Development of an ephemeral fluvial system in continental fault-bounded basin – an example from the Early Permian Krajanów Formation of the Intra-Sudetic Basin (NE Bohemian Massif)

Ephemeral fluvial systems dominated by seasonal discharge fluctuations and episodic events of rapid flood flows are typical for arid to semi-arid climatic conditions. Dryland fluvial systems have been described from many ancient and modern, predominantly tectonically-controlled sedimentary basins across the globe. The author shows the results of detailed sedimentological analysis and palaeoenvironmental interpretation of the Early Permian (Rotliegend) Krajanów Formation exposed within a continental, fault-bounded Intra-Sudetic Basin (ISB), NE Bohemian Massif. This basin started its development in the middle Viséan (Turnau et al., 2002), as an narrow, intramontane trough and underwent complex evolution from Early Carboniferous to Late Cretaceous. The maximum stratigraphic thickness of the basin infill reaches about 11 000 metres (Nemec et al., 1982). During the Early Permian the ISB constituted a semi-enclosed, south-western outlier of the Polish Rotliegend Basin (Southern Permian Basin of Central Europe). The Permian sedimentary-volanogenic succession of the ISB exhibits distinct, large-scale cyclic structure and comprises successive, fining-upwards continental megacyclothems (megasequences) up to 600 metres thick (Awdankiewicz et al., 2003). Such megacyclic structure is thought to have originated from tectonic activity and is attributed to relatively rapid, fault-controlled subsidence of the basin floor (Nemec et al., 1982; Wojewoda and Mastalerz, 1989).

The Krajanów Formation is composed of fluvial, playa-like and lacustrine deposits which form one of such fining-upwards cyclothems and attain up to 300 m in thickness. Sediments of the lowermost part of the Formation are represented by coarse-grained fluvial red-bed assemblage. Early investigations described these sediments as fluvial in origin. The upper part of the Formation distinguished as the Upper Anthracosia Shale, is characterized by the mudstone-dominated siliciclastics interbedded with fine-grained calcareous deposits which acummulated in a floodplain-to-ephemeral („terminal”) lacustrine setting.

High-resolution sedimentological logging and facies analysis indicate that the Early Permian fluvial system in the study area was dominated by ephemeral fluvial processes influenced strongly by semi-arid to arid climate. Rapid (catastrophic?) flood events led to episodic sedimentation of poorly channelized, laterally extensive sheet-like bodies of sandstone as well as vertically and laterally amalgamated fluvial channel infills, with abundant upper-flow regime structures. The overbank deposits are poorly preserved due to the frequent lateral shifting of the channels. Soft sediment deformational structures formed due to events of river bank collapse as well as debris-flow facies point to high-energy, waning flows. It is concluded that deposition occurred on a broad, terminal-type alluvial fans, probably in their proximal- to medial sub-environments. Petrographic composition and measured paleocurrent directions show that the sediment was sourced from the framing massifs – the Sowie Mts. Block to the east and a hypothetical Southern Massif to the south/south-east.

The research was funded by the Polish National Science Centre (Grant 2017/26/M/ST10/00646).

References

Awdankiewicz, M., Kurowski, L., Mastalerz, K., Raczyński, P., 2003: Geolines 16, 165–183;

Nemec, W., Porębski, S.J., Teisseyre, A.K., 1982: Veröff. Zentralinst. Erde, Potsdam, 267–278;

Turnau, E., Żelaźniewicz, A., Franke, W., 2002: Geologia Sudetica 34, 9–16;

Wojewoda, J., Mastalerz, K., 1989: Przegląd Geologiczny 432, 173–180.