Mineralogical and geochemical trends of the Vaca Muerta-Quintuco system in the Puerta Curaco section, Neuquén Basin

Resumen

The deposits of the Vaca Muerta-Quintuco system (Tithonian-Valanginian) have been analyzed in the Puerta Curaco section to study the mineralogical and geochemical variations through the progressive transition from a carbonate ramp (Vaca Muerta Formation) to a mixed shelf (Quintuco Formation). A lithostratigraphic log, gamma-ray measurements (GR), mineralogical (XRD) and geochemical analysis (XRF) allowed to define five intervals: I1-I5. I1 and I2 belong to the Vaca Muerta Formation. They are dominated by carbonate sedimentation and present the highest values of organic proxies of the system. The total GR values are strongly dependent of the U content and the V/V+Ni ratio suggests alternating anoxic and euxinic conditions of the sea bottom. I3 is developed in the transition between both units and exhibits a sudden increase in the clay mineral content and a decrease in the organic proxies, associated to a sea level fall. I4 and I5 belong to the Quintuco Formation. I4 is characterized by a renewed increase in the carbonate sedimentation while I5 records the highest clastic input to the basin, evidencing the settling of the mixed shelf. In the Quintuco Formation, the V/V+Ni ratio suggests anoxia/dysoxia at the sea-bottom. In addition, a decrease in the organic proxies and an excellent correlation between total GR and K and Th is recorded. Decreasing organic matter content in the Quintuco Formation is attributed to a dilution phenomenon caused by the increasing detrital input and not by changes in the oxygenation of the sea-bottom.