Side-scan sonar and subbottom profiling in the northern Adriatic Sea

Abstract

Side-scan sonar and subbottom profiling, coupled with bottom control through diving and sampling, have revealed new seafloor features in the northern Adriatic Sea and contributed to a better understanding of previously researched morphological and sedimentological features. Reef rock was found to be more significant than the previously known submerged beachrock exposures. Calcareous algae, madreporaria, bryozoa and serpulidae are the dominant reef builders. Single exposures of organic rock protruding above the seafloor range in size from a few cubic decimeters to several hundred thousand cubic meters. Such reefs have been tentatively divided into three types: (a) reef “sensu strictu” where organisms have built the entire feature; (b) cap reef, where the reef rock represents only a hard cap over a sedimentary core; and (c) coating reef, where the organic rock represents a thin coating over other rock types (mainly beachrock). Subbottom reflections both under and within features which at the surface appear to be reefs, confirm the existence of (b). Large beachrock exposures have been found between Lignano and Grado. In at least one case, beachrock seems to have supported reef growth. Other calcareous sandstones have been found in the area, but their thickness coupled with the absence of sedimentary structures and shell remains make it questionable whether the rock should be regarded as beachrock. A discontinuous, dense mattress of dead Posidonia roots has been found covering a large area (ca. 5 by 30 km) northeast of Venice in water depths ranging from 15 to 23 m. Such Posidonia mats are usually covered by a few centimeters of sand and have been cut and eroded by wave-induced turbulence to form “terraces” and a generally irregular bottom morphology. Mattresses of Posidonia roots form a protective cover along the crests of sandwaves south of Caorle in 20 m of water and protect them from wave erosion. Periodic lineations of very coarse sediment spaced 15 to 20 m apart on an otherwise fine, sandy bottom, have been found 20 miles east of Venice in 29 m of water. These are the result of sediment sorting due to exceptional storm waves coming from a southeasterly direction.