Controls on the accumulation of Early Cretaceous organic-rich fine-grained deposits in a mixed marine siliciclastic-carbonate distal settings of the Neuquén Basin, central-western Argentina

Abstract

Deposition of black shales occurred under specific conditions that need to combine in a precise way the rate of production and the rates of destruction and dilution of organic matter. Whereas good progress has been made in the determination of the role of production and destruction of organic matter, the role of dilution of the organic components still remains comparatively understudied. In mixed siliciclastic-carbonate marine systems, the dilution of organic matter by non-organic components could be detrital and/or biogenic in origin, adding more complexity to the understanding of these particular types of black shales. In the present contribution, we studied the mixed siliciclastic-carbonate distal deposits of the late Hauterivian Agua de la Mula Member (Agrio Formation) in the Neuquén Basin. Through a combination of sedimentological, petrographic, SEM, and geochemical analyses we define the origin of the fine-grained components, reconstruct the environmental evolution, and determine the role of the production, preservation, and dilution of the organic matter. The results indicate that the unit is predominantly composed of calcite (>47%), whereas illite is the major component among the clay minerals. Thin sections and SEM studies suggest that the carbonate fraction is dominantly composed of nannofossils, i.e. biogenic in origin, and subordinately micarbs (diagenetic, but with a potential biogenic precursor). Thus, the composition of the distal deposits is mostly controlled by the input of detrital illite. The organic geochemistry and XRF data reveal that the most appropriate conditions for the accumulation of organic matter are the transgressive intervals, near the base, with a predominant effect of the rate of the production over the preservation. On the other hand, the dilution of organic matter is mainly driven by the rate of production of biogenic calcite. Interestingly, previous studies showed that in more proximal settings (but still offshore to basin realms) the dilution is controlled by siliciclastic components. Finally, organic geochemistry suggests that most of the organic matter produced is marine in origin (type II), but towards the top of the section it switches to mixed marine and terrestrial (type III to IV), likely related to a progradation of the shallower system and the consequent increase in the input of terrestrial-derived components.