How far south did Cenozoic tropical carbonate platforms develop in the South Atlantic Ocean?

Tropical carbonate platforms accompanied the latitudinal shifts of the tropical belt throughout the Cenozoic. Their flat-topped geometries were influenced by a variety of processes, including climate and sea level changes, biofacies composition, tectonics and fluctuations in continental runoff. Within this scenario, tropical carbonate platforms grew across the global oceans between the Oligocene-Pliocene interval. Global evidence indicates that tropical platforms expanded toward higher latitudes in the early Miocene but underwent a collapse during the late Miocene and Pliocene. However, the occurrence of carbonate platforms in the South Atlantic during the Neogene is still poorly studied, creating a gap in the understanding of how warm periods influence the development of carbonate environments. Our study aims to characterize the sedimentological composition and depositional architecture of Neogene carbonates in the Santos Basin, to reconstruct the paleoenvironmental evolution and depositional history of South Atlantic tropical platforms across that interval. We use an extensive dataset of seismic data (2D and 3D) and wells to characterize the lithostratigraphy, the geometries (e.g. Isolated Carbonate Platform, rimmed shelf, homoclinal ramps and channels), and the build-up patterns (aggradation, progradation and backstepping) of carbonate platforms presented in the region. Lithostratigraphic results reveal the accumulation of a ~500 m thick calcarenite succession. Seismic interpretation shows that the shelf break was delineated by flat-topped platforms in the southern part of the Santos Basin (~28°S). Seismic analyses then indicate that middle Miocene smaller carbonate build-ups of up to 30 km in length coalesced via progradation into a 90 km long Quaternary carbonate platform. Within this interval, the transition from an aggradational to a progradational buildup is accompanied by a geometric transformation from isolated carbonate platforms to an extensive rimmed shelf. This change is supported by an isopach map which shows an intense sedimentary infilling of the inter-platform seaway and of the upper slope. In summary, the occurrence of Cenozoic carbonate platforms in the Santos Basin is divided into four stages: 1) onset of carbonate deposition occurred during the late Oligocene, driven by the late Oligocene Warming and the Oligocene-Miocene Transition which led to the establishment of a mixed siliciclastic-carbonate sedimentary environment; 2) carbonate buildups reached their aggradational peak during the middle Miocene Climate Optimum; 3) in the late Miocene, carbonate platforms evolved to a progradational stage in response to the long-term sea-level fall of the Miocene Climate Transition; 4) in the Quaternary, the development of mixed siliciclastic-carbonate homoclinal ramps buried the tropical carbonate platforms. This study reveals that the development and demise of tropical carbonate platforms in the Santos Basin were linked to global paleoclimate events, which also governed sea-level changes, the intensification of ocean currents, and sea-surface temperatures. Finally, the findings in this study demonstrate a 10° southward migration of tropical carbonate zones across the Santos Basin during the Neogene when compared to the modern, extending to 28°S latitudes whereas nowadays that environmental boundary is placed along the Abrolhos Shelf at 18°S.