(UTh)/He Geochronology of Fe and FeAl Lateritic Duricrusts and Bauxites in Central-Northern Amazonia: Implications for Paleoclimate and Landscape Evolution

Over the past 100 million years, intense lateritization has resulted in the widespread development of Fe, FeAl, and bauxite lateritic duricrusts across Amazonia. In Central-Northern Amazonia, Fe-rich lateritic duricrusts formed around 44 Ma, with peaks at 11–10 Ma and 5–4 Ma, while bauxite formation occurred between 25 and 15 Ma. Expanding this interpretation to the Amazonian region, two main bauxitization episodes have been identified: (i) between 30 and 15 Ma and (ii) post-15 Ma, most pronounced at ∼3–4 Ma. These lateritic duricrusts suggest that lateritization persisted from the Eocene to the Pliocene. This indicates a predominantly humid, well-drained climate conducive to goethite (bauxite) formation, overprinted by drier ferruginization pulses associated with global climatic fluctuations.

These lateritic duricrusts have acted as effective erosion-resistant surfaces, forming three distinct geomorphic levels. Surface 1 (S1, >240 m a.s.l.) and Surface 2 (S2, 100–240 m a.s.l.), characterized by extensive bauxite and FeAl lateritic duricrusts, developed at ∼20 Ma and between ∼17 and ∼ 11–5 Ma, respectively. Surface 3 (S3, <100 m a.s.l.) formed under intense ferruginous lateritization at ∼4 Ma. A progressive decrease in goethite ages and Al content from S1 to S3 reflects increasing landscape dissection. Furthermore, (UTh)/He data reveal a significant fluvial incision rate of 28 m/Ma, suggesting pronounced landscape rejuvenation, particularly between 10 and 3 Ma, which overprints older weathering phases preserved in southern Amazonia. Neotectonic activity facilitated the preservation of complete lateritic profiles on hilltops and plateaus, whereas truncated profiles with stone lines characterize valley slopes.