Submarine canyons of north-western Sicily (Southern Tyrrhenian Sea): Variability in morphology, sedimentary processes and evolution on a tectonically active margin
Introduction
Submarine canyons are widespread complex features along continental margins and are pathways for sediment transport to the deep basins (Piper and Savoye, 1993, Wynn et al., 2007, Palanques et al., 2012), They play a key role in hydrography, biogeochemistry and carbon cycle dynamics (Epping et al., 2002, Canals et al., 2006, Huvenne et al., 2011). Despite their importance, the processes of start-up, evolution and maintenance of submarine canyons remain poorly understood. Based on a wide range of canyon dimensions and morphologies observed in different geologic settings, the sedimentary processes responsible for shaping submarine canyons may be summarized in terms of top–down and bottom–up processes (Shepard, 1981, Twichell and Roberts, 1982, Piper and Savoye, 1993). The top–down processes mainly refer to turbidity and hyperpycnal flows occurring along the shelf margin which generate axial incision down to the slope, in alternating erosive and depositional dynamics (Baztan et al., 2005). At a global scale, top–down processes generally occur in moderate to high sediment-supply margins, alimented by coastal and fluvial sedimentary inputs, and are more frequent and active during the glacial phases (Canals et al., 2000, Baztan et al., 2005, Harris and Whiteway, 2011). Bottom–up processes are governed by the development of stacked mass-movements retrograding through the lower slope to the shelf margin and are more commonly observed in sediment-undersupplied margins, in tectonically active high-gradient bedrock slopes (Lo Iacono et al., 2011, Biscara et al., 2012, Micallef et al., 2012). Commonly, both of these mechanisms contribute to canyon shaping and development, with top–down turbidity processes producing axial incision from the shelf, and bottom–up retrograding landslides widening the canyons and producing downslope debris flows (Pratson and Coakley, 1996, Lo Iacono et al., 2011). Moreover, active tectonics play a relevant role in controlling the evolution and the morphology of submarine canyons, above all along continental margins affected by tilting and vertical movements (Mountjoy et al., 2009, Ratzov et al., 2012).
Offshore north-western Sicily (southern Tyrrhenian Sea) swath-bathymetry and high resolution seismic data have unveiled a dense network of submarine canyons within the depth range of 80–2100 m, across a wide range of morphologies and stratigraphic settings. The main aims of this work are: (1) to describe the morphologies of submarine canyons mapped in the Gulfs of Palermo and Castellammare, (2) to identify the main sedimentary and geological processes which contributed to canyon formation, and (3) to infer on the main sedimentary processes which can have contributed in maintaining some of the submarine canyons during the last glacial–interglacial phase.
Section snippets
Geological setting
The northern Sicily continental margin, in the southern Tyrrhenian Sea, extends from the north Sicily coastal belt to the Marsili bathyal plain (Fig. 1a). The margin is located in the transitional area between the Sicilian–Maghrebian chain to the south and the Tyrrhenian back-arc basin to the north (Kastens et al., 1988, Scarascia et al., 1994). This region originated as a consequence of a complex interaction of compressional events, crustal thinning and strike-slip faulting (Pepe et al., 2005
Materials and methods
Swath-bathymetry Multi Beam (MB) data were acquired during three different oceanographic cruises in 2001, 2004 (CARG cruises) and 2009 (MaGIC cruise) (Fig. 1) (www.isprambiente.it; Chiocci and Ridente, 2011). The MB system of the 2001 cruise was a Reson SeaBat 8111, generating 105 beams at a frequency of 100 kHz, with an operational depth range of 35–800 m. The MB system of the 2004 and 2009 cruises was a Reson SeaBat 8160, generating 126 beams at a frequency of 50 kHz, for an operational depth
Results
Throughout the study area, we describe two canyon systems with different morphologies and sizes: the canyons of the Gulf of Palermo and the canyons of the Gulf of Castellammare (Fig. 1A). The two gulfs are bounded by promontories, made up of deformed Meso-Cenozoic sedimentary rocks (Agate et al., 1993) and by a central coastal plain, where the Meso-Cenozoic bedrock is buried, unconformably overlain by the Quaternary deposits. Both study areas are characterized by a variable extension of the
Discussion
Our data demonstrate that the Gulfs of Castellammare and Palermo display different submarine canyon landscapes (Table 2), despite them belonging to the same margin (South-Tyrrhenian border), the same inland geologic setting (Tertiary Sicilian Thrust Belt) and being exposed to similar oceanographic conditions. In the following sections we discuss bathymetric and seismic evidence related to the geological processes which govern these differences.
Conclusions
The application of Multibeam bathymetry and high resolution seismics revealed the presence of several submarine canyons in the Gulf of Palermo and the Gulf of Castellammare, along north-western Sicily. Despite the two canyon systems belonging to the same continental margin and being located few kilometers apart, they display relevant differences in the morphologic characteristics and sedimentary processes. Retrograding mass movements breaching the Plio-Quaternary succession from the lower slope
Acknowledgments
We gratefully acknowledge the FP7 2011–2013 EU Marie Curie Action “Geo-Habit”: GA29874 (Dr. Claudio Lo Iacono) and the Italian National Research Projects MaGIC (Marine Geological Hazard along the Italian Coast) funded by the Italian Civil Protection Department and CARG (Geological Maps of Italy) funded by the ISPRA-Italian Geological Survey. We also acknowledge the Grup de Recerca de la Generalitat de Catalunya B-CSI (2009 SGR 146). We are finally very grateful to the two reviewers, whose
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