Análisis de la distribución y composición de las biofacies de la Formación San Juan (Ordovícico Temprano), Precordillera argentina

Abstract

Several paleoecologic studies have been carried out on the Ordovician limestones of the San Juan Forrnation, Argentine Precordillera. However, neither an integrated analysis of the fossil associations nor a complete study of their stratigraphic distribution have been performed. The aim of this contribution is to analyze biofacies distribution and composition along the complete stratigraphic record of the San Juan Formation. Seven main biofacies have been recognized: 1) Archeoscyphia-Calathium-Girvanella biofacies 2) Archaeorthis biofacies 3) Leptellinid biofacies 4) stromatoporoid biofacies 5) Petroria-Paralenorthis biofacies 6) Demosponge biofacies and 7) Nileid biofacies. Taking into account generic and species composition of biofacies and associated biozones, a high turnover is evident, showing no recurrence. However, at higher taxonomic levels or considering guilds composition recurrence of biofacies follows sea level fluctuations ("guilds tracking"). This pattern of highly variable biozone content and the absence of recurrent biofacies may imply the absence of refuge areas possibly related with the isolation of the Argentine Precordillera. Changing environmental conditions may eliminate a high percentage of the genera and only few survivors (eurytopic elements) persist. The other components in the subsequent biofacies are immigrants and new endemic clades. A clear change in biofacies framework is verified since the late Arenig with the development of stromatoporoid reef mounds in central sections and the nileid biofacies both in northern and southern sections. This event reflects the tectonic shift of the basin and the generation of subsiding depocenters. The paleogeographic history of the basin controls the highly variable composition of biofacies and biozones, causing the changing taxonomic composition of the immigrants, and origination of endemic forms. Distribution of the biofacies is closely related to tectonic evolution and sea level fluctuations. A drastic change in basin background triggers a one way pattern of biofacies development since the late Arenig both in northern and southern areas. Sea level changes periodically restart