Ichnofabrics from muddy shallow-water bottom current deposits, Upper Jurassic to Lower Cretaceous Vaca Muerta Formation, Argentina

PAZ, M.; MÁNGANO, M. G.; BUATOIS, L.A.; DESJARDINS, P.; NOTTA, R.; GONZÁLEZ TOMASSINI, F.; CARMONA, N.B.

Workshop; 16th International Ichnofabric Workshop; 2021

Bottom currents induced by thermohaline-, wind-, or tide-driven circulation may occur inshallow waters (50-300 m water depths), such as in outer shelves, upper slopes and shallowsills. The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation in Argentina shows driftdeposits located in bottomsets and foresets of a mixed carbonate-siliciclastic, shelf-margin,subaqueous clinoform system that can be analyzed to enhance understanding of ichnofabricsassociated with shallow-water bottom current sedimentation. In addition, this formationrepresents one of the most important unconventional reservoirs in the world, displaying highTOC levels. The drift deposits were recorded in cores from five wells (660.5 m total) locatedin areas that are currently explored and developed by oil and gas companies. These depositsare composed of massive, cross-bedded and parallel, low angle- and wavy-laminated crinoidalmudstone; crinoid-rich lenses; coarse mudstone laminae encased in fine mudstone; massive,parallel-, low-angle and current-ripple cross-laminated, fine to coarse mudstone; and massive,normal-graded and composite beds of calcareous, fine to coarse mudstone.In these deposits, four ichnofabrics are recognized: Palaeophycus heberti,Phycosiphon incertum, Nereites isp. and Equilibrichnia-Fugichnia ichnofabrics. ThePalaeophycus heberti ichnofabric consists of shallow-tier Palaeophycus heberti, Planolitesisp., Crininicaminus isp., and rare Palaeophycus isp. overprinted onto a mottled backgroundrecording irregular biodeformational structures. The Phycosiphon incertum ichnofabric isrepresented by shallow-tier Phycosiphon incertum, Nereites isp., Planolites isp.,Palaeophycus isp., and rare Lockeia? isp. The Nereites isp. ichnofabric is dominated byshallow-tier structures such as Nereites isp., Phycosiphon incertum, Planolites isp., andPalaeophycus isp. The Equilibrichnia-Fugichnia ichnofabric is characterized by very shallowtier Lockeia siliquaria, bowl-shaped structures referred to as Lockeia? isp., U- and V-shaped,nested vertical to highly inclined structures interpreted as equilibrium and escape structures,and rare Skolithos? isp.These ichnofabrics occur in three different facies. The Palaeophycus hebertiichnofabric is dominant in the crinoidal mudstone facies, forming highly bioturbated intervals.The Palaeophycus heberti, Nereites isp. and Phycosiphon incertum ichnofabrics occur in thefine to coarse mudstone facies, constituting highly, moderately, and sparsely bioturbatedintervals respectively. In this facies, m-thick successions display an upward decrease and thenincrease in bioturbation index that may be similar to the bigradational succession ofcontourites. The Equilibrichnia-Fugichnia ichnofabric was mostly recorded in the fine tocoarse mudstone and calcareous mudstone facies and less commonly in fine to coarsemudstone facies, forming distinctive bioturbated intervals within scarcely bioturbated,laminated mudstone successions.Bioturbation was evidently controlled by food distribution, oxygenation,hydrodynamic energy, and water turbidity. Food was delivered at the surface or in suspension,promoting deposit-feeding (Nereites isp. and Phycosiphon incertum ichnofabrics),suspension-feeding (Equilibrichnia-Fugichnia ichnofabric), or mixed feeding (Palaeophycusheberti ichnofabric) strategies in the infauna, respectively. Oxygen levels increased duringbottom-current activity compared with other deposits (e.g., basin and slope), yet relativelysmall burrow diameters indicate that dysoxic conditions were dominant. Hydrodynamicenergy controlled bioturbation intensity, resulting in lower degrees of bioturbation duringhigh-energy events. Suspension feeding strategies suggest low turbidity. The present exampleincreases our understanding of environmental controls of shallow-water bottom currents,supporting previous suggestions that bottom-current activity increases backgroundoxygenation of bottom waters. Moreover, it indicates that traction structures in drift depositscan be preserved when bioturbation is suppressed by high hydrodynamic energy and lowoxygen conditions.