When distinction matters: Carbonate shrubs from the Aptian Barra Velha Formation of Brazilian's Pre-salt

Branched, bush- or shrub-like crystals are some of the most striking components of carbonate facies from the Cretaceous South Atlantic Pre-salt carbonate reservoirs. Many different analogues have been proposed to help to understand them, including travertines, lacustrine mounds and alkaline lake deposits, amongst others. Interpretation and genesis of these special facies involve discussions about biogenic vs. abiogenic formation of these branched components, relation with Mg-clays, role of microbes and growth of spherulites, amongst others. The present work shows some examples at microscale from a drilled core from Santos Basin (Brazil). The analyses of the shrub-like crystals present in the studied section allowed the distinction between four different types: (i) Type 1: fan-shaped fibrous-radial crystals; (ii) Type 2: fan-shaped, branched fibrous-radial crystals; (iii) Type 3: dendrite crystals; and (iv) Type 4: laminated crusts. All these components show similarities with their counterparts present in travertines, suggesting similar conditions controlling crystal growth in both Pre-salt and travertines. However, microfacies relationships show clear differences between the Pre-salt and those from travertine in the literature. Such Pre-salt microfacies presented here show a clear relationship between the development of the so called shrub-like crystals and the environment's energy, allowing the establishment of a proximal–distal pattern within a lacustrine environment as well as the identification of microfacies that could be related to (subaqueous?) hydrothermal vents. Distinction between the different types of branched components described here as well as their relationships with other microfacies are critical issues for the differentiation of the processes involved and the interpretation of the environments of formation (e.g., spring-related and proximal–distal environments within a lake). The observations made on samples from a drilled core of Brazilian's Pre-salt allow the discussion on some topics and interpretations involving the Pre-salt facies, providing potential research guidelines on these challenging continental carbonates.