Chachao Formation
Stratigraphic range: Valanginian
~140–136 Ma

PreЄ

Type| Geological formation
Unit of| Mendoza Group
Underlies| Agrio Formation
Overlies| Vaca Muerta
Lithology
Primary| Limestone
Other| Mudstone
Location
Coordinates|  [ ⚑ ] : 35°00′S 69°42′W﻿ / ﻿35.0°S 69.7°W﻿ / -35.0; -69.7
Paleocoordinates|  [ ⚑ ] 33°48′S 33°00′W﻿ / ﻿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.

## Contents

* 1 Description
* 2 Fossil content
* 3 References
* 3.1 Bibliography

## 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
* 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.[1]

## References

1. ↑ 1.0 1.1 Chachao Coquina - Taphofacies 3 at Fossilworks.org
2. ↑ Chachao Formation at Fossilworks.org

### Bibliography

* Gómez Dacal, Alejandro R.; Gómez Peral, Lucía E.; Spalletti, Luis A.; Sial, Alcides N.; Siccardi, Aron; Poiré, Daniel G. (2018), "First record of the Valanginian positive carbon isotope anomaly in the Mendoza shelf, Neuquén Basin, Argentina: palaeoclimatic implications", Andean Geology 45 (2): 111–129, doi:10.5027/andgeoV45n2-3059, http://www.andeangeology.cl/index.php/revista1/article/view/V45n2-3059/pdf, retrieved 2019-02-22
* Palma, R. M.; Lanes, S. G. (2001), "Shell Bed Stacking Patterns in the Chachao Formation (Early Valanginian) in Malargüe Area, Mendoza Province, Neuquén Basin, Argentina", Carbonates and Evaporites 16 (2): 168–180, doi:10.1007/BF03175834, Bibcode: 2001CarEv..16..168P
* Aguirre Urreta, M. B.; Álvarez, P. P. (1999), "The Berriasian genus Groebericeras in Argentina and the problem of its age", Scripta Geologica 3 (Special Issue): 15–29
* Palma, R. M.; Angeleri, M. P. (1992), "Early Cretaceous serpulid limestones: Chachao Formation, Neuquen basin, Argentina", Facies 27 (1): 175–178, doi:10.1007/BF02536810, Bibcode: 1992Faci...27..175P
* Aguirre Urreta, M.B (1989), "The Cretaceous decapod Crustacea of Argentina and the Antarctic Peninsula", Palaeontology 32: 499–552

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