Chachao Formation
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.
| Chachao Formation Stratigraphic range: Valanginian ~140–136 Ma | |
|---|---|
| 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 |
| Approximate paleocoordinates | 33.8°S 33.0°W |
| Region | Mendoza Province |
| Country |  Argentina |
| Extent | Neuquén Basin |
  Chachao Formation (Argentina) | |
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.[1]
Fossil content
The formation has provided the following fossils:[2]
- Decapods
- 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
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. 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.