The preglacial fluvial deposits in the southern part of the Upper Nysa Depression, central Sudetes Mts, southwestern Poland

Waldemar Sroka, Anna Kowalska

Abstract


Three fluvial series have been distinguished in the Upper Nysa Depression, Kłodzko Basin, central Sudetes. The oldest, the Červeny Potok series, of probable Late Pliocene age, contains fluvial facies, mainly gravelly but also sandy and silty. It is quartz-rich and characterized by strong chemical decomposition of gneiss clasts. The Lichkov series, of probable Early to early Middle Pleistocene age, contains similar lithofacies to the Červeny Potok series, with locally abundant debris-flow deposits. The Boboszów series, of probable late Middle Pleistocene age, consists of a monotonous series of fluvial gravels. The pebbles in both, the Lichkov and Boboszów series are gneiss-rich and are characterised by a lack of significant chemical clast decomposition. The coarse-grained fluvial sediments of the southern part of the Upper Nysa Depression were mainly deposited in gravel-dominated braided rivers of Donjek and Scott type, and locally on alluvial fans. The material was transported from N or NE to S or SW, with the source area only in the adjacent metamorphic massif, despite the fact that the series lie on Late Cretaceous rocks. The studied area constituted a small intramontane depression subjected to limited subsidence, surrounded by episodically active faults. The variability of sedimentary process reflected climatic changes rather than tectonic activity. The northern part of the studied area, which now belongs to the Baltic Sea drainage area, was drained to the south, either to the North or Black Seas, during the Late Pliocene to Middle Pleistocene. The capture of this area by the Nysa Kłodzka river took place in the Late Pleistocene as a result of upstream erosion reflecting glacio-isostatic rebound and fault activity after the early Saalian glaciation.

Keywords


gravel-bearing rivers, stratigraphy, fluvial palaeoenvironments.

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