Age and palaeoenvironmental constraints on the earliest dinosaur-bearing strata of the Densuș-Ciula Formation (Hațeg Basin, Romania): Evidence of their late Campanian-early Maastrichtian syntectonic deposition

The Haţeg Basin is famous for its rich uppermost Cretaceous continental vertebrate assemblages, with some of the most important ones originating from the Densuş-Ciula Formation. The present study aims to provide a more accurate picture of the geological and palaeoenvironmental context of this important dinosaur-bearing succession through complex spatial analysis based on detailed geological mapping combined with results of zircon U–Pb geochronology, sedimentology, vertebrate palaeontology and micropalaeontology investigations. This integrated stratigraphical survey revealed that the important environmental shift from marine to continental deposition in western Haţeg Basin occurred significantly earlier (by middle late Campanian) than hitherto considered, and that the lower Densuș-Ciula Formation – previously thought to be restricted to the Maastrichtian – covers a good part of the upper Campanian as well. Sedimentological investigations, aimed to characterize palaeoenvironmental changes during basin evolution, identify two, vertically superimposed fining-upward successions within the lower Densuș-Ciula Formation, reconstructed as alluvial fan environments linked to distinct stages of basin tectonic evolution. Contrary to previous interpretations as a post-orogenic molasse, the lower Densuș-Ciula Formation is here re-interpreted as a largely syntectonic unit deposited in a transtensional, dextral strike-slip basin initiated during the late Campanian. The stratigraphic positions and palaeoenvironmental settings of all major vertebrate sites from this area are re-assessed using our new age constraints and improved tectonic-stratigraphic-sedimentologic framework, documenting a significantly earlier start of the accumulation of vertebrate-bearing deposits than thought before, and challenging previous ideas about the timing of assembly, isolation and evolution of the Haţeg Island faunas.