The role of oxygen and substrate on trace-fossil distribution in the fine-grained Vaca Muerta Formation (Upper Jurassic-Lower Cretaceous, Argentina)

In fine-grained deposits, benthic oxygen-deficiency and soupy substrates during deposition, combined with high compaction during diagenesis, preclude preservation of discrete bioturbation structures, hindering a clear assessment of paleoecologic conditions. The Vaca Muerta Formation of Argentina represents an excellent example to characterize trace fossils in a fine-grained succession, due to the abundance of fresh surfaces in cores available for evaluation. An ichnologic analysis was applied to cores from nine wells and one outcrop, which consisted in the description of ichnotaxa and other bioturbation metrics. This formation (ca. 150 m-thick) is composed of fine to medium mudstone, interbedded with coarse mudstone, tuff, limestone and bindstone, deposited in an oxygen-deficient, mixed carbonate-siliciclastic system. Indistinct bioturbation structures generate irregular-laminated (cryptobioturbation) and massive fabrics in fine to medium mudstone, associated with lower dysoxic to oxic conditions in soupy substrates. Discrete bioturbation structures were subdivided into nine ichnocoenoses. The Coprulus oblongus, Alcyonidiopsis longobardiae, Teichichnus rectus, and Teichichnus rectus-Phycosiphon incertum ichnocoenoses constitute oxygen-related ichnocoenoses (ORI) that occur in tuff, reflecting a gradient of lower dysoxic to oxic conditions in soft substrates. The Palaeophycus heberti-Crininicaminus isp., Nereites isp., equilibrichnia-fugichnia, and Planolites isp. ichnocoenoses are observed in heterolithic fine to coarse mudstone, and coarse mudstone, and are interpreted as produced under upper dysoxic to oxic conditions in softgrounds. The Thalassinoides isp. ichnocoenosis occurs in mudstone underlying tuff or intraclastic wackestone and represents oxic conditions in firmgrounds (Glossifungites ichnofacies). In previous paleoecologic studies, it has been argued that decreased benthic diversity could be produced by dysoxia or low substrate consistency. This analysis demonstrates the relationship between ORIs and cryptobioturbation, which helps to disentangle oxygen and substrate environmental controls and create a strong paleoecologic framework that can be applied to other ancient fine-grained depositional systems.