Geophysical characterization of an active fluid escape structure (the Zannone Giant Pockmark, central Tyrrhenian Sea)

An active giant pockmark located offshore Zannone Island (central Tyrrhenian Sea, Italy), is here analyzed by integration of high resolution seismic profiles, very high resolution multibeam bathymetry and ground-truth data. The active fluid escape structure is located on the outer shelf at 110-130 m water depth, and affects the Late Quaternary lowstand and highstand deposits resting on the rocky bedrock. Evidence of active seepage includes both continuous and intermittent bubble release from the seafloor and a well-defined plume rising 70 m above the seafloor and a variety of fluid-escape features characterizes the area (e.g. craters, pockmarks, lithified pavements, mounds and bacterial mats). The Zannone giant pockmark (ZGP) is about 900 long and 500 m wide and represents the first evidence of an active shallow-water seepage area in the central Tyrrhenian Sea (Italy) and the first record of a morphologically complex giant pockmark in the entire Mediterranean Sea. Some speculations on processes originating the observed features are explored, including possible occurrence of multiple eruption events, processes of fluidization-liquefaction and minor slides that may have modified the original morphology. Factors peculiar to the study area (shallow depth, thin sedimentary cover resting on a faulted rocky basement, seeping occurring through sandy sediments) appear to have been key to the formation and morphology of the ZGP.