AN UNUSUAL ICHNOFAUNA FROM OXYGEN-DEFICIENT, ORGANIC-RICH MUDSTONE AND VOLCANIC TUFF OF THE UPPER JURASSIC-LOWER CRETACEOUS VACA MUERTA FORMATION (ARGENTINA)

Ancient oxygen-deficient benthic marine environments are characterized by trace fossils which are relatively smaller, occur shallower within the substrate, and have low diversity. Ichnologic studies of these environments suggest Chondrites and Zoophycos are the last trace fossils to disappear during deoxygenation events. The present study reports a succession deposited under dysoxia lacking Chondrites and showing scarce Zoophycos, from the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation of Argentina. The study analyzed one outcrop and cores from nine wells. The Formation consists of fine-grained, mixed carbonate-siliciclastic deposits originated in marginal-marine, basin, contourite drift, slope, and outer ramp environments. Most bioturbation comprises biodeformational structures lacking well-defined outlines in fine to medium mudstone. Discrete trace fossils can be typically found in volcanic tuff, and less commonly in crinoidal fine mudstone, intraclastic wackestone, and fine to medium mudstone. Typical ichnogenera include (in order of decreased ichnogenera abundance), Teichichnus, Alcyonidiopsis, Coprulus, Phycosiphon, Planolites, Lockeia, Thalassinoides, Palaeophycus, Nereites, and Crininicaminus, whereas Zoophycos, Diplocraterion, and ?Skolithos are rare. These trace fossils occur in low-diversity assemblages in different depositional environments. Several environmental factors controlled trace fossil distribution. Substrate consistency considerably affected the production of ichnotaxa, as biodeformational structures were formed in fine to medium mud soupgrounds, whereas discrete ichnotaxa were emplaced in volcanic ash or bioclastic mud sofgrounds, loosegrounds, or firmgrounds. Shallow-water contour currents dictated the extent of oxygenation in bottom waters. Oxygen levels determined the size, bioturbation depth and intensity, and ichnodiversity of trace fossils. The unusual absence/scarcity of Chondrites and Zoophycos may be related either to a lack of strong seasonality in food-rich environments, or to salinity variations, which may have precluded the establishment of specialized feeding styles. This contribution greatly expands our understanding of organism-sediment interactions in muddy, oxygen-deficient basins.