Strukturalny zapis procesów akumulacyjno-deflacyjnych w wydmie wa owej we wschodniej cz ci Kotliny Sandomierskiej (Ukraina)

During the studies of aeolian forms in the central part of the Lubaczów-Jaworów Plateau, in the borderland between Poland and Ukraine (Fig. 1), a special attention was given to lithologic features of deposits building a ridge dune, which is situated to the NE of Wola Ar amowa village. These deposits are characterized by distinct cyclic variability. This fact determined us to undertake investigations in order to reconstruct aerodynamic conditions under which such deposit variability had appeared. These investigations included: a) geomorphological mapping of the ridge dune and its immediate vicinity, b) identification of texture and structure of deposits building the form, c) measurements of scale and frequency of lithofacies, d) measurements of structural directional elements, and e) identification of primary deposition environments according to the classification by Hunter (1977) and Borówka (2001), described according to the suggestions published by Zieliński and Issmer (2008). Based on the lithologic features of dune building material (Figs 2, 3), we distinguished two deflation-accumulation cycles. The first one is recorded within the south-eastern slope of the dune, and it is characterized by the occurrence of sand sets with high-angle inclined stratification, which are separated by reactivation surfaces (Figs 2D, 3B). Sand was deposited on the leeward slope by wind from a predominant direction. The formation of reactivation surfaces is regarded as a result of the leeward slope exposure to winter strong wind from a direction completely different from the predominant one (McKee 1966; Borówka 1980; Hunter and Rubin 1983; Zieliński et al. in press). Therefore, we can assume that this change of wind direction occurred in annual rhythm. The second cycle was most completely recorded in the crest zone of the dune. It consists, from bottom to top, of the following lithofacies: RC→MR→PB→DT (Figs 2D, 3A). This cycle evidences wind from a slightly variable direction but with increasing velocity that resulted in increased deposition, and after exceeding the threshold value — increased deflation (Fig. 4). The nature of deposition can be compared with the described in literature wedge stratification (McKee 1966; McKee et al. 1971; Izmai ow 2001). During this cycle the aerodynamic conditions changed in considerably shortened periods — from several days to several weeks. The authors suppose that this variability was associated with deepening, travelling to the east, and filling up of dynamic lows.