| Ischigualasto Formation | |
|---|---|
| Stratigraphic range: Late Carnian - Early Norian | |
 Valle Pintado, Ischigualasto Formation | |
| Type | Geological formation |
| Unit of | Agua de la Peña Group |
| Sub-units | Quebrada de la Sal, Valle de la Luna, Cancha de Bochas & La Peña Members |
| Underlies | Los Colorados Formation |
| Overlies | Los Rastros Formation |
| Thickness | Up to 1,059 m (3,474 ft) |
| Lithology | |
| Primary | Sandstone, mudstone |
| Other | Tuff, conglomerate, siltstone, basalt |
| Location | |
| Coordinates | 29°36′S 68°06′W / 29.6°S 68.1°W |
| Approximate paleocoordinates | 46°00′S 40°12′W / 46.0°S 40.2°W |
| Region | La Rioja Province & San Juan Province |
| Country | Argentina |
| Extent | Ischigualasto-Villa Unión Basin |
| Type section | |
| Named for | Cacán: "Place where the moon alights" |
| Named by | Frenguelli (1944) |
  Ischigualasto Formation (Argentina) | |
The Ischigualasto Formation is a Late Triassic geological formation in the Ischigualasto-Villa Unión Basin of southwestern La Rioja Province and northeastern San Juan Province in northwestern Argentina. The formation dates to the late Carnian and early Norian stages of the Late Triassic (around 231.7 to 225 Ma), according to radiometric dating of ash beds.
The Ischigualasto Formation is part of the Agua de la Peña Group, overlying the Los Rastros Formation and overlain by the Los Colorados Formation. The formation is typically subdivided into four members, from old to young; La Peña, Cancha de Bochas, Valle de la Luna and Quebrada de la Sal. The sandstones, mudstones, conglomerates and tuffs of the formation were deposited in an fluvial (river-dominated) floodplain environment, characterized by cool temperatures and strongly seasonal rainfall. The formation is most well-studied in Ischigualasto Provincial Park, a protected area established in San Juan Province in 1967 and designated a UNESCO World Heritage Site in 2000. Sites in La Rioja Province, such as Cerro Las Lajas and Cerro Bola, are less investigated but potentially even more stratigraphically extensive.
The Ischigualasto Formation is an important paleontological unit, considered a konzentrat-lagerstätte due to its density of fossils. It preserves a diverse assortment of Late Triassic synapsids, temnospondyls, and reptiles, including some of the earliest unambiguous dinosaur fossils. Herbivorous rhynchosaurs and cynodonts (especially the rhynchosaur Hyperodapedon sanjuanensis and the cynodont Exaeretodon argentinus) are by far the predominant findings among the tetrapod fossils in the formation. Herrerasaurus ischigualastensis is both the most numerous dinosaur and most abundant carnivore of the formation. Another important dinosaur with primitive characteristics is Eoraptor lunensis, found in Ischigualasto in the early 1990s. Coprolites, burrows, petrified wood, and plant compressions have also been found in the formation.