engleski

RECENT AND SUBMERGED TOMBOLO ON PRVIĆ ISLAND, KVARNER AREA

The tombolo is a depositional geomorphological form (sandbar, barrier or spit) that joins an island or a barrier with the mainland or another island, resulting from longshore drift or the migration of an offshore bar toward the coast (WARD, 2004). Two tombolos, recent and submerged, are on the south-western coast of Prvić Island (Kvarner area, the north-eastern channel part of the Adriatic Sea). Three geomorphological factors are important for the origin of these tombolos: specific geological fabric, oceanographic conditions and sea-level change during Holocene. Prvić Island is mostly formed in carbonate rocks consisting of Upper Cretaceous and Palaeogene carbonate rocks, and partially of Palaeogene siliciclastic rocks (MAMUŽIĆ et al., 1969). Most of the island has a typical karstic landscape. The investigated area close to Cape Pipa is located in the central part of the south-western coast. The coastal relief and geologic fabric are very different in relation to other coastal parts of this island. There Palaeogene flysch bedrock is partially covered by Quaternary sediments. This sediment body has a form of irregular triangle with a base length of approximately 50 m. The height of this deposit is 10 to 12 m. The stratification is well visible in the sediment body, where silty sand prevails. Some layers contain angular fragments and blocks from carbonate rock mass. Cape vertex itself is formed of collapsed blocks of talus breccia and provide an obstacle to wave motion. Beach sediment body connects the cape vertex with the coast. Wind waves from the northwest (tramuntana) and southeast (jugo) generate longshore currents, which are responsible for coastal erosion and accumulation of sediment in the beach bodies. Recent tombolo is in a state of equilibrium in the present climatic and oceanographic condition. Much bigger triangular tombolo form is clearly visible on the sea bottom, southeast from Cape Pipa. Paleo-coastline is approximately 300 m long. Unusual dark parallel lines are clearly visible on the ortho-photo image, where these submerged outcrops look like artificially built walls. The exploration by scuba-diving has revealed that they are outcrops of more resistant sub-vertical layers within the flysch rock mass. Concave traces of ancient coast are clearly visible on the western side and less noticeable on the south-eastern side. This submerged tombolo form was formed behind another obstacle, and it is located at depths up to 10 m. The Njivice Rock is built of relative resistant carbonate breccia. The conditions for forming a submerged tombolo could have occurred during the sea-level rise during the late Pleistocene and Holocene (SURIĆ, 2009 ; TSIMPLIS et al., 2009). The sea has flooded the submerged tombolo probably in the last phase of rapid sea-level rise at the beginning of Holocene. A large part of less resistant Quaternary sediments and fine-grained flysch rock mass were eroded. Njivice Rock played an important role in the formation of the tombolo. This rock provided an obstacle to waves in the shallow and flattened area. Two generations of tombolos co-existing at close proximity is a unique phenomenon in the Adriatic Sea, and possible in the Mediterranean Sea. This work has been supported in part by the University of Rijeka (Uniri-tehnic-18-97 1232) and GEOSEKVA (HRZZ IP-2016-06-1854).

tombolo, marine erosion, sea-level, Holocene, Adriatic Sea

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