Sedimentary carbonate outcrops of the Pirabas Formation at the Bragantina Platform in North Brazil represent the exposed portion of the entire carbonate succession in the marine equatorial platform from the subsurface Ilha de Santana Formation (Cretaceous/Maastrichtian-Miocene/Aquitanian) in the Pará-Maranhão Basin and from the Formation Pirabas (Miocene/Aquitanian-Serravallian) in the Barreirinhas Basin, both from the Humberto de Campos Group. The transgressive deposit, flooding, and advance of the carbonate platform were investigated through the study of outcrops of the Pirabas Formation (type locality in the Ilha de Fortaleza, Pará state) and the analogous carbonate of the upper section of the Ilha de Santana Formation of the well-log 1- MAS-16-MA (510 to 660 meters below the seabed). The stratigraphic analyses were based on petrography, microCT, microfossil assemblages (foraminifers, ostracods, and bryozoans) and index species (Amphistegina, Archaias, Pyrgo, Quinqueloculina, Pirabasoporella, Nellia, Skylonia, and Alpheus), and biofacies approach. The boundary between the Ilha de Santana Formation (Aquitanian/Burdigalian at 510–660 m section of 1-MAS-16-MA) and the Pirabas Formation (Burdigalian/Serravallian at the Ilha de Fortaleza outcrop) suggests that shallow-water sedimentary facies are similar to those deposited in the marginal basins and marks the start of siliciclastic supplies to the inner platform and the reduction of coralline algal carbonate factories.
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