San Juan Formation, Argentina

The San Juan Formation (Formación San Juan) is a geologic formation in Argentina. The formation comprising limestones, mudstones and marls was deposited in a shallow marine reefal environment and preserves many fossils dating back to the Ordovician period. The formation overlies the La Silla Formation and crops out in the Precordillera of San Juan Province.

The oldest calcareous microfossils, known as calcisphers or calcitarchs, were found in the Early Ordovician (Floian) strata of this formation. Trilobites, gastropods and echinoderm Nuia sibiria have been collected from the same deposits.