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

Abstract

The Vrgoračko polje is a karst field with a surface area of 37 km 2 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, coarsegrained 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 clay and 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 Vrgoračko polje and affected subsequent depositional facies. According to 14 C 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 Vrgoračko polje could be considered to represent a typical Quaternary lacustrine sedimentation pattern for other Dinaric karst poljes. described in the Cerkničko polje, Slovenian Dinaric karst (VAL-VASOR, 1689; PLENIČAR, 1954; SMREKAR, 2000). Similarly, in the karstic Lake Banyoles (NE Spain), MORELLÓN et al. (2014) reported intense groundwater inflow that leads to the fluidization and re-suspension of previously deposited sediments. Seasonal autumn flooding of the Vrgoračko polje may (re)establish a lacustrine environment that lasts through the winter as confirmed by zonal vegetation around the polje (Fig. 1). Likewise, seasonal runoff through estavelles and ponors into the Neretva River (E of the Vrgoračko polje), and the artificial Prigon tunnel draining into the Baćinska Lakes (S of the Vrgoračko polje), leave the Vrgoračko polje dry throughout the summer. The Matica River is the only permanent stream that meanders from its springs in the NW, splits to the south and the east and then sinks (Fig. 1). The altitude difference between the springs, estavelles and ponors is approximately 8 metres. T...

Palaeoenvironmental reconstruction of the Milna valley on the island of Vis (Central Adriatic) during the late Holocene

“…It is clear that due to deforestation the temporal river (torrent) had enough energy to incise a relatively deep bed, as the sediments were soft. When the energy of the torrent flow was suddenly reduced (stopped), pebbles remained as they were (lodged in underlying soft silty sediment, as presented in Posilović et al, (2018)). These pebbles are mostly composed of dolomite (rarely limestone) with a specific gravity of 2.8-2.9 g cm -3 .…”