Facies architecture and REE+Y constraints on the evolution of a rift-related lacustrine carbonate system (lower Cretaceous, Santos Basin, SE Brazil)

: Continental carbonate systems represent heterogeneous depositional environments shaped by autogenic and allogenic processes. As such, their imprint in the geological record remains poorly understood. The Barra Velha Formation (Lower Aptian) in the Santos Basin, particularly the Mero Field, provides a natural laboratory for investigating the dynamics of lacustrine carbonate systems, which record interactions among chemical precipitation, microbial activity, sedimentary reworking, and diagenetic overprinting. This study adopts a multiscale framework integrating petrographic analyses and elemental geochemistry (REE+Y and trace elements) to unravel the physicochemical conditions, sedimentary processes, and paleoenvironmental evolution of these carbonate successions. Shrubstone, spherulestone, mudstone, reworked facies (calcarenitic and calciruditic), and microbialitic facies were identified and organized into three main successions reflecting chemical precipitation, sedimentary reworking, and microbial growth. Stratigraphic variations in REE+Y systematics and trace-element signatures indicate fluctuations in water chemistry and hydrological conditions, episodic microbial activity, phases of chemical homogenization driven by sedimentary reworking, and increasing oxygenation during lake infilling. Recurrent terrigenous input and evidence for persistent hydrothermal influence, potentially operating during carbonate precipitation and subsequent diagenesis, are also documented. Multiple diagenetic processes, including mineral replacement, carbonate cementation, compaction, dissolution, and recrystallization, modulate primary geochemical signals. Integration of facies and geochemical data enables the recognition of five depositional intervals, whose architecture is primarily governed by tectonic activity and climatic variability, expressed through lake-level fluctuations, shifts in water chemistry, terrigenous input, microbial activity, and hydrothermal circulation. The Barra Velha Formation provides a key example of how alkaline lacustrine carbonate systems develop complex facies architectures and geochemical heterogeneity through the interaction of water chemistry, microbial activity, sedimentary processes, hydrothermal circulation, and diagenetic overprinting in tectonically active rift settings.