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Los Llanos Formation
Los Llanos Formation
Stratigraphic range: Campanian; ~84–70 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Underlies	exposed to surface
Overlies	Unconformity with Early Permian Los Sauces & Patquía Formations or Ordovician granitic basement
Thickness	Up to ~150 m (490 ft)
Lithology
Primary	Sandstone, conglomerate
Other	Paleosols
Location
Coordinates	30°36′S 66°36′W / 30.6°S 66.6°W
Approximate paleocoordinates	33°48′S 39°30′W / 33.8°S 39.5°W
Region	La Rioja Province
Country	Argentina
Extent	Sanagasta geological park [es], Sierra de Los Llanos, Sierra de Chepes, Sierra Ulapes, Sierra de Velasco & Sierra Brava
Type section
Named for	Sierra de Los Llanos
Named by	Bodenbender
Year defined	1911
	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 Permian Los 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 fluvial environment. The formation crops out in the Sanagasta geological park [es], 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 crocodyliform Llanosuchus, described in 2016 from the formation, was named after Los Llanos Formation.

Description

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 Permian Los Sauces Formation in the Sierra de Velasco and the Permian Patquía Formation or Ordovician granitic basement 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 fluvial environment, 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

The formation has provided fossilized nesting sites attributed to a titanosaurid dinosaur. 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
- Aeolosaurini indet.
- Titanosauria indet.
- Abelisauridae indet.
Reptiles
- Crocodyliforms
  - Peirosauridae indet.
  - Llanosuchus tamaensis^[13]
Oofossils
- Faveoloolithidae indet.^[14]
Invertebrates
- Ostracods
  - Ilyocypris triebeli^[15]
  - Wolburgiopsis neocretacea^[15]
Flora

References

^ Bodenbender, 2011
^ Hechenleitner, 2017, p.23
^ Hechenleitner, 2017, p.24
^ Hechenleitner, 2017, p.25
^ Chiesa et al., 2019, p.292
^ ^a ^b ^c ^d De Sosa et al., 2017, p.34
^ Hechenleitner, 2017, p.26
^ ^a ^b Hechenleitner, 2017, p.29
^ Hechenleitner, 2017, p.34
^ Hechenleitner, 2017, p.35
^ Hechenleitner et al., 2015, p.6
^ Los Llanos Formation at Fossilworks.org
^ Fiorelli et al., 2016
^ Grellet-Tinner & Fiorelli, 2010
^ ^a ^b Carignano et al., 2013

Bibliography

Chiesa, Jorge (2019), "Estratigrafía, paleontología y paleoambientes del Plioceno de la región Cuyo de Argentina (San Luis, Mendoza, San Juan y La Rioja)", Opera Iilloana, 52: 287–347
Hechenleitner, E. M (2017), Dinosaurios saurópodos de la Formación Los Llanos (Cretácico, La Rioja, Argentina): ocurrencia, caracterización sistemática y tafonómica, bioestratigrafía, análisis paleoambientales y sus implicancias geológico-regionales (PhD thesis), Universidad Nacional de La Plata, pp. 1–161
De Sosa Tomas, Andrea; Carignano, Ana Paula; Fiorelli, Lucas Ernesto; Hechenleitner, E. Martín (2017), Carofitas del cretácico tardío de la Formación Los Llanos, La Rioja - Reporte preliminar, V Jornadas de las Ciencias de la Tierra "Dr. Eduardo Musacchio", pp. 34–36, ISBN 978-987-1937-79-0
Fiorelli, Lucas E.; Leardi, Juan M.; Hechenleitner, E. Martín; Pol, Diego; Basilici, Giorgio; Grellet-Tinner, Gerald (2016), "A new Late Cretaceous crocodyliform from the western margin of Gondwana (La Rioja Province, Argentina)", Cretaceous Research, 60: 194–209, Bibcode:2016CrRes..60..194F, doi:10.1016/j.cretres.2015.12.003, hdl:11336/43823
Hechenleitner, M. E.; Fiorelli, L.E.; Grellet-Tinner, G.; Leuzinger, L.; Basilici, G.; Taborda, J. R. A.; de la Vega, S. R.; Bustamante, C. A. (2016), "A new Upper Cretaceous titanosaur nesting site from La Rioja (NW Argentina), with implications for titanosaur nesting strategies", Palaeontology, 59 (3): 433–446, Bibcode:2016Palgy..59..433H, doi:10.1111/pala.12234, hdl:11336/44234
Hechenleitner, E. Martín; Grellet Tinner, Gerald; Fiorelli, Lucas E. (2015), "What do giant titanosaur dinosaurs and modern Australasian megapodes have in common?" (PDF), PeerJ, 3:e1341: 1–32, retrieved 2018-05-24
Carignano, Ana Paula; Hechenleitner, E. Martín; Fiorelli, Lucas Ernesto (2013), "Hallazgo de ostrácodos (Crustacea) Cretácicos continentales en la Formación Los Llanos, localidad de Tama, La Rioja", Ameghiniana, 50: R39, retrieved 2020-06-19
Grellet-Tinner, G.; Fiorelli, L. E. (2010), "A new Argentinean nesting site showing neosauropod dinosaur reproduction in a Cretaceous hydrothermal environment", Nature Communications, 1 (3): 1–8, Bibcode:2010NatCo...1...32G, doi:10.1038/ncomms1031, PMID 20975685
Bodenbender, G (1911), "Constitución geológica de la parte meridional de La Rioja y regiones limítrofes", Boletín de la Academia Nacional de Ciencias, 19: 2–11