The Agrio Formation is an

Early Cretaceous The Early Cretaceous (geochronological name) or the Lower Cretaceous ( chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pr ...

geologic 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 ...

that is up to thick and is located in the southern

Mendoza Province Mendoza, officially 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 republic ...

and northern-central Neuquén Province, 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 ba ...

of northwestern

Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and ...

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, t ...

.Weaver, C. E. (1931). Paleontology of the Jurassic and cretaceous of west central Argentina. In ''Memoires of the University of Washington'' (Vol. 1, pp. 1–599). This formation is the youngest one of the

Mendoza Group Mendoza may refer to: Places Argentina * Greater Mendoza, the name given to the large urban conurbation around the city of Mendoza * Mendoza, Argentina, the capital of the province of Mendoza * Mendoza Department, subdivision of the Mendoza Provi ...

, overlying the Mulichinco and

Bajada Colorada Formation The Bajada Colorada Formation is a geologic formation of the southern Neuquén Province in the Neuquén Basin of northern Patagonia, Argentina. The formation belongs to the Mendoza Group and is Berriasian, Late Berriasian to Valanginian, Early Val ...

s and overlain by the Huitrín and

La Amarga Formation The La Amarga Formation is a geologic formation with outcrops in the Argentine provinces of Río Negro, Neuquén, and Mendoza. It is the oldest Cretaceous terrestrial formation in the Neuquén Basin. The type locality is La Amarga Arroyo a ...

s. It is dated to the Late Valanginian to Early Hauterivian,Voglino, 2017, p.39 Late Valanginian to Early Barremian,Voglino, 2017, p.49 or

Hauterivian The Hauterivian is, in the geologic timescale, an age in the Early Cretaceous Epoch or a stage in the Lower Cretaceous Series. It spans the time between 132.9 ± 2 Ma and 129.4 ± 1.5 Ma (million years ago). The Hauterivian is preceded by t ...

to earliest

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ag ...

.Gómez Dacal et al., 2018, p.113 The Agrio Formation is considered the third most important

source rock In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been deposi ...

in the hydrocarbon-rich

, after the Vaca Muerta Formation and

Los Molles Formation The Los Molles Formation is a geologic formation of Early to Middle Jurassic age, located at northern and central part of Neuquén Basin at Mendoza Shelf in Argentina. It is overlain by the Niyeu–Lajas Formation.McIlroy et al., 2005 Descript ...

. Similarly to these older units, it is potentially a source of

shale gas Shale gas is an unconventional natural gas that is found trapped within shale formations. Since the 1990s a combination of horizontal drilling and hydraulic fracturing has made large volumes of shale gas more economical to produce, and some ...

. This formation has provided fossils of

ichthyosaur Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, altho ...

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 cuttle ...

s, gastropods,

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, biv ...

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

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secre ...

s and fish. The newly described species of fish, '' Tranawuen agrioensis'', the ammonite '' Holcoptychites agrioensis'', and the bivalve '' Pholadomya agrioensis'' have been named after the formation.

Description

The Agrio Formation was first described by Weaver in 1931 and its three members, from bottom to top: Pilmatué, Avilé and Agua de la Mula Members, were defined by Leanza and Hugo in 2001.Archuby, F. M. (2009). ''Taphonomy and palaeoecology of benthic macroinvertebrates from the Agua de la Mula Member of the Agrio Formation, Neuquén Basin (Neuquén province, Argentina): sequence stratigraphic significance'' ürzburg University http://www.opus-bayern.de/uni-wuerzburg/volltexte/2009/3717/index.html The formation crops out in an approximately wide band from north to south along 70° longitude west, from 37° to 39° south in the northern-central part of the Neuquén Basin.Voglino, 2017, p.50 The southern termination of the formation is the Huincul High, formed by the

Huincul Fault The Huincul Fault or Huincul Fault Zone ( es, Falla de Huincul, Zona de falla Huincul) is an east-to-west-oriented, continental-scale fault that extends from the Neuquén Basin eastwards into the Argentine Shelf. To the west, it has been propo ...

. Towards the east in the basin, the formation grades into the Centenario Formation.

Stratigraphy

The Agrio Formation is included in the

, representing its youngest formation. In the east of the Neuquén Basin, the formation rests upon continental clastic deposits of the

Mulichinco Formation The Mulichinco Formation is a geological formation in Argentina. It is Valanginian in age and is predominantly terrestrial, being deposited at a time of marine regression in the Neuquén Basin, and predominantly consists of siliciclastic rocks. ...

, with the contact between the two formations characterized by a regional transgressive surface. Towards the west, the formation unconformably overlies the

.Olivo et al., 2016, p.218Gallina et al., 2014, p.2 In its eastern part, the Agrio Formation is overlain by the clastic, carbonaceous and evaporitic deposits of the Huitrín Formation and in the western area by the

. The total thickness of the Agrio Formation reaches up to , with the Pilmatué Member having a thickness of and the Agua de la Mula Member reaching .Voglino, 2017, p.51

Lithologies

Formación Agrio (Cretácico inferior) cerca de Pilmatué

The Agrio Formation is primarily composed of

pelitic A pelite (Greek: ''pelos'', "clay") or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock ...

rocks with intercalations of

limestone 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 wh ...

sandstone 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 ...

s and rare fine conglomerates. The Pilmatué and Agua de la Mula Members are characterized by thick successions of

black 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, especiall ...

s with intercalating limestones and sandstones. The Avilé Member comprises sandstones and

claystone 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 sm ...

s with conglomerates.

Depositional environment

The Agrio Formation was deposited in a post-rift setting of the Neuquén Basin, probably representing a tectonic regime of

thermal subsidence In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal contraction: as mantle material cools ...

. The sediments of the lower and upper members of the formation are marine in character, interpreted as the combination of thermal subsidence and a

eustatic sea level The eustatic sea level is the distance from the center of the earth to the sea surface. An increase of the eustatic sea level can be generated by decreasing glaciation, increasing spreading rates of the mid-ocean ridges or more mid-oceanic ridges. ...

rise. The middle Avilé member was deposited in a

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluv ...

environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...

. Within the marine Pilmatué Member, a succession of approximately thick, described as "San Eduardo Beds", is recognized as deposited in a wave-dominated

deltaic A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rare ...

setting with hyperdense currents. This sequence is overlain by about thick limestones deposits in a

reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic component, abiotic processes—deposition (geology), deposition of ...

al environment. The presence of the newly described gastropod '' Eunerinea mendozana'' led researchers to estimate

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

conditions for the Agrio Formation.Cataldo, 2012

Fossil content

The formation has provided many fossils of

s,Aguirre Urreta, 1998 gastropods,

echinoid Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...

crinoid Crinoids are marine animals that make up the Class (biology), class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or coma ...

s and nano and

microfossils A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

( calcareous nanofossils,

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, typic ...

foraminifers 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 an ...

).Kauffman & Leanza, 2004Agrio Formation
at

Fossilworks Fossilworks is 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 cre ...

.org In

2018 File:2018 Events Collage.png, From top left, clockwise: The 2018 Winter Olympics opening ceremony in PyeongChang, South Korea; Protests erupt following the Assassination of Jamal Khashoggi; March for Our Lives protests take place across the United ...

remains not determined to the genus level were described from the Agrio Formation, suggesting the possibility of

viviparity Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...

of these marine reptiles in the

epeiric sea An inland sea (also known as an epeiric sea or an epicontinental sea) is a continental body of water which is very large and is either completely surrounded by dry land or connected to an ocean by a river, strait, or "arm of the sea". An inland s ...

of the Neuquén Basin. The finds were notable as well because of a relative lack of abundance of ichthyosaur fossils from the Valanginian to Hauterivian worldwide.Lazo et al., 2018 Fossil fish of '' Gyrodus huiliches'', and '' Tranawuen agrioensis'' were described from the formation in

2019 File:2019 collage v1.png, From top left, clockwise: Hong Kong protests turn to widespread riots and civil disobedience; House of Representatives votes to adopt articles of impeachment against Donald Trump; CRISPR gene editing first used to experim ...

.Gouiric Cavalli et al., 2019 The decapod '' Palaeohomarus pacificus'',Aguirre Urreta et al., 2012 and ammonites '' Curacoites rotundus'' and '' Sabaudiella riverorum'' were described from the formation in

2012 File:2012 Events Collage V3.png, From left, clockwise: The passenger cruise ship Costa Concordia lies capsized after the Costa Concordia disaster; Damage to Casino Pier in Seaside Heights, New Jersey as a result of Hurricane Sandy; People gather ...

,Aguirre Urreta & Rawson, 2012 the gastropods ''

Ampullina pichinka ''Ampullina'' is an extinct taxonomic genus of deep-water sea snails, marine gastropod molluscs in the clade Caenogastropoda. These sea snails were epifaunal grazers. They lived from the Middle Triassic period to the Lower Pliocene age. Speci ...

'' and '' Mesalia? kushea'' in 2016,Cataldo & Lazo, 2016 and the ammonite '' Comahueites aequalicostatus'' in 2018.Aguirre Urreta & Rawson, 2018 Newly described species of fish, '' Tranawuen agrioensis'',

, '' Holcoptychites agrioensis'',Lazo, 2003 and the bivalve '' Pholadomya agrioensis'' were named after the formation. The first known brittle stars in the Southern Hemisphere and

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 ...

age have been identified in Agrio Formation.Fernández ''et al''., 2019 However, fossils are not complete enough to define species.

Petroleum geology

The Agrio Formation is considered the third-most important

of the hydrocarbon-rich Neuquén Basin, after the older

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 r ...

and

s.Geologic Map, 2007, p.142 Two levels of organic-rich sediments exist in the formation, related to the marine transgressions of the late Valanginian and the late Hauterivian, in the Pilmatué and Agua de la Mula Members respectively. The marly shales of the Pilmatué Member reach up to thickness in the western Neuquén Basin, while the same facies in the Agua de la Piedra Member is less than thick. The organic properties of the formation are similar to the Vaca Muerta, with a TOC value averaging 2.5%, with some levels up to 5%. The kerogen types are II to II/III.Voglino, 2017, p.39

Gallery

Fragmento de amonoide (Formación Agrio, Cretácico inferior, cerro Marucho).jpg, Ammonite from the Agrio Formation Bivalvo (Formación Agrio, Cretácico inferior, cerro Santa Elena).jpg, Bivalve from the formation Fosíl de Ptychomya koeneni.jpg, Bivalve ('' Ptychomya koeneni'')

References

Bibliography

;General * * * * * * * * * * * * ;Paleontology * * * * * * * * * * ;Geologic map * {{cite LSA , last=Rodríguez , first=María F. , last2=Leanza , first2=Héctor A. , last3=Salvarredy Aranguren , first3=Matías , year=2007 , title=Hoja Geológica 3969-II, NEUQUÉN, provincias del Neuquén, Río Negro y La Pampa 1:250,000 , url=https://www.researchgate.net/publication/263009433_Hoja_Geologica_3969-II_NEUQUEN_provincias_del_Neuquen_Rio_Negro_y_La_Pampa , publisher=Instituto de Geología y Recursos Minerales , pages=1–165 , accessdate=2019-02-23 Geologic formations of Argentina Lower Cretaceous Series of South America Cretaceous Argentina Aptian Stage Barremian Stage Hauterivian Stage Valanginian Stage Shale formations Conglomerate formations Sandstone formations Limestone formations Deep marine deposits Shallow marine deposits Deltaic deposits Fluvial deposits Reef deposits Source rock formations Shale gas Formations Geology of Mendoza Province Geology of Neuquén Province Geology of Patagonia