CONICET | Buscador de Institutos y Recursos Humanos

The Ñirihuau basin developed during the Miocene between 41° and 43°30 S on the eastern slope of the North Patagonian Andes, in western Argentina. Its infill includes the clastic and volcaniclastic Ñirihuau Formation (2211 Ma), which preserves an extensive lacustrine system that characterizes the middle section of the unit, of Middle Miocene age (~15-13 Ma; Langhian-Serravallian). This lacustrine system is constituted by great thicknesses (up to 600 m) of tabular mudstones and siltstones with tabular to lentiform sandstones, scarce tuff levels and several carbonate-rich intervals. We present detailed petrographic, cathodoluminescence, and geochemical data of the lacustrine carbonate beds from two stratigraphic logs of the middle section of the Ñirihuau Formation along the Arroyo Las Bayas locality; and of isolated carbonate levels from the Ñirihuau Formation of two other locations (Cerro Carbon and Arroyo del Medio). The stratigraphic logs are situated on each flank of the David syncline, while the other two levels are located towards the north of the basin. Eight carbonate microfacies were identified: intraclastic carbonate, homogeneous carbonate, disrupted micrite, birds-eye-micrite, bioclastic mudstone, siliciclastic mudstone, fenestral micrite, and biogenic carbonate. The observed micro- and macrofacies, are interpreted to represent a lacustrine association with two well differentiated main features: one of low energy (biogenically induced precipitation) and another of higher energy (tractive transport and accumulation of bioclasts). Most microfacies analyzed show very low luminescence in the micrite matrix indicating a low Mn content, without visible zonation and a few diagenetic features. Bright orange luminescence is observed mainly in the cracks filled by spar and in the bioclasts. This indicates low diagenetic effects in the carbonate-rich deposits probably corresponding to very early diagenesis stages (eogenesis) what supports a conservative primary signature of carbon and oxygen stable isotopes.

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