Understanding the origin of the airborne geophysical signatures of the Jalapão geomorphological units, Urucuia Basin, Central–Northern Brazil

Abstract

This study was carried out in order to understand the airborne geophysical signatures observed in the Jalapão region, Urucuia Basin, eastern Tocantins state. Airborne geophysical data (magnetic and gamma-ray spectrometry) and satellite images (Advanced Spaceborne Thermal Emission and Reflection Radiometer – ASTER) were processed and interpreted. I It was possible to characterize the main geomorphological units of the Jalapão region (residual landforms and dissected surfaces). The residual landforms (e.g., Serra do Espirito Santo and Occidental Bahiano Plateau) have high concentrations of equivalent U (eU) and equivalent Th (eTh), and low K concentrations. ASTER image analysis suggests that these concentrations can be correlated with the products of chemical weathering (Fe oxides, clay, and weathering-resistant opaque minerals and other accessories). The dissected surfaces (mainly Jalapão Surface), however, is deficient in these radioelements, probably due to the quartz-rich nature of the related soils. Furthermore, the Jalapão Surface is practically non-magnetic, whereas residual landforms exhibit magnetic signals of low amplitude and high frequency, and are of unknown origin. The results suggest that the sources of these magnetic anomalies are shallow (≈ 270 m) and could be associated with ferrimagnetic minerals present in the alteration zone and/or in the ferruginous cements of the Serra das Araras Formation sandstones. The depth to basement in the region of this basin (1.44 km) rules out the possibility that the magnetic sources could be related to residual landforms.