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 691 metres (2,267 ft), narrowing to 397 metres (1,302 ft) in the eastern area.^[2][3] 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.^[4] 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 1,059 metres (3,474 feet) thick in a small geographic area.^[3] Another well-exposed outcrop in La Rioja is Cerro Bola, which preserves nearly 800 metres (2,600 feet) of sediment.^[5]

The Ischigualasto-Villa Unión Basin is the remnant of an ancient half-graben which preserves a 3.5-kilometre (11,000 ft)^[6]-thick series of Triassic sediments.^[7] The Ischigualasto Formation represents the second syn-rift period within the basin, meaning that its sediments were implaced during a brief interval when rifting was reactivated to widen the basin further. The rifting was accommodated by movement along Valle Fértil, a dormant fault at the northwest corner of the basin.^[2]

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.^[8]

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.^[2][9] Thin bentonite tuffs are scattered through the entire formation, derived from volcanic ash.^[2][10][3]

Subdivisions

The formation is subdivided four members in Ischigualasto Provincial Park, with each member showing a distinctive pattern of sedimentology and fossil content.^[2] These members were informally delineated in 2006^[11] and formally named in 2009.^[2] From top (youngest and stratigraphically highest) to bottom (oldest and stratigraphically lowest), they are:^[2][12]

• Quebrada de la Sal Member (Unit IV): ~60 metres (200 ft). 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.^[2] 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.^[11]
• Valle de la Luna Member (UNIT III): ~450 metres (1,480 ft). 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.^[2] 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.^[11]
• Cancha de Bochas Member (UNIT II): ~130 metres (430 ft). 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.^[2] 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.^[11]
• La Peña Member (UNIT I): ~50 metres (160 ft). A coarse-grained member with uncommon but well-preserved fossils. Its base is defined by the first massive channel conglomerate layer above the Los Rastros Formation. Multi-channel fluvial sandstone is common and tannish-grey in color, though isolated single-channel and point bar deposits are seemingly absent, in contrast to the rest of the formation. Crevasse splay deposits are present but thin and silty, and floodplain mudstone is greenish-gray, sandy, and rich in smectite.^[2] Paleosols are uncommon; the western basin has type A and very rare type D paleosols while the eastern basin mostly has type B.^[11]

The outcrop at Cerro Las Lajas is instead divided into three subunits: a fossiliferous lower section (11 to 310 meters above the base of the outcrop) with meandering river deposits, a fossil-poor middle section (310 to 740 meters above base) with high-humidity paleosols, and an upper section (740 to 1070 meters above base) with braided river deposits and welded tuff beds, but no fossils.^[3]

Radiometric dating

Interlayered volcanic ash layers above the base and below the top of the Ischigualasto Formation in Ischigualasto Provincial Park provide precise chronostratigraphic control on the formation's duration. The two ash beds have yielded ages of 231.4 ± 0.3 Ma and 225.9 ± 0.9 Ma, respectively. These age estimates were derived from ⁴⁰Ar/³⁹Ar radiometric dating on sediment samples first analyzed in 1993^[10] and recalibrated in 2011.^[13][14] The formation as a whole spans the Carnian-Norian boundary (at approximately 227 Ma), though the greatest diversity occurs in the older Carnian-age members of the formation (the La Peña and Cancha de Bochas members).^[12] A major faunal turnover and loss of diversity occurs in the early part of the Valle de la Luna Member, shortly before the start of the Norian.^[12] According to U-Pb dating, a 2021 study published an age of 228.91 ± 0.14 Ma for an ash bed at this level.^[15]

A 2020 study dated three tuffs at Cerro Las Lajas via U-Pb dating. The oldest tuff is at 107 meters above the base of the outcrop, shortly before the earliest fossils at the site. It has an age of 229.25 ± 0.1 Ma. The second tuff, in the middle of the fossiliferous section at 160 meters above base, is 228.97 ± 0.22 Ma. The third tuff is close to the top of the formation, 1035 meters above base with an age of 221.82 ± 0.1 Ma. These values estimate that the formation was deposited between 230.2 ± 1.9 Ma and 221.4 ± 1.2 at Cerro Las Lajas.^[3]

Small stratigraphic gaps may have been present at Cerro Las Lajas, lending some uncertainty to the thickness of the exposure and the precise level of the tuff beds.^[15][5] As a results, some studies suggest that only 700 meters are exposed at the site.^[15][5] Even accounting for this possibility, deposition of the Ischigualasto Formation at Cerro Las Lajas seems to end several million years later than at Ischigualasto Provincial Park. The age estimates at either site may be erroneous, or the transition between the Ischigualasto and Los Colorados formations is asynchronous, with some areas acquiring sediments characteristic of the latter formation at an earlier time than other areas.^[5][16] Likewise, sediments of the Los Rastros Formation may have been superseded later in Cerro Las Lajas relative to Ischigualasto Provincial Park.^[5]

Biostratigraphy

At Ischigualasto Provincial Park, the Ischigualasto Formation is split into three biozones based on the composition and abundance of tetrapod fossils. The earliest biozone, and by far the most fossiliferous, is known as the Hyperodapedon-Exaeretodon-Herrerasaurus biozone^[15][17][18] (formerly the Scaphonyx-Exaeretodon-Herrerasaurus biozone,^[14][12] using an older synonym for Hyperodapedon). This biozone is characterized by the abundance of the rhynchosaur Hyperodapedon, the cynodont Exaeretodon, and the carnivorous dinosaur Herrerasaurus. It lasts from the La Peña member through the Cancha de Bochas member up as far as the earliest layers of the Valle de la Luna Member.^[14][12]

The second biozone is the Exaeretodon biozone, which occupies most of the Valle de la Luna Member. Hyperodapedon and Herrerasaurus disappear from the fossil record in the park, but Exaeretodon is slightly more abundant.^[14][12] Diversity and fossil abundance as a whole is significantly lower than the previous biozone. Only a few reptiles continue to persist through both biozones, namely Aetosauroides, Saurosuchus, and Proterochampsa.^[12] The third and final biozone is the Jachaleria biozone in the Quebrada de la Sal Member. Fossils are extremely rare; as the name implies, the only identifiable tetrapod from this member is the dicynodont Jachaleria. This biozone continues into the Los Colorados Formation.^[14][12]

At Cerro Las Lajas, two alternative biozones are used. The earliest biozone is the Hyperodapedon biozone, named for the prevalence of Hyperodapedon fossils. This biozone continues until about 260 meters above the base of the outcrop (equivalent to around 228 Ma). It is followed by the Teyumbaita biozone, which lacks Hyperodapedon and instead has abundant fossils of another rhynchosaur, Teyumbaita. Teyumbaita stops occurring around 350 meters above base, and the youngest fossils in the outcrop are about 400 meters above base. The Hyperodapedon and Teyumbaita biozones are suggested to be equivalent to the entire Hyperodapedon-Exaeretodon-Herrerasaurus biozone, with Teyumbaita first appearing in layers equivalent to the early Valle de la Luna member.^[3]

Regional and global correlations

Palynomorph and tetrapod biostratigraphy agree with a late Carnian to early Norian age for the formation.^{[19][20][3][16]} In terms of geological formations outside of Argentina, the Ischigualasto Formation is most easily correlated with the Upper Santa Maria Formation of the Paraná Basin in southeastern Brazil. Both formations show a pattern of abundant Hyperodapedon occurrences, followed by a decline in the genus along with a surge of Exaeretodon fossils. Proterochampsa, Aetosauroides, and herrerasaurids are also prevalent in both formations.^[21][22][23] Langer (2005) reestablished the term "Ischigualastian" for this shared faunal zone in South America, based on Bonaparte (1966)'s characterization of the Ischigualasto Formation as an interval with unique fossils. The Ischigualastian is defined by the presence of Hyperodapedon and Exaeretodon, after the dominance of the dicynodont Dinodontosaurus and prior to the first fossils of Jachaleria.^[21] Radiometric dating further supports this proposal, as the Upper Santa Maria Formation was also deposited during the late Carnian.^[24][25]

Outside of South America, faunal correlations are more tenuous. The Land Vertebrate Faunachron (LVF) system, initiated by Lucas (1998), intends to correlate Triassic formations on a global scale via tetrapod biostratigraphy. Lucas placed the Ischigualasto Formation within the Adamanian LVF, an biozone defined primarily by species and assemblages in southwest North America.^[26] His correlation was justified by the reported presence of several taxa shared between both continents: Ischigualastia, Saurosuchus,^[27] herrerasaurids, and the aetosaur Stagonolepis.^[26] Later investigations have disputed many of these claims. A proposed Ischigualastia femur from the Santa Rosa Formation of New Mexico may instead by referrable to Eubrachiosaurus or another unnamed stahleckeriid dicynodont.^[28][29] "Saurosuchus" fragments from Arizona are undiagnostic to the genus level, and cannot be taken as evidence that the genus ranged up to North America.^[30] Lucas's conception of Stagonolepis is unconventionally broad, lumping together not just the original European fossils, but also Aetosauroides^[31][32] and the North American genus Calyptosuchus, in defiance of many other specialists.^[21][33]

As an Ischigualastian index taxon, Hyperodapedon has been reported from the Pebbly Arkose Formation of Zimbabwe, the Tunduru beds of Tanzania, and the Lower Maleri Formation of India.^[21][34][35] It has also been reported from more northerly formations, such as the Lossiemouth Sandstone of Scotland, the Middle Wolfville Formation of Nova Scotia, and the Popo Agie Formation of Wyoming. Stagonolepis was initially discovered in the Lossiemouth Sandstone, providing another possible avenue of correlation with the Ischigualasto Formation.^[21] Some of the Hyperodapedon species have subsequently been placed into their own genus: the Tanzanian species is also known as Supradapedon,^[36] the Nova Scotian species is Oryctorhynchus,^[37] and the Wyoming species is Beesiiwo.^[38] Fossils from the "Isalo II" beds of Madagascar are broadly comparable to the Ischigualasto Formation, and the native Madagascan rhynchosaur, Isalorhynchus, has occasionally been considered a species of Hyperodapedon.^[21]

The Ischigualasto Formation is home to one of the oldest crocodylomorphs, Trialestes. This may facilitate correlation to other formations with very early crocodylomorphs: the Pekin Formation of North Carolina and the Lower Maleri Formation in India.^[34] Faunal composition through the "Ischigualastian" interval may be more closely tied to paleolatitude rather than geographical closeness. South America's late Carnian-early Norian ecosystems roughly resemble Africa, Europe, India, and northern North America, but strongly differ from southern North America.^[39]

Vertebrate preservation

According to a 2012 survey, vertebrate fossils in Ischigualasto Provincial Park present several distinct modes of preservation influenced by their location and stratigraphic level within the park.^[45] Fossils are rare in the center of the basin and most common along the margins, especially the southern edge. Nearly 88% of fossils are from floodplain deposits, with the remainder from active channel fills. There is little consistency in the distribution of particular animal groups relative to their place of burial, though well-preserved skulls of small cynodonts are unusually common in channel sandstone.^[45]

Around 4% of vertebrate fossils in the survey are recovered from the La Peña Member. Vertebrates from this member tend to be slightly weathered and splintered, with associated or semi-articulated bone clusters more common than fully articulated skeletons. Fossils are covered with a hematite crust, and internally recrystallized by hematite, apatite, calcite, and barite. Carcasses were buried in place by waterlogged floodplain and river sediments, though more fragmentary specimens were probably exposed for longer. Higher-quality preservation would have been facilitated by a high level of anoxic groundwater or dense vegetation to protect the bones from the elements.^[45]

Nearly 65% of vertebrate fossils are from the Cancha de Bochas Member, but preservation is overall worse than in the previous member. A larger portion of the fossils are isolated and/or highly weathered bones, though many articulated skeletons with excellent preservation have also been found. Calcite is by far the most common mineral in fossils both externally and internally, while hematite is uncommon, occurring as an internal crust layer close to the original bone. The drier climate induced a patchier distribution of carcasses, with most animals dying alone in dry areas exposed to scavengers. The few exceptional fossils may have been mummified in areas with a high density of animals, such as watering holes, where scavengers cannot keep up with the accumulation of carcasses. The proportion of fossils per sediment layer increases due to a lower sedimentation rate and a reduced influence from rivers, which otherwise would have washed the remains out of the basin. Soil chemistry shifts from anoxic to alkaline, encouraging calcite formation while discouraging hematite.^[45]

The lower Valle de la Luna Member preserves 26% of vertebrate fossils in the survey. Preservation is even worse than the Cancha de Bochas Member, as the proportion of highly weathered and isolated fossils continues to increase. Hematite regains its position as the most common mineral in fossils, and calcite is rare. Finally, 5% of vertebrate fossils are from the middle-upper Valle de la Luna Member. The same trend of decreasing fossil quality continues, with hematite and (to a lesser extent) barite as the predominant minerals. Higher subsidence and sedimentation in the middle of the basin decreased the relative density of fossil accumulation. The opposite is true for the edge of the basin, where carcasses would have been isolated and exposed to scavengers and weathering processes for prolonged periods. Preservation was jeopardized further by high humidity and groundwater acidity. Vertebrate fossils practically never occur in abandoned channel and marsh deposits, despite the frequency of plant fossils in these environments. The few fossil fragments from the Quebrada de la Sal Member are strongly weathered and covered by a very thick hematite crust.^[45]

Plant preservation

Plant preservation in the Ischigualasto Formation can follow nine different taphonomic pathways, each associated with a particular depositional environment:^[44]

• (A) Fibrous clay-filled root traces occupy smaller and more fine-grained sandstone beds, which were only exposed long enough for herbaceous plants to take hold. This type of preservation is common, especially in the La Peña Member and the middle part of the basin.^[44]
• (B) Root halos, ring-shaped discolorations indicative of altered soil chemistry around root traces. They are found in red paleosols affected by microbial activity in upland environments. They occur from the Cancha de Bochas member upwards, with the thickest root halos in the Valle de la Luna Member.^[44]
• (C) Silicified roots and stumps of woody shrubs are only found in thick braided channel sandstone deposits. The plants responsible for the silicified fossils would have inhabited dry sandbars and abandoned channels, only to be inundated and buried by new sediment and volcanic ash washed down by seasonal floods. This type of preservation is known from every member, but it is rare.^[44]
• (D) Silicified roots and trunks of large woody trees, often standing perpendicular to the bedding plane. The trunks occupy abandoned channel deposits in the mid-upper Valle de la Luna Member, with the wood stabilized in position by interbedded layers of mudstone and sandstone. Rhexoxylon piatnitzkyi is the most common species of petrified wood in the formation.^[44]
• (E) Horizontal silicified tree trunks and branches, parallel to the bedding plane. These trunks are found in exceptionally large sandstone beds in the mid-upper Valle de la Luna Member. They were probably riparian trees which were toppled, stripped of their roots, and buried during massive floods.^[44]
• (F) Horizontal foliage impressions and cuticle, parallel to the bedding plane. These plant fossils are the most diverse and diagnostic in the formation, deposited in thin flattened layers of fine-grained sediment. The original environment was calm stagnant water, probably a shallow marsh occupying an abandoned channel near an active river. Only found in the mid-upper Valle de la Luna Member.^[44]
• (G) Cuticle oriented oblique to the bedding plane, similar to the horizontal impressions but significantly more fragmentary. These are only found in trough cross-bedded sandstone, representing freshly detached foliage slowly settling onto a streambed by waning flood waters. Only found in the mid-upper Valle de la Luna Member.^[44]
• (H) Palynomorphs (pollen, spores, and other microscopic plant fossils). Found in marsh mudstone alongside horizontal foliage impressions and cuticle.^[44]
• (I) Eroded charcoal fragments, found alongside horizontal silicified trunks (in major flood deposits) or oblique cuticle (in trough cross-bedded sandstone beds).^[44]

Some fossilized wood in the formation display particular styles of decay and degradation, helping to estimate the occurrence of various bacteria and fungi groups in the Triassic Period which otherwise lack fossil evidence.^[46][47]

Trace fossils

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.^[48]

Two different morphotypes of animal burrows are found in levee and crevasse splay deposits in the Cancha de Bochas member.^[49][50] One type of burrow is a horizontal mesh of tunnels and enlarged chambers which occasionally meet with vertical shafts branching off at right angles. Though the tunnel system extends up to 2 cubic meters, the tunnel diameters do not exceed 15 cm, while chambers only reach 25 cm in width. They were almost certainly created by small terrestrial vertebrates, presumably cynodonts.^[49][50] The other tunnel morphotype is a calcareous web of narrow, elliptical burrows (average diameters of 7 cm) branching out from a long vertical shaft. It probably represents plant root casts enlarged further by small animals.^[50]

Reptiles

Synapsids

Temnospondyls

Plants