The Ischigualasto Formation is a Late Triassic fossiliferous formation and Lagerstätte in the Ischigualasto-Villa Unión Basin of the southwestern La Rioja Province and northeastern San Juan Province in northwestern Argentina. The formation dates to the Carnian and Norian ages and ranges between 231.7 and 225 Ma, based on ash bed dating. The up to thick formation is part of the Agua de la Peña Group, overlies Los Rastros Formation and is overlain by Los Colorados Formation. The formation is 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 a humid alluvial to fluvial floodplain environment, characterized by strongly seasonal rainfall. The Ischigualasto Formation is an important paleontological unit and considered a Lagerstätte, as it preserves several genera of early dinosaurs, other archosaurs, synapsids, and temnospondyls of the Late Triassic. Coprolites and fossil wood also have been found in the formation. The formation crops out in the in 1967 established Ischigualasto Provincial Park, which was designated a UNESCO World Heritage Site in 2000. Etymology The name Ischigualasto is derived from the extinct Cacán language, spoken by an indigenous group referred to as the Diaguita by the Spanish conquistadors and means "place where the moon alights". The genus Ischigualastia and the species Herrerasaurus ischigualastensis, Pseudochampsa ischigualastensis, Pelorocephalus ischigualastensis and Protojuniperoxylon ischigualastianus were named after the formation. Geology The formation represents the second syn-rift period in the Ischigualasto-Villa Unión Basin, where the total thickness of Triassic sediments amounts to . The formation is exposed in the Ischigualasto Provincial Park of La Rioja and San Juan Provinces. In the neighboring Talampaya National Park, the formation is thin and covered by recent sediments. The formation, part of the Agua de la Peña Group, overlies Los Rastros Formation and is overlain by Los Colorados Formation. The Ischigualasto Formation strongly contrasts with the bounding formation in color. The total thickness amounts to . The Ischigualasto Formation comprises a sequence of fluvial channel sandstones with well-drained floodplain sandstones and mudstones, dominated by rivers and strongly seasonal rainfall has been estimated at time of deposition. The formation dates to the Carnian Pluvial Event. Interlayered volcanic ash layers above the base and below the top of the formation provide chronostratigraphic control and have yielded ages of 231.4 ± 0.3 Ma and 225.9 ± 0.9 Ma