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 expo ...

in the

Mendoza Province Mendoza (), officially the Province of Mendoza, is a province of Argentina, in the western central part of the country in the Cuyo region. It borders San Juan to the north, La Pampa and Neuquén to the south, San Luis to the east, and the r ...

in northern

Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

. It is

Valanginian In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 137.05 ± 0.2 Ma and 132.6 ± 0.2 Ma (million years ago). The Valanginian Stage succeeds the Berriasian Stage of the Lower Cretac ...

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

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

in the

Neuquén Basin Neuquén Basin () 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 basin bounds to the wes ...

, and predominantly consists of

carbonate A carbonate is a salt of carbonic acid, (), 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 group ...

rocks. The formation belongs to the Mendoza Group, sediments deposited on the Mendoza Shelf. The formation overlies the

Vaca Muerta The Vaca Muerta Formation, commonly known as Vaca Muerta ( Spanish for ''dead cow''), is a geologic formation of Late Jurassic to Early Cretaceous age, located in the Neuquén Basin in northern Patagonia, Argentina. It is well known as the host ...

and is overlain by the Agrio Formation.

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 no ...

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 Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

s, ammonites ('' Olcostephanus curacoensis''), and

ichnofossil A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...

s 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 was a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. History Fossilworks was cr ...

.org

Fossil content

The formation has provided the following fossils:Chachao Formation
at

.org *

Decapods The Decapoda or decapods, from Ancient Greek δεκάς (''dekás''), meaning "ten", and πούς (''poús''), meaning "foot", is a large order (biology), order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfis ...

** '' Callianassa aff. peruviana'' *

Ammonite Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

s ** '' Groebericeras bifrons'' ** '' Olcostephanus curacoensis'' * Polychaeta ** '' Parsimonia sp.'' ** '' Sarcinella occidentalis'' *

Bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

s ** '' Exogyra caudoni'' ** '' Cucullaea sp.'' ** '' Exogyra (Aetostreon)'' ** '' Amphidonte (Ceratostreon)'' ** '' Eriphyla sp.'' ** '' Myoconcha sp.'' ** '' Pinna sp.'' ** '' Pecten sp.'' ** '' Ptychomya sp.'' ** '' Trigonia sp.'' *

Ichnofossil A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...

s ** '' 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 as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geo ...

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