Sierra Bayas Group is a group of sedimentary rock formations in Buenos Aires Province, Argentina, that deposited in Neoproterozoic times. The group crops out in the central and northwestern parts of the Tandilia System, a chain of hills made up by of ancient rocks. The nearby Cerro Negro Formation aside, Sierra Bayas Group contains the oldest sedimentary formations in Argentina that have not been subjected to in a significant degree the geological processes of metamorphism and deformation.