Shallow seep-related seafloor features along the Malta plateau (Sicily channel – Mediterranean Sea): Morphologies and geo-environmental control of their distribution

Abstract

Between 140 and 170 m water depth, more than 100 small-scale domes and peculiar ridges were mapped a few miles offshore of south-eastern Sicily along the Malta plateau (eastern Mediterranean Sea), Swath bathymetric data along with a dense grid of side scan sonar and seismic profiles were acquired in an area extending over 100 km2. Gravity cores, water samples and video observations were also collected at selected sites. Mapped domes were found from 50 to 200 m wide and no more than 5 m high occurring on the seafloor, isolated or arranged in clusters. Ridges consisted of large tabular sub-elongated structures, elevated from 5 to 10 m from the surrounding seafloor, and had flat tops on which numerous closeset, small cones occurred, appearing in video observation as carbonate buildings strongly colonized by gorgonians. Characteristic acoustic signatures (i.e. blank areas and/or turbidity zones and enhanced reflections in seismic records), measured gas anomalies in seawater samples and detected plumes on echosounder profiles suggest that both the domes and ridges are influenced by active seeps. In addition, their spatial distribution reflected patterns of tectonic lineaments produced by the late Miocene to present-day geo-dynamic evolution of the Malta plateau, which is also an important hydrocarbon province. Results from gravity cores suggest that mud extrusion seems to be the main process responsible for the origin of the domes, which are formed by gray mud with only a few centimeters of biogenic sand at the top, indicating that recent bioclastic material is not a major contributor to mound building. However, at present, active degassing appears to be the main process that controls the morphological and sedimentological expression of both the domes and ridges. Therefore, a quiescent or intermittent active stage for mud extrusion is considered.