Los Llanos Formation
Los Llanos Formation
Stratigraphic range: Campanian~
TypeGeological formation
Underliesexposed to surface
OverliesUnconformity with Early PermianLos Sauces & Patquía Formations or Ordoviciangraniticbasement
ThicknessUp to ~150 m (490 ft)
Lithology
PrimarySandstone, conglomerate
OtherPaleosols
Location
Coordinates30°36′S66°36′W / 30.6°S 66.6°W / -30.6; -66.6
Approximate paleocoordinates33°48′S39°30′W / 33.8°S 39.5°W / -33.8; -39.5
RegionLa Rioja Province
CountryArgentina
ExtentSanagasta geological park, Sierra de Los Llanos, Sierra de Chepes, Sierra Ulapes, Sierra de Velasco & Sierra Brava
Type section
Named forSierra de Los Llanos
Named byBodenbender
Year defined1911
Los Llanos Formation is located in Argentina
Los Llanos Formation
Los Llanos Formation (Argentina)

Los Llanos Formation is a geological formation in the La Rioja Province, northwestern Argentina whose strata date back to the Campanian stage of the Late Cretaceous.

Los Llanos Formation over the years has been controversially described as ranging from Late Cretaceous to Miocene, but the Miocene succession was assigned to Las Mulitas Formation in 2019. The formation rests on top of the Early PermianLos Sauces and Patquía Formations and in parts on Ordovician crystalline basement. The maximum thickness is estimated at 150 metres (490 ft).

The sandstones and conglomerates of the formation were deposited ij a fluvialenvironment. The formation crops out in the Sanagasta geological park, where more than 90 titanosaurid nesting sites were discovered in Los Llanos Formation. The sites were encountered on top of areas characterized by hydrothermal activity as geysers and other vents, suggesting a preferred location for the incubation of the dinosaur eggs. Apart from fossil eggs, the formation has provided fossil flora and ostracods. The crocodyliformLlanosuchus, described in 2016 from the formation, was named after Los Llanos Formation.

Description

Paleogeography of the Late Cretaceous with distribution of titanosaur nesting sites

The Los Llanos Formation was first formally described by Bodenbender in 1911.[1] The formation crops out in very localized patches in the south of La Rioja Province.[2] Los Llanos Formation is found on the western and eastern slopes of the eponymous Sierra de Los Llanos, in the Sierra de Chepes, Sierra Ulapes, Sierra de Velasco and Sierra Brava.[3]

Between the original definition of "Los Estratos de Los Llanos" (Los Llanos Beds) by Bodenbender and later analysis during the twentieth century, a number of controversies arose. Rusconi (1936), Bordas (1941), Pascual (1954), Guiñazú (1962) and Zuzek (1978) assigned a Miocene age to the formation, based on mammal fossils found in the strata.[4] This unit is presently known as the Las Mulitas Formation.[5] The occurrence of typical Cretaceous charophytes confirms the assigned age for Los Llanos Formation.[6]

Los Llanos Formation unconformably overlies the PermianLos Sauces Formation in the Sierra de Velasco and the Permian Patquía Formation or Ordoviciangraniticbasement in other places. The maximum recorded thickness in the Sanagasta Geologic Park is 4 metres (13 ft),[7] while in the region of Tama the formation reaches an estimated thickness of 150 metres (490 ft).[8]

It comprises lightgrey and orange sandstones with basal conglomerate beds,[8] deposited in a fluvialenvironment, which in the Sanagasta park is associated with hydrothermal activity. [9] The more than 90 sauropod nests were found in direct association with these hydrothermal vents, suggesting a preferred incubation location of the mother dinosaurs.[10]

Paleontological significance

Interpretation of nest excavation and egg laying by a titanosaur

The formation has provided fossilized nesting sites attributed to a titanosauriddinosaur. Research conducted in 2015 by Hechenleitner et al. include a comparison with the Cretaceous Sanpetru Formation of Hațeg paleo-island in Romania, the Tremp Formation of the Spanish Pre-Pyrenees and the Boseong Formation of the Gyeongsang Basin in South Korea.[11]

Fossil content

The following fossils were reported from the formation:[12]

Dinosaurs

Color key
TaxonReclassified taxonTaxon falsely reported as presentDubious taxon or junior synonymIchnotaxonOotaxonMorphotaxon
Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited.
Dinosaurs of the Los Llanos Formation
GenusSpeciesLocalityMaterialNotesImages
Aeolosaurini indet.
Vitosaura
Titanosauria indet.
Abelisauridae indet. An abelisaur
VitosauraV. colozacani[13]Tama village A partial skeleton An abelisaur

Crocodyliformes

Crocodyliforms of the Los Llanos Formation
GenusSpeciesLocalityMaterialNotesImages
LlanosuchusL. tamaensis[14]
Peirosauridae indet.

Oofossils

Egg fossils of the Los Llanos Formation
GenusSpeciesLocalityMaterialNotesImages
Faveoloolithidae[15]indet. Egg fossils Dinosaur of uncertain affinities

Invertebrates

Crocodyliforms of the Los Llanos Formation
GenusSpeciesLocalityMaterialNotesImages
IlyocyprisI. triebeli[16]An ostracod
WolburgiopsisW. neocretacea[16]An ostracod

Flora

Flora of the Los Llanos Formation
GenusSpeciesLocalityMaterialNotesImages
GobicharaG. (Pseudoharrisichara) groeberi[6]
LychnothamnusL. (Pseudoharrisichara) tenuis[6]A stonewort
L. (Pseudoharrisichara) walpurgica[6]

See also

References

  1. ^Bodenbender, 2011
  2. ^Hechenleitner, 2017, p.23
  3. ^Hechenleitner, 2017, p.24
  4. ^Hechenleitner, 2017, p.25
  5. ^Chiesa et al., 2019, p.292
  6. ^ abcdDe Sosa et al., 2017, p.34
  7. ^Hechenleitner, 2017, p.26
  8. ^ abHechenleitner, 2017, p.29
  9. ^Hechenleitner, 2017, p.34
  10. ^Hechenleitner, 2017, p.35
  11. ^Hechenleitner et al., 2015, p.6
  12. ^Los Llanos Formation at Fossilworks.org
  13. ^Jiménez Velandia, Harold; Ezcurra, Martín D.; Hechenleitner, E. Martín; Basilici, Giorgio; Martinelli, Agustín G.; Fiorelli, Lucas E. (2025). "A new Late Cretaceous abelisaurid species from La Rioja Province, northwestern Argentina". Ameghiniana. 65 (1): 1–23. ISSN 0002-7014.
  14. ^Fiorelli et al., 2016
  15. ^Grellet-Tinner & Fiorelli, 2010
  16. ^ abCarignano et al., 2013

Bibliography

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