Erosional versus aggradational canyons along a tectonically-active margin: The northeastern Ligurian margin (western Mediterranean Sea)
Résumé
Submarine canyons are usually described as erosive conduits incising the continental slope through retrogressive sediment failures and active erosion by gravity flows. Only a few studies have revealed that canyon deepening is possible under conditions of net sediment deposition.
In the present study, we used bathymetry/backscatter data, chirp and seismic-reflection profiles collected within the framework of the MALISAR project to investigate nine active canyons belonging to one canyon system built in the eastern part of the Ligurian margin (western Mediterranean) since the Messinian Salinity Crisis. By comparing their planform pattern and architecture, we identified two sets of canyons located along the western and eastern segments of the margin. Sub-surface data showed that despite their contrasting sizes, both western and eastern canyons exhibit V-shaped cross sections, concave-up longitudinal bathymetric profiles and their thalwegs are marked by erosive structures or coarse-grained deposits. Their present-day activity could thus be controlled by similar sedimentary processes dominated by erosive sandy turbidity currents originating from canyon breaches or other mass-wasting processes. However, according to the seismic profiles, the Plio-Quaternary evolution of the two sets of canyons differed dramatically. The western canyons are mainly aggradational systems, with the progressive building of thick levees composed of sigmoidal shaly lateral accretion packages and the amalgamation of paleo-thalwegs infilled with coarse-grained deposits. In contrast, eastern canyons developed as by-passing conduits during the Plio-Quaternary. It is important note that the present-day morphology of the eastern canyons was not caused by erosion of previous deposits, as described for other large canyons, but by the gradual building of 700-m-thick adjacent levee-like accumulations.
Here, both the spatial and temporal distribution of the tectonic activity is thought to have strongly constrained the building style of the canyons. More particularly, aggradational canyons have thick infillings consisting of several amalgamated units separated by erosive surfaces. This contribution could thus be of interest for both academia and industry as it helps understand the processes and factors controlling the formation of canyons and represents an unusual analogue model for potential reservoirs derived from continental slope processes.