The formation and evolution of Al‐rich protoliths in the southern Ribeira belt, Brazil: Insights from remnants of a Precambrian paleoweathering surface

Abstract

AbstractBauxitic clays, clayey bauxites and bauxites are residual products formed from the weathering of aluminosilicate‐rich parent rocks under wet and warm conditions. These deposits show a complex interplay between remobilisation of pre‐existing kaolinitic soils and depositional micro‐environments.In the southern Ribeira belt, Brazil, a paleoweathering surface developed on the continental shelf of the Capiru Group passive margin strata and on its basement/source rocks. This weathering cycle gave rise to a paleosol profile on the continental domain and a karstic paleotopography on the emerged carbonate shelf. The surface and karstic nets were later caped and filled with Al‐rich materials, typical of remobilised soils, bauxitic clays, clayey bauxites and pods of karst‐type bauxites.The age of the disconformity is Pre‐Brazilian/Pan‐African, implying a Neoproterozoic age for the weathering surface. In this study, we investigate the relationship among disconformities, paleosols and karst topography to better understand the stratigraphic control, chemical composition and mineralogy of the Al‐rich materials. We interpret this horizon as the remnants of a (meta‐)paleoweathering surface that can serve as a valuable key bed for stratigraphic correlation and paleogeographic reconstructions.We propose that the triggering process for the genesis of the paleosol profiles, bauxitic material and bauxite pods was a coupled mechanism between favourable climate and uplift along the northern margin of the Curitiba–Angola craton, creating a karstic topography that allowed the down‐plain transport of Al‐rich materials. In a later stage, these deposits were metamorphosed, developing a series of Al silicates in unusual paragenesis, typical of Al‐rich protoliths.The presence of these paragenesis, along with the recognition of the disconformity–paleoweathering surface pair, could be a useful tool to understand the paleoenvironmental and paleogeographic evolution of highly deformed terrains.