Miocene heterozoan carbonate systems from the western Atlantic equatorial margin in South America: The Pirabas formation

Outcrops of Neogene carbonates of the North Brazilian platform occur exclusively along the equatorial coast and represent some of the few existing examples of Neogene carbonate systems of the Atlantic coast of South America. The Pirabas Formation (early-middle Miocene) is the northernmost part of this platform. Although onshore mainly consists of small and scattered outcrops, it considerably extends in the subsurface keeping record of relevant geological and paleontological episodes of tropical South American history. Intending to improve the knowledge of South American carbonate and providing a solid basis for future comparisons between the Pirabas Basin and other, largely subsurface, Cenozoic basins, this research investigates the Aricuru outcrops by combining a standard petrographical and paleontological approach with advanced microCT analyses. The Aricuru area is characterized by mixed siliciclastic-carbonate sand-sized deposits, which probably deposited after the early Miocene according to the palynological assemblage. The bioclastic fraction of the rock is dominated by benthic foraminifera (mostly soritids, amphisteginids, small rotaliids, and small miliolids, typical of marginal marine environments), bryozoans, calcareous algae (Halimeda), echinoderms and mollusks. The abundant siliciclastic fraction together with the taxonomic composition of the foraminiferal, echinodermal, crustacean, ichnofossil and fish assemblages indicates deposition in a tropical coastal environment featuring both protected and more exposed sectors. The carbonate system probably developed under abundant nutrient supply, which fostered heterotroph suspension feeders over hermatypic corals. The demise of this system was most likely caused by a growth in siliciclastic input due to increased rainfall in the coastal area. The sedimentary evolution of the Pirabas Formation is similar to the one of the Foz do Amazonas Basin, and fits well with the general evolutionary trend of Cenozoic carbonate factories of the region, indicating the potential of the Pirabas subsurface record for understanding other Cenozoic basins, their paleoenvironmental significance, and their potential as oil, gas and water reservoirs.