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Preliminary Paleolimnological and Geomorphological Research of the Prološko Blato Area in Imotsko Polje

The Prološko Blato area is seasonally flooded, north-western part of the Imotsko polje (karst polje) situated in the central part of Dinaric karst. It is made of Quaternary lacustrine sedimentary sequence surrounded by Cretaceous carbonates. Fluctuating water level turns the environment from wetland to grassland. This transition represents a period of erosion and formation of distinctive erosional channels, which are then suitable for mapping and sampling of sediments. The eastern part of the Prološko Blato called Prološko Lake, is according to BONACCI & ROJE-BONACCI (2000) an estavelle. This is a typical Dinaric karst landscape that contains numerous geomorphological features: springs, streams, and dolines. Later ones seem to be the most significant in the context of climate change and environmental protection of this remarkable karst phenomenon. The preliminary field research included: core drilling, sediments sampling, side-scan sonar measuring, and digital photogrammetry by ShapeMetriX3D (3G SOFTWARE & MEASUREMENT GMBH, 2007). Laboratory work included: high- resolution core analysis, radiocarbon dating, geochemical, mineralogical and grain-size analysis, as well as micropaleontological analysis of ostracods. Additionally, geomorphological analysis of the Prološko blato, its catchment area, and surrounding dolines was conducted using various spatial input data. The lacustrine sequence is about 7 meters (or more) thick and divided into lacustrine deposits with alluvial influence at the bottom and carbonate deposits with clays at the top. Carbonate deposits belong to the Holocene and can be characterized as lacustrine chalk that started to form approximately 9 600 cal yr. BP. It is rich in calcium and well- preserved fossils of ostracods, gastropods, and charophytes. The lower part of the Holocene section is a dark siliciclastic mud determined as black and brown clays with great lateral continuity. Their formation is related to strong erosional input, evident in the significant increase of magnetic susceptibility, higher content of siliciclastic elements and lack of ostracods. They were forming between 11 250 and 9 600 cal yr. BP. Beneath the clays come lacustrine deposits with clear alluvial influence belonging to the period of intensive erosion and deposition of poorly sorted sands and silts, with numerous gravel lenses defined as alluvial paleochannels. It is presumed that alluvial deposits belong to the Late Pleistocene period. These are followed by the thick bluish-gray clay, again rich in ostracods. The Prološko Blato can be divided into the paleolake in the western, and modern lake in the eastern part. Its central basin was probably in the western part, evident in the absence of alluvial paleochannels and increasing thickness of lacustrine sediments. The Prološko Lake is categorized as a collapse type of doline (FORD & WILLIAMS, 2007) filled with water, i.e. karst lake, having a steep north-eastern sidewall with characteristic red to orange color. Its formation may be related to the strong tectonic activity of the area (DRAGIČEVIĆ et al., 1999), combined with the dissolution processes. Existence of a paleolake with the constant water level was presumably disrupted by the collapse of Prološko doline. This event may have been crucial in the development of the present hydrogeological system and can be used for future predictions, giving the fact that dolines are still forming both in lowland and highland part of the Imotsko polje, particularly subsidence type of dolines (FORD & WILLIAMS, 2007 ; MARŠIĆ, 2011). These hypotheses will be investigated with more detailed digital photogrammetry and core drilling. For further understanding of the underground karst features and paleolake extent, geophysical methods (electrical resistivity tomography, georadar, and multibeam sonar) will be of great importance.

Dinaric karst, karst lake, paleolake, paleolimnology, geomorphology

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