Vertical displacement and taphonomic filtering of nannofossils by bioturbation in the Cretaceous-Paleogene boundary section at Caravaca, SE Spain

At the Caravaca section, SE Spain, the position of the Cretaceous–Palaeogene (K–Pg) boundary is well-defined, with multidisciplinary datasets from a thin rusty layer at the base of the 10 cm dark boundary layer. Nannoplankton assemblages containing the Danian taxon Neochiastozygus sp. and enriched in Thoracosphaera spp. are displaced below the K–Pg boundary into the trace fossils Zoophycos, Thalassinoides and Chondrites. These trace fossils are filled with dark-coloured sediments of the dark boundary layer. The nannofossil assemblage from the 1-cm thick interval below the boundary, enriched in Thoracosphaera spp. and Braarudosphaera spp., may have been displaced by Chondrites tracemakers, the traces of which are abundant in this interval. The downward transport of the Danian nannofossils into the Maastrichtian by the tracemakers seems to be one of the common mechanisms responsible for their apparent appearance below the K–Pg boundary. The dark boundary layer contains very rare Danian specimens and abundant Maastrichtian nannofossils. The Maastrichtian taxa were most likely conveyed up on to the seafloor by tracemakers during the Danian. The redistribution of nannoplankton down and up across the rusty layer (K–Pg), challenges the usefulness of nannofossils for high-resolution stratigraphy of the K–Pg boundary.