Bottom currents in the jurassic-cretaceous vaca muerta formation black shales (argentina): an example of benthos response to oxygen delivery in oxygendeficient environments

• Autores: Paz, Maximiliano; Mángano, Gabriela; Buatois, Luis; Carmona, Noelia Beatriz; Desjardins, Patricio; Reijenstein, H.; Gonzalez Tomassini, Federico
• Año de publicación: 2018
• Idioma: inglés
• Tipo de recurso: documento de conferencia
• Materia: Ciencias Exactas y Naturales; Vaca Muerta; bottom currents; Ciencias Exactas y Naturales
• Institución: Universidad Nacional de Río Negro

Fil: Paz, Maximiliano. University of Saskatchewan, Department of Geological Sciences, 114 Science Place, S7N 5E2 Saskatoon, Canada
Fil: Mángano, Gabriela. Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2, Canada,
Fil: Buatois, Luis. Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2, Canada,
Fil: Carmona, Noelia Beatriz. Universidad Nacional de Río Negro, Instituto de Investigación en Paleobiología y Geología, Río Negro, Argentina.
Fil: Desjardins, Patricio. Shell Exploration and Production Company, 150N Dairy Ashford St, TX, 77019, Houston, USA
Fil: Reijenstein, H. Chevron, 1400 Smith, Houston, USA
Fil: Gonzalez Tomassini, Federico. Phoenix Global Resources, Alem 855, C1001AAQ, Buenos Aires, Argentina
The Vaca Muerta Formation represents marine bottomset and foreset facies of a mixed carbonatesiliciclastic ramp developed during Late Jurassic-Early Cretaceous in the Neuquén Basin, Argentina. Mudstone, marl and limestone deposited in low-energy, oxygen-deficient settings constitute the main lithologies of this formation. The present contribution describes mudstone and minor very fine-grained sandstone lithofacies showing variable intensities of bioturbation in cores from two wells (239 m thick in total) from the central Neuquén Basin area. Nine cycles of oxygenation events occur within outer to mid ramp settings and are subdivided in low- and highenergy. Low-energy cycles (30-50 cm thick) are characterized by very thin-bedded, parallel- to ripple cross-laminated, coarse to medium mudstone couplets. Intervals grade from highly bioturbated to sparsely bioturbated, showing Teichichnus and biodeformational structures. Highenergy cycles (1-10 m thick) show typical coarsening-upward trends in the lower part to finingupward patterns in the upper part. Their bases consist of a mottled mudstone and minor sandstone, grading towards sharp-based, thin-bedded, current-ripple cross-laminated mudstone. The cycles end with a mottled interval capped by cryptobioturbated mudstone. Bioturbation index displays a distinctive decrease to increase pattern, showing particularly small occurrences of Asterosoma, Bergaueria, Conichnus, Cylindrichnus, Palaeophycus, Planolites, Phycosiphon, Skolithos and escape trace fossils. Equilibrium behavior is inferred for Bergaueria and Conichnus. The cycles record bottom current activity at the upper slope, with hydrodynamic energy and oxygenation being limiting factors for the benthos and, therefore, controlling bioturbation. The cycles display increasing to decreasing energy conditions that show an inverse correlation with bioturbation index driven by shorter colonization windows in the higher energy settings (middle parts of the cycles). Bottom currents delivered oxygen to bottom waters, supporting a moderately diverse ichnofauna. Waning of bottom currents towards the end of the cycles generated progressive deoxygenation of the seafloor. Cryptobioturbation on top indicates that smaller organisms were able to thrive in lower oxygen conditions, documenting the last stage of the oxygenation event.