Erosional versus aggradational canyons along a tectonically-active margin: The northeastern Ligurian margin (western Mediterranean Sea)
Introduction
Submarine canyons are the most common conduits for erosion-derived particles, organic matter and nutrients transported from land to the base of continental margins (Normark and Carlson, 2003, Shepard, 1981, Twichell and Roberts, 1982). Submarine erosion is thought to be the main process involved in canyon development (Baztan et al., 2005, Popescu et al., 2004, Twichell and Roberts, 1982) through (1) repeated and continuous action of gravity flows including debris flows, turbidity currents linked to breaching of canyon heads or walls (Pratson and Coakley, 1996, Sultan et al., 2007) or flood events (hyperpycnal flows), (2) retrogressive erosion within the canyon heads related to the triggering of submarine landslides, and (3) up-and-down currents related to tides or internal waves, and cascading currents which are also now recognised to play a key role in local seafloor erosion, in depositing substantial volumes of sandy particles and in maintaining thalweg morphology (Gaudin et al., 2006, Mulder et al., 2012, Trincardi et al., 2007).
Many authors have distinguished between submarine canyons based on their morphological/architectural characteristics, and submarine canyons are often divided into two groups, erosional and depositional canyons (Goff, 2001, Jobe et al., 2011). Erosional canyons deeply incise the continental shelf and are characterised by terraces and numerous failure scars, and are often associated with sand-prone continental slopes (Adams and Schlager, 2000). The main mechanisms involved in their development are thought to be erosive turbidity currents (linked to coarse-grained gravity flows) and mass-transport processes (Paull et al., 2013, Shepard, 1981). They are often associated with coastal mountain belts (Covault et al., 2011, Gervais et al., 2006) or large fluvial systems (Babonneau et al., 2002, Green and Uken, 2008, Shepard, 1981). Depositional canyons do not reach the shelf edge; they are more aggradational, often have a U-shaped cross section, smooth inner walls lacking terraces or slumps (Jobe et al., 2011). They are associated with mud-rich infilling originating from hemipelagic deposition, dilute turbidity currents and bottom currents. So, canyons classically interpreted as net-erosional environments may in fact develop under net depositional conditions (Straub and Mohrig, 2009, Zhu et al., 2010). Both erosive and aggradational canyons can be found in both passive and active margins and the factors which control their behaviour are still the subject of debate.
In the present study, based on bathymetry/backscatter data, chirp and seismic-reflection profiles collected within the framework of the MALISAR project, we investigated nine erosive and aggradational canyons built since the Messinian Salinity Crisis in the eastern part of the Ligurian margin (western Mediterranean). Here, the spatial and temporal distribution of the tectonics activity is thought to strongly constrain the building style of the canyons. More particularly, aggradational canyons have thick infillings consisting of several amalgamated units separated by erosive surfaces. This paper could thus interest both academia and industry, as it helps understand the processes and factors controlling the formation of canyons and provides an unusual analogue model for potential reservoirs derived from continental-slope processes.
Section snippets
Geological setting
The Gulf of Genova (GOG) is located in the northernmost part of the Ligurian Sea (north-western Mediterranean), and extends between the cities of Imperia to the west and La Spezia to the east (Fig. 1). In its western part, it is bordered by the E-W trending Ligurian Alps, which in turn, are connected to the NW-SE Apennine chain by the Voltri Massif (Fig. 1).
Methods and data
The Ligurian margin was investigated during the Malisar1-2-3 campaigns in 2006, 2007 and 2008, which revealed the margin physiography between the cities of Nice (France) and Genova (Italy). Bathymetric surveys and backscatter data were collected over the whole margin at a depth of between 100 and 2,500 m in the water using the SIMRAD EM300 multibeam system. Raw data were processed and merged to build a digital terrain model (DTM) with a spatial resolution of 25 m (Fig. 1 and others). This DTM
Messinian and post Messinian architecture of the Gulf of Genova margin
To better constrain and understand some of the factors that might have controlled both the location and evolution of the present-day canyons described in the following paragraphs, several geological markers were first mapped to reconstruct the post-Messinian GOG margin settings.
For the Messinian period, in the western part of the GOG, the Messinian Erosional Surface was mapped on the upper continental slope. It can be identified as a widespread erosional surface or an angular unconformity above
Characteristics of the canyons along the north-eastern Ligurian margin
Between the cities of Imperia and La Spezia, one of the most striking features shaping the margin of the GOG is a canyon system, composed of nine canyons coalescing into the Genova Valley to the south (Fig. 1). From west to east, they are the Mercula, Laigueglia, Centa, Pora, Noli, Sansobbia, Polcevera, Bisagno and Levante canyons (Fig. 2). Combining observations from bathymetric data, chirp and seismic-reflection profiles allowed us to distinguish two set of canyons. Based on the morphology,
Discussion
From the analysis of seafloor (multibeam bathymetry, seafloor reflectivity) and seismic data (3.5 kHz, high-resolution seismic profiles), nine canyons were identified along the GOG margin. These canyons evolved from Messinian erosive conduits excavated during the dramatic fall in sea level. In the following paragraph, we discuss the evolution/building of these canyons during the Plio-Quaternary and the factors that could have controlled these features, along with their implications for reservoir
Conclusions
A series of submarine canyons was built on the continental margin of the GOG during the Plio-Quaternary. These canyons started from Messinian paleo-valleys cut during the Messinian Salinity Crisis. Based on a large dataset discussed here, including multibeam bathymetric data, backscatter data, 3.5 kHz profiles and seismic reflection profiles, we analysed the present day morphology and the architectural development of these canyons during the Plio-Quaternary. Based on the sub-surface data,
Acknowledgements
The authors thank the captain and crew of the RV Le Suroit and L'Europe and all the scientists and technicians who participated in the MALISAR cruises. Quentin Soulet is a PhD student funded by TOTAL and the ANRT. Critical reviews by Bilal Haq and Sara Lafuerza helped to improve a preliminary version of the manuscript. The authors thank Hervé Gillet and an anonymous reviewer for their constructive comments and suggestions that clarified the manuscript.
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