An unusual oxygen-deficient ichnofauna from the Vaca Muerta Formation: implications for the ichnofacies model

In most oxygen-deficient ancient successions, Chondrites and Zoophycos are typical and recurrent trace fossils that are the last to disappear during deoxygenation events. Here, we report an unusual case of an oxygen-deficient ichnofauna lacking Chondrites and showing scarce Zoophycos, from the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation of Argentina. One outcrop and cores from nine wells were analysed. The succession comprises fine-grained, mixed carbonate-siliciclastic deposits accumulated in marginal-marine, basin, drift, slope, and outer ramp environments. It displays abundant biodeformational structures mostly producing irregular-laminated and massive fabrics in fine to medium mudstone. However, discrete trace fossils can be found within interbedded coarser-grained intervals, namely (in order of decreased ichnogenera abundance), Teichichnus, Alcyonidiopsis, Coprulus, Phycosiphon, Planolites, Lockeia, Thalassinoides, Palaeophycus, Nereites, Crininicaminus, Zoophycos, Diplocraterion, and ?Skolithos. Considering all ichnotaxa, the Vaca Muerta Formation may be seen as hosting a relatively diverse ichnofauna, yet, this is misleading because most occurrences are of low diversity. This atypical ichnofauna may be explained by either a lack of strong seasonality and existence of background food-rich environments, or by salinity variations, precluding the specialized feeding style of the Chondrites and Zoophycos producers. The dominance of feeding trace fossils with spreite (Teichichnus) and the low ichnodiversity suggest that the studied ichnofauna belongs to the Zoophycos Ichnofacies.