Cerro Azul Formation

The Cerro Azul Formation (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.

Description
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 m, although the unit reaches about 180 m 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.

Fossil content
The following fossils have been recovered from the formation:

Locations:
 * La Pampa


 * Barrancas Coloradas - Cerro Azul Formation - La Pampa
 * Caleufú - Cerro Azul Formation - La Pampa
 * Cerro El Chancho - Cerro Azul Formation - La Pampa
 * Cerro Patagua - Cerro Azul Formation - La Pampa
 * Estancia Don Mariano - Cerro Azul Formation - La Pampa
 * Estancia El Recado - Cerro Azul Formation - La Pampa
 * Estancia Ré - Cerro Azul Formation - La Pampa
 * El Guanaco - Cerro Azul Formation - La Pampa
 * Huayquerías de San Carlos - Cerro Azul Formation - La Pampa
 * Laguna Chillhué - Cerro Azul Formation - La Pampa
 * Laguna Guatraché - Cerro Azul Formation - La Pampa
 * Quehué - Cerro Azul Formation - La Pampa
 * Salinas Grandes de Hidalgo - Cerro Azul Formation - La Pampa
 * Telen - Cerro Azul Formation - La Pampa


 * Buenos Aires


 * Bajo Giuliani - Cerro Azul Formation - Buenos Aires
 * Barrancas de Sarmiento - Cerro Azul Formation - Buenos Aires
 * Eduardo Castex - Cerro Azul Formation - Buenos Aires
 * Estancia los Médanos - Cerro Azul Formation - Buenos Aires
 * Estancia Quiñi-Malal - Cerro Azul Formation - Buenos Aires
 * Grünbein Cantera Seminario - Cerro Azul Formation - Buenos Aires
 * Grünbein Cantera Seminario Level 2 - Cerro Azul Formation - Buenos Aires
 * Laguna Epecuén - Cerro Azul Formation - Buenos Aires
 * Laguna La Paraguaya - Cerro Azul Formation - Buenos Aires
 * Puesto Colorado - Cerro Azul Formation - Buenos Aires
 * Ruta 14 - Cerro Azul Formation - Buenos Aires
 * Arroyo Guaminí - Epecuén Formation - Buenos Aires
 * Carhué - Epecuén Formation - Buenos Aires
 * Laguna de los Paraguayos - Epecuén Formation - Buenos Aires
 * Robilote Field - SE of Epecuén lagoon & E of Laguna de Epecuén - Epecuén Formation - Buenos Aires
 * Salinas Grandes de Hidalgo - Epecuén Formation - Buenos Aires