The basin stretches across an area of approximately 180,000 square kilometres (69,000 sq mi), of which 37,000 square kilometres (14,000 sq mi) onshore in the southern Buenos Aires Province and the easternmost Río Negro Province extending offshore in the South Atlantic Ocean.

The Middle to Late Miocene Gran Bajo del Gualicho Formation contains vertebrate fossils of the cetacean Preaulophyseter gualichensis.

The Colorado Basin stretches across an approximate area of 180,000 square kilometres (69,000 sq mi) with about 37,000 square kilometres (14,000 sq mi) onshore, underlying the southernmost Buenos Aires Province and the southeasternmost Río Negro Province.

[9] The basin started forming in the Middle to Late Jurassic with the break-up of Pangea and the formation of the Southern Atlantic.

[14] The early Mesozoic succession is missing, and the main rifting phase happened in the Barremian and Aptian, around 120 million years ago, represented in the Fortín Formation.

[15] The post-rift sag stage dates to the early Late Cretaceous leading to the deposition of the alluvial and fluvial sediments of the Colorado Formation.

[14] The later Cenozoic succession is characteristic of a terrestrial passive margin setting and comprises the Paleocene Elvira,[14] Middle Miocene Barranca Final Formation,[16] and the Middle to Late Miocene Gran Bajo de Gualicho Formation.

[20] This sequence is covered by the Late Miocene to Early Pliocene eolian and fluvial Río Negro Formation,[21][22] outcropping in a thin band along the eponymous river.

[26] The Cerro Azul Formation contains fossils of the rodents Chasichimys bonaerense,[27] Neocavia pampeana,[28] Reigechimys plesiodon and R. simplex,[29] the armadillos Chasicotatus ameghinoi,[23] and Macrochorobates scalabrinii,[27] and the opossum Zygolestes tatei,[30] among other mammals.

[31] The marine Gran Bajo de Gualicho Formation contains many bivalve, gastropod and echinoid fossils,[32] and the cetacean Preaulophyseter gualichensis.