Central Amazonia FR

The Central Amazonia area in northern South America where the Negro River and the Solimões River join to form the largest river on Earth (Amazon) near the city of Manaus is a key area to study the geological evolution of the area. This area is at the border between the western lowlands, where a wide Cenozoic to Quaternary basin was filled with Andean-derived sediments, and the eastern Precambrian shields and Paleozoic-Mesozoic sedimentary cover.

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Cretaceous Alter do Chão and Cenozoic Iranduba and Novo Remanso sandstones formations are particularly rich in supergene geological formations such as laterites.

Lateritic profiles are well developed over the Alter do Chão in Central Amazonia, where samples from laterites at different area in Central Amazonia have been collected. Deep profiles up to more than ten meters have been developed on this cretaceous sedimentary sandstone formation. On the panoramic photo, one can distinguish the rich white saprolite and the red ferruginous duricrust above.
Two main fieldtrips have been conducted in collaboration with the University of Brasilia and Sao Paulo, allowing to collect various samples from the saprolite rich in clays and iron-oxides to the iron lateritic duricrust in three different sites.

We conduct a complete study on the different samples to determine the mineralogy, elemental geochemistry, traditional (d)O-(d)D and non-traditional (d)56Fe and (d)30Si isotopic composition. The clays and iron oxides were also dated with EPR and (U-Th)/He methods. A first study that have measured the 30Si isotopic composition on dated kaolinite is just published by Guinoiseau et al. (2021). The study reveals that the clays present in this lateritic formed during two distinct weathering episodes. We used numerical models to show that the distribution of the isotopes of Si depends on the rapidity at which rainwater percolates downward through soil porosity. So, the combination of isotopic and dating techniques can reveal the duration and the environmental conditions prevailing during weathering episodes forming tropical soils.
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