Abstract:The paper presents the results of the latest radiocarbon dating and the palynological analyses of organic sediments found in the alluvia of the Wisłok River valley between the towns of Łańcut and Przeworsk. The study conducted in the gravel pit made possible the dating of several alluvial fills of the 7-8 m high Holocene terrace and the 5-6 m high floodplain. The oldest channel alluvia and palaeochannel sediments of the 7-8 m high terrace were dated at 10 100-9300 BP (11 960-10 500 cal BP). According to the anthracological analysis the top cover of clay overbank alluvia, which bears charcoal fragments, was deposited in the Subboreal and/or the Subatlantic Phases. In the sequence of the 5-6 m high floodplain, the bottom fills of palaeochannels, dated at 10 195-9885 cal BP (probability level: 68.2%; GdS-3233: 8900 ± 95 BP ) and 11 095-10 755 cal BP (probability level: 68.2%; GdS-3228: 9575 ± 95 BP), were truncated by erosion and covered by alluvia of palaeomeanders, which were active in the 19 th century. Preservation of the erosional bench of the Early Holocene organic sediments indicates the predomination of lateral migration of the river channel during the last 200 years and the formation of wide erosional floodplain that has been aggraded with recent flood alluvia. In the 20 th century the floodplain aggradation was simultaneous with the deepening of the Wisłok riverbed.
<p>Reconstruction of fluvial style changes in the San River in the Subcarpathian Basins is based on<br>geomorphological and sedimentological analyses. The time control of alluvial fills and temporal<br>changes in the river channel are derived from radiocarbon and optically stimulated luminescence<br>dating combined with independent pollen-based biochronostratigraphy. The results showed that the<br>alluvial plain of the braided (BR) or braided-meandering (BR-M?) river was abandoned before<br>12,800 cal BP. Large meanders (LM) were cut off in the older part of the Younger Dryas (YD; ca.<br>12,600 cal BP), and in the younger part of this period (ca. 12,450 cal BP). The small meanders (SM)<br>developed at the end of the YD and were abandoned at the onset of the Preboreal (PB; ca.11,550<br>cal BP). The erosion phase at the YD-PB transition, reported from many valleys in Central Europe,<br>was not confirmed in the study area. The full cycle of San River channel transformation (BR (BRM?)-><br>LM->SM); was estimated to be approximately 1200 years. According to the palynological data,<br>open pine forests with birch that survived from the end of the Aller&#248;d dominated the landscape of<br>the river valley during the YD cooling and did not undergo major changes during the warming in the<br>early PB. Therefore, we assume that the influence of vegetation changes in the San River channel<br>pattern transformation was nonsignificant. The location of the studied palaeochannels in the<br>floodbasin filled with silty clayey deposits may have influenced the formation of relatively narrow<br>and deep channels, than that of much the wider and shallower meanders from the YD, situated<br>several kilometres downstream of the surveyed sites.</p>
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