engleski

Dynamics of the Vrgoračko polje Quaternary sedimentation

Sedimentology and its dynamics in the Vrgoračko polje area during the Pleistocene and Holocene are investigated. The Vrgoračko polje is a karst field situated at the southern edge of the Dalmatian Zagora enclosing the area of 37 km2, elevation above the sea level is between 20 and 28 m. The most important water stream of this area is the Matica River. Its water originates from the springs in the north-west of Vrgoračko polje and flows into the Bačinska lakes through the Staševica, Krotuša, Crni Vir and Krtinovac swallow hole and artificial Prigon channel. Before the digging of the additional drainage canals, it used to be a lake during most of the year. Data were collected from the drilling cores and from the outcrops in the field. In the karst field depression Pleistocene and Holocene clay and silt were deposited in lake and swamp environment, reaching maximal thickness of 15 meters. These sediments were continuously fed by limestone Cretaceous rocks from surrounding slopes. The older Pleistocene sediments are represented with clay silt and clay in the south-east part of the lake, whereas during the Holocene lake lime sediment is deposited the north-west area. Clay sediments are intensively coloured by iron and manganese oxide and hydroxide impregnations. Whitish lake lime is almost completely composed from charachean algae and with rare remains of ostracodes and molluscs. Between older clays and younger lake lime is layer representing an archive of intensive dynamics of erosion and resedimentation as a consequence of periodical strong current flows. Carbonate intercalations in the clay sediments in the southern part of the lake were deposited due to extensive erosion of the lake lime from the north. Quick deposition rates on the top of still unsolidified clay sediment are marked by Soft Sediment Deformations (SSD) (water escape and convolution). The lake was periodically completelly or partially dryed up, giving the place for pedogenetic processes to appear. The erosion of the lake lime from the west exposed older Pleistocene and Holocene clay and silt sediments. This material was much better parent material for pedogenesis, than lake lime on the south. As a consequence, today we have much thicker soil horizon on the north-west polje area (>1m), than on the south-east (0.2-0.3 m). Recently, soil and older sediment erosion is still present, but in much smaller extent.

Vrgoračko polje ; soft sediment deformations (SSD) ; lake lime

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