The formation is named after Guaratiba, a neighbourhood in the west of Rio de Janeiro.
Description
The Guaratiba Group is thick and subdivided into three formations, four if the underlying basalts of the Camboriú Formation are included, and five if the salt layer of the Ariri Formation is included, from old to young the Piçarras, Itapema, and Barra Velha Formations.
Subdivision
;Piçarras Formation The Piçarras Formation is thick and consists of clastic and carbonate rocks. The formation includes reddish polymictic conglomerates, with clasts of basalt and quartz in a clay-sandy matrix. It also includes white, reddish lacustrine coquinas and sandstones, siltstones and shales of stevensite composition. Its age, based on the ostracod assemblages, is Hauterivian to Aptian. The conglomerates and sandstones of the formation are representative of an alluvialenvironment. The coquinas represent a shallow lacustrine environment. Similar to the Atafona Formation of the Campos Basin, the sandstones, stevensite-bearing siltstones and shales represent an alkaline lacustrine environment affected by volcanic activity. The shales represent deeper lacustrine waters in more distal areas. The alternation of the two facies implies a series of alluvial progradation-retractions into the Early Cretaceouscarbonate lakes. The low textural and compositional maturity of conglomerates and sandstones implies the basin was supplied from areas close to the basin margins. ;Itapema Formation The Itapema Formation is several hundreds of metres thick and consists of calcirudites and dark shales. The calcirudite limestones consist of fragmented bivalve shells, frequentlydolomitized and silicified. In more distal sections, the formation consists of dark organic matterrich shales. In the well 1-RSJ-625, the formation includes of radioactive shales interbedded with carbonates. These facies are thought to represent a lacustrine environment. The organic matter-rich shales are one of the main source rocks of the Santos Basin. This formation is correlative with the Coqueiros Formation in the Campos Basin. The age of the Itapema Formation is Barremian to Aptian. ;Barra Velha Formation The Barra Velha Formation is approximately thick. In the proximal sections, the formation comprises limestones of stromatolites and laminated microbialites. In the distal sections, it is composed of shales. Interbedded with the laminated microbialites there are limestones with packstone and grainstone textures made up of algal clasts and bioclasts. The carbonates frequently are partly or completely dolomitized. These facies represent a transitional continental and shallow marine environment. The age of this formation has been estimated to be Late Barremian to Aptian. It is correlative with the Macabu Formation in the Campos Basin, as both are typified by laminated microbialites and stromatolites. These limestones are one of the sub-salt reservoirs in the Santos Basin, most notably of the giant Lula and initially reported as supergiant Lapa Fields.
Petroleum geology
The formation hosts the important pre-salt reservoir rocks of the Santos Basin.
