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

Abstract

Non-perennial to ephemeral fluvial systems dominated by seasonal discharge fluctuations and episodic rapid flood-flow events are typical of 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. This study provides detailed sedimentological analysis and palaeoenvironmental interpretation of the lowermost part of the early Permian (?Asselian) Krajanów Formation exposed within the continental, fault-bounded Intra-Sudetic Basin (ISB), located on the NE periphery of Bohemian Massif. High-resolution sedimentological logging and facies analysis indicate that the early Permian fluvial system in this area was dominated by ephemeral fluvial processes influenced strongly by a semi-arid to arid climate. Rapid (?catastrophic) flood events led to episodic sedimentation of vertically and laterally amalgamated fluvial channel infills, with abundant upper flow regime structures as well as poorly channelized, laterally extensive sheet-like bodies of sandstone. The overbank deposits are poorly preserved due to frequent lateral shifting of the channels. Soft-sediment deformation 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 broad, terminal-type alluvial fans, probably in their proximal- to medial segments within a distributive fluvial system of the Permian Intra-Sudetic Basin. Petrographic composition and measured palaeocurrent directions show that the sediment was sourced from the neighbouring massifs – the Sowie Mts. Massif to the east, the Bardo Structure and a hypothetical Southern Massif to the south/south-east.