Chañares Formation

The Chañares Formation is a geologic formation of the Ischigualasto-Villa Unión Basin, located in La Rioja Provence, Argentina. The claystones and tuffs of the formation date to the Carnian stage of the Late Triassic and were deposited in a fluvial to lacustrine environment.

Chañares Formation
Stratigraphic range: Carnian
~237–235 Ma
↓
TypeGeologic formation
Unit ofAgua de la Peña Group
UnderliesIschichuca Formation
OverliesTarjados Formation
Thickness750 m (2,460 ft) (includes Ischichuca Fm.)
Lithology
PrimaryClaystone
OtherTuff
Location
Coordinates29.8°S 67.8°W / -29.8; -67.8
Approximate paleocoordinates49.9°S 37.8°W / -49.9; -37.8
RegionLa Rioja Province
Country Argentina
ExtentIschigualasto-Villa Unión Basin
Type section
Named forChañares River
Chañares Formation (Argentina)

The formation represents the onset of the first syn-rift phase in the Triassic rift basin and is the lowermost stratigraphic unit of the Agua de la Peña Group, unconformably overlying the Tarjados Formation of the Paganzo Group. The Chañares Formation is overlain by the Ischichuca Formation and both formations have a combined maximum thickness of 750 metres (2,460 ft).

The Chañares Formation has provided a rich faunal assemblage, including many of the earliest crocodylomorph fossils, as Tropidosuchus, Chanaresuchus, and Gualosuchus, as well as other archosaurs; Lewisuchus admixtus, Lagerpeton, Marasuchus lilloensis, Gracilisuchus, Luperosuchus and Pseudolagosuchus major. Cynodonts are represented by Probainognathus and Massetognathus and other therapsids include Dinodontosaurus.

Description

The Chañares Formation is the lowermost unit of the Agua de la Peña Group, representing the onset of the first syn-rift phase of the basin. The combined thickness of the overlying Ischichuca Formation and the Chañares Formation is 750 metres (2,460 ft).[1] It was originally thought to be formed during the Ladinian age of the Middle Triassic epoch, in the Triassic Period of the Mesozoic Era; Marsicano et al. (2016) dated it to early Carnian (235–234 Ma), Late Triassic.[2] 2020 U-Pb dating of the lower Los Rastros Formation overlying the Ischichuca Formation yielded an age of 234.47 ± 0.44 Ma, making the Chañares Formation lowermost Carnian.[3]

The formation overlies the Tarjados Formation and is overlain by the Ischichuca Formation. The formation is exposed in the Ischigualasto Provincial Park, a World Heritage site in Argentina.[4] The formation of claystones and tuffs was deposited in a fluvial to lacustrine environment.[5][6]

Fossil content

Some localities are well known for their abundance of tetrapod fossils. Theraspids include the kannemeyeriid Dinodontosaurus, and cynodonts such as Probainognathus and Massetognathus, the latter being the most abundant tetrapod taxon in the formation represented by Massetognathus pascuali and M. teruggii.[7]

Other notable tetrapods present from the formation are the archosaurs. Ornithodirans include Lewisuchus,[8][9] Lagerpeton,[10] and Marasuchus.[11]

Other archosaurs include Gracilisuchus[12] and Luperosuchus.[13] Three proterochampsids are known from the formation, Tropidosuchus, Chanaresuchus, and Gualosuchus. They are very similar in appearance, but differ in size and cranial proportions.[14] Tropidosuchus had an estimate body mass of about 3 kilograms (6.6 lb), Gualosuchus of approximately 10 kilograms (22 lb), and Chanaresuchus weighed about 20 kilograms (44 lb).[5]

In 2018, plant remains and palynomorphs preserved in the coprolites produced by large dicynodonts, and a study on their implications for inferring the diet of dicynodonts from the formation were described.[15] Also tetrapod burrows, likely produced by small eucynodonts, were described.[16]

Other fossils

Other fossils found in the formation include:[6][17]

See also

References
  1. Aceituno Cieri et al., 2015, p.60
  2. Marsicano et al., 2016
  3. Mancuso et al., 2020
  4. Kent et al, 2014, p.7959
  5. Arcucci et al., 2019, p.3
  6. Chañares Tetrapod Assemblage at Fossilworks.org
  7. Romer, 1967, III
  8. Romer, 1972, XIV
  9. Bittencourt, 2014, p.2
  10. Romer, 1972, X
  11. Sereno & Arcucci, 1994
  12. Romer, 1972, XIII
  13. Romer, 1971, VIII
  14. Romer, 1971, XI
  15. Pérez et al., 2018
  16. Fiorelli et al., 2018
  17. Agua Escondida at Fossilworks.org

Bibliography
The Chañares Triassic reptile fauna
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