Biostratigraphy and Paleoenvironments of the Pirabas Formation (Neogene, Pará State-Brazil)

The Pirabas Formation (Neogene, eastern Brazilian Amazonia) is the only marine Miocene unit in Brazil that crops out on the edge of the equatorial shelf. The formation has a distinctive mixture of carbonate and siliciclastic sediments. The paleodiversity of the deposit includes assemblages of foraminifera, ostracods, decapod crustaceans, mollusks, bryozoans, corals, echinoderms, fish, and sirenids. Microfossils are abundant and palynological studies can contribute to accurate paleoenvironmental reconstructions and biostratigraphic descriptions of the Pirabas formation. Here, we present 17 samples from eight outcrops and one quarry belonging to the Pirabas Formation with a goal to understand their depositional environments and relative ages. We recorded 9 species of dinoflagellate cysts, 7 spore species morphotypes, 59 pollen species morphotypes, 17 species of diatoms, and one species of silicoflagellate. We clustered the samples into two depositional groups, with one group formed by the Capanema quarry, Ponta do Castelo and Fazenda outcrops, deposited in a mesotrophic, shallow, marine environment, with a late early-Miocene age. The second one, formed by Atalaia, Aricuru, Maçarico, and Baunilha Grande outcrops, was deposited in a mesotrophic coastal environment with nearby mangroves; these samples may date to the late middle to late Miocene.