Microfacies of the San Juan Formation (lower Darriwilian) in the Rio Francia, Central Precordillera, Argentina

In this contribution, we describe and interpret the microfacies of the upper levels of the San Juan Formation in the Río Francia section of the Central Precordillera, Argentina, for the first time. Several studies in this section identified the Lenodus antivariabilis, Lenodus variabilis, Lenodus crassus, and Lenodus pseudoplanus zones, thereby limiting this stratigraphic interval to the early Darriwilian age. Five microfacies were recognized: M1 burrowed bioclastic wackestone, M2 bioclastic mudstone-wackestone, M3 intrabioclastic grainstone, M4 crinoidal wackestone-packstone, and M5 bioclastic wackestone-packstone. These microfacies indicate a shallow subtidal inner platform environment with variations in hydrodynamic energy ranging from low to moderate conditions below the fair-weather wave base level. The vertical arrangement of these sub-environments suggests a gradual transgressive setting during the lower Darriwilian for the carbonate beds of the San Juan Formation. The Baños de Talacasto, Ancha Creek, and Don Braulio sections are correlated with the section studied in this work, where conodont zones were documented, and microfacies analyses revealed shallow subtidal environments with low-energy conditions and occasional high-energy episodes. Differences in redox conditions were observed between the Don Braulio section (anoxic) and the Río Francia section (well-oxygenated), interpreted as the result of a rapid sea-level rise, though diachronic across the platform. Microfacies analysis, combined with precise conodont biostratigraphy, provides a powerful tool for reconstructing the Middle Ordovician carbonate platform.