The Frasnian/Famennian boundary interval in the South Polish–Moravian shelf basins: integrated event-stratigraphical approach

The Late Devonian mass extinction event near the Frasnian/Famennian (F/F) boundary has been analysed using conodont biostratigraphy and biofacies, sedimentology, magnetic susceptibility and geochemistry in reference sections of the South Polish–Moravian shelf (Holy Cross Mountains, Cracow and Brno areas). High-resolution biostratigraphic study revealed difficulties in the precise recognition of this ‘natural’ stage boundary, but confirmed the occurrence of a major (third-order) sequence boundary in the F/F transition in an active synsedimentary tectonic setting, marked by erosional discontinuities, hardgrounds and brecciation or omission surfaces. Conclusive evidence of an extraterrestrial impact has not been found. Among Earth-bound factors, the main devastating role in the shelf habitats is ascribed to fluctuating anoxia and/or nutrient dynamics in a disturbed greenhouse climatic setting. The long-term facies changes were determined by a conspicuous break in carbonate production, accompanied by replacement of mature stromatoporoid–coral reefs by pioneer shelly-crinoid banks, microbial mounds and localised oolitic bars. The key F/F passage interval was marked by intermittent but generally accelerated periplatform ooze/debris input and severe storm events, as well as by probably highly fluctuating oxygenation and biological overproduction, best manifested in radiolarian–silicisponge and cephalopod acmes. Eutrophication phenomena, at least partly stimulated by various hydrothermal and volcanic processes, were one of the major biogeochemical processes during this climax of the Late Devonian biotic crisis. However, significant oceanographic perturbations were also linked with the onset of the transgressive–hypoxic Upper Kellwasser Event, still within the latest Frasnian linguiformis Zone, i.e., well before the F/F conodont collapse. Implied variations in the redox state of seawater through the Kellwasser timespan, probably in regionally highly variable temporal scales, are in agreement with recent palaeoecological and biogeochemical inferences, in particular suggesting recovering oxygenation just prior to the F/F boundary in the other Laurussian intrashelf basins. All the data support a long-time, multicausal Earth-bound crisis instead a worldwide cosmic catastrophe.