Holocene palaeoenvironmental development of Prokljan Lake (Krka River, Croatia): Evolution from a calcareous tufa barrier system to a karst estuary

The upstream part of the karst estuary of the Krka River (Prokljan Lake) on the Eastern Adriatic Coast was studied by combining high-resolution seismic data, analyses on sediment cores, and radiocarbon dating to reconstruct the late Quaternary stratigraphy of the estuary. The sedimentary infill consists of up to 20 m of freshwater and estuarine deposits divided into four seismostratigraphic units corresponding to several stages of development since the last glacial period. The lowstand stage is assumed to correspond to fluvial incision and sediment bypass associated with glacial periods. The postglacial transgressive stage is marked by the growth of calcareous tufa barriers and fluvio-lacustrine deposition. The onset of marine influence in the estuary (Prokljan Lake) was recorded at the beginning of the Holocene (>10,500 cal y BP) with the deposition of brackish sediments with a strong freshwater influence. Calcareous tufa barriers prevented direct rapid flooding of Prokljan Lake in the period from >10,500 cal y to ∼7200 cal y BP. Finally, the transgressive brackish deposition changed to estuarine/marine highstand deposition in a salt-wedge estuary during the late Holocene (∼4000–3000 cal y BP). The late Quaternary deposition in this unique karst setting showed dramatic changes. It was primarily influenced by rapid sea-level rise, climate changes, estuarine geomorphology and relatively low sediment supply.