Ichnology of the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation, Neuquén Basin, Argentina: Delineating environmental changes in fine-grained depositional systems.

GONZÁLEZ TOMASSINI, F.,; MÁNGANO, M.G.; PAZ, M.; DESJARDINS, P.,; REIJENSTEIN, H.; PONCE JUAN JOSE; BUATOIS, L.A.

Idanha-a-Nova,

Congreso; Ichnia 2016; 2016

ICHNIA

The Vaca Muerta Formation represents the bottomsets and foresets facies of a clinoform system, only locally recording more proximal deposits. This formation consists of a mixed siliciclastic-carbonate system mainly composed by shallowing-upwards cycles, at several scales, of mudstone, marlstone and limestone with high organic matter content. An integrated sedimentologic and ichnologic analysis in two outcrops and one core, located at the southern and central zone of the basin, respectively, allowed an initial assessment on the main controls on bioturbation. At the basal contact of this formation, a coarse- to fine-grained sandstone shoreface, sparsely bioturbated, with Palaeophycus and Ophiomorpha, sharply overlies eolian dune deposits of the Tordillo Formation. The section continues with cyclic successions of parallel-laminated mudstone, limestone and marl, with minor intercalations of fine- to medium-grained sandstone and tuff levels. Mudstone and marl near the base tend to be sparsely bioturbated (BI=1-2), containing Planolites, Thalassinoides, Teichichnus, Phycosiphon and Zoophycos. The middle part of these cycles commonly alternates between unbioturbated and bioturbated zones with a variable BI. The upper part of the cycles contains a similar trace-fossil assemblage to that in the mudstone and marl facies intervals, albeit with a higher degree of bioturbation (BI=3-5). This ichnofauna records (1) the presence of the Skolithos Ichnofacies in the shoreface, (2) oxygen-depleted conditions in the fine-grained sediments, with an abundance of unburrowed anoxic levels, and (3) the onset of more oxygenated waters in the upper section, due to sedimentation in relative higher-energy environments affected by oscillatory flows and bottom currents.