The formation reaching a thickness of 100 metres (330 ft) is the defining unit for the Itaboraian South American land mammal age (SALMA), dating to the Early Eocene, approximately 53 to 50 Ma.

The up to 100 metres (330 ft) thick formation has provided many fossil mammals of various groups among which the marsupials and related metatherians dominate, birds, snakes, crocodiles, amphibians, and several species of gastropods.

[5] Between 1933 and 1984, a local cement company exploited the rocks in the area and their workers discovered the first fossil remains in the formation.

[6] The now abandoned and largely inaccessible limestone quarries of this locality have yielded a diverse mammalian fauna from early late Paleocene fissure fillings.

Another very important source of data is palynological analysis of a coal-bearing horizon (lignite) interlayered with alluvial fan deposits at the northern border of the Itaboraí Basin, suggesting a Paleocene to Eocene age.

[24] Despite its relatively small size, the São José de Itaboraí Basin comprises a diversified fossil assemblage.

[7] The relative fossil diversity of the Itaboraí Formation at family level consists of 44% mammals, 23% mollusks, 14% reptiles (lizards, chelonians, crocodyliforms), 7% birds, 5% amphibians and 7% plants.

The records of Itaboraí are the oldest for the genera Austrodiscus, Brachypodella, Bulimulus, Cecilioides, Cyclodontina, Eoborus, Gastrocopta, Leiostracus, Plagiodontes and Temesa.