The Río Mayer Formation is sequence of sedimentary rocks of Early Cretaceous age that form part of the northern Magallanes Basin (Austral Basin) in Santa Cruz Province, Argentina.

The Río Mayer Formation and it lateral stratigraphic equivalents form part of the post-rift sequence following the major back-arc rifting event in the Jurassic, which was associated with the formation of a significant back-arc basin, greater than 100 km wide, the Rocas Verdes Basin.

The rifting was accompanied by eruption of a thick sequence of silica-rich volcanic rocks, forming the Chon Aike Large Igneous Province.

[4] This overall fine-grained sequence is generally highly deformed, with the development of tight asymmetric folds, such as the overturned syncline that outcrops on the Loma del Pliegue Tumbado (literally the hill of the recumbent fold) and the nearby Loma de las Pizarras, west of El Chaltén.

The boundaries with the older Jurassic rhyolites of the El Quemado Complex are all marked with thrust faults.