Neogene isolated carbonate platform of the Rio Grande rise (southwest Atlantic ocean)

Abstract

Isolated carbonate platform (ICP) deposits have been extensively studied due to their high sensitivity to sea-level variations and hydrocarbon reservoir potential. Oceanic seamounts/plateaus such as the Rio Grande Rise (RGR) off south-east Brazil, are significant sites for investigating drowned ICPs, as they record the existence of former oceanic volcanic islands. The Geological Survey of Brazil dredged hundreds of samples from the RGR summit, comprising a condensed section with phosphatization, ferromanganese (Fe–Mn) crusts, and associated carbonate rocks. Our study investigates the sedimentary facies of carbonate substrate rocks collected from the borders of the Cruzeiro do Sul Lineament of RGR. Eight facies were identified: Planktonic foraminifera mudstone (Pfm), Planktonic foraminifera wackstone (Pfw), Bioclastic wackstone (Bw), Red algae packstone (Rp), Red algae grainstone (Rg), Foraminifera grainstone (Fg), Red algae bindstone (Rb), and Bioclastic rudstone (Br). Based on facies descriptions and fossil index ages, a paleoenvironmental reconstruction was proposed, indicating the development of a carbonate isolated platform during tectonic quiescence in the Oligocene-Miocene associated with a volcanic island that emerged during the RGR exposure in the Eocene. These deposits likely represent a weakly protected carbonate platform featuring a lagoon associated with barrier coralgal reefs, algal shoals, and patch reefs. A semi-drowning phase during the Pliocene-Pleistocene, characterized by the strong influence of ocean currents and deposition of mixed planktonic and shallow carbonate particles, ensued until complete drowning in the Pleistocene-Recent, marked by pelagic/reworked sedimentation. Platform shrinking and cessation may correlate with the Miocene onset of modern carbonate platforms influenced by currents, leading to the present-day substrate composed of carbonate and Fe–Mn duricrust covered by dunes of winnowed pelagic particles. The RGR's former ICP resembles the south-west Atlantic islands/atolls.