Chañares Formation

The Chañares Formation is a Carnian-age geologic formation of the Ischigualasto-Villa Unión Basin, located in La Rioja Province, Argentina. It is characterized by drab-colored fine-grained volcaniclastic claystones, siltstones, and sandstones which were deposited in a fluvial to lacustrine environment. The formation is most prominently exposed within Talampaya National Park, a UNESCO World Heritage Site within La Rioja Province.

The Chañares formation is the lowermost stratigraphic unit of the Agua de la Peña Group, overlying the Tarjados Formation of the Paganzo Group, and underlying the Los Rastros Formation. Though previously considered Ladinian in age, U-Pb dating has determined that most or all of the Chañares Formation dates to the early Carnian stage of the Late Triassic.

The Chañares Formation has provided a diverse and well-preserved faunal assemblage which has been studied intensively since the 1960s. The most common reptiles were proterochampsids (Chanaresuchus, Tropidosuchus, and Gualosuchus), which lived alongside true archosaurs such as Lewisuchus, Lagerpeton, Marasuchus, Gracilisuchus, and Luperosuchus. Cynodonts were abundant, represented by the medium-sized traversodontid Massetognathus, as well as smaller carnivores such as Chiniquodon and Probainognathus. The largest animal in the ecosystem was the giant dicynodont Dinodontosaurus. An older faunal assemblage, distinguished by the large erpetosuchid Tarjadia, has been discovered in the earliest part of the formation. The formation as a whole is considered one of the best sources of Carnian-age tetrapods in South America, along with the slightly younger Ischigualasto Formation which lies above the Los Rastros Formation.