Evolution of the Holocene alluvial fan of the River Stryi in the foreland of the Eastern Carpathians (Western Ukraine)

Abstract

The research reconstructs the development of the alluvial fan of the River Stryi in the Holocene and dates the phases of increased river activity and their correlation with other valleys in the upper Dniester and Vistula river basins. The age of the palaeochannels and the stratigraphy of alluvial fills are based on radiocarbon dating supported by the results of palynological analyses. The oldest (fossil) palaeochannel of the Stryi was dated to the Younger Dryas. The large-radius meander was abandoned at the end of the Boreal Phase during the period of flood intensification at about 8700 BP. The uneven top of the gravels (cut by palaeotroughs of a depth ranging from 2 to 4 m) indicates that during periods of increased discharges (Younger Dryas, end of the Boreal Phase, late Atlantic and beginning of the Subboreal) the troughs were cut and filled with channel sands. From the beginning of the Subboreal Phase (5400-5300 BP), aggradation of the valley bottom has been predominant, which is manifested not only by the overbuilding of the floodplain but also by the gradual elevation of the channel. The accumulation of sandy-silty overbank sediments dated to about 8300, 5400 and 2800 BP correlates well with the beginning of humid phases and floods in the upper Vistula and upper Dniester basins as well as floods recorded in lakes in the northern foreland of the Alps.