engleski

Sources and distribution of mixed cool-water carbonate-siliciclastic surface sediment in the Eastern Adriatic Sea

The Adriatic Sea is microtidal, semi-enclosed northernmost Mediterranean basin. Its eastern side is bordered by steep and rugged karst coast, with some flysch outcrops. Shelf coastal zones of the eastern Adriatic, including channel area, receive small amounts of terrigenous material due to the poorly developed surface network of mostly karstic rivers. As lacking substantial terrigenous input, eastern Adriatic shelf is peculiar depositional environment significantly influenced by in situ biogenic carbonate production. First 5 cm of surface sediment at bottom depths up to 155 m were collected from 248 sampling points along the eastern Adriatic and studied for grain size and carbonate share. Grain size distribution was determined by combination of wet sieving and sedigraph, while the concentration of carbonates was obtained by the gas volumetry. Forty selected samples were analyzed for semiquantitative mineral composition using X-ray diffractometry. Coarse-grained fraction (gravel and sand) of all sediment samples was microscopically examined to identify skeletal and non-skeletal grains. Seabed of the eastern Adriatic shelf is covered by mixed carbonate-siliciclastic sediments, mostly sands, containing 6-99 % of carbonates. Carbonate component is mostly of biogenic origin and found prevalently in coarse-grained fraction, consisting mainly of recent and, to a lesser extent, of relict biogenic particles. Composition of bioclasts reveals the dominance of bivalve shells, gastropods, bryozoa, foraminifera and red algae, with small contribution of echinoderms, serpulids, arthropods and sponges remnants. Based on the dominance of certain group of skeletal grains three skeletal associations were identified: foramol, bryomol and rhodalgal, all three characteristic for non-tropical carbonates. Additionally, low-Mg calcite dominates over high-Mg calcite and aragonite, which is also one of important characteristics of non-tropical carbonates. Only few samples contained carbonate lithoclasts of terrigenous origin in gravel fraction. Considering their good roundness and sampling water depth below wave base, these lithoclast were presumed to be relict, deposited and shaped during a sea-level lowstand.

Adriatic Sea; cool-water carbonates; mixed carbonate-siliciclastic shelf

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