Ichnological analysis of the Bidart and Sopelana Cretaceous/Paleogene (K/Pg) boundary sections (Basque Basin, W Pyrenees): Refining eco-sedimentary environment

Ichnological analysis of two Cretaceous–Paleogene boundary sections at Bidart (SE France) and Sopelana (N Spain) has been conducted in order to refining eco-sedimentary environment, and to a make comparison with previous interpretations based on microfossils. In both sections, trace fossil assemblage is low diverse, consisting of Chondrites, Planolites, Thalassinoides, Trichichnus, Zoophycos, and ?Phycosiphon, ascribed to the Zoophycos ichnofacies, however, with distinct differences. In the Bidart section, early Danian dark-filled trace fossil assemblage is more abundant in large Thalassinoides, Zoophycos and larger Chondrites, and less abundant in Trichichnus and small Chondrites in comparison to the Sopelana section. Sopelana is thus interpreted as a more offshore, deeper section than Bidart although both were located in the upper bathyal zone of the basin. Oxygenation below the sediment–water interface was enough to favour a generalized total bioturbation of sediments. The presence of chemichnia and trace fossils which tracemakers sequestered sediments from the sea floor in deep burrows (Zoophycos) suggests at least periodical food deficiency. The distribution and content of trace fossils across the K/Pg boundary do not change significantly, revealing minor incidence of the K/Pg boundary event on the macrobenthic environment. The K/Pg boundary transition, including the boundary layer, is nearly totally bioturbated; hence a redistribution of microbiotic and abiotic components by tracemakers is possible.