Evidences of mud volcanoes in the Palermo and Termini Gulf (N Sicily offshore)

The occurrence of mud volcanoes and mud domes is, for the first time, documented in the continental shelf and upper slope of the Palermo and Termini Gulf (north Sicily offshore region). The study area belongs to the northern Sicily continental margin (southern Tyrrhenian Sea). Along this margin, morphology of the high-gradient continental slope is irregular due to the presence of structural highs, slope failures and canyons, and is interrupted by a flat area at a depth of 1,500 m. Mud volcano activity generally formed in areas characterised by high sedimentation rates and/or as consequences of regional tectonics (often compressional), overpressure in the sediments and gas generated at different depths. Materials and methods This study is based on unpublished, near-vertical seismic profiles shot in 2001 in the N Sicily offshore region, coupled with multichannel seismic profiles and multibeam data. The latter have been acquired using the 50-100 kHz Seabat instrument, with operating depths ranging from 50 to 1500 m, and processed with the software PDS 2000. Seismostratigraphic analysis tools and methods were used to identify mud volcanoes and to make classification on the basis of their morpho-acoustic characteristics. The detailed 3D bathymetric chart was used to define the top view morphologic features and their areal distribution. Results The new data show the occurrence of three types of structures with highly contrasting seismic and morphologic signatures: • dome-type structures (type 1), elevating 5 to 30 m from the seafloor, showing transparent to chaotic internal facies with mound external geometry (locally with a central depression), and deforming the adjacent reflectors; • concave-upwards structures (type 2), depressed 5 to 20 m from the seafloor, showing at depth layered concave-upwards pattern or locally chaotic convex-upwards pattern. Their top view is a circular aligned areas known as pockmarks; • buried structures (type 3), showing chaotic to transparent facies, and deforming the adjacent reflectors; these structures appear to be buried by recent deposits. Type 1 structures are recognized in the whole investigated shelf areas and are prevalent in the Palermo Gulf western margin (Pellegrino Mt. offshore); type 2 formed near the shelf margin, whilst the type 3 structures occur only in the Termini Gulf. In the we recognise type 1 structures only. Our data integrated with stratigraphic and structural data available in literature for the North Sicily offshore suggest that mound structures (type 1) area associated with fault planes, that are widespread offshore Pellegrino Mt. We interpreted these structures as mud domes associated with diagenetic carbonates and fluid venting activity. The pockmark structures (type 2) could be the result of both fault and landslide structures, as they appear aligned along straight direction and occur in proximity of the slope, associated with slope instabilities. The buried structures (type 3) occur in the Termini offshore where the higher sedimentation rate supports the sealing of the mud flow.