engleski

Shift from siliciclastic to carbonate sedimentation in the Raša river estuary (Istria, Croatia)

A 136 cm long core was sampled at the station located at the seaward end of the Raša River estuary (Raša Bay, east coast of the Istria peninsula, 44° 57' 02 N, 14° 03' 66 E, 40 m deep). Granulometric and mineralogical composition, carbonate content, terrigenous and biogenous content and radionuclide activity were determined. Analyses were done on 4 cm long subsamples, and radionuclide activity on 16 cm long subsamples. Moreover, a peat sample was 14C dated. Most of the core contains equivalent share of carbonate and noncarbonate components. However, the deepest part of the core (below 121.5 cm), contains only quartz. The two parts of the core are divided by thin peat layer (118-121.5 cm). In the uppermost 40 cm of the core in the fraction >63 mm (34-73 %) the most important particles/fragments are of biogenous origin (foraminifera, shells, snails and ostracods). This is also reflected in the mineralogical composition (considerable share of aragonite and high Mg-calcite). In the interval 40-118 cm clayey silt prevails (<32 mm, 71-92 %), and in the fraction >63 mm terrigenous particles (rock fragments, dolomite, limestone) are dominant, while plant remains are ubiquitous. In addition, marine foraminifera are also present in a considerable amount. The most important minerals are quartz, calcite and dolomite. Increased 238U activity in the sediment deeper than 32 cm indicates on more terrigenous influences and possibly reductive conditions during the sedimentation process. The 14C analysis of the peat layer dated it to 9994 &plusmn ; ; ; 195 yBP (cal. 9200 &#8211; 10000 yBC). In the deepest part of the core (121.5-136 cm) a sandy silt containing 42-65 % of fraction >32 mm is found, and quartz is the predominant mineral. The Raša river estuary was formed by the Holocene sea&#8211; level rise, when the river valley, carved into carbonate rocks, was flooded by the sea. The sediment below the peat, very probably, is a riverine sediment deposited before the Holocene, presumably during or after the last glacial maximum (LGM). The conditions for the peat layer formation have been established at the end of the Younger Dryas, while the marine sedimentation started after the sea has flooded the area, now 40 m below the present sea-level (presumably at the beginning of the Holocene). A relatively thin layer of the Holocene sediments (~1.2m) indicates a very efficient deposition of terrigenous riverine material in the inner-upper estuary.

sedimentation; Raša river estuary

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