DROWNED KARST FEATURES IDENTIFIED ON THE ADRIATIC SEA CONTINENTAL SHELF (ITALY)

Drowned karst features have been recently discovered on the Adriatic continental shelf at water depths of 70 m and, possibly, at 160 m. The most relevant features are (i) circular depressions interpreted to represent ‘dolines’ formed at times of lowered sea level in the Pleistocene and, subordinately, (ii) buttered surfaces interpreted as erosive remnants. Such ‘dolines’ have been extensively explored and imaged by means of multibeam bathymetry, chirp sonar, side scan sonar, CTD cast and ROV survey during oceanographic missions ARCO and ARCADIA of RV Urania in December 2008 and March 2010 in the frame of EU Hermes and Hermione programmes. A field of topographic depressions of variable depth punctuates the continental shelf offshore the Gargano Promontory between 50-130 m. The best developed ‘dolina’ in this field occurs in 73.6 m of depth at 41°59.62’N-16°15.15’E (‘oyster pit’: ca. 150 m in diameter, ca. 25 m deep). One large feature (ca. 400 m in diameter, ca. 30 m deep) is an isolated hole located at 42°27.39’N-16°01.31’E ,160 m below the sea-level and is possibly a karst feature as well. Bottom sampling (large volume grab, gravity corer) recorded the stratigraphic history of these sites. The smaller and shallower one (‘oyster pit’), formed at the expenses of Mesozoic-Cenozoic carbonate-rich units belonging to the Apulian foreland, is sediment-starved during the Holocene sea level high stand and fed by multiple events of coarse-grained shell-rich mass gravity inputs. At present, the ‘oyster pit’ benthic life is remarkably dominated by aggregations of the deep oyster Neopycnodonte cochlear and hexactinellid sponges. The largest and deepest putative ‘doline’ acts as a sediment sink intercepting Holocene highstand fine-grained sediment and its muddy bottom is colonized by sparse benthic life.