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

Abstract

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