Neotectonic conditions of sedimentation and erosion in small fluvial basins of the Roztocze Tomaszowskie (south-eastern Poland)

摘要

The impact of neotectonic activity on sedimentation in small fluvial basins of the Roztocze has not been studied until now. Investigations into the lithology and genesis of the valley deposits and morphometric measurements aimed at determining the intensity of recent tectonic movements indicate that the Quaternary tectonic activity in the Roztocze Tomaszowskie influences the dynamics of geomorphic processes. The results are especially interesting as far as the north-eastern part of the mesoregion comprising the Roztocze Wieprz Valley is concerned. The valley is composed of three segments, which vary according to the intensity of tectonic activity, relief features, Quaternary development and the system of Weichselian terraces. The upper course of the valley corresponds to the area of the Upper Wieprz Trough; the middle course is a narrow subsequent valley between Majdan Wielki and Zwierzyniec, while the lower course occupies the northern part of a parallel fault graben. The Upper Wieprz Trough is delimited by edges of tectonic origin. The drainage basin shape index of the Trough and its mountain-front sinuosity indicate a relatively intense recent subsidence of its northern part and a limited one of its south-eastern part. This system of neotectonic activity has persisted since the beginning of the Holsteinian s.s. till modern times, as the Trough is filled with a thick series of deposits from that period. Quaternary deposits are mostly of lacustrine and fluvial origin, which additionally supports the hypothesis of sedimentation occurring at the time of the development of the fault graben. The middle course of the Roztocze Wieprz Valley is characterised by a narrow floor and numerous knickpoints. The variable tectonic activity of this part of Roztocze accounts for the fact that the Wieprz now runs across several microblocs, some subsiding, others - uplifting. This has led to the diverse development of the Wieprz channel. Variable tectonic activity also occurred in the Weichselian, which is indicated by the different altitude of Weichselian terraces in front of and behind the fault and a step system of alluvial fans in the tributary valleys. Moreover, tectonic movements influenced the type and intensity of Quaternary deposits sedimentation in the Zwierzyniec-Szczebrzeszyn fault graben. By the eastern edge of the graben, on a rotationally tilted tectonic scarp, the thickness of deposits is high, decreasing westward, in line with the gently inclined surface of the scarp. In the Weichselian, the subsidence of the floor of the parallel graben gave rise to a sequence of alluvial fans in the Zwierzyniec area.