The Ischigualasto Formation is a Late Triassic geological formation in the Ischigualasto-Villa Unión Basin of northwestern Argentina (southwestern La Rioja Province and northeastern San Juan Province). According to radiometric dating of ash beds, the formation dates to the late Carnian and early Norian stages of the Late Triassic (around 231.7 to 225 million years ago, though the exact duration may vary between different parts of the basin). The late Carnian portion is particularly rich in fossils, including early dinosaurs.
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 a 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.
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". El lugar donde se posa la luna The genus Ischigualastia and the species Herrerasaurus ischigualastensis, Pseudochampsa ischigualastensis, Pelorocephalus ischigualastensis and Protojuniperoxylon ischigualastianus were named after the formation.
Geology
Regional setting
The Ischigualasto Formation is found in the Ischigualasto-Villa Unión Basin, along the border between La Rioja and San Juan Provinces, in the badlands of Western Argentina. The formation is most widely exposed and studied in Ischigualasto Provincial Park (IPP), an Argentine national park and UNESCO World Heritage Site which occupies a large portion of the basin in San Juan province. In the western part of the park, the maximum thickness of the formation amounts to , narrowing to in the eastern area. Conversely, the formation is thin and mostly covered by recent sediments in Talampaya National Park, which neighbors Ischigualasto Provincial Park in La Rioja to the north.Balabusic et al., 2001, p.26 Not every exposure in La Rioja is so limited, however. The previously neglected Cerro Las Lajas area, at the northwest tip of the basin, preserves an outcrop up to thick in a small geographic area. Another well-exposed outcrop in La Rioja is Cerro Bola, which preserves nearly of sediment.
The Ischigualasto-Villa Unión Basin is the remnant of an ancient half-graben which preserves a Schencman, 2015, p.220-thick series of Triassic sediments.Aceituno Cieri et al., 2015, p.60 The Ischigualasto Formation represents the second syn-rift period within the basin, meaning that its sediments were emplaced during a brief interval when rifting was reactivated to widen the basin further. The rifting was accommodated by movement on Valle Fértil, a dormant fault along the western edge of the basin.
Stratigraphically, the formation is part of the Agua de la Peña Group, a name encompassing Late Triassic formations within the Ischigualasto-Villa Unión Basin. The Ischigualasto Formation overlies the Los Rastros Formation, a mid-Carnian unit dominated by green mudstone. It is subsequently overlain by the Los Colorados Formation, which is primarily Norian-age red sandstone. The comparatively pale color of the Ischigualasto Formation's sediments strongly contrasts with its predecessor and successor, helping to distinguish the three formations in the field.Monetta et al., 2000, p.644
Sedimentology The Ischigualasto Formation was a river-dominated environment as indicated by its rock types: coarse fluvial (river channel) sandstone beds, and finer floodplain deposits of sandstones, mudstones, and paleosols. Thin bentonite tuffs are scattered through the entire formation, derived from volcanic ash.
Sandstone beds
The coarsest rocks in the formation are channel lag pebble-cobble river conglomerates (i.e., gravel beds, the first river sediments to sink and become buried). These form the lowermost layer of many sandstone beds, eroding into underlying fine-grained sediments. Flat, ribbon-like layers are the most common variety of river sandstone, and some layers preserve underwater ripples or dunes. Slanted sandstone layers indicate sediment accretion, either downstream or laterally (in the form of point bars). Sandstone beds occupy a range of sizes and proportions, from relatively small single-channel deposits (2 meters thick, 10 meters wide), up to massive complexes of multiple superimposed braided rivers (20 meters thick, 2000 meters wide). Sandstone and siltstone also occur in the surrounding floodplain, in the form of crevasse splays, abandoned channels, and levee deposits.
The ancient rivers and streams mainly flowed towards the north or northeast from volcanic highlands flanking the southern part of a prehistoric valley. At the southeastern edge of the modern basin, where the Ischigualasto Formation is somewhat thinner, flow indicators trend towards a northwestward orientation, possibly redirected by basalt flows. The sandstone beds also tend to be smaller and less numerous in the eastern part of the basin, while paleosols are more voluminous.
Mudstones and paleosols
Most floodplain sediments were fine-grained. Many of the coarser mudstone layers are overbank deposits, laid down over abandoned channels and crevasse splays. The finest sediments are preserved as paleosols (fossilized soil), of which there were eight varieties, labelled A through H. Type A paleosols are sandy soils rich in quartz and mica, with indistinct mottling, root casts, and no internal structure. They were probably protosols or entisols, young soils developing along streambanks where siliciclastic grains outnumber organic material. The other paleosols are mostly found among claystone or mudstone beds rather than sandy layers. Type B paleosols have high proportions of smectite and mica, frequent slickensides, wedge-shaped crevices (mudcracks in cross-section), and strong redoximorphic features such as gleying and mottling. They can be classified are a type of vertisol, emplaced in a warm seasonal climate with a high water table during much of the year. Type C paleosols are similar to type B, yet differ in their abundance of calcareous nodules and a paucity of redoximorphic features. Though also vertisols, they correlate with a less humid climate, moistened by periodic rainfall rather than groundwater.
Type D paleosols have high smectite and kaolinite, fine mottling, and distinctively numerous hematite nodules and clay films (argillic horizons). They are classified as argillisols or alfisols, buried in relatively dry and stable environments such as forests, away from the influence of water. Type E paleosols are simple blocky soils packed with calcareous structures and a distinct carbonate horizon alongside smectite and mica. They are calcisols, perpetually dry and stable desert soils forming in a climate with little precipitation. Type F paleosols are intermediate between type D and type E, since they have both clay- and carbonate-rich horizons and all three major clay minerals: smectite, mica, and kaolinite.
Type G and H paleosols are distinct in that they are derived from volcanic rocks, rather than sediments. Type G paleosols are colorful and sandy soils with distinct horizons rich in smectite, silica, and iron oxides. They develop above feldspar-based ash beds and are among the most laterally extensive soil type in the formation, since volcanic ash covers a broad area without regard to climate or water availability. Type H paleosols, on the other hand, are a rare variety of granular paleosol developing above basalt lavas which have been degraded and weathered during the process of soil formation.
Subdivisions
The formation is subdivided four members in Ischigualasto Provincial Park, with each member showing a distinctive pattern of sedimentology and fossil content. These members were informally delineated in 2006 and formally named in 2009. From top (youngest and stratigraphically highest) to bottom (oldest and stratigraphically lowest), they are: Quebrada de la Sal Member (Unit IV): ~. A coarse-grained member nearly lacking in fossil content. Most sediments are brown or red in color, though grey or mottled mudstones still occur sporadically, unlike the overlying Los Colorados Formation. The upper extent of the member (and the Ischigualasto Formation as a whole) is defined by the last grey mudstone bed in the Ischigualasto-Villa Unión Basin. All variety of sandstone features can be found in this member. Paleosols are limited and similar to those of the La Peña member, with type A and to a lesser extent type D as the most common varieties. Type B, F, and G paleosols are present but exceedingly rare. Valle de la Luna Member (UNIT III): ~. The thickest member in the formation. Dark grey smectite-rich floodplain mudstone is the most common lithology, especially in the eastern part of the basin. Sandstone features are common and well-developed in the western part of the basin, as are abandoned channel overbank deposits with a high proportion of calcareous sediments. Tetrapod fossils continue to decrease in prevalence relative to earlier members, but plant fossils are locally abundant in abandoned channel deposits. The western basin has a high proportion of type A, D, and G paleosols, the central basin has more type B, and the eastern basin is mostly type A and B. Type C, E, and F paleosols are practically absent outside of the eastern basin. Cancha de Bochas Member (UNIT II): ~. A fine-grained member mainly composed of mottled red/green/gray mudstone. Fossils are abundant, especially in the earliest layers of the member. Calcareous nodules are also abundant and used to distinguish this member from the La Peña and Valle de la Luna members. Slickensides, root casts, and other hallmarks of soil alteration are frequently encountered as well. Both single-channel and multi-channel sandstone beds are present, as are crevasse splay and levee deposits. Volcanic rocks include not just tuff, but also a thick basalt extrusion in the eastern part of the basin. This member hosts every type of paleosol except for type G. It is also the only member where type H paleosols are present. Type A is most common overall, type C is the most common in the middle, and types E, F, and B are most frequently found in the eastern area.