The Chachao Formation is a

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

in the Mendoza Province in northern Patagonian

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

. It is

Valanginian In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 139.8 ± 3.0 Ma and 132.9 ± 2.0 Ma (million years ago). The Valanginian Stage succeeds the Berriasian Stage of the Lower Cretaceou ...

in age and is predominantly marine, being deposited at a time of

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

in the

Neuquén Basin Neuquén Basin ( es, Cuenca Neuquina) is a sedimentary basin covering most of Neuquén Province in Argentina. The basin originated in the Jurassic and developed through alternating continental and marine conditions well into the Tertiary. The bas ...

, and predominantly consists of

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...

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 The Agrio Formation is an Early Cretaceous geologic formation that is up to thick and is located in the southern Mendoza Province and northern-central Neuquén Province, in the Neuquén Basin of northwestern Patagonia, Argentina.Weaver, C. E. (1 ...

Description

A typical feature of the Chachao Formation is the dominance of

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not al ...

s, 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

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

s, ammonites ('' Olcostephanus curacoensis''), and ichnofossils such as '' Thalassinoides sp.''

Serpulid The Serpulidae are a family of sessile, tube-building annelid worms in the class Polychaeta. The members of this family differ from other sabellid tube worms in that they have a specialized operculum that blocks the entrance of their tubes wh ...

s 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.Chachao Coquina - Taphofacies 3
at

Fossilworks Fossilworks is a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database The Paleobiology Database is an online resource for information on the distribution and classification of fossil animals ...

.org

Fossil content

The formation has provided the following fossils:Chachao Formation
at

.org * Decapods ** '' Callianassa aff. peruviana'' *

Ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

s ** '' Groebericeras bifrons'' ** '' Olcostephanus curacoensis'' *

Polychaeta Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are m ...

** '' Parsimonia sp.'' ** '' Sarcinella occidentalis'' *

Bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

s ** '' 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

* * * * * {{citation , first=M.B , last=Aguirre Urreta , year=1989 , title=The Cretaceous decapod Crustacea of Argentina and the Antarctic Peninsula , journal=

Palaeontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

, volume=32 , pages=499–552 Geologic formations of Argentina Lower Cretaceous Series of South America Cretaceous Argentina Valanginian Stage Limestone formations Mudstone formations Open marine deposits Shallow marine deposits Formations Fossiliferous stratigraphic units of South America Paleontology in Argentina Geology of Mendoza Province