Morpho-sedimentary characteristics of the volcaniclastic apron around Stromboli volcano (Italy)

Submarine portions of Stromboli volcano account for about 98% of the whole extent of the volcanic edifice and are mostly covered by volcaniclastic sediments that made up a modern volcaniclastic apron. The architecture and main sedimentary processes acting within the apron are depicted through the integration of geophysical evidence (multibeam, long-range side scan sonar and seismic data) and seafloor sampling. The apron shows, on the whole, a large variability both across and along slope of morphologies and deposits related to mass-wasting and reworking processes, passing into areas covered by hemipelagic sedimentation. A large spectrum of erosive-depositional features was recognized on the surface of the apron. On shallower areas (i.e. submerged shelves, shore platforms and depositional terraces), submarine features related to wave action and sea-level fluctuations are present; they mainly act as temporary storing and reworking areas for volcaniclastic material derived from subaerial portions. Unconfined cohesionless inertial flows commonly dominate the upper slope, where gradients are very high (up to 35°) and grain-to-grain and grain-to-bed collisions are favoured within the flows. These flows tend to evolve into turbidity currents on the lower slope, in response to a marked decrease of slope gradients below ∼ 8°, generating various erosive-depositional features, such as erosive furrows, troughs, channel-levee features and coarse-grained sediment waves. The most active area of the apron lies offshore Sciara del Fuoco (on the NW flank), where a large amount of coarse-grained volcaniclastic material, derived by the persistent Strombolian activity, is collected and deposited by means of high- and low-density turbidity currents on the lower slope down to 2600 m bsl, more than 20 km far from the island. Conversely, other areas are mostly dominated by deposition via settling of suspended sediments, related both to hemipelagic sedimentation and to the dispersion of fine tephras by the prevailing wind and marine currents. Such processes interfinger and overlie in space and time, giving rise to a very complex morpho-sedimentary facies distribution. However, the structural bilateral symmetry of Stromboli volcano allows defining distinct morpho-sedimentary zones, where one (or more) type of processes dominates, and related different stratal architecture of the apron. The unbuttressed portions of the island (NW and SE) are characterized by the emplacement of wide and thick debris avalanche deposits (volume in the order of 1–2 km3 for each event) related to large-scale sector collapses, representing the most important and efficient mass-wasting processes making up the apron. Such deposits are embedded within different volcaniclastic sequences and can be eroded or exhumed by successive gravity flows. The result is a highly complex (both spatially and vertically) succession of facies, where it is often difficult to depict an arranged series of processes. Conversely, on buttressed flanks (SW and NE) a more ordered evolution of processes and deposits making up the apron is observed, giving rise to a less complex apron architecture.