| Cerro Azul Formation | |
|---|---|
| Stratigraphic range: Tortonian (Huayquerian) ~ | |
 Fossil burrow fills in the Cerro Azul Formation | |
| Type | Geological formation |
| Underlies | alluvium |
| Overlies | Arroyo Chasicó Formation or Crystalline basement |
| Thickness | 54 m (177 ft) (outcrop) 180 m (590 ft) (subsurface) |
| Lithology | |
| Primary | Siltstone, sandstone |
| Other | Conglomerate |
| Location | |
| Coordinates | 35°42′S 64°42′W / 35.7°S 64.7°W |
| Approximate paleocoordinates | 35°54′S 62°42′W / 35.9°S 62.7°W |
| Region | Buenos Aires & La Pampa Provinces |
| Country | Argentina |
| Extent | Colorado Basin |
| Type section | |
| Named for | Cerro Azul, Epecuén Lake |
| Named by | Linares et al. |
| Year defined | 1980 |
 Outcrop map of the Cerro Azul Formation | |
The Cerro Azul Formation (Spanish: Formación Cerro Azul), also described as Epecuén Formation, is a geological formation of Late Miocene (Tortonian, or Huayquerian in the SALMA classification) age in the Colorado Basin of the Buenos Aires and La Pampa Provinces in northeastern Argentina.[1][2]
The fluvial and aeolian siltstones, sandstones and tuffs of the formation contain many mammals, such as Thylacosmilus and Macrauchenia, reptiles, amphibians and fossils of terror birds as well as Argentavis, the largest flying bird ever discovered.