Próba rekonstrukcji warunków paleohydrodynamicznych Wisły na odcinku Tarnobrzeg-Sandomierz

Abstract

Paleohydrodynamic condition of Wisła River between Tarnobrzeg and Sandomierz The fossil valley is preserved below the recent Wisła sediments. Its maximum depth amounts to 18 m and the bottom gradient is cognate to the recent one. The valley is filled with three sedimentary units. In stratigraphical order they are: gravel, sandy-gravel and sandy series. Diagonal bedding is present in all these sediments. In the first series and in the lower part of the second numerous frost structures are found. The radiocarbon analysis of plant remains present in the lowermost series shows that they were accumulated in the Baltic glaciation stage (40.700 + 2.000 years BP). The same age is indicated by the bone remains. Bad sorting and large content of gravel fraction indicate rather considerable velocity of the currents. Low roundness index of grains and realtively large number of crystalline rocks suggest the weathering cover as source material for the gravel. Diagonal bedding shows small scattering of flow direction, which is characteristic for braided rivers. In this series no fossil river beds were found. In the sandy series the grains are well rounded and sorted. Considerable scattering of paleocurrent directions and preserved dead channels are indicative of meandering waters. By C14 analysis of peat filling one of the dead channels the series was found to belong to preboreal period of Holocene (9.070 + 90 years BP). The youngest stage of development is noted in a dead channel undercutting the slope of the valley where oak stems 6 m deep from the surface were found. Their age was estimated to be 1850 + 35 years BP. In the recent and fossil Wisła valleys which do not differ much in the bottom gradient, quite different sediments were accumulated: sand and muds in the former and gravel in the latter. The author concludes that the total yearly outlet of the recent Wisła is adequate to the outlet existing in the Baltic glaciation stage and the difference consists only on its seasonal distribution. In the periglacial climate of the last glaciation the accumulation of gravels and sands were caused by the concentration of high waters in the short period of spring thaws.