The Onzole Formation is an Early Pliocene (Montehermosan to Chapadmalalan in the SALMA classification) geologic formation in the Borbón Basin of northwestern Ecuador.

The formation consists of a shallow marine sandstone member containing many fish fossils, among which megalodon, and a deep water member comprising tuffaceous shales and mudstones containing gastropods, bivalves and scaphopods.

[1][2] The formation consists of the Esmeraldas Member, which is a unit comprising highly foraminiferal tuffaceous calcareous shale.

[3] The Esmeraldas Member is covered by the Súa Member comprising burrowed, bioturbated, silty sandstones deposited in a coastal environment.

The formation has provided bivalve, gastropod, and scaphopod fossils and the following vertebrates:[4][5][6]