Palaeoenvironment of the Miocene Pirabas Formation mixed carbonate–siliciclastic deposits, Northern Brazil: Insights from skeletal assemblages

Abstract

The Bragantina Platform is an onshore basin of the Brazilian equatorial margin, including the Pirabas Formation, the youngest bioclastic-dominated formation in the area (early–middle Miocene), testifying to the final demise of carbonate factories along this margin area and the transition to a siliciclastic-dominated sedimentation. Although the greatest part of the Pirabas Formation is overlain by the Barreiras Formation and the post-Barreiras deposits, the outcrops provide the opportunity to investigate these Neogene successions that are the onshore equivalent of the large offshore basins (e.g. Amapá Formation: Foz do Amazonas and Ilha de Santana Formation: Pará-Maranhão) of the margin. The latter are deemed to represent an important target for reservoir models. Aiming to thoroughly describe the Pirabas Formation deposits and provide a quantitative and simple approach to analysing the outcropping successions, this study examines seven different successions and a quarry using palaeontological and petrographical methods. Similar to the modern Brazilian equatorial margin, the siliciclastic fraction in the Pirabas Formation rocks decreases as the distance from the coast increases, whereas the bioclastic material is found in greater amounts offshore. However, while the carbonate production close to the Amazon River mouth currently occurs hundreds of kilometres offshore, the carbonate factories in coastal along the Pirabas platform are also located in coastal waters. This indicates that the terrigenous input over the carbonate rocks was still lower than that found at present. The combining analysis of the skeletal and foraminiferal assemblages enabled a separation of protected embayments (characterised by seagrass-related assemblages), exposed areas (characterised by bioclastic shoals) and mangrove forests (characterised by dark fine-grained sediments). This accurate approach serves as an unprecedented reference for the northeastern part of the South American equatorial margin and lays the foundation for future research.