The Cenozoic deposits of the ancient landscapes of Quadrilátero Ferrífero highlands, Southeastern Brazil : sedimentation, pedogenesis and landscape evolution.

Abstract

The Quadrilátero Ferrífero (QF), Southeastern Brazil, is a very important tropical highland region in the world’s geological context for its large and diverse Archean and Proterozoic rocks with great reserves of gold, iron, manganese, aluminum and industrial rocks, on a apparently stable geological structure. There, Cenozoic deposits perched on highland valleys show unclear genetic relationships with the underlying bedrock, with no apparent regional correlation. We studied five representative Cenozoic deposits (BR356, Água Limpa, Padre Domingos, Pau Branco and Casa de Pedra) on the highlands of the Serra da Moeda syncline, Western QF, to answer the question of their sedimentological origin, and investigate their pedological evolution. Field sampling was complemented by macromorphological, mineralogical and micromorphological analysis supported by X-ray diffraction (XRD), differential thermal analysis (DTA), infrared analysis (IR), scanning electron microscopy

(SEM), micropobre and transmission electron microscopy (TEM). The evolution of these isolated highland de- posits comprises a deep-weathered source area, a tectonic activity, besides sedimentological and pedological

processes, during and after the deposition. The deposits overlie deep saprolites of Precambrian rocks (Piracicaba and Itabira Groups), representing unconformable contact. The onset of the deposition was marked by torrential, coarse colluvial and large blocks landslides into the small basins generated by reactivating tectonic events during the Oligocene, producing local grabens. These tectonic basins were filled by cohesive debris and mudflow from the adjacent and previously laterized cover, developed under the hot and humid climate in the Eocene. The cover reveals an upside-down lateritic profile where morphology and kaolinite crystal properties (values of size of coherent scattering thickness ranging from 135 Å to 162 Å) in the bottom is related to the pedolith horizons of the former lateritic cover. Later, due to climate changes (during and after the Miocene), renewed weathering on these pre-weathered sediments occurred, characterized by alternating ferruginization and Fe-losses, with the superimposed generation of new pedogenic features such as nodules, ferruginous duricrusts and mottling (redox features). In addition to demonstrating that the Brazilian platform was not stable during the Cenozoic, these deposits reveal the role of polygenetic tropical pedological processes in their formation and transformation.