The Cerro Azul Formation (Spanish: Formación Cerro Azul), also described as Epecuén Formation, is a geological formation of Late Miocene (Tortonian, or Huayquerian in the SALMA classification) age in the Colorado Basin of the Buenos Aires and La Pampa Provinces in northeastern Argentina.

The fluvial and aeolian siltstones, sandstones and tuffs of the formation contain many mammals, such as Thylacosmilus and Macrauchenia, reptiles, amphibians and fossils of terror birds as well as Argentavis, the largest flying bird ever discovered.

The Cerro Azul Formation crops out in patches in the southwestern Buenos Aires Province and southeastern Pampa Province. The Epecuén Formation has been correlated to the Cerro Azul Formation in the early 2000s. The Cerro Azul and Epecuén Formations were named after the Cerro Azul ("Blue Hill") and Epecuén Lake where the formation crops out. The formation overlies crystalline basement or the Arroyo Chasicó Formation. The mammal assemblage of the Cerro Azul-Epecuén unit is the most diverse for the Huayquerian Late Miocene age, possibly ranging into the Pliocene. The formation is considered contemporaneous with the Río Negro Formation of the Colorado Basin.

The unit is characterized by a monotonous succession of loess containing moderately developed paleosols. In particular, the formation is considered as representing the interval between 10 and 5.7 Ma. The maximum exposed thickness in outcrop is 54 metres (177 ft), although the unit reaches about 180 metres (590 ft) in the subsurface.

The Cerro Azul Formation deposits were described by Linares et al. in 1980. They are discontinuous along the whole occupied area in the provinces of La Pampa and Buenos Aires. They are composed of silts, sandy silts and very thin silty sands, reddish and brown colored, with a homogeneous and compact general aspect, and frequent carbonate nodules and evidences of pedogenic processes. Visconti et al. (2010) interpreted them as eolian deposits characterized by loessic materials, with a high percentage of lithic fragments and volcaniclastic sediments.

The sediments and their fauna belong to a sedimentary and faunal cycle, which followed the withdrawal (around 10 Ma) of a widespread marine transgression that extended from central Argentina, to western Uruguay and southern Paraguay and Brazil, the "Paraná Sea" or mar paranense in Spanish.

Large cylindrical sediment-filled structures, 115 of which interpreted as mammal burrows occur within the loess-paleosol sequence of the formation.

The following fossils have been recovered from the formation:

Locations:

La Pampa

Buenos Aires

Group	Clade	Taxa	Location	Images	Notes
Mammals	Carnivora	Cyonasua brevirostris	Salinas de Hidalgo
		C. prebrevirostris	Salinas de Hidalgo
		Cyonasua sp.	Telén
	Sparassodonta	Borhyaenidium musteloides	Salinas de Hidalgo
		Thylacosmilus atrox	El Guanaco Quehué Salinas de Hidalgo
		Borhyaenidae indet.	Telén
	Rodents	Abrocoma antiqua	Estancia Ré
		Cardiatherium aff. orientalis	Laguna Chillhué Laguna Guatraché
		Chasichimys bonaerense	Cerro Patagua
		C. scagliai	Laguna Chillhué
		Neocavia pampeana	Estancia El Recado
		Lagostomus pretrichodactyla	Salinas de Hidalgo
		Neophanomys pristinus	Estancia Don Mariano Quehué
		Palaeoctodon aff. simplicidens	Bajo Giuliani
		Pampamys emmonsae	Laguna Chillhué Bajo Giuliani Estancia Ré
		Phtoramys hidalguense	Barrancas Coloradas Salinas de Hidalgo
		Proctenomys simpsoni	El Guanaco Barrancas Coloradas
		Reigechimys octodontiformis	Bajo Giuliani Estancia Ré
		R. plesiodon	Laguna Chillhué
		R. simplex	Bajo Giuliani
		Tetrastylus araucanus	Salinas de Hidalgo
		Theridomysops parvulus	Telén
		Xenodontomys ellipticus	Grünbein Cantera Seminario Barrancas de Sarmiento
		Phtoramys cf. hidalguense	Grünbein Cantera Seminario
		Cardiomys sp.	Salinas de Hidalgo
		Chasichimys sp.	Bajo Giuliani
		Dolicavia sp.	Grünbein Cantera Seminario
		Lagostomus (Lagostomopsis) sp.	Grünbein Cantera Seminario Barrancas de Sarmiento Estancia Ré Salinas de Hidalgo
		Neocavia sp.	Grünbein Cantera Seminario
		Neophanomys sp.	Estancia Ré
		Orthomyctera sp.	Grünbein Cantera Seminario Salinas de Hidalgo
		Palaeocavia sp.	Grünbein Cantera Seminario Barrancas de Sarmiento Estancia Ré
		Phtoramys sp.	Estancia Ré
		Protabrocoma sp.	Telén
		Tetrastylus sp.	Telén
		aff. Clyomys sp.	Barrancas Coloradas
		Caviidae indet.	Barrancas de Sarmiento
		Ctenomyidae indet.	Telén
		Echimyidae indet.	Telén
		Hydrochoeridae indet.	Salinas de Hidalgo
		Octodontoidea indet.	Estancia Ré
	Xenarthra	Aspidocalyptus castroi	Robilote Field Salinas de Hidalgo
		Coscinocercus brachyurus	Robilote Field Salinas de Hidalgo
		C. marcalaini	Robilote Field Salinas de Hidalgo
		Hoplophractus tapinocefalus	Laguna de los Paraguayos Salinas de Hidalgo
		Plohophorus araucanus	Salinas de Hidalgo
		Proscelidodon gracillimus	Estancia Don Mariano Huayquerías de San Carlos Laguna Epecuén Bajo Giuliani Laguna Chillhué Loventué Salinas Grandes de Hidalgo Quehué Barrancas Coloradas Estancia Ré Arroyo Guaminí Laguna de los Paraguayos Carhué
		Aspidocalyptus sp.	Telén
		Coscinocercus sp.	Telén
		Eosclerocalyptus sp.	Laguna La Paraguaya
		Eumysops sp.	Laguna Chillhué
		Hoplophorus sp.	Telén
		Hoplophractus sp.	Telén
		Plesiomegatherium sp.	Salinas de Hidalgo
		cf. Aspidocalyptus sp.	Grünbein Cantera Relleno Sanitario Level 2
		cf. Pseudoplohophorus sp.	Grünbein Cantera Relleno Sanitario Level 2
		Glyptodontidae indet.	Laguna La Paraguaya Salinas Grandes de Hidalgo
		Mylodontidae indet.	Telén
	Cingulata	Chasicotatus ameghinoi	Laguna Chillhué Estancia Don Mariano Cerro Patagua Cerro El Chancho Cerro de los Guanacos Puesto Colorado Estancia Quiñi-Malal Bajo Giuliani El Guanaco Telen El Recado Laguna Guatraché Loventué Quehué Barrancas Coloradas Estancia Ré Salinas de Hidalgo
		Chasicotatus cf. peiranoi	Grünbein Cantera Relleno Sanitario Level 2
		Chorobates villosissimus	Laguna Chillhué Estancia Don Mariano Grünbein Cantera Relleno Sanitario Level 2 Puesto Colorado Ruta 14 Estancia Quiñi-Malal El Guanaco Telen El Recado Loventué Quehué Estancia Ré Salinas de Hidalgo
		Doellotatus chapadmalensis	El Recado Salinas de Hidalgo
		D. inornatus	Laguna Chillhué Estancia Don Mariano Grünbein Cantera Seminario Puesto Colorado Ruta 14 El Guanaco El Recado Laguna Guatraché Loventué Quehué Barrancas Coloradas Estancia Ré Telén Salinas de Hidalgo
		D. cf. praecursor	Grünbein Cantera Seminario
Macrochorobates chapalmalensis		El Guanaco Barrancas Coloradas
M. scalabrinii		Laguna Chillhué Estancia Don Mariano Telen Loventué Quehué Salinas de Hidalgo
Macroeuphractus morenoi		Grünbein Cantera Relleno Sanitario Level 2 Puesto Colorado Laguna Chillhué Telen El Recado Laguna Guatraché Loventué Quehué Barrancas Coloradas Estancia Ré Salinas de Hidalgo
M. retusus		Salinas de Hidalgo
Proeuphractus limpidus		Laguna Chillhué Telén
Vetelia perforata		Cerro Patagua Cerro de los Guanacos Telén
Zaedyus pichiy		Laguna Chillhué Telen Quehué
Chasicotatus sp.		El Recado
Chorobates sp.		Grünbein Cantera Seminario
Doellotatus sp.		Laguna La Paraguaya
Macrochorobates sp.		Grünbein Cantera Relleno Sanitario Level 2 El Recado
Paleuphractus sp.		Salinas de Hidalgo
Proeuphractus sp.		Laguna Chillhué Cerro Patagua Telen El Recado Laguna Guatraché Quehué Salinas de Hidalgo
Didelphimorphia	Hyperdidelphys pattersoni	El Guanaco Telén
	Thylamys pinei	Laguna Chillhué
	T. zetti	Estancia Ré Arroyo Guaminí
	Thylatheridium dolgopolae	El Guanaco Quehué Telén
	T. hudsoni	Bajo Giuliani Telén
	Zygolestes tatei	Bajo Giuliani
	Hyperdidelphys sp.	Salinas Grandes de Hidalgo
	Thylatheridium sp.	Estancia Ré Telén
	Didelphinae indet.	Telén
	Lutreolina sp.	Salinas de Hidalgo
	Sparassocynidae indet.	Telen
Ungulates	Diplasiotherium pampa	Bajo Giuliani Laguna Chillhué Salinas de Hidalgo
	Eoauchenia cingulata	Robilote Field Salinas de Hidalgo
	E. primitiva	El Guanaco
	Epecuenia thoatherioides	Robilote Field Salinas de Hidalgo
	Huayqueriana cristata	Robilote Field
	Macrauchenia latidens	Salinas de Hidalgo
	cf. Brachytherium cuspidatum	Telén Salinas de Hidalgo
	Neobrachytherium sp.	Bajo Giuliani Salinas de Hidalgo
	Promacrauchenia sp.	Grünbein Cantera Relleno Sanitario Level 2 Salinas de Hidalgo
	Proterotheriidae indet.	Bajo Giuliani Telén
	Litopterna indet.	Telén
Notoungulata	Hemihegetotherium achataleptum	Cerro Patagua Eduardo Castex Cerro de los Guanacos Puesto Colorado Bajo Giuliani Laguna Chillhué Loventué Salinas de Hidalgo
	H. lazai	Salinas de Hidalgo
	Paedotherium bonaerense	Barrancas de Sarmiento
	Paedotherium minor	Laguna La Paraguaya Estancia Ré Salinas de Hidalgo
	Paedotherium cf. minor	Grünbein Cantera Seminario
	Pisanodon nazari	Salinas de Hidalgo
	Pseudotypotherium carhuense	Robilote Field Salinas de Hidalgo
	Tremacyllus impressus	Grünbein Cantera Seminario Bajo Giuliani Laguna Chillhué Salinas Grandes de Hidalgo Quehué Barrancas Coloradas
	Typotheriodon grandis	Robilote Field
	Typotheriopsis minimus	Robilote Field
	Paedotherium sp.	Bajo Giuliani Laguna Chillhué Estancia Ré
	Mesotheriinae indet.	Laguna La Paraguaya Telén
Polydolopimorphia	Argyrolagus rusconii	Bajo Giuliani El Guanaco Barrancas Coloradas
	Argyrolagidae indet.	Telén
Paucituberculata	Pliolestes venetus	Bajo Giuliani Laguna Chillhué
Birds	Phorusrhacidae	Devincenzia pozzi	Carhué
		Procariama simplex	Salinas de Hidalgo
		Phorusrhacidae indet.	Laguna Guatraché Carhué
	Cathartidae	Argentavis magnificens	Carhué
	Tinamiformes	Eudromia sp.	Salinas de Hidalgo
		Nothura sp.	Bajo Giuliani El Guanaco Salinas de Hidalgo
	Rheidae	Rhea sp.	Salinas de Hidalgo
Reptiles	Squamata	Tupinambis sp.	Telén
	Testudines	Chelidae indet.	Telén
		Testudinidae indet.	Salinas de Hidalgo
Amphibians	Ceratophrynidae	Lepidobatrachus sp.	Quehué

Huayquerian correlations in South America

Formation	Cerro Azul	Ituzaingó	Paraná	Camacho	Raigón	Andalhuala	Chiquimil	Las Flores	Maimará	Palo	Pebas	Muyu	Rosa	Saldungaray	Salicas	Urumaco	Map
Basin	Colorado	Paraná				Hualfín		Tontal	Andes	Salta	Amazon	Huasi	Altiplano	BA	Velasco	Falcón	Cerro Azul Formation (South America)
Country	Argentina			Uruguay		Argentina					Brazil  Peru	Bolivia		Argentina		Venezuela
Cardiatherium
Lagostomus
Macroeuphractus
Proeuphractus
Pronothrotherium
Pseudotypotherium
Thylacosmilus
Xotodon
Macraucheniidae
Primates
Rodents
Reptiles
Birds
Terror birds
Flora
Environments	Aeolian-fluvial	Fluvio-deltaic			Fluvial						Fluvio-lacustrine	Fluvial			Fluvio-lacustrine	Fluvio-deltaic	Huayquerian volcanoclastics Huayquerian fauna Huayquerian flora
Volcanic						Yes

Geology

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Paleontology

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• Sostillo, Renata; Montalvo, Claudia I.; Verzi, Diego H. (2014), "A new species of Reigechimys (Rodentia, Echimyidae) from the Late Miocene of central Argentina and the evolutionary pattern of the lineage", Ameghiniana, 51 (4): 284–294, doi:10.5710/AMGH.24.04.2014.2741, retrieved 2019-02-12
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