Los Llanos Formation

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

Not to be confused with Llanos Basin.
Los Llanos Formation
Stratigraphic range: Campanian
~84–70 Ma
TypeGeological formation
Underliesexposed to surface
OverliesUnconformity with Early Permian Los Sauces & Patquía Formations or Ordovician granitic basement
ThicknessUp to ~150 m (490 ft)
Lithology
PrimarySandstone, conglomerate
OtherPaleosols
Location
Coordinates30.6°S 66.6°W / -30.6; -66.6
Approximate paleocoordinates33.8°S 39.5°W / -33.8; -39.5
RegionLa Rioja Province
Country Argentina
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 (Argentina)

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, 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
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 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
Interpretation of nest excavation and egg laying by a titanosaur

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]

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. De Sosa et al., 2017, p.34
  7. Hechenleitner, 2017, p.26
  8. Hechenleitner, 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. Fiorelli et al., 2016
  14. Grellet-Tinner & Fiorelli, 2010
  15. Carignano et al., 2013

Bibliography
  • Chiesa, Jorge et al. 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), 1-161. Universidad Nacional de La Plata.
  • De Sosa Tomas, Andrea; Ana Paula Carignano; Lucas Ernesto Fiorelli, and E. Martín Hechenleitner. 2017. Carofitas del cretácico tardío de la Formación Los Llanos, La Rioja - Reporte preliminar, 34-36. V Jornadas de las Ciencias de la Tierra "Dr. Eduardo Musacchio". ISBN 978-987-1937-79-0
  • Fiorelli, Lucas E.; Juan M. Leardi; E. Martín Hechenleitner; Diego Pol; Giorgio Basilici, and Gerald Grellet-Tinner. 2016. A new Late Cretaceous crocodyliform from the western margin of Gondwana (La Rioja Province, Argentina). Cretaceous Research 60. 194–209. doi:10.1016/j.cretres.2015.12.003
  • Hechenleitner, M. E.; L.E. Fiorelli; G. Grellet-Tinner; L. Leuzinger; G. Basilici; J. R. A. Taborda; S. R. de la Vega, and C. A. Bustamante. 2016. A new Upper Cretaceous titanosaur nesting site from La Rioja (NW Argentina), with implications for titanosaur nesting strategies. Palaeontology 59. 433-446.
  • Hechenleitner, E. Martín; Gerald Grellet Tinner, and Lucas E. Fiorelli. 2015. What do giant titanosaur dinosaurs and modern Australasian megapodes have in common?. PeerJ 3:e1341. 1–32. Accessed 2018-05-24.
  • Carignano, Ana Paula; E. Martín Hechenleitner, and Lucas Ernesto Fiorelli. 2013. Hallazgo de ostrácodos (Crustacea) Cretácicos continentales en la Formación Los Llanos, localidad de Tama, La Rioja. Ameghiniana 50. R39. Accessed 2020-06-19.
  • Grellet-Tinner, G., and L. E. Fiorelli. 2010. A new Argentinean nesting site showing neosauropod dinosaur reproduction in a Cretaceous hydrothermal environment. Nature Communications 1. 1-8.
  • 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.
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