Post-Cretaceous–Paleogene slumping in the Subsilesian Unit of the Outer Western Carpathians: Biostratigraphic, sedimentary and magnetic records from the Bystřice section

Abstract

. The Bystřice section, which was previously interpreted as the continuous Cretaceous–Paleogene transition, has been newly studied using biostratigraphy (planktonic foraminifers, calcareous nannofossils), magnetic properties, and geochemistry. Biostratigraphy has confirmed the presence of the upper Maastrichtian (UC20d TP nannozone; Abathomphalus mayaroensis foraminifer zone) and Selandian (NP5–NP7 nannozones; P3b–P4b foraminifer zones). Moreover, the Danian is completely absent. Strong remagnetisation of the rocks did not enable magnetostratigraphy of the section. The magnetic fabric indicates tectonic disturbance of the section. The studied strata consist predominantly of paraconglomerates, which are interpreted as slumps. The slumps contain pebbles and blocks of diverse exotic rocks, intraclasts, and reworked carbonate concretions enclosed within a marly matrix. A few thick paraconglomerate bodies are separated by bedded grey silty marls, sequences of medium-rhythmic sandstone turbidites, and conglomerate. Frequent slump folds indicate synsedimentary deformation. Submarine landslides are manifested by folded and thrusted sandstone beds, breccia of partly-lithified sandstones, and characteristic failure planes. In the lower part of the section, marls and paraconglomerates with Maastrichtian microfossils are interbedded with marls containing Selandian microfossils. It is most likely that the whole of the studied sequence was deposited during the Selandian, and that Maastrichtian marls and paraconglomerates represent submarine mass flows. The deposition took place on the basin slope in the bathyal zone. The geochemical proxy parameters indicate more reducing setting, higher input of terrestrial phytodetrite, as well as higher surface-water temperatures in the Maastrichtian, which is confirmed also by occurrences of low-latitude nanno - plankton. The Selandian sediments contain a higher share of aquatic organic matter. The pristane/phytane ratio indicates an oxygenated water column, and carbonate δ 13 C and δ 18 O isotopes point to lower surface-water temperatures.