engleski

Postglacial sedimentary environments and Mid- Holocene marine flooding of a costal karst lake in Pirovac Bay (Dalmatia)

Many modern epicontinental seas were dry land before their marine flooding by the mid- Holocene glacioeustatic sea-level rise, whereas, the Adriatic Sea was submerged by semi-enclosed seas and lakes. The sedimentary record in marine embayments holds important information on Holocene environmental and sea- level changes. Eastern Adriatic shelf along Croatian coast presents karst relief with indented coasts containing numerous islands and bays. The submerged landscape of such coasts in central Dalmatia is studied and it involves the evolution of Pirovac Bay and adjacent Lake Vrana. The Pirovac Bay karst depression is partially separated from the Adriatic Sea by barriers and barrier islands. The change from a postglacial floodplain/alluvial deposits and lake sedimentation to the present-day marine bay (Pirovac Bay) and to a freshwater-brackish lake (Lake Vrana) is investigated using the combination of seismic reflection profiling and sediment cores (grain size and geochemical analysis, 14C dating). A dense grid of seismic profiles in the Pirovac Bay and Lake Vrana has imaged the Quaternary sediment sequence. The accumulation of the sediments in Pirovac bay is the highest in the central deepest part of the depression where more than 12 m of sediments is observed. The sediment thickness decreases going northwards and westwards in the bay as a consequence of significant input of material from the southeastern part of the catchment. This part of the catchment makes approx. 70% of the whole Pirovac bay catchment on land. The sediment core taken from the central part of the bay at 25 m water depth provided 7.2 m of sediments, comprises several seismic units: karstified bedrock, Pleistocene colluvial sediments, floodplain/alluvial sediments, lake and marine sediments. The Pleistocene colluvial sediments are well preserved in the deepest part of the bay and in the Murter Channel, and additionally they appear as outcrops on the shores of islands, mostly in the southwestern part of the area. Lake (high parallel internal reflectors) sediments have a total thickness of ~3.5 m in the deepest part of the bay, while on the edges lake sediments are not present. Our data reveal that the accumulation of the floodplain/alluvial sediments in the Pirovac Bay occurred in the period from 14 to 12 ka cal yr BP. The lake environment existed from 12 to 8 ka cal yr BP, receiving the water through the permeable underground from Lake Vrana. The raising of the sea level in that period caused deepening of the lake in Pirovac Bay and formation of the Lake Vrana at 9.1 ka cal yr BP. The flooding of the lake in Pirovac bay occurred at the onset of Mid-Holocene (8 ka cal yr BP) due to sea level rise and overflow of sea water over the sills that outline the Pirovac bay karst depression (6 mbsl). The lake in Pirovac Bay and Lake Vrana coexisted for approx. 1.000 years during the Early Holocene (9 to 8 ka cal yr BP), before the karst lake in Pirovac Bay was flooded by the sea. The sedimentary environments in the bay and the formation of both lakes are primarily forced by postglacial sea-level change. The present lake environment in Lake Vrana could be an analogue stage (slightly brackish) in the evolution the lake in Pirovac Bay in its terminal phase at the end of Early-Holocene, with a sea intrusion phase through the karstified bedrock, before the formation of a bay during the Mid-Holocene.

Holocene, submerged landscape, marine sediments, geophysics, sea level rise

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