Osady organiczne w utworach rzecznych interglacjału mazowieckiego z Żabieńca na południe od Warszawy

Abstract

W Żabieńcu i Siedliskach, 20 km na południe od Warszawy, w czwartym cyklu sedymentacyjnym utworów rzecznych interglacjału mazowieckiego stwierdzono osady organiczne o miąższościach 0,55 m i 0,60 m. Skład florystyczny tych osadów, zbadany przez Z. Janczyk-Kopikową, wskazuje na tworzenie się ich podczas ochłodzenia sygnalizującego zbliżanie się lądolodu zlodowacenia środkowopolskiego, kiedy to panował klimat surowy, subarktyczny.ORGANIC SEDIMENTS IN MASOVIAN (GREAT) INTERGLACIAL FLUVIAL SERIES FROM ŻABIENIEC SOUTH OF WARSAWThe Masovian Interglacial in which organic sediments were found (Fig. 1) comprises fluvial deposits of the Vistula river, developed in four sedimentary cycles 40 m thick (Fig. 2). The river deposits are mainly represented by various-grained sands with gravels of crystalline rocks, sandstones, quartzes and Mesozoic limestones and flints up to 5 cm in diameter. They differ from underlying Preglacial deposits in relatively good rounding of grains (up to 52% of well-rounded grains in the former and 43% of angulated quartz grains in the latter). The Preglacial deposits are preserved only at Łbiska, where they are 19 m thick. The main heavy minerals of the deposits studied include garnet (27 to 48%), amphiboles (11 to 24%) and zircon (2 to 29%). The deposits of two lower sedimentary cycles are 7-10 m thick whilst the upper cycles - about a dozen meters thick. A layer of organic sediments 0.6 m in thickness was found in the middle of the fourth cycle at Żabieniec (borehole No II) and Siedliska (borehole No VI). The pollen analysis carried out by Z. Janczyk-Kopikowa made possible reconstruction of the type of vegetational cover and climatic conditions prevailing in the Żabieniec area in times of formation of the organic sediments in one of old river-beds in the Vistula river valley. Table 1 and Figure 3 show the results of the analysis. The pollen spectrum obtained for sample No 15 taken at depth of 17.8 m indicates severe subarctic climate conditions. The absence of forest was typical of the area and grasses (Gramineae) and sedges (Cyperaceae) were predominating. The grasses and sedges were accompanied by plants of the families Chenopodiaceae, Compositae, Rosaceae as well as Artemisia and Thalictrum. A total contribution of herbaceous plants (NAP) approached 78% of the whole vegetation.Peat was formed under milder but still cool climate conditions which is evidenced by the spectra obtained fur samples No 3-13. The spectra are characterized by the predominance of pine (Pinus) , accompanied by birch (Betula), spruce (Picea) and alder (Alnus). Pollens of oak (Quercus), linden (Tilia), hornbeam (Carpinus) and hazel (Corylus) were found to occur in negligible amounts. The spectra are typical of swampy pine forest. The contribution of herbaceous plants (NAP) and mainly of sedges increases in top parts of the deposit. This indicates a successive phase of climatex deterioration.The palynological profile of organic sediments from Żabieniec characterizes the times of the Masovian Interglacial for which paleobotanic record is still very scarce.