Quaternary Depositional Environments in the Vrgoračko Polje/Lake (SE Croatia)

The Vrgoracko polje is a karst field with a surface area of 37 km2 and an altitude of between 20 and 28 m above sea level, situated at the southern edge of the Dalmatian Zagora. During the Quaternary the polje was flooded for variable periods of time and a lacustrine environment was established. A multidisciplinary study of drill-cores, outcrops and geoelectric measurements recognised five main sedimentary facies: laminated sediment, redeposited sediment, coarse grained carbonate debris, littoral clay and lacustrine chalk. Based on the facies analysis, depositional environments developed during the Holocene include aquatic lacustrine littoral and deeper-water environments. The terrestrial environment is represented by a desiccated lake phase. The littoral clay facies (filling depressions and caverns in the karst relief) is laterally equivalent to the deep-water laminated facies (varves?). A stratigraphic break between littoral claya nd lacustrine chalk could be time-equivalent to disturbed laminated sediments deposited in deeper-water and to local intercalations of coarse-grained carbonate debris in shallow-water facies sediments. These features could have been the result of a neotectonic event (earthquake), which triggered debris flows of colluvial material from slopes around the lake, and this could also have changed the hydrological regime of the Vrgoracko polje and affected subsequent depositional facies. According to 14C dating, deposition of the lacustrine chalk started at the beginning of the Mid-Holocene Warm Period (7686±36 aBP) with a sedimentation rate of approximately 0.51 mm a-1 during the Middle, and 0.58 mm a-1 during the Late Holocene to today. Calculated carbonate production was estimated at 1050 gm-2 a-1. A temporary phase of subaerial exposure of the lake is indicated by desiccation cracks and two bioturbated palaeosol horizons. The described depositional environments and sediment facies found in the Vrgoracko polje could be considered to represent a typical Quaternary lacustrine sedimentation pattern for other Dinaric karst poljes.