Integration of geochemical and geophysical data to characterize and map lateritic regolith: an example in the Brazilian Amazon

Abstract

In tropical regions like the Amazon, there are extensive lateritic covers that have not been properly mapped, despite their economic-bearing minerals and their importance related to denudation and regolith landscape studies. To consolidate tools for regolith cartography, geochemical and geophysical data (airborne gamma ray spectrometry and magnetometry) were integrated. Regional indexes (including the weathering intensity index, WII; lateritic index, LI; and mafic index, MI) were generated and applied, which allowed for the identification of regolith properties. The WII highlighted areas with high weathering levels located between 149 and 300 m and between 500 and 627 m of altitude that are related to the lower planation surface and upper planation surfaces, respectively. The LI ratified the WII and highlighted areas with high Th/K and U/K ratios, associated with lateritic duricrusts. The correlation between LI and MI showed lateritic duricrusts related to mafic and felsic substrates, especially on altitudes below 300 m, which confirmed the geochemical data. All of these results lead to the reinterpretation of areas previously considered to be sedimentary as residuals associated with oxisols and lateritic duricrusts, and this allows us to propose that the regolith mapping techniques and model generation (weathering intensity and lateritic indexes) have a good level of reliability.