Ischigualasto Formation

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 age 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 respectively.
The formation is approximately coeval with the upper Santa Maria Formation of the Paraná Basin in southeastern Brazil, the Pekin Formation of the United States and the lower Maleri Formation of India.
Subdivision.
The Ischigualasto Formation is subdivided into four members, from top to bottom:
Paleontological significance.
The Ischigualasto Formation is highly fossiliferous and its unique paleontological characteristics made it a Lagerstätte; a stratigraphic unit containing a diverse faunal assemblage. The paleontological importance led to the establishment of the Ischigualasto Provincial Park, a UNESCO World Heritage Site since 2000.
The Ischigualasto Formation contains Late Triassic (Carnian) deposits (231.4 -225.9 million years before the present), with some of the oldest known dinosaur remains, which are the world's foremost with regards to quality, number and importance.
Rhynchosaurs and cynodonts (especially rhynchosaur "Hyperodapedon" and cynodont "Exaeretodon") are by far the predominant findings among the tetrapod fossils in the park. A study from 1993 found dinosaur specimens to comprise only 6% of the total tetrapod sample; subsequent discoveries increased this number to approximately 11% of all findings. Carnivorous dinosaurs are the most common terrestrial carnivores of the Ischigualasto Formation, with herrerasaurids comprising 72% of all recovered terrestrial carnivores. The carnivorous archosaur "Herrerasaurus" is the most numerous of these dinosaur fossils. Another important putative dinosaur with primitive characteristics is "Eoraptor lunensis", found in Ischigualasto in the early 1990s.
Petrified tree trunks of "Protojuniperoxylon ischigualastianus" of more than tall attest to a rich vegetation at that time. Fossil ferns and horsetails have also been found in the formation.
Coprolites were found in Valle Pintado in the upper part of the formation. Analysis of the coprolites revealed that plant remains were absent and bone material and apatite were sparse. The most likely candidate to have produced these fossil feces has been suggest as the most common reptile in the formation; "Herrerasaurus".
Fossil content.
Dinosaurs.
The fossils of an undescribed species of theropod are present in San Juan Province.