CONICET | Buscador de Institutos y Recursos Humanos

The Avilé Member of the Agrio Formation is a non-marine unit developed due to a major relative sea-level fall that almost desiccated the back-arc Neuquén Basin during Hauterivian times. It has a wedge shape ranging from a few metres in marginal areas to over 150 m in the central basin, and its lower boundary is a major regressive surface that puts non-marine deposits on top of deep-marine shales all across its extension. The upper limit of the Avilé Member is a transgressive surface that marks the re-establishment of the deep-marine accumulation in vast areas of the basin. Internally, the Avilé Member is dominated by a complex interaction among non-marine depositional systems. Five major associations were identified representing different settings, including bedload and mixed-load dominated fluvial systems, aeolian systems and lacustrine systems (normal and hypersaline). Two different non-marine palaeogeographic settings are defined from the regional distribution and the vertical evolution of sedimentary environments. The western sector is characterised by a northward flowing fluvial system that shows a longitudinal evolution from bedload dominated sandy systems down to mixed-load, high sinuosity systems. Downstream, the fluvial system evolved into an open, clastic lacustrine system, where coarsening upwards cycles are inferred to represent lacustrine deltaic progradation. Upstream, the unit records an alternation of bedload and mixed-load fluvial systems, reflecting high-frequency changes in the accommodation/supply relation. Eventually, the fluvial system evolves into a lacustrine system indicating a vertical evolution to conditions dominated by an increase in accommodation or a drastic decrease in sediment supply. In the eastern sector, a different lateral and vertical array of sedimentary environments is recorded. A hypersaline lacustrine system with reduced clastic input is developed in downstream areas, which evolved from fluvial deposits that indicate the complete desiccation of this part of the basin. This system is not fed by a well-developed fluvial network, but it laterally relates to a dry aeolian system. A vertical cyclicity is also defined in these deposits recorded as coarsening-upward cycles in the lacustrine system and super surface formation in the aeolian environment. On top, a fluvial system is developed represented by amalgamated channels upstream and isolated channels with preservation of floodplain deposits in downstream areas. The differences between the sedimentary environments developed in the eastern and western sectors of the study area reflect a strong compartmentalization of non-marine systems. Most of the water and clastic input was routed towards the western area, while in the east a shallow hypersaline system was developed with reduced clastic input and laterally related to an aeolian system. Both areas seem to react effectively to short-term changes in accommodation/supply conditions developing an internal cyclicity that can be associated with high-frequency changes in base level. Also these two sectors seem to evolve into wetter conditions associated with higher base levels that may indicate a gradual climate change or the influence of a transgressive trend that ends with the complete flooding of the non-marine system. However, while in the western sector the unit ends with the accumulation of lacustrine deposits, in the eastern area both the aeolian and the lacustrine system are truncated by the development of a fluvial system. This contrasting response it is likely to represent the re-routing of the main fluvial system into the eastern sector. The differences observed within this lowstand wedge indicate that facies and stratigraphic organization within lowstands can be more complicated than during highstand and transgressive periods. Therefore, models based on the distribution of marine deposits may not predict the variability of non-marine deposits observed during lowstand periods.