Microbialites and Biosignatures in the Barra Velha Formation, Santos Basin—Brazil

The Barremian-Aptian carbonate succession of the pre-salt interval in the Santos Basin has been extensively studied since the discovery of major hydrocarbon accumulations. In this context, lacustrine carbonates have been commonly attributed to physicochemical processes due to limited evidence of microbial activity. However, core and sidewall samples from the Barra Velha Formation (Mero Field) revealed a thick microbialite interval, which contrasts with previous interpretations and highlights the need to reassess microbial contributions. Using an integrated multiscale approach combining petrographic and lithogeochemical analyses, this study identifies biosignatures and evaluates microbial roles in facies development and taphonomic pathways. Reticulated microbialite, thrombolitic, microcolumnar stromatolitic, dendritic tufa-like, and microbial mat–related facies were documented, representing distinct paleoenvironmental conditions and physicochemical–microbial interactions. Microbial control on carbonate precipitation occurred through (i) bioinduced processes, related to microbial metabolism, extracellular polymer secretion, and organic matter degradation, and (ii) bioinfluenced processes, where organic matrices conditioned mineral morphology and composition without direct metabolic mediation. Early taphonomic processes, including encrustation, impregnation, and replacement of organic matrices by calcite and amorphous silica, were crucial for biosignature preservation. These results demonstrate that microbial activity strongly influenced the facies architecture of the Barra Velha Formation, revealing a complex interplay between biological and physicochemical processes.