The Roca Formation is a

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

to Paleogene lithostratigraphic unit, located 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 ...

.Roca Formation
at Fossilworks.org It crops out in the Argentinian provinces of Río Negro,

Neuquén Neuquén (; arn, Nehuenken) is the capital city of the Argentine province of Neuquén and of the Confluencia Department, located in the east of the province. It occupies a strip of land west of the confluence of the Limay and Neuquén river ...

La Pampa La Pampa () is a sparsely populated province of 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 ...

, and Mendoza. Its deposition is

diachronous In geology, a diachronism (Greek ''dia'', "through" + ''chronos'', "time" + ''-ism''), or diachronous deposit, is a sedimentary rock formation in which the material, although of a similar nature, varies in age with the place where it was deposited. ...

, beginning during the

Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval ...

in the north of its distribution, and later moving to the south, where its strata reached the Late

Danian The Danian is the oldest age or lowest stage of the Paleocene Epoch or Series, of the Paleogene Period or System, and of the Cenozoic Era or Erathem. The beginning of the Danian (and the end of the preceding Maastrichtian) is at the Cretace ...

. It lies transitionally above the Jagüel Formation, and the top of the formation is marked by a regional

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

due to an

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

and Oligocene

orogenic Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...

pulse. These two units belong to the

Malargüe Group The Malargüe Group is a group of geologic formations of the Neuquén Basin of the Mendoza, Neuquén, Río Negro and La Pampa Provinces in northern Patagonia, Argentina. The formations of the Malargüe Group range in age between the middle Camp ...

. The marine sediments of the Jagüel and Roca Formations were deposited during a transgression from the

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

, beginning in the Maastrichtian and ending in the Danian. The

stratotype A stratotype or type section in geology is the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. If the stratigraphic unit is layered, it is called a stratotype, whereas the stan ...

of the Roca Formation is located north of

General Roca A general officer is an officer of high rank in the armies, and in some nations' air forces, space forces, and marines or naval infantry. In some usages the term "general officer" refers to a rank above colonel."general, adj. and n.". OED O ...

, Río Negro (39º40´S, 67º32´W). The fossiliferous beds of the Roca Formation were discovered by G. Rohde Windhausen (1914), who was also the first author to describe these sediments. Schiller (1922) took samples of one section along the Zanjón Roca, from the northern part of General Roca to ''Horno de Cal'' (

lime kiln A lime kiln is a kiln used for the calcination of limestone ( calcium carbonate) to produce the form of lime called quicklime (calcium oxide). The chemical equation for this reaction is : CaCO3 + heat → CaO + CO2 This reaction can take p ...

). This author proposed to name the lime kiln as the "classic area", and the westward cliffs from the lime kiln as the "model area" of these beds. The lithological composition of this type locality contains gray-yellowish and highly fossiliferous limestones, with greenish claystones and marls, and abundant gypsum at the top. The basal and middle sections are approximately thick (Weber, 1972).

Lithology

In its type locality, the Roca Formation is divided into three sections. The first section, that is, the base, is approximately . It consists of intercalated bioclastic

limestones Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms when t ...

and green claystones. The calcareous materials are highly fossiliferous with textures like

wackestone Under the Dunham classification (Dunham, 1962Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: W.E. Ham (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir. Amer ...

and

packstone Under the Dunham classification (Dunham, 1962Dunham, R.J. (1962) Classification of carbonate rocks according to depositional texture. In: Classification of Carbonate Rocks (Ed. W.E. Ham), Am. Assoc. Pet. Geol. Mem., 1, 108–121.) system of limesto ...

without an obvious orientation of the shells. The

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 ...

are composed of

montmorillonite Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite gro ...

with calcite and

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

. The second section (middle) is not over and consists mainly of limestones. Such section has very thin strata of varied yellow limestones and siltstones, interbedded with yellow porous and coarse-grained limestones. The limestones of the middle section are almost devoid of fossil invertebrates. The third section (upper) is . It is mainly composed of white

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywal ...

in large crystals. It has lenticular intercalations of greenish siltstones with wave-formed ripples and fragments of gypsum. The faunal character on the fossils of marine invertebrates in the Roca Formation suggests a transgression from the Atlantic Ocean. By contrast, the marine invasions in

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 ...

and

Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...

during the

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

and Early Cretaceous were caused by an inflow of seawater from the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...

.Aguirre-Urreta, M.B., S. Casadío, M. Cichowolski, D.G. Lazo y D.L. Rodríguez, 2008. Afinidades paleobiogeográficas de los invertebrados cretácicos de la Cuenca Neuquina. ''Ameghiniana'' 45 (3): 593-613.

Fossil content

The first section of the Roca Formation has abundant fossiliferous content, including bivalves,

gastropods 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. The ...

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...

ns, echinoderms, crustaceans,

ostracod Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typi ...

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

, and calcareous

nanoplankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

, as well as remains of fish.

Bivalves

* '' Pycnodonte (Phygraea) burckhardti'' (Boehm) * '' Pycnodonte (Phygraea) sarmientoi'' Casadío 1998 * '' Gryphaeostrea callophyla'' (Ihering) * '' Ostrea wilckensi'' Ihering * '' Ostrea neuquena'' Ihering * '' Cubitostrea ameghinoi'' (Ihering) * '' Nucula (Leionucula) dynastes'' Ihering * '' Neilo cf. N. ornata'' (Sowerby) * '' Cucullaea rocana'' Ihering * '' Chlamys patagonensis negroina'' Ihering * '' Musculus rionegrensis'' (Ihering) * '' Arca ameghinorum'' Ihering (=''Venericardia ameghinorum'') * '' Venericardia iheringi'' (Boehm) * '' V. feruglioi'' Petersen * '' Aphrodina burckhardti'' (Ihering)

Gastropods 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. The ...

* ''"Aporrhais"'' spp. * '' Turritella burckhardti'' Ihering * ''Turritella aff. T. malaspina'' Ihering

Nautoloids

* '' Hercoglossa romeroi'' (Ihering) * '' Cimomia camachoi'' Masiuk

Decapods The Decapoda or decapods (literally "ten-footed") are an order (biology), order of crustaceans within the class Malacostraca, including many familiar groups, such as crabs, lobsters, crayfish, Caridea, shrimp and Dendrobranchiata, prawns. Most ...

* '' Callianassa burckhardti'' Boehm

Echinoida

* '' Linthia joannisboehmi'' Oppenheim * '' Nucleopygus salgadoi'' Parm

References

{{reflist Geologic formations of Argentina Malargüe Group Maastrichtian Stage of South America Cretaceous Argentina Cretaceous–Paleogene boundary Paleocene Series of South America Danian Stage Paleogene Argentina Limestone formations Siltstone formations Shale formations Shallow marine deposits Paleontology in Argentina