and the augmenting distance between the river mouth and the canyon head. A drastic change in the configuration of the shelf occurred when the sea-level raised enough to flood the entire continental shelf. The along-shelf sediment transport between the shelf areas to the north and south of the canyon head was then restored and new sediment bodies were formed between the coastline and the canyon tip. At present, these sediment bodies constitute the primary source of coarse sediment into the BC. 5. These results confirm that the Blanes submarine canyon head is highly dynamic and sensitive to a variety of processes that enhance the transport of sediment from the shelf into the canyon, particularly during major storms.

Acknowledgments This work is a contribution to the Spanish RTD projects DOS MARES (CTM2010-21810-C03-01/MAR), and VALORPLAT (CTM2011-14623-E) and GRACCIE-CONSOLIDER (CSD200700067). Generalitat de Catalunya support through its grant 2009SGR-1305 is also acknowledged. Ruth Durán thanks the Spanish Ministry of Science and Innovation for a Juan de la Cierva research contract. A. Micallef was supported by a Marie Curie Intra-European Fellowship PIEF-GA-2009-252702 within the 7th Framework Programme of the European Commission.

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