Depositional model and sedimentation controls of a complex hybrid carbonate–siliciclastic ramp (Albian) – SW Campos Basin, Brazil

Hybrid deposits form from the interaction between carbonate deposition and coeval siliciclastic input, influenced by spatial and temporal dynamics. This interaction occurs across various mixing scales, resulting in a high vertical and lateral lithological variability. Understanding this heterogeneity is crucial for comprehending and modelling hybrid reservoirs. The Albian succession of the SE Brazilian margin records the early evolution of the South Atlantic Ocean. In the Campos Basin, this interval is represented by a retrograding succession of shallow-marine carbonates (Quissamã Formation) overlain by deep marine calcilutites and shales. We use cored wells, petrographic thin sections and geophysical logs to detail the Albian in the SW Campos Basin, to characterize its depositional model and stratigraphical succession. We identified four facies associations that define different parts of a hybrid carbonate–siliciclastic ramp: carbonate shoal (subdivided into shallower and deeper portions), hybrid carbonate-siliciclastic shoal, and coastal environment. Carbonate shoals are composed of calcarenites and calcirudites with oolites, oncolites, peloids, bioclasts and intraclasts. Hybrid shoals also present calcarenites and calcirudites but include a significant siliciclastic fraction. Coastal environments are dominantly composed of hybrid arenites, locally dolomitized. Carbonate-siliciclastic mixing occurs at different scales: compositional mixing at bed scale, and strata mixing at lithofacies and stratigraphic scales. This sediment mixture results from the interaction between waves, currents, base-level variation cycles, and salt tectonics. Isolith maps of siliciclastic and carbonate facies of the Quissamã Formation reveal four NE-trending shoals aligned on a NW-SE trend. Chronostratigraphic analysis indicates that the shoals were non-contemporary, with the most distal shoal forming earlier, indicating a retrograding, backstepping pattern for the Albian in the Campos Basin.