Cerro Azul Formation, Geological formation in La Pampa and Buenos Aires provinces, Argentina

The Cerro Azul Formation is a geological layer in northeastern Argentina spanning La Pampa and Buenos Aires provinces. It consists of river deposits with siltstone, sandstone, and volcanic ash layers that together reach several hundred meters in depth.

This layer formed after the retreat of the Parana Sea roughly 10 million years ago and was first systematically documented by scientists in the 1980s. The deposits reveal important environmental changes across Argentina over extended geological time periods.

The names Cerro Azul ("Blue Hill") and Epecuén Lake refer to the outcrops where the formation appears. Its mammal assemblage is the most diverse known for the Huayquerian Late Miocene, and the unit is correlated with the Río Negro Formation of the Colorado Basin.

Researchers reach this layer via country roads connecting various study sites including Barrancas Coloradas, Cerro Patagua, and Estancia Re. The locations require good planning since they are spread out and clear orientation is necessary.

The formation's loess-paleosol sequence holds 115 large cylindrical structures interpreted as mammal burrows, filled with sediment. Its mammal assemblage is the most diverse known for the Huayquerian Late Miocene, possibly ranging into the Pliocene. The unit is correlated with the Río Negro Formation of the Colorado Basin.