Chachao Formation

Chachao Formation
Stratigraphic range: Valanginian ~
Type	Geological formation
Unit of	Mendoza Group
Underlies	Agrio Formation
Overlies	Vaca Muerta
Lithology
Primary	Limestone
Other	Mudstone
Location
Coordinates	35.0°S 69.7°W / -35.0; -69.7
Approximate paleocoordinates	33.8°S 33.0°W / -33.8; -33.0
Region	Mendoza Province
Country	Argentina
Extent	Neuquén Basin
Chachao Formation (Argentina)

The Chachao Formation is a geological formation in the Mendoza Province in northern Patagonian Argentina. It is Valanginian in age and is predominantly marine, being deposited at a time of marine transgression in the Neuquén Basin, and predominantly consists of carbonate rocks.

The formation belongs to the Mendoza Group, sediments deposited on the Mendoza Shelf. The formation overlies the Vaca Muerta and is overlain by the Agrio Formation.

Description

A typical feature of the Chachao Formation is the dominance of oysters, many of them quite large e.g. Aetostreon latissimun, and others small, e.g. Ceratostreon minos. Different kinds of semi-infaunal soft bottom dwellers and swimming bivalves were recognized. Additional forms are represented by gastropods, ammonites (Olcostephanus curacoensis), and ichnofossils such as Thalassinoides sp. Serpulids are represented by the colonial Sarcinella sp., and the solitary form Parsimonia sp. The highly diverse fauna of the analyzed succession, made up with numerous stenohaline elements such as echinoderm, bryozoa, ammonite as well as serpulids and typical marine pelecypods indicate a fully marine environment.

Fossil content

The formation has provided the following fossils:

• Decapods
  • Callianassa aff. peruviana
• Ammonites
  • Groebericeras bifrons
  • Olcostephanus curacoensis
• Polychaeta
  • Parsimonia sp.
  • Sarcinella occidentalis
• Bivalves
  • Exogyra caudoni
  • Cucullaea sp.
  • Exogyra (Aetostreon)
  • Amphidonte (Ceratostreon)
  • Eriphyla sp.
  • Myoconcha sp.
  • Pinna sp.
  • Pecten sp.
  • Ptychomya sp.
  • Trigonia sp.
• Ichnofossils
  • Thalassinoides sp.

References

Bibliography

• Gómez Dacal, Alejandro R.; Lucía E. Gómez Peral; Luis A. Spalletti; Alcides N. Sial; Aron Siccardi, and Daniel G. Poiré. 2018. First record of the Valanginian positive carbon isotope anomaly in the Mendoza shelf, Neuquén Basin, Argentina: palaeoclimatic implications. Andean Geology 45. 111–129. Accessed 2019-02-22.
• Palma, R. M., and S. G. Lanes. 2001. Shell Bed Stacking Patterns in the Chachao Formation (Early Valanginian) in Malargüe Area, Mendoza Province, Neuquen Basin, Argintina. Carbonates and Evaporites 16. 168-180. .
• Aguirre Urreta, M. B., and P. P. Álvarez. 1999. The Berriasian genus Groebericeras in Argentina and the problem of its age. Scripta Geologica 3(Special Issue). 15-29. .
• Palma, R. M., and M. P. Angeleri. 1992. Early Cretaceous serpulid limestones: Chachao Formation, Neuquen basin, Argentina. Facies 27. 175-178. .
• Aguirre Urreta, M.B. 1989. The Cretaceous decapod Crustacea of Argentina and the Antarctic Peninsula. Palaeontology 32. 499-552. .