Paleoproterozoic proto-rift evolution in the Western São Francisco Paleocontinent, Central Brazil: the Água Morna Formation

The Água Morna Formation, the basal unit of the Araí Group in central Brazil, records an important episode of Paleoproterozoic–Mesoproterozoic intracratonic rifting within the São Francisco–Congo paleoplate and is overlain by the Arraias Formation. Despite its significance, its depositional environment, provenance, and geotectonic context remain poorly constrained due to limited geochronological and sedimentological data. This study integrates detrital zircon U–Pb geochronology, petrographic point-counting, and major element geochemistry to refine its depositional history and tectonic setting. Results indicate that the Água Morna Formation developed as a shallow intracontinental sag basin formed by flexural downwarping during the early Statherian proto-rift stage, with a maximum depositional age of 2039 ± 10 Ma. Field and petrographic evidence indicate deposition under continental conditions dominated by a sand-rich braided fluvial system, with planar and trough cross-stratification, massive to stratified sandstones, and local conglomeratic interbeds. Petrographic and geochemical data indicate overall mature quartz-rich sandstones, whereas localized pulses of less mature sediments reflect episodic input from proximal basement sources. Geochemical signatures marked by contrasting SiO₂, Al₂O₃, and TiO₂ contents confirm a mixture of mature and relatively immature detritus. Detrital zircon U–Pb ages indicate dominant Rhyacian sources in the Aurumina Terrane, with additional Siderian input from the Almas Terrane. An erosive unconformity at the top of the Água Morna Formation marks the transition to active rifting, represented by the overlying Arraias Formation, which records increased crustal thinning, brittle faulting, and limited volcanism. Overall, the integrated evidence confirms that the Água Morna Formation represents a proto-rift unit within a magma-poor rift system, consistent with modern rifted margin models. These results provide new constraints for regional correlations with other Statherian–Calymmian intracratonic basins in central and eastern Brazil and improve understanding of Paleoproterozoic–Mesoproterozoic extensional processes related to the Columbia supercontinent.