The Chichinales 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 ...

Río Negro Province Río Negro (, ''Black River'') is a province of Argentina, located in northern Patagonia. Neighboring provinces are from the south clockwise Chubut, Neuquén, Mendoza, La Pampa and Buenos Aires. To the east lies the Atlantic Ocean. Its capit ...

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 ...

which dates from the Late Oligocene to the Early Miocene, around 23 to 17.5 million years ago. It predominantly consists of

pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

deposits, which were deposited in a

semi-arid A semi-arid climate, semi-desert climate, or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration, but not as low as a desert climate. There are different kinds of semi-ar ...

environment. It is divided up into a number of members. The diverse fauna of the Chichinales Formation, including a variety of turtles and birds, also includes many mammals such as

South American native ungulates South American native ungulates, commonly abbreviated as SANUs, are extinct ungulate-like mammals of controversial affinities that were indigenous to South America prior to the Great American Biotic Interchange. They comprise five major groups co ...

(

notoungulates Notoungulata is an extinct order of mammalian ungulates that inhabited South America from the early Paleocene to the Holocene, living from approximately 61 million to 11,000 years ago. Notoungulates were morphologically diverse, with forms resem ...

and

litopterns Litopterna (from grc, λῑτή πτέρνα "smooth heel") is an extinction, extinct order of fossil hoofed mammals from the Cenozoic era. The order is one of the five great orders of Meridiungulata, South American ungulates that were endemic to ...

) as well as

armadillo Armadillos (meaning "little armored ones" in Spanish) are New World placental mammals in the order Cingulata. The Chlamyphoridae and Dasypodidae are the only surviving families in the order, which is part of the superorder Xenarthra, along wi ...

s, and caviomorph rodents. It is named after the type locality and extends over a large part of the province of Río Negro. It outcrops in the valle de Río Negro, in the towns of

General Roca, Río Negro General Roca is a city in the northeast of the Argentine province of Río Negro, northern Patagonia. It was founded on September 1, 1879, by Colonel Lorenzo Vintter, on the order of War Minister Julio A. Roca, during the Conquest of the Desert. ...

and Villa Regina, bordering the southern bank of the Río Negro. Part of the most important outcrops are located within the Paso Córdova Natural Protected Area. The Chichinales Formation, broadly speaking, is made up of grayish

sandstones Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) b ...

, whitish

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

s and greenish

claystones Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too smal ...

with paleo soil levels. It is divided into 3 members: lower, middle and upper. The lower one is composed of grayish brown sandstones with low-angle

cross bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...

, alternating with light brown tuffs, tuffaceous sandstones, sandstones with carbonate cement and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

s. In this member, the presence of "

opal Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms ...

ized" fossil logs is common. The middle member is formed by

paleosol In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science. In geolo ...

s and sequences of siltstones, shales and fine sands that in some sectors form

lenses A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

in the form of fluvial channels. Finally, the upper member is represented by more homogeneous levels of tuffs, whitish gray tuffites, siltstones, green claystones and tuffaceous sandstones This member, in some sectors, is laterally interdigitated with marginal marine levels of the

Gran Bajo del Gualicho Formation Gran may refer to: People *Grandmother, affectionately known as "gran" *Gran (name) Places * Gran, the historical German name for Esztergom, a city and the primatial metropolitan see of Hungary * Gran, Norway, a municipality in Innlandet count ...

, which dates from the Late Oligocene to the Early Pliocene. The Chichinales Formation is correlated with the contemporaneous fluvial and

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

tuffaceous Collón Curá Formation of the central Neuquén Basin. The sedimentological and paleontological evidence provided by the Chichinales Formation suggests the existence of open environments with low relief, restricted wooded areas with temporary water bodies and warm temperate climate, in agreement with other Colhuehuapian localities of Chubut Province. In this context, it is possible to interpret, at least tentatively, that the differences between the set of taxa represented in the Cerro Bandera Formation and those of the Chichinales Formation are due to contrasts between the local environmental conditions of each unit rather than to differences in temporal range. It is likely that such differences are the result of the particular conditions that prevailed in the Neuquén region during the Early Miocene, which would be a consequence of its proximity to the Andean zone rather than its latitudinal location. CC-BY_icon

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Paleoflora

Paleofauna

Reptiles

Lepidosaurs

Crocodylomorphs

Testudines

Birds

Mammals

Meridiungulates

Xenarthrans

Rodents

Metatherians

References

{{Reflist Geologic formations of Argentina Colhuehuapian Deseadan Burdigalian Aquitanian (stage) Chattian Miocene Series of South America Oligocene Series of South America Neogene Argentina Paleogene Argentina Tuff formations Sandstone formations Fluvial deposits Lacustrine deposits Deltaic deposits Fossiliferous stratigraphic units of South America Paleontology in Argentina Formations Geology of Río Negro Province Geology of Patagonia