Geomorphology and sedimentary features in the Central Portuguese submarine canyons, Western Iberian margin
Introduction
Submarine canyons are deep and steep incisions on the seafloor of continental shelves and slopes that act both as temporary stores of sediment and carbon (van Weering et al., 2001, van Weering et al., 2002, Oliveira et al., 2007) and as conduits for the transport of large amounts of sediment from the land to the deep sea, the latter driven by dense-water cascading and gravity-driven sediment flow events (Shepard and Dill, 1966, Gardner, 1989, Canals et al., 2006, de Stigter et al., 2007, Puig et al., in press). Submarine canyons are often associated with sand-rich turbidites and are thus investigated as modern analogues for deep-water hydrocarbon reservoirs (Wynn et al., 2000, Stow and Mayall, 2000, Kenyon et al., 2002). They are hotspots of biodiversity (Vetter and Dayton, 1998, Genin, 2004, Mortensen and Buhl-Mortensen, 2005), but pollutants may endanger canyon ecosystems when transported with sediment to the deep depositional systems (Gascó and Antón, 1997, Puig et al., 1999, Paull et al., 2006).
Since the early studies on submarine canyons, their geomorphology has been largely ignored, mainly due to their complex terrain and the difficulty of studying the seafloor (Daly, 1936, Shepard, 1963, Normark and Piper, 1969). However, with the advent of new geophysical techniques, a great effort has been made in the description and interpretation of the geomorphology of submarine canyons (Liu et al., 1993, Hagen et al., 1996, Gardner et al., 2003, Popescu et al., 2004, Antobreh and Krastel, 2006, Lastras et al., 2007, Mitchell et al., 2007) and evolution through time (Laursen and Normark, 2002, Smith et al., 2007).
Submarine canyons are generally V-shaped in profile and display tributary canyon systems in plan view, including steep, gullied slopes. Canyons may pass downslope into shallower, U-shaped channels, and both canyon and channel floors can have bedforms, like gravel and sand waves, and erosional scours. Their evolution is controlled by factors including structural fabric, regional and salt tectonism, sea-level variation and sediment supply (Laursen and Normark, 2002, Soh and Tokuyama, 2002, dos Reis et al., 2005), and may involve processes such as large-scale erosion, sediment destabilisation, axial incision or thalweg infilling (Baztan et al., 2005, Canals et al., 2006).
Submarine canyons dissecting the European continental margins have been the target of the EC-EUROSTRATAFORM and HERMES projects. These projects investigated the driving forces that control ocean margin strata formation and ecosystems (Weaver et al., 2004). Among those of the European margins, the Nazaré, Cascais and Setúbal–Lisbon canyons on the Western Iberian margin have been studied and monitored in great detail during several cruises, gathering a significant amount of high quality, multidisciplinary data (e.g., de Stigter et al., 2007, Oliveira et al., 2007, Arzola et al., 2008).
In spite of this large effort, a general, world-wide accepted valid theoretical context of the location, evolution and activity of submarine canyons is not yet available. From this point of view, this paper aims to present the results of the interpretation of seafloor geophysical data obtained from the Nazaré, Cascais and Setúbal–Lisbon canyons, adding new examples to the knowledge of submarine canyons in light of multibeam bathymetry and deep-towed side-scan sonar data. We describe the key geomorphologic features observed and discuss the different nature of processes occurring in these canyons and their causes, and compare them with other canyons known: their riverine influence, their activity during high-stands and low-stands, how their location is controlled by the tectonism, and how they relate to deep-sea deposits through channel-lobe transition zones.
Section snippets
Regional setting
The Western Iberian (or Portuguese) margin (Fig. 1) is a non-volcanic rifted continental margin that evolved during separation from North America in the late Aptian (Driscoll et al., 1995). The development of the margin is influenced by the complex Iberian plate tectonic history (Srivastava et al., 1990) causing the reactivation of ENE- to NW-trending late-Variscan onshore and offshore strike-slip fault systems (Pinheiro et al., 1996). The margin consists of a nearly flat, relatively narrow
Materials and methods
The present study is based on a compilation of swath bathymetry and side-scan sonar data obtained during several cruises during the last fifteen years. Swath bathymetry data were obtained using the Simrad EM-12S model onboard RRS Charles Darwin during cruises CD157 and CD179 in May–June 2004 and April 2006, respectively. Previously, swath bathymetry data from the head of the Setúbal canyon were obtained in the EM-12D test cruise Espichel onboard Ifremer's R/V L'Atalante in 1991. The EM-12S and
Results: the Central Portuguese canyons
This study is devoted to the Nazaré, Cascais and Setúbal–Lisbon canyons (Fig. 1), which severely indent the 50 km-wide continental shelf.
Discussion
The first complete detailed mapping of the Central Portuguese canyons reveals substantial differences in their morphologies, and also reveals a number of landforms that can be attributed to erosion, sediment transport and sediment deposition (Fig. 9 and Table 2).
Conclusions
The complete detailed mapping of the Central Portuguese canyons, located in the Western Iberian margin, reveals a wide variety of morphologies, both erosional and depositional. The upper and middle canyon parts are deeply entrenched into the continental shelf and slope, and display V-shaped, terraced valleys and gullied walls, whereas their distal parts are occupied by either channel-lobe transition zones with scours and sediment waves, or by a base-of-slope channel.
Turbidity currents
Acknowledgements
This research was supported by the HERMES project, EC contract no GOCE-CT-2005-511234, funded by the European Commission's Sixth Framework Programme under the priority ‘Sustainable Development, Global Change and Ecosystems’, the EUROSTRATAFORM project (EVK3-CT-2002-00079) from the European Commission's Fifth Framework Programme, and a Generalitat de Catalunya “Grups de Recerca Consolidats” grant (2005 SGR-00152). The authors thank the masters, crew and technicians involved in sea-going work,
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