engleski

Coastal typology and related seabed sediments along the eastern Adriatic coast (Croatia)

The Eastern Adriatic Coast (EAC) makes a naturally continuous geological and geomorphological zone comprising the area from the Gulf of Trieste to the Albanian coast. In recent years researchers have become interested in coastal area, primarily in beaches, in the context of sustainable coastal management. However, fundamental coastal and seabed research along the EAC is generally insufficient, regarding either natural or anthropogenic processes govern coastal/beach development and functioning. Most of the EAC is composed of the Mesozoic carbonates (>90% in length), while Eocene flysch coasts are of subordinate share (6%). This carbonate-flysch ratio is even more pronounced in favour of carbonates in Croatia. Croatian coasts in carbonates are distinctive because of numerous and various forms of submerged karst forms. In general three types of coasts in carbonates can be found. The first includes steep, inaccessible, and most often tectonically formed island and mainland coasts. The second type comprises accessible carbonate coasts with naturally exposed upper surfaces of limestone strata, and the third type includes numerous, but small carbonate gravel beaches. These beaches are generally formed in two ways: one by wave abrasion of tectonically crushed and weakened carbonate rocks, and the other by accumulation of carbonate material in bays that occasionally experience torrential flows. In general, carbonate gravel beaches are numerous, but their length along carbonate coasts is rather small. As a result of flysch being susceptible to mechanical weathering, high percentage of flysch coasts are beaches. Due to its mixed lithology (alternation of breccias, sandstones, siltstones and marls) flysch is weathered differentially. Cliffs usually form where sandstone prevails, while earthflows and mass movements occur in marls. The development of muddy, sandy and gravel beaches as well as mixed beaches is controlled by the lithification degree, variable grain size, general composition of flysch assemblage, and the exposition of coast towards wave action. Other types of coast are rare: i) sandy beaches develop where Palaeogene and Pleistocene sands and sandstones outcrop, and ii) muddy coasts where Quaternary river sediment face the sea. Unique and exotic coasts in gabbroic rocks occur only on two islets, with quite individual coastal processes. Due to the prevalent karstic relief, river supply is rather low along the eastern Adriatic, leaving the place for in situ biogenous carbonate production. As a result, seabed is veneered by mostly coarse-grained mixed carbonate-siliciclastic sediments. Unlike biogenic carbonates, siliciclastic material resides mostly in fine-grained fraction. Detailed inspection of mud fraction from sediment collected along the channel area has shown the presence of terrigenous minerals (quartz, mica, plagioclase, amphibole and chlorite), typical for mineral composition of flysch. Mud fraction was further qualitatively examined for its carbonate nannoplankton content. Dominant share of Eocene nannoplankton species has corroborated the assumption that siliciclastic fine-grained sediment particles originate from flysch. Due to the small extent of flysch outcrops in the coastal region today, it is likely that considerable amount of siliciclastic component found in surface seabed sediments has been derived in the same way but from present-day submerged flysch outcrops, weathered under subaerial conditions during the sea-level low-stand. The largest share of the flysch-derived fine-grained material in surface sediments is found in the Kvarner region and along channels of the Middle and South Dalmatia (eg. Brač Channel, Hvar Channel, and Neretva Channel). At the same time, those are areas where the highest amounts of finest flysch material are weathered along the coast (eg. Kaštela-Ploče flysch zone) and transported to the deeper seafloor.

Adriatic Sea; carbonate sediments; flysch; karstic coast

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