engleski

Late Quaternary Karst Paleohydrology, Geomorphology and Sedimentology of Prološko Blato paleolake in Imotsko polje, Croatia

Prološko Blato represents a remarkable karst phenomenon situated in the central part of the Dinaric karst, i.e. in the north-western part of the Imotsko polje. Multidisciplinary research enabled the classification of this karstified area and produced several hypotheses regarding its formation and development. Prološko blato is a seasonally flooded karst landscape developed within Cretaceous limestones and dolomites covered with a thick sequence of Quaternary paleolake sediments in its central and western part. The complexity of the Prološko Blato arises from its eastern part (the Prološko Lake), where a present day lake was formed probably due to combined processes of dissolution, tectonic activity and collapsing of both the limestone bedrock and overlying paleolake sediments. Thus, the Prološko Lake was determined as collapsed sinkhole (Ford & Williams, 2007), whose underground cracks and cavities serve as springs in winter, and sinks in the summer period, i.e. as estavelle (Bonacci & Roje- Bonacci, 2000). The repetitive water fluctuations are a major modern modifier of the landscape, eroding the loose Quaternary paleolake sediments and leaving distinctive erosional channels suitable for geological mapping and sampling. The Quaternary paleolake consists of a thick sequence of lake sediments divided into lower Pleistocene and upper Holocene part. The Pleistocene deposits consist of coarser-grained lake sediments (sand, sandy silt, silt) with a clear alluvial influence marked by gravel paleochannels. The Holocene deposits consist of lake sediments whose deposition started approximately 11, 250 cal yr BP. The sedimentation is characterised with dark siliciclastic mud in the base of the Holocene sequence abruptly followed by light calcareous lake sediments (lake marl). The lower Pleistocene deposits probably formed as a result of an intense fluvial, possibly fluvioglacial processes related to the snow melting in the Bosnian mountains situated north-east from the Imotsko polje. The fluvioglacial processes probably ended with the deposition of the dark siliciclastic mud in the early Holocene, as seen from mineralogical, geochemical and magnetic properties which suggest strong erosional input from the catchment area. This is followed by a calm period of the paleolake's history (approximately 9 600 cal yr. BP) marked with the deposition of lake carbonates determined as lacustrine chalk (lake marl), rich in calcium and well-preserved fossils of ostracods, gastropods, and charophytes. The paleolake with relatively constant water level existed throughout the Holocene until it was disrupted by the collapse of the sinkhole in its eastern part, which probably led to the modern hydrological regime with the seasonally controlled water level, i.e. drought versus swampy-like environment. This hypothesis needs to be further investigated, but a recent analysis of Sentinel satellite images showed a distinctive pattern of water withdrawal along the western and northern edge during the summer periods which suggests potential subsidence and collapse in the future in this part of the area. Further investigations are planned with an emphasis on drilling and geophysics to determine the paleolake's extent, its relationship with the modern Prološko Lake and underground karst features. Giving the fact that the Prološko Blato is of great importance to the local community, its status of a protected landscape within the vulnerable Dinaric karst, as well as tourism, sports and recreation, future research will undoubtedly serve this exceptional karst phenomenon in the terms of environmental protection and managing for sustainable use.

Dinaric karst, karst lake, sinkholes, lake sediments

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